CN1610850B - Free space optical system for wavelength switching and spectral monitoring applications - Google Patents

Abstract

The present invention provides a novel method and apparatus for using wavelength dispersive devices, such as diffraction gratings (220), to spatially separate a multi-wavelength optical signal and a reference signal by wavelength into a plurality of spectral channels and a reference spectral component in a spectral array, and the plurality of spectral channels and a reference spectral component having a predetermined relative alignment. By aligning the reference spectral components at a predetermined location, a plurality of spectral channels may be simultaneously incident at a designated location, such as an array of beam receiving elements distributed according to the spectral array. The reference spectral content may further be maintained at a predetermined position by means of servo control (260), thereby ensuring that the plurality of spectral channels remain aligned at the specified position. The present invention can be used to build a new family of servo-based optical systems, including spectral power monitors and optical multiplexers/demultiplexers, for WDM optical networking applications.

Description

Free-Space Optical Systems for Wavelength Switching and Spectrum Monitoring Applications

Cross References to Related Applications

This application claims priority to U.S. Patent Application No. 09/961,565, filed September 20, 2001; this application claims priority to U.S. Patent Application No. 09/992,778, filed November 13, 2001, which is A continuation of U.S. Patent Application No. 09/961,565; and this application claims priority to U.S. Patent Application No. 10/022,303, filed December 14, 2001, which is a continuation of U.S. Patent Application No. 09/992,778 parts, and all of the above patent applications are hereby incorporated by reference in their entirety.

field of invention

The present invention relates generally to optical systems. More specifically, the present invention relates to free-space optical systems for wavelength switching and spectral power monitoring applications, with alignment compensation and polarization diversity schemes. The present invention can be used to build a variety of optical devices, such as spectral power monitors, multiplexers and demultiplexers, and optical add/drop multiplexers, all of which are suitable for WDM optical networking applications, and can be implemented through hardware and /or software control actively aligns them, and they further may use polarization diversity schemes.

Background of the invention

As all-optical communication networks become more common, an urgent problem for manufacturers of optical networking equipment is to provide robust, versatile, and cost-effective optical components and subsystems.

Existing optical communication networks usually use wavelength division multiplexing (WDM), because it enables multiple information (or data) channels to be transmitted simultaneously on one optical fiber by using different optical wavelengths, thereby significantly improving The information bandwidth of optical fiber. The ubiquity of WDM creates a special need for a series of optical systems that can decompose a multi-wavelength optical signal into a spatial array of spectral channels by wavelength so that these spectral channels can be analyzed by an array of optical power sensors. Probing individually, as in the case of a spectrum monitor; directing these spectral channels to an array of input/output ports (i.e., fiber optics), as in the case of an optical multiplexer/demultiplexer; or by microreflection The mirror array dynamically routes the channels according to a predetermined scheme. In such an optical system, it is important to maintain the necessary alignment between the spectral channel and the designated beam receiving device (i.e. optical power sensor or micromirror) during operation, and to maintain the necessary alignment for e.g. thermal and mechanical disturbances Robustness to environmental influences.

However, in the art, traditional optics typically use precision alignment, which means tight manufacturing tolerances and painstaking alignment efforts during assembly, making these devices expensive, bulky and complex to operate . Furthermore, no measures are provided for maintaining the necessary alignment during operation; and no mechanisms are implemented for overcoming alignment shifts due to environmental influences such as thermal and mechanical disturbances. Taken together, these drawbacks make existing optical devices expensive, bulky and complex to operate, as well as prone to performance degradation.

In order to meet the requirements of high bandwidth (capacity), Dense Wavelength Division Multiplexing (DWDM) has also been widely used in optical communication networks. Accompanying the application of DWDM technology is the need for a new generation of optical components and subsystems, including optical spectrum (or channel) power monitors. A particularly desirable feature of these new optical spectral power monitors is the ability to resolve multiple spectral channels occupying a wide spectral range (eg, C-band or L-band) and increasingly narrowly spaced in frequency (eg, 50 or 25 GHz). It is also desirable that these optical frequency power monitors have fast response times, robust performance, and cost-effective construction.

Conventional spectral power monitors typically use a structure in which a diffraction grating separates a multi-wavelength optical signal into a spatial array of spectral channels by wavelength and is incident on an optical power sensor array. By detecting the electrical signal generated by the optical power sensor, the optical power spectrum of the multi-wavelength optical signal can be obtained. To provide increased spectral resolution in such a system, a diffraction grating with sufficient dispersion capability is required. However, highly dispersive diffraction gratings known in the art, such as holographic gratings, are polarization sensitive, which makes them unsuitable for optical spectral power monitors using the structures described above.

In view of the above problems, it would be a desirable and significant advance in the art to provide a new series of optical devices which, in a simple, robust and cost-effective construction, have the ability to be Actively controlled light alignment, and/or they use polarization diversity schemes to reduce and/or eliminate polarization sensitivity.

Summary of the invention

The present invention provides optical systems using active alignment compensation and polarization diversity schemes. The present invention may incorporate optical devices such as spectral power monitors, multiplexers and demultiplexers, and optical add/drop multiplexers, all of which are suitable for WDM and DWDM optical networking applications and may be controlled by hardware and/or software They are actively aligned, and/or they can use polarization diversity schemes.

In one embodiment, the present invention provides a method and apparatus for servo-based spectral array alignment in an optical system. The optical device of this embodiment of the present invention includes an input port for providing a multi-wavelength optical signal and a reference signal; a wavelength disperser for spatially separating the multi-wavelength optical signal and the reference signal according to wavelength Separated into a plurality of spectral channels and a reference spectral component in a spectral array, and the plurality of spectral channels and a reference spectral component have a predetermined relative arrangement; a beam receiving array includes a reference wavelength sensing element and multiple a beam receiving element, they are located in a specific position so as to receive the corresponding reference spectral component and spectral channel; and a servo control unit, used to keep the reference spectral component at a predetermined position on the reference wavelength sensing element, thereby ensuring the Specific alignment between spectral channels and beam receiving elements.

In the present invention, a "spectrum channel" is characterized by a unique center wavelength and associated bandwidth, and can carry a unique information signal such as in WDM optical networking applications. A "reference signal" (and correspondingly a "reference spectral component") generally refers to any optical signal characterized in that it has a defined (and stable) center wavelength. Also, the terms "reference signal" (or "reference spectral component") and "calibration signal" (or "calibration spectral component") may be used interchangeably in this description.

A beam receiving element is to be understood broadly in the present invention as any optical element corresponding to at least one spectral channel. By way of example, the beam receiving element may be an optical power sensor, an optical fiber, a micromirror, a converging lens or a light modulator. The beam receiving elements may be configured to have a one-to-one correspondence with spectral channels. It is also possible to configure the beam receiving elements such that each subset of beam receiving elements corresponds to a plurality of spectral channels.

In the spectrum power monitoring device of the present invention, the light sensing array (i.e. photodiode array) can be configured in such a way that the power level of the spectrum channel incident on the photodiode array can be related to the power level by a predetermined conversion matrix thereby The resulting electrical signal, which can be obtained by calibration of this matrix. Also, selected two (or more) adjacent channel sensing elements in the light sensing array may be utilized in such a way as to provide a reference position sensing element.

In one embodiment of the invention, the alignment compensation unit is servo-based, and one form of the alignment compensation unit may comprise an alignment adjustment element and a processing element for adjusting the alignment of the spectral channel and the reference spectral component. The alignment adjustment element may be an actuation device coupled to the light sensing array for moving the array, thereby adjusting the relative alignment between the spectral array and the underlying light sensing array. The processing element is used to monitor the real-time incidence position of the reference spectral component incident on the reference position sensing element and provide control of the alignment adjustment element accordingly. The alignment compensation unit maintains the reference spectral component at a predetermined position on the reference position sensing element through servo control, thereby ensuring the necessary alignment between the spectral array and the photo-sensing array below it. Such a servo-based alignment compensation unit allows the spectral power monitoring device of the present invention to actively correct for any shift in alignment that may occur during operation (e.g. caused by environmental disturbances such as thermal and mechanical disturbances). impact), thereby increasing the robustness of the device. An additional benefit of using such a servo-based alignment compensation unit is the relaxed manufacturing tolerances and precision during initial assembly, which makes the spectral power monitoring device of the present invention structurally simpler and more cost effective.

Alternatively, the alignment adjustment element may be a beam control device, such as a dynamically adjustable mirror with an input port and a wavelength disperser in optical communication, for adjusting the alignment of the input multi-wavelength optical signal and the reference signal. The alignment adjustment element may also be an actuation device coupled to the wavelength disperser (eg a diffraction grating) for moving (eg rotating) the wavelength disperser and thereby adjusting the alignment of the spectral channels and the reference spectral components. In case a converging lens is used as beam concentrator in the optical arrangement of the invention, alignment adjustment elements may also be present in the form of suitable drive devices coupled to this converging lens for controlling the spectral channel and reference spectral components The incident position on the beam receiving array.

Furthermore, the optical device of the present invention can use one or more auxiliary reference signals and corresponding auxiliary reference wavelength sensing elements to compensate for the above-mentioned functions of the reference spectral components. Therefore, the servo control unit may advantageously utilize a combination of the above alignment adjustment methods to actively control the position and spacing of the spectral array, thereby ensuring a more robust alignment between the spectral channels and the respective beam receiving elements.

In an alternative embodiment of the invention, the alignment compensation unit is software based and may take the form of a signal processor in communication with the light sensing array. The alignment compensation unit comprises a predetermined calibration table comprising a plurality of transformation matrices, each transformation matrix correlates the electrical signal output from the light sensing array with respect to the incident spectral channel at a particular incidence position of the reference spectral component Power level. The alignment compensation unit monitors the real-time incidence position of the reference spectral component incident on the reference position sensing element. At each incident position of the reference spectral components thus detected, the alignment compensation unit processes the electrical signals generated by the spectral channels incident on the light sensing array and looks up the corresponding transformation matrix from the calibration table, thereby providing the spectral The power level of the channel. The spectral power monitoring device thus constructed effectively compensates for any shifts in alignment that may occur during operation by means of software control, without involving arbitrary "moving" drives. This makes the spectrum power monitoring device of the present invention have a simpler structure and more robust performance.

The spectral power monitoring apparatus of the present invention may further employ a polarization diversity scheme for attenuating any undesired polarization-dependent effects caused by one or more polarization-sensitive elements in the system. This can be achieved by placing a polarization splitting element (such as a polarization beam splitter) and a polarization rotating element (such as a half-wave plate) along the optical path between the input port and the wavelength disperser. The polarization splitting element is used to split the input multi-wavelength optical signal (and reference signal) into first and second polarization components, and the polarization rotation element in turn rotates the polarization direction of the second polarization component by 90 degrees. For example, where the wavelength disperser is a diffraction grating that provides higher diffraction efficiency for the p (or TM) polarization (perpendicular to the concave lines of the grating) than for the s (or TE) polarization (orthogonal to the p polarization) In the case of , the above-mentioned first and second polarization components correspond to the p-polarization and s-polarization components of the multi-wavelength optical signal (and reference signal), respectively. The wavelength disperser splits the first and second polarization components by wavelength into first and second sets of beams, respectively, which are then incident on the light sensing array. The first and second light beams (from both polarization components) associated with each spectral channel may be incident on the light sensing array at substantially the same location. Such a polarization diversity scheme has the advantage of maximizing diffraction efficiency and thus minimizing insertion loss of the system.

The application of a servo control unit and/or active alignment compensation hardware and/or software enables the optical device of the present invention to actively correct alignment shifts caused by, for example, thermal and mechanical instabilities during operation, And thus more robust in performance. An additional benefit of using such alignment compensation is relaxed manufacturing tolerances and precision during initial assembly, which allows for a more adaptable and cost-effective construction of the optical device of the present invention.

According to an aspect of the present invention, a method for performing spectral alignment of multi-wavelength optical signals is provided. The method of the present invention makes it necessary to combine a multi-wavelength optical signal and a reference signal; separate the multi-wavelength optical signal and the reference signal into a plurality of spectral channels and a reference spectral component according to the wavelength, wherein the plurality of spectral channels and the reference spectral the components have a predetermined relative arrangement; causing the reference spectral component to be incident at a predetermined position so that the spectral channel is incident at the designated position according to the predetermined relative arrangement; and maintaining the reference spectral component at the predetermined position by servo control, This ensures that the spectral channels remain aligned at the designated locations.

In the above method of the present invention, a servo control mechanism can be implemented by monitoring the real-time incident position of the reference spectral component and adjusting the alignment of the reference spectral component and the spectral channel accordingly, so that the incident position of the reference component remains at a predetermined position and the spectrum channels remain at their respective assigned locations.

The method of the present invention may further comprise the step of converging the spectral channels and reference spectral components to corresponding converging points. The method of the present invention may additionally comprise the step of optically detecting the spectral channel at a designated location so as to provide a power spectrum of the detected spectral channel; the step of redirecting the spectral channel so as to dynamically route the spectral channel according to a predetermined scheme ; or the step of modulating one or more characteristics of the spectral channel.

According to one aspect of the present invention, a method of using software-based alignment compensation in spectral power monitoring is provided. The method comprises the steps of: providing a multi-wavelength optical signal and a reference signal; spatially separating the multi-wavelength optical signal and the reference signal into a plurality of spectral channels and a reference spectral component according to wavelengths, wherein the plurality of spectral channels The reference spectral component and the reference spectral component have a predetermined relative arrangement; the reference spectral component and the spectral channel are incident on the optical power sensor array; and the incident position of the reference spectral component is determined, and a corresponding transformation matrix is searched from a predetermined calibration table, wherein the A matrix relates output signals from the optical power sensor array to power levels of spectral channels incident on the optical power sensor array, thereby providing a power spectrum of the multi-wavelength optical signal.

According to another aspect of the present invention, a scheme for spectrum power monitoring using a polarization diversity scheme is provided. The optical spectrum power monitoring device of the present invention comprises an input port for a multi-wavelength optical signal; a polarization splitting element for separating the multi-wavelength optical signal into first and second polarization components; a rotation of the second polarization component a 90 degree polarization rotation element; a wavelength disperser that separates the first and second polarization components into first and second sets of light beams, respectively, by wavelength; and positioned to receive the first and second sets of light beams The optical power sensor array (herein referred to as "optical sensing array").

In cases where it is desired that the first and second light beams associated with the same wavelength be incident on the light sensing array at substantially the same location (or within the same optical power sensor), an additional polarization rotation can be employed elements such that the first and second sets of light beams have orthogonal polarization directions when incident on the light sensing array. This eliminates any intensity interference fringes caused by the spatial overlap of the beams. The auxiliary polarization rotation element can be placed between the wavelength disperser and the light sensing array such that the polarization directions of the first and second sets of light beams are both subjected to a 90 degree rotation before being incident on the light sensing array.

Alternatively, a modulation component may be used in the present invention for modulating the first and second sets of light beams before they are incident on the light sensing array. The first and second sets of light beams may be modulated to reach the light sensing array in a time-division multiplexed sequence. Alternatively, the first and second sets of light beams may be modulated in a frequency division multiplexed manner such that the first and second sets of light beams incident on the light sensing array carry different "dither" modulated signals. In either of the above two cases, the use of such a modulating component allows the first and second sets of light beams to be detected separately, whereby the correlation to each positive signal in the input multi-wavelength optical signal can be obtained independently. Optical power spectrum (optical power level as a function of wavelength) of the intersecting polarization components. The modulation component can be placed in the optical path between the polarization splitting element and the polarization rotator element and the wavelength disperser, thereby controlling the first and second polarization components. Alternatively, the modulation component may be placed between the wavelength disperser and the light sensing array, thereby controlling the first and second sets of light beams.

The modulation component may comprise a liquid crystal shutter element, a MEMS (micro-electromechanical system) shutter element, or an electro-optical intensity modulation element as known in the art. The modulation component may also be provided by an optical chopper, such as a rotating plate provided with at least one aperture, configured to introduce different modulations to the two incident optical signals.

The application of the above polarization diversity scheme enables the optical spectrum power monitoring device of the present invention to minimize the insertion loss while utilizing a simple and cost-effective structure (such as by advantageously utilizing the highly dispersive diffraction grating commonly available in the art ) provides enhanced spectral resolution. Furthermore, by introducing different modulations to the first and second sets of light beams before they are incident on the light sensing array, it is possible to separately determine the optical power spectrum associated with each polarization component in the input multi-wavelength optical signal, which in may be desirable in some applications.

Likewise, a series of novel servo-based optical systems, including spectral power monitors and optical multiplexers/demultiplexers, can be constructed according to the present invention to meet the challenging requirements of optical networking applications.

The novel features of the invention, as well as the invention itself, can be understood from the following drawings and detailed description.

Brief description of the drawings

Figure 1 provides an illustration of an exemplary power spectrum of a reference spectral component and a plurality of spectral channels in accordance with the present invention;

2A-2D describe a first embodiment of an optical device according to the invention;

3A-3B describe second and third embodiments of an optical device according to the invention;

4A-4C describe a fourth embodiment of an optical device according to the invention;

5A-5B depict two flowcharts showing a method of performing spectral alignment of multi-wavelength optical signals according to the present invention.

6A-6C depict another embodiment of a spectral power monitoring device according to the present invention, using a servo-based alignment compensation unit;

7A-7B depict another embodiment of a spectrum power monitoring device according to the present invention, using a software-based alignment compensation unit;

Figure 8 depicts an embodiment of an optical spectrum power monitoring device using a polarization diversity scheme according to the present invention;

Figure 9 depicts another embodiment of an optical spectrum power monitoring device using a polarization diversity scheme according to the present invention; and

FIG. 10 depicts another embodiment of an optical spectrum power monitoring device using a polarization diversity scheme according to the present invention.

Detailed description of the invention

FIG. 1 depicts an exemplary power spectrum, ie, a graph of optical power P as a function of wavelength λ for a reference spectral component λ _C and a plurality of spectral channels λ ₁ to λ _N. In this detailed description and appended claims, a "spectral channel" is characterized by a unique center wavelength (eg, λ _i ) and associated bandwidth, as shown in FIG. 1 . Each spectrum channel can carry a unique information signal such as in WDM optical networking applications. A "reference spectral component" (or "reference signal"), characterized by a wavelength λ _C , usually has a well-defined (and stable) central wavelength that does not substantially overlap with the wavelength of any of the spectral channels under consideration any optical signal. In Fig. 1, by way of example, reference spectral components are shown having a wavelength [lambda] _c shorter than the wavelength of the spectral channel. In general, the spectral channels need not be evenly spaced in wavelength (or frequency).

The following discussion describes the present invention using active alignment compensation and polarization diversity schemes in the following manner: (i) Section I describes a method for implementing active alignment compensation in an optical system using a servo-based system (ii) Section II describes other hardware and software solutions for active alignment compensation in optical systems; and (iii) Section III describes polarization diversity schemes that can be applied to the present invention.

I. Servo-Based Active Alignment Compensation

Figure 2A depicts a first embodiment of an optical device according to the invention. By way of example, showing the principle and overall structure of the present invention, the optical device 200 includes an input port 210 for multi-wavelength optical signals, which can be in the form of a fiber collimator; an alignment adjustment element, which can In the form of a control mirror 260-1 and a wavelength disperser 220, which may be a diffraction grating; a beam concentrator 230 in the form of a converging lens; and a reference wavelength sensing element 240 and a plurality of beam receiving elements Beam receiving arrays from 250-1 to 250-N. In this detailed description and the appended claims, a beam receiving element is broadly understood to encompass any optical element that receives one or more spectral channels. It can be, for example, an optical power sensor, a micromirror, an optical fiber, a converging lens, or a light modulator, as will be described in detail later.

The optical device 200 of FIG. 2A can work as follows. The input port 210 transmits a multi-wavelength optical signal comprising wavelengths _λ1 to _λN and a reference signal comprising wavelength _λC . These optical signals are then guided to the diffraction grating 220 by the control mirror 260-1. The diffraction grating 220 angularly separates the multi-wavelength optical signal and the reference signal into a plurality of spectral channels λ ₁ to λ _N and a reference spectral component λ _C with a predetermined relative arrangement according to the wavelength. The converging lens 230 converges the reference spectral components and spectral channels to corresponding converging points, such as converging to a spectrum array according to the predetermined relative arrangement. The beam receiving array including the reference wavelength sensing element 240 and the beam receiving elements 250-1 to 250-N is arranged in such a manner that once the reference spectral component λ _C is incident on the reference wavelength sensing element at a predetermined position x ₀ element 240, then spectral channels λ ₁ to λ _N are incident on beam receiving elements 250-1 to 250-N at designated positions x ₁ to x _N , respectively.

It should be noted that the embodiment of Figure 2A and subsequent figures are shown in schematic form and are for illustration only. The various elements and beams are not drawn to scale. In general, there can be any number of spectral channels in the optical arrangement of the invention (as long as their number is equivalent to the number of beam receiving elements located therebelow). Also, the points of convergence of the diffracted beams incident on the beam receiving array shown in FIG. 2A (and subsequent figures) may not be uniformly spaced.

The optical device 200 of FIG. 2A may further include a servo control unit 260, which in one form may include a control mirror 260-1 and a processing element 260-2. The control mirror 260-1 dynamically adjusts the alignment of the multi-wavelength optical signal and the reference signal, thereby controlling the incident position of the spectrum channel and the reference spectrum component incident on the beam receiving array. Processing element 260-2 monitors the real-time location of incidence of reference spectral component _λC on reference wavelength sensor element 240 and accordingly provides feedback (or servo) control of control mirror 260-1 such that reference spectral component _λC remains at The predetermined positions _xo and thus the spectral channels _λi to _λN remain at the specified positions _x1 to _xN . Likewise, the servo control unit 260 enables the optical device of the present invention to actively correct for misalignment caused by environmental influences such as thermal and/or mechanical instabilities during operation, thereby improving performance. robust. An additional benefit of using such a servo control unit is the relaxed manufacturing tolerances and precision during initial assembly, so that this allows for a more adaptable and cost-effective construction of the optical device of the present invention.

In the above embodiments, the reference wavelength sensing element 240 may be a position sensitive detector, a quadrant detector, a split detector, or any other position sensitive device known in the art which enables The electrical (such as current or voltage) signal can monitor the real-time incident position (one-dimensional or two-dimensional) of the beam. By way of example, FIG. 2B shows a schematic diagram of a position sensitive detector 240-A with a light beam 241 incident thereon. The incident position of the beam 241 in the x-direction can be deduced by detecting a pair of output signals, such as the current signals _Ix1 and _Ix2 , where the relative magnitudes of _Ix1 and _Ix2 provide a measure of the beam spot in the x-direction. Metrics. Similarly, the incident position of the light beam 241 in the y direction can be obtained by measuring another pair of current signals I _y1 and I _y2 . Furthermore, by following an appropriate normalized differential detection scheme (e.g. by measuring (I _x1 -I _x2 )/(I _x1 +I _x2 ) and/or (I _y1 -I _y2 )/(I _y1 +I _y2 )) The detection output signal, as commonly practiced in the art, can monitor the real-time incident position of the light beam 241 for deviation from a specified position, such as the center point O on the position sensitive detector 240-A. Those skilled in the art also know how to use other types of position sensitive devices known in the art to provide the reference wavelength sensing element in the present invention.

Control mirror 260-1 of FIG. 2A can be a dynamically adjustable mirror that can rotate about one or two axes. For example, the control mirror may be a silicon micromechanical mirror with an appropriate drive mechanism; it may be obtained by coupling drive devices known in the art to the mirror or to the beam deflecting element. Processing element 260-2 of FIG. 2A may contain electronic circuitry, controllers, and signal processing routines for processing output signals received from reference wavelength sensing element 240 (e.g., from position sensitive detector 240-A of FIG. 2B ). current signal) and the real-time incident position of the reference spectral component λ _C is obtained through the detected signal. The processing element 260-2 correspondingly generates appropriate control signals to be applied to an alignment adjustment element (such as the control mirror 260-1 of FIG. 2A ) to adjust the alignment of the reference spectral components and spectral channels in such a way that That is, the reference spectral component λ _C is kept at a predetermined position x _o . Electronic circuits and associated signal processing algorithms/software for processing elements in servo control systems are well known in the fields of electronic engineering and servo control systems.

2C-2D depict two exemplary methods of adjusting the alignment of the beams in the embodiment of FIG. 2A. In Fig. 2C, the first light beam 271 containing the wavelength _λi is incident on the diffraction grating 220 at an incident angle _θin (which can represent a spectrum channel), and is diffracted by the diffraction grating 220 to become the first diffracted beam 272, and is transmitted at the diffraction angle θ _out exits, as determined by the following grating equation:

${\mathrm{<span\; class="notranslate">\; sin</span>}\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; in</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; sin</span>}\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; out</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; m</span>}\frac{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}{\mathrm{<span\; class="notranslate">\; d</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

where m is the diffraction order and d is the grating pitch (i.e. the interval between two adjacent grooves on the grating). Both θ _in and θ _out are measured relative to the normal to the diffraction grating 220 . If the angle of incidence of the beam onto the diffraction grating 220 changes Δθ _in , as indicated by the second beam 273 , then the diffraction angle of the diffracted beam correspondingly changes Δθ _out , as indicated by the second diffracted beam 274 . Therefore, changing the incident angle of the multi-wavelength optical signal and the reference signal on the grating 220, for example, through the action of the control mirror 260-1 in FIG. The reference spectral component λ _C is incident on a predetermined position x _o , and the spectral channels λ ₁ to λ _N are incident on specified positions x _t to x _N .

In addition to (or in conjunction with) changing the angle of incidence of the light beam on the diffraction grating 220 (e.g., by means of control mirror 260-1 in FIG. 2A), the diffraction grating 220 itself can be rotated, thereby achieving a similar alignment function. , as shown in Figure 2D. In this case, the first light beam 281 including the wavelength λ _i is incident on the diffraction grating 220 and is diffracted by the diffraction grating 220 to become the first diffracted light beam 282 . Rotating the diffraction grating 220 by the angle Δθ _g , as indicated by the thus rotated diffraction grating 221 , actually changes the diffraction angle by Δθ _out , as indicated by the second diffracted beam 283 . Diffraction gratings can be implemented by coupling the grating to a suitable drive device such as a voice coil actuator, stepper motor, solenoid actuator, piezoelectric actuator, or any type of drive known in the art rotation. In turn, the drive device can be controlled by a processing element in the servo control unit.

In the embodiment of FIG. 2A, the angular dispersion D of the diffraction grating 220 can be obtained by the diffraction equation in equation (1):

$\mathrm{<span\; class="notranslate">\; D.</span>}<span\; class="notranslate">\; =</span>\frac{{<span\; class="notranslate">\; \&PartialD;</span>\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; out</span>}}}{<span\; class="notranslate">\; \&PartialD;</span>\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; cos</span>}{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; out</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

Let the focal length of the converging lens 230 be f. The pitch P of the spectral array formed by the diffracted beam, that is, the interval between any two adjacent spectral points, can be expressed as:

$\mathrm{<span\; class="notranslate">\; P</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; f\&Delta;\&lambda;</span>}\frac{{<span\; class="notranslate">\; \&PartialD;</span>\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; out</span>}}}{<span\; class="notranslate">\; \&PartialD;</span>\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; f\&Delta;\&lambda;m</span>}}{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="1"\; label="d"\; filenames="H02823085XE00021.png"\; state="\{\{state\}\}">d</figure-callout></span>}}\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; out</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

where Δλ is the wavelength difference between two adjacent spectral channels. Equation (3) shows that the pitch of the spectral array generally varies with the diffraction angle θ _out unless θ _out is zero. The rate of change of P with respect to _θout can be expressed as:

$\frac{<span\; class="notranslate">\; \&PartialD;</span>\mathrm{<span\; class="notranslate">\; P</span>}}{{<span\; class="notranslate">\; \&PartialD;</span>\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; out</span>}}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; f\&Delta;\&lambda;m</span>}}{\mathrm{<span\; class="notranslate">\; d</span>}}\frac{{\mathrm{<span\; class="notranslate">\; sin</span>}\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; out</span>}}}{{\mathrm{<span\; class="notranslate">\; cos</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; out</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

In Figures 2C and 2D above, since the alignment adjustment is achieved by changing the angle of incidence θ _in and thus the angle of diffraction θ _out , equation (3) indicates that the pitch of the spectral array can be changed when the alignment adjustment occurs , especially at large values of the diffraction angle θ _out . Therefore, in the embodiment of FIG. 2A (where the alignment adjustment method shown in FIG. 2C or 2D can be implemented), the components of the beam receiving elements should be configured in such a way that they can tolerate variations in the pitch of the spectral array . (For example, the dimensions of the beam receiving elements are such that a change in the pitch of the spectral array does not substantially change the correspondence between the spectral channels and the respective beam receiving elements, so that a change in the pitch of the spectral array is insignificant in practice.) The embodiment of FIG. 2A is also desirable in applications where the variation in the pitch of the spectral array described above is so small (as in the case where the diffraction angle θ _out approaches zero) that it is practically insignificant .

In addition to performing alignment adjustment by changing the incident angles of the input multi-wavelength optical signal and the reference signal as shown in FIG. 2C or 2D , it is also possible to translate and 2A and/or rotate to move the reference wavelength sensing element 240 and the beam receiving elements 250-1 to 250-N as a whole so that the reference spectral components and spectral channels can be incident on designated positions. Alternatively, the converging lens 230 in the embodiment of FIG. 2A can be moved, such as shifted or translated, so as to control the incident position of the diffracted beam.

Figure 3A shows a second embodiment of the optical device of the present invention. By way of example, optical device 300 utilizes the structure and many elements in the embodiment of Figure 2A, as those indicated with like numerals. In this case, for the convenience of operation, the beam receiving array including the reference wavelength sensing element 240 and the beam receiving elements 250-1 to 250-N can be integrated into a single structure, such as by installing or Manufactured on a substrate. Servo control unit 360 may include alignment adjustment element 360-1, which may be a linear drive device coupled to the beam receiving array, and processing element 360-2. The drive device 360-1 is configured such that it can cause the beam receiving array to move as a whole, so that the reference wavelength sensing element 240 and the beam receiving elements 250-1 to 250-N move as a whole, for example along substantially The translation is performed in a direction transverse to the direction of propagation of the incident beam, thereby adjusting the relative alignment between the spectral array formed by the diffracted beam and the underlying beam receiving array. As in the embodiment of FIG. 2A , the processing element 360-2 is used to monitor the real-time incident position of the reference spectral component λ _C incident on the reference wavelength sensing element 240, and accordingly provide the servo control of the drive device 360-1, so that The reference spectral component λ _C is held at a predetermined position _xo and the spectral channels λ ₁ to λ _N are held at respective designated positions x ₁ to x _N .

Figure 3B shows a third embodiment of the optical device of the present invention. By way of example, optical device 350 utilizes the structure and many elements used in the embodiment of Figure 3A, as those indicated with like numerals. An alternative servo control unit 365 may be implemented comprising an alignment adjustment element 365-1 in the form of a drive device coupled to the converging lens 230, and a processing element 365-2. The driving device 365-1 causes the converging lens to move, such as shift, rotate or translate, thereby controlling the incident positions of the diffracted beams incident on the reference wavelength sensing element 240 and the beam receiving elements 250-1 to 250-N, respectively. As is the case in the embodiment of FIG. 3A , the processing element 365-2 monitors the real-time incidence position of the reference spectral component _λc incident on the reference wavelength sensing element 240 and accordingly provides servo control of the drive device 365-1 by This keeps the reference spectral component λ _C at the predetermined position _xo and the spectral channels λ ₁ to λ _N at the respective specified positions x ₁ to x _N .

The driving device 360-1 in the embodiment of Figure 3A, or the driving device 365-1 in the embodiment of Figure 3B can be a stepper motor, a solenoid actuator, a piezoelectric actuator, a voice coil actuator , or any type of actuating device known in the art. Processing element 360-2 of FIG. 3A, or processing element 365-2 of FIG. 3B may be substantially similar in structure and operation to processing element 260-2 of FIG. 2A. The advantage of the embodiment of Fig. 3A or 3B is obvious, and this is because the alignment adjustment method under it does not substantially change the spacing of the spectral array, that is, only adjusts the relative alignment between the spectral array and the beam receiving array. allow. It should be appreciated that aberrations and other imperfections can be substantially eliminated by simply taking care in the design of the converging lens 230 in the embodiment of FIG. 3B. As can be appreciated from this detailed description, those skilled in the art know how to design an appropriate alignment adjustment method and corresponding servo control system in accordance with the present invention to suit a given application.

In the embodiment of Fig. 2A, 3A or 3B, the input fiber 201 that is coupled to the fiber collimator used as input port 210 can provide a multi-wavelength optical signal containing wavelengths _λ1 to _λN , and can be provided by a reference light source 202 A reference signal λ _c is provided. An optical combiner 203 , which in one form may be a fiber fused coupler, may be used to couple the reference light source 202 to the input fiber 201 so that both the multi-wavelength optical signal and the reference signal are directed into the input port 210 . Thus, the optical device 200 has an independent, internally generated reference light source. Alternatively, the multi-wavelength optical signal itself may contain spectral components used as reference signals (eg traffic channels in optical networks), as in WDM optical networking applications. In such a case, it is not necessary to implement internal reference light source 202 and fiber coupler 203 .

In the present invention, one or more auxiliary reference signals and corresponding reference wavelength sensing elements may be additionally used to complement the above-mentioned function of the reference spectral component λ _c . Figure 4A depicts a fourth embodiment of the optical device of the present invention. By way of example, optical device 400 utilizes the structure and many elements used in the embodiments of Figures 2A and 3A, as those indicated with like numerals. In addition, the auxiliary reference light source 402 can be coupled to the input optical fiber 201 through the auxiliary optical multiplexer 403, so that the auxiliary reference signal containing the wavelength λ _c ' can be coupled to the input port 210, wherein the multiplexer can be a fiber coupler . Then, the auxiliary reference signal λ _c ′, as well as the multi-wavelength optical signal and the reference signal λ _c can be guided onto the diffraction grating 220 by controlling the mirror 260-1. The wavelength λ _c ' of the auxiliary reference signal can be selected to be larger than the wavelength of the spectrum channel, so that when diffraction occurs, the auxiliary reference signal λ _c ' is incident on the auxiliary reference wavelength sensing element 441 at a specified position x _o '. Likewise, the reference spectral component λ _c , the spectral channels λ ₁ to λ _N , and the auxiliary reference signal λ _c ′ form a spectral array with a predetermined relative arrangement. Therefore, the reference wavelength sensing element 240, the beam receiving elements 250-1 to 250-N, and the auxiliary reference wavelength sensing element 441 form a beam receiving array configured to receive a spectral array. The beam receiving array can be integrated in a single structure, for example by mounting or fabricating the constituent elements on a substrate.

Auxiliary reference wavelength sensing element 441 may be a position sensitive detector, a split detector, a quadrant detector, or any other type of position sensitive device known in the art. Those skilled in the art should understand that the above-mentioned reference signal and auxiliary reference signal can also be referred to as the first and second reference signals; and correspondingly, the reference wavelength sensing element and the auxiliary reference wavelength sensing element can be referred to as the first first and second reference wavelength sensing elements. Furthermore, a wavelength disperser, such as a diffraction grating 220, can direct the (first) reference spectral component λ _c in the first reference signal and the (second) reference spectral component λ _c ′ in the second reference signal to respectively First and second reference wavelength sensing elements are located at first and second predetermined locations.

The embodiment of FIG. 4A may further include a driving device 460-1 and a processing element 460-2. By way of example, the driving device 460-1 may be coupled to the above-mentioned beam receiving array, thereby causing the beam receiving array as a whole, thus the reference wavelength sensing element 240, the beam receiving elements 250-1 to 250-N and the auxiliary reference wavelength Sensing element 441 as a whole moves, eg translates in a direction substantially transverse to the direction of propagation of the spectral channel and/or rotates as indicated by curved arrow 470 . For example, the drive device 460-1 may cause the beam receiving array to rotate about an axis point at a predetermined position _xo . Likewise, the drive device 460-1 may be primarily used to adjust the relative alignment between the spectral array formed by the diffracted beams and the beam receiving array beneath it. The processing element 460-2 may monitor the real-time incidence position of the reference spectral component _λc incident on the reference wavelength sensing element 240 and accordingly provide servo control of the drive device 460-1 such that the reference spectral component _λc remains at the predetermined position At x _o , the spectral channels λ ₁ to λ _N remain at the respective specified positions x ₁ to x _N .

The processing element 460 - 2 may additionally monitor the real-time incidence position of the auxiliary reference signal λ _c ′ incident on the auxiliary reference wavelength sensing element 441 . Such information is useful for monitoring misalignment between spectral channels and respective beam receiving elements, which misalignment may not be reflected by the incident position of the reference spectral component λ _c . By way of example, FIG. 4B shows a situation where the reference spectral component λ _c remains at a predetermined position x _o , while the incident position of the auxiliary reference signal λ _c ' incident on the auxiliary reference wavelength sensing element 441 along The x-direction deviates from the specified position x _o ', where the deviation may be caused by a pitch variation of the spectrum array. The xy plane in this figure (and in Figure 4C) is shown substantially transverse to the direction of propagation of the spectral channel. As indicated in the discussion above, the pitch of the spectral array generally varies with the diffraction angle and thus with the angle of incidence of the optical signal onto the diffraction grating (see e.g. equations (3) and (4) above )). Therefore, the processing element 460-2 can use the detected deviation of the auxiliary reference signal λ _c ' from the specified position x _o ' to control the control mirror 260-1 in such a way as to bring the auxiliary reference signal λ _c ' back to the specified position x o ' The position x _o ' of , such as adjusting the incident angles of the input multi-wavelength optical signal, the reference signal and the auxiliary reference signal incident on the diffraction grating 220 by an alignment adjustment method similar to that described in FIG. 2C . The alignment of the reference spectral component λ _c and the auxiliary reference signal λ _c ′ at the respective positions x _o , x _{o ′} indicates the necessary alignment between the spectral channels and the respective beam receiving elements.

By way of example, Fig. 4C shows another situation where the reference spectral component λ _c remains at a predetermined position x _o , while the incident position of the auxiliary reference signal λ _c ' incident on the auxiliary reference wavelength sensing element 441 deviates from The predetermined position x _o ', as shown, may be caused by rotational movement of the beam receiving array relative to the spectral array (or vice versa). Therefore, the processing element 460-2 can use the detected deviation of the auxiliary reference signal λ _c ' from the specified position x _o ' to control the drive device 460-1 in such a way as to bring the auxiliary reference signal λ _c ' back to the specified position x o '. At the position x _o ', for example by rotating the beam receiving array relative to the spectral array, thereby restoring the necessary alignment between the spectral channels and the respective beam receiving elements.

Those skilled in the art will appreciate that the embodiments of Figures 4B-4C are provided as examples to illustrate the basic principles of the invention. In practical situations, the deviation of the auxiliary reference signal _λc ' from the specified position may be due to various factors, such as a combination of spacing variation of the spectral array and rotational movement of the beam receiving array (relative to the spectral array). Therefore, the processing element 460-2 can control the driving device 460-1 and the control mirror 260-1 in a coordinated manner, thereby bringing the auxiliary reference signal λ _c ' back to the specified position, while at the same time making the reference spectral component λ _c remains at the predetermined position _xo , thereby restoring the necessary alignment between the spectral channel and the beam receiving element. Moreover, instead of (or in combination with) the function of controlling the mirror 260-1, the control of the pitch of the spectrum array can be realized by means of the alignment adjustment method described in the embodiment of FIG. 2D. Instead of (or in combination with) the alignment function of the driving device 460-1, it is possible to adjust the alignment relationship between the spectral array and the beam receiving array below it by connecting a suitable driving device to the converging lens 230, as shown in FIG. 3B described in the examples. In addition, the incident position of the auxiliary reference signal λ _c ' can be maintained at a predetermined position by servo control, and the incident position of the reference spectral component λ _c can be monitored periodically or continuously; or it can be based on appropriate signal processing and The servo control scheme actively controls the incidence positions of the reference spectral components and auxiliary reference signals.

In the embodiment of FIG. 4A , the servo control unit may generally include a first alignment adjustment element (such as a drive device 460 ) for adjusting the relative alignment between the spectral array formed by the diffracted beam and the beam receiving array below it. -1, or a suitable driving device coupled to the converging lens 230); a second alignment adjustment element (such as a control mirror 260-1, or a suitable driving device coupled to the diffraction grating 220) for controlling the pitch of the spectral array and a processing element (eg, processing element 460 - 2 ) in communication with the first and second alignment adjustment elements and the reference wavelength sensing element 240 and the auxiliary reference wavelength sensing element 441 . The processing element 460-2 can monitor the incident positions of the reference spectral component _λc and the auxiliary reference signal _λc ' incident on the reference wavelength sensing element 240 and the auxiliary reference wavelength sensing element 441, respectively, and provide first and second The control of the alignment adjustment elements keeps the reference spectral component λ _c and the auxiliary reference signal λ _c ′ at their respective assigned positions and thereby ensures the necessary alignment between the spectral channels and the respective beam receiving elements.

Likewise, the optical arrangement of FIG. 4A advantageously utilizes a combination of appropriate alignment adjustment methods to actively control the position and spacing of the spectral array, so the optical arrangement is more robust in performance.

In general, the one or more auxiliary reference signals in the present invention may be any optical signal having a defined (and stable) central wavelength that does not substantially overlap with the wavelengths of the spectral channel and the reference spectral component _λc . In the embodiment of FIG. 4A , by way of example, the wavelength λ _c ' of the auxiliary reference signal is shown to be larger than the wavelength of the spectral channel, while the wavelength λ _c of the reference spectral component is smaller than the wavelength of the spectral channel, and both reference signals are Provided by the internal reference light source, as shown. It should be noted that the two reference sources in Figure 4A can be coupled to the input fiber through a single optical combiner (such as a 3x1 fiber coupler); or the reference signal and the auxiliary reference signal can be provided by a single reference source that can provide multiple reference signals To provide, where the reference signal is coupled to the input fiber through an optical combiner. Alternatively, the multi-wavelength optical signal may itself contain one or more spectral components (eg, one or more traffic channels in an optical network) that can be used as one or more reference signals. A person skilled in the art knows how to implement a suitable reference signal in an optical arrangement according to the invention, so as to suit a given application.

In the above embodiments, the diffraction grating 220 may be a ruled diffraction grating, a holographic diffraction grating, an echelle grating, or a dispersion prism, all of which are commonly used in the art to separate multi-wavelength signals by wavelength. By way of example, the wavelength disperser in the above embodiments is shown in the form of a reflective diffraction grating. Those skilled in the art can understand that in the optical device of the present invention, a transmission diffraction grating or a dispersion prism can be used instead. The beam concentrator may also be a collection of converging lenses, or any other suitable beam converging device known in the art. The converging function can also be provided by using a curved diffraction grating that performs the dual functions of wavelength separation and beam converging. It should be noted that in applications where the spectral channel and reference spectral components are completely separated, a beam converger such as the converging lens 230 in the embodiments described above may not be used.

Furthermore, the beam receiving elements 250-1 to 250-N may be optical power sensors, such as photodetectors in the form of pn photodetectors, pin (p-intrinsic-n) photodetectors, or avalanche photodetectors (APDs). device. The optical device thus constructed constitutes a spectral power monitor with servo control capability, thereby providing the characteristic power spectrum of the spectral channel of interest. Beam receiving elements 250-1 to 250-N can also be micromirrors (eg, silicon micromachined mirrors), each of which can be individually controlled (eg, can be rotated about one or two axes) to dynamically follow a predetermined scheme Routing spectrum channels. Alternatively, the beam receiving elements 250-1 to 250-N can also be an array of optical fibers, and the spectral channels are guided into the array of optical fibers. Optical devices thus constructed constitute demultiplexers, or multiplexers when reversing the direction of propagation of the beam. The beam receiving elements 250-1 to 250-N may additionally be in the form of beam shaping elements, such as converging lenses, so as to project spectral channels to desired locations. Beam receiving elements 250-1 through 250-N may also be in the form of an array of light modulators, such as liquid crystal light modulators or light attenuators, for modulating one or more characteristics (e.g., amplitude and/or phase).

In Fig. 2A, 3A, 3B or 4A, by way of example, beam receiving elements are shown in one-to-one correspondence with spectral channels. There may be an application where subsets of beam receiving elements each correspond to multiple spectral channels, or where multiple beam receiving elements are assigned to a single spectral channel. For example, where optical power sensors are used as beam receiving elements, each of one or more optical power sensors may be designated for receiving multiple spectral channels, thereby providing an overview of the received spectral channels. Complete power measurement.

It is known that the diffraction efficiency of diffraction gratings is generally polarization-dependent, and that for gratings with a large number of grooves (per unit length), this polarization-dependent effect can become significant. Thus, where a diffraction grating is used as a wavelength disperser, as in the case of the embodiments of FIGS. The mechanism is discussed in Section III below. By way of example, a polarization diversity scheme may be implemented. In this scheme, an input multi-wavelength optical signal (and one or more reference signals) is first decomposed into a P-polarized part and an S-polarized part. Assuming that the P polarization direction is the preferred direction of the diffraction grating (that is, the diffraction efficiency is higher for P polarization than for S polarization), then the S polarization part is rotated by 90 degrees, so that the optical signals incident on the diffraction grating have P polarization. This polarization diversity scheme has the advantage of maximizing diffraction efficiency. Alternatively, a suitable polarization-sensitive element (such as a leaky beam splitter) can be used to polarize P among the input multi-wavelength optical signals (and one or more reference signals) before these signals are incident on the diffraction grating The portion is attenuated by a predetermined ratio relative to the S-polarized portion, thereby compensating for the discrimination of different polarization states caused by the diffraction grating. Apparatus and methods for these polarization diversity schemes are discussed in more detail below in Section III.

The invention further provides a spectrum alignment method of multi-wavelength optical signals. As an example illustrating the general principles of the invention, FIG. 5A shows an exemplary flowchart outlining the method of the invention. Method 500 entails combining the multi-wavelength optical signal and the reference (or calibration) signal, as indicated in step 510; spatially separating the multi-wavelength optical signal and the reference (or calibration) signal by wavelength into a plurality of spectral channels and a a reference (or calibration) spectral component, wherein the plurality of spectral channels and a reference (or calibration) spectral component have a predetermined arrangement, as indicated in step 520; making the reference (or calibration) spectral component incident on at a predetermined position so that the spectral channels are incident on the designated position in a predetermined relative arrangement, as indicated in step 530; the reference (or calibration) spectral component is maintained at the predetermined position by servo control, whereby It is ensured that the spectral channels remain aligned at the designated locations, as indicated in step 540 .

The above-described method of the present invention takes advantage of the fact that reference spectral components and spectral channels, each characterized by a unique center wavelength, form a spectral array with a predetermined relative arrangement. In this way, aligning the reference frequency components at predetermined locations ensures that the spectral channels are simultaneously incident at the designated locations according to the spectral array. This provides a simple and efficient method of aligning spectral arrays formed by multi-wavelength optical signals. The spectral channels thus aligned can then be individually manipulated eg by the beam receiving elements, as described above.

FIG. 5B illustrates an exemplary embodiment of the servo control operation described in step 540 of FIG. 5A in further detail. This embodiment entails monitoring the real-time incident location of the reference (or calibration) spectral component, as indicated in step 540-A; and adjusting the calibration of the reference (or calibration) spectral component and the spectral channel accordingly, so that the reference (or calibration) Calibration) spectral components remain at predetermined locations, and thereby ensure that spectral channels remain aligned at specified locations, as described in step 540-B.

The method 500 of FIG. 5A (or FIG. 5B ) may further comprise the step of converging the spectral channels and reference (or calibration) spectral components to corresponding convergence points, as indicated in step 550 . The method 500 of FIG. 5A (or FIG. 5B ) may additionally include the step of optically detecting the spectral channel at a designated location, thereby providing the power spectrum of the detected spectral channel; routing a spectral channel; or the step of modulating one or more characteristics of a spectral channel.

II. Other Hardware and Software for Active Alignment Compensation in Optical Systems

FIG. 6A depicts an exemplary embodiment of a spectrum power monitoring device according to the present invention. By way of example, the principle and overall structure of the present invention are described. The spectrum power monitoring device 600 includes an input port 610 for multi-wavelength optical signals, which can appear in the form of a fiber collimator; a wavelength disperser 620, It can be in the form of a diffraction grating; a beam converger 630, which can be a converging lens; an optical power sensor array 640 (referred to herein as a "light sensor array"), which provides a reference position sensor element 640-C and multiple channel sensing elements 640-1 through 640-N. Light sensing array 640 may be integrated into a single structure (eg, by mounting or fabricating the constituent elements on a single substrate).

The spectrum power monitoring device 600 in FIG. 6A can work in the following manner. Input port 610 transmits a multi-wavelength optical signal comprising wavelengths _λ1 to _λN and a reference signal comprising wavelength _λc . The diffraction grating 620 angularly separates the incident multi-wavelength optical signal and the reference signal into a plurality of spectral channels λ ₁ to λ _N and a reference spectral component λ _c with a predetermined relative arrangement according to the wavelength. The converging lens 630 converges the reference spectral component λ _c and the spectral channels λ ₁ to λ _N to corresponding convergence points, such as a spatial array (or "spectral array") with a predetermined relative arrangement. The light sensing array 640 can be placed such that when the reference spectral component λ _c is incident on the reference position sensing element 640-C at a predetermined position x _o , the spectral channels λ ₁ to λ _N are respectively at the specified position x ₁ -x _N is incident on channel sense elements 640-1 through 640-N.

It should be noted that the embodiment of Figure 6A and the following figures are shown in schematic diagram form for purposes of example. The various elements and beams are not drawn to scale. In general, there may be any number of spectral channels in the spectral power monitoring apparatus of the present invention as long as the number of channel sensing elements used in the system is sufficient to determine the power levels of the spectral channels with the desired accuracy. Moreover, the points of convergence of the diffracted beams incident on the photo-sensing array shown in FIG. 6A (and subsequent figures) may not be evenly spaced and need not have a one-to-one correspondence with the underlying channel sensing elements, As will be described in detail later.

The spectral power monitoring apparatus 600 of FIG. 6A may further include a servo-based alignment compensation unit 660 , which in one form may include a drive device 660 - 1 coupled to a light sensing array 640 and a processing element 660 - 2 . The drive device 660-1 is configured to cause movement of the light sensing array 640 as a whole so that the reference position sensing element 640-C and the channel sensing elements 640-1 to 640-N collectively perform movement (e.g., translation and and/or rotation), thereby adjusting the relative alignment between the spectral array formed by the diffracted light beams and the underlying light sensing array 640. The processing element 660-2 monitors the real-time incidence position of the reference spectral component _λc incident on the reference position sensing element 640-C, and accordingly provides servo (or feedback) control of the drive device 660-1 such that the reference spectral component λ _c remains at the predetermined position _x0 , and thus the spectral channels _λ1 to _λN remain at the specified positions _x1 to _xN . The servo-based alignment compensation unit thus described enables the optical device of the present invention to actively correct alignment shifts that may occur during operation (for example due to environmental influences such as thermal and/or mechanical disturbances), thus Increased robustness of the device. An additional benefit of using such an alignment compensation unit is the loose manufacturing tolerances and precision during initial assembly, thus allowing for a simpler and more cost effective construction of the spectral power monitoring device of the present invention.

By way of example, FIG. 6B shows how a photodiode array may be implemented as light sensing array 640 in the embodiment of FIG. 6A. FIG. 6B shows an enlarged view of an exemplary segment of a photodiode array 640A comprising a plurality of adjacent light sensing elements having different photoresponse characteristics, as differentiated by shaded and non-shaded regions as shown. . As an example, the photoresponse function R _i (x), shown by the solid line in the figure, represents the non-shaded photosensitive region 640-i and its two adjacent shaded regions 640-iH and 640-jH characteristics of the photoresponse. Similarly, the photoresponse function R _j (x), as shown by the dashed line in the figure, represents the adjacent non-shaded photosensitive region 640-j and its two adjacent shaded regions 640-jH and 640 Characterization of the photoresponse of -kH. The optical response function relates the optical power incident on the optical sensing element to the resulting electrical (eg, voltage) signal, as will be discussed in further detail below. By way of example, each photoresponse function in Figure 6B is shown to be nearly constant in the corresponding non-shaded region and decrease in a nearly linear fashion as one moves away from the non-shaded region into the adjacent shaded region , thus appearing as a trapezoid-like shape in terms of overall characteristics. Likewise, photodiode array 640A has a continuous overall photoresponse function; that is, there are no "dead zones" (or photoinsensitive regions) on photodiode array 640A. Photodiode arrays having the characteristics thus described are commercially available, for example, from Sensors Unlimited, Inc., Princeton, New Jersey.

As an example, the photodiode arrays described in the ensuing discussion include appropriate detection circuitry so that the output signal can be in the form of a voltage signal. It can be understood that the basic principles and working methods of the present invention are also applicable to other photodiode arrays or optical power sensor arrays, and their output signals are in the form of current signals. It can also be understood that the subscript i, j, or k in this detailed description may be any integer between 1 and N.

The photodiode array 640A in FIG. 6B may be configured such that a voltage signal is output through the non-shaded region. By way of example, the voltage signal V _i output from the non-shaded region 640-i can be obtained by the following formula:

V _i =∫R _i (x)I(x, y)dxdy (1)

where the integration occurs over the unshaded region 640-i and its adjacent shaded regions 640-iH and 640-jH, and I(x,y) is our interest incident on the xy plane specified in FIG. 6B The light intensity on the area. The photoresponse function R _i (x) is predetermined and based on the characteristics of the photodiode array used. Thus, the voltage signal _Vi takes into account the total optical power incident on the non-shaded region 640-i and its adjacent shaded regions 640-iH and 640-jH. Similarly, the voltage signal V _j output from the unshaded region 640-j is related to the total optical power incident on the unshaded region 640-j and its adjacent shaded regions 640-jH and 640-kH. Moreover, because two spatially adjacent photoresponse functions, such as R _i (x) and R _j (x), are intertwined, it can be obtained from the measured voltage signals V _i and V _j incident on the shaded region The power level of the light beam above, for example, the shaded region 640-iH sandwiched between the non-shaded regions 640-i and 640-j. The same applies to other parts in the photodiode array 640A. Therefore, each unshaded area and its adjacent shaded areas, for example, the unshaded area 640-i and its adjacent shaded areas 640-iH and 640-jH constitute a channel sensing element in the present invention ( or pixels).

Alternatively, two adjacent channel sensing elements in photodiode array 640A may be used as "split detectors" to provide reference position sensing elements for reference spectral component _λc (as in the embodiment of FIG. 6A ). position sensing element 640-C). This can be achieved by measuring the voltage signals V ₁ , V ₂ output from the non-shaded regions 640-1, 640-2, respectively, using a suitable normalized differential detection scheme known in the art, for example by monitoring the position error signal (V ₁ -V ₂ )/(V ₁ +V ₂ ). Such a normalized differential detection scheme has the advantage of improving the signal-to-noise ratio (SNR) of the detection through general mode suppression of amplitude noise. As an example, the incident position of the reference spectral component _λc may be located on the shaded region 640-2-H sandwiched between two adjacent unshaded regions 640-1, 640-2, so that from the unshaded region 640- 1. The voltage signals V ₁ and V ₂ output by 640-2 change in an almost linear manner with the position of the reference spectral component λ _c respectively. In this case, a single channel sensing element, such as the channel sensing element associated with either of the non-shaded regions 640-1, 640-2, may be used as the reference position sensing element.

The spectral power monitoring device 600 of FIG. 6A can be configured in such a way that the spectral channels are incident to the non-shaded area of the photodiode array 640A in a one-to-one correspondence; and the spectral points formed by the spectral channels are limited in the respective non-shaded areas , as shown in Figure 6B. By way of example, non-shaded regions 640-i may be designated for spectral channels λ _i , while non-shaded regions 640-j may be designated for spectral channels λ _j . In this way, the voltage signals output from the non-shaded regions are respectively proportional to the power levels of their corresponding spectral channels, because in each non-shaded region (such as the non-shaded region 640-i), only A photoresponse function is controlled (eg R _i (x)). For example, the voltage signal V _i is directly proportional to the power level of the spectral channel λ _i incident on the non-shaded region 640-i, and the related scaling factor can be obtained through calibration, as will be described in detail later. Such a configuration also takes advantage of the uniform photoresponse properties in the unshaded regions, which makes any deviations in the positions of incidence of the spectral channels in the corresponding unshaded regions practically insignificant. Moreover, the processing element 660-2 in the embodiment of FIG. 6A can measure the aforementioned voltage signals V ₁ , V ₂ using a suitable differential detection scheme known in the art so that the actual value of the reference spectral component λ _c can be easily monitored. The deviation of the incident position from the specified position. In turn, the processing element 660-2 can use the detected deviation of the incident position of the reference spectral component _λc to generate an appropriate control signal to be applied to the driving device 660-1 so as to keep the reference spectral component _λc at a specified position, thereby ensuring the necessary alignment between the spectral channel and the corresponding channel sensing element. Likewise, the embodiment of FIG. 6B provides an embodiment of the light sensor array 640 in FIG. 6A.

In some applications, it may be difficult to confine the spectral points of a spectral channel to non-shaded regions in the corresponding channel sensing elements (as in the manner described in FIG. 6B ). Spectral arrays formed by diffracted beams may also have non-uniform spacing, meaning that the spacing between any two adjacent spectral points may not be constant. Either of these two schemes can lead to a situation where each of the one or more channel sensing elements receives more than one spectral channel, and in some cases spectral points can overlap. Figure 2C depicts an exemplary embodiment of how the photodiode array described in Figure 6B can be used in such an application.

By way of example, the photodiode array 640B shown in FIG. 2C may be substantially similar in configuration and operation to the photodiode array 640A of FIG. 6B, and thus like numerals are used to designate elements therein. For purposes of illustration and clarity, only 3 spectral channels λ _i , λ _j , λ _k are explicitly shown; and these spectral channels are shown arranged such that one or more channel sensing elements can each receive more than A spectrum channel. For example, channel sensing elements associated with unshaded region 640-i receive at least spectral channels λ _i , λ _j ; similarly, channel sensing elements associated with unshaded region 640-j receive at least spectral channels λ _j , λ _k . Based on equation (1), the voltage signal V _i output from the non-shaded region 640-i can generally be expressed as:

${\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; \&Integral;</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; [</span>\underset{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; N</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; the\; y</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span>\mathrm{<span\; class="notranslate">\; dxdy</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

where I _n (x, y) is the light intensity of the associated spectral channel λ _n (n = 1 to N) in our region of interest. Those skilled in the art will understand that equation (2) is applicable to any spectral channel we are interested in (that is, i=1 to N in the above). Thus, if the power levels P ₁ to _PN of the spectral channels incident on photodiode array 640B are represented by optical power vector (P) and the resulting voltage signal V of photodiode array 640B is represented by voltage vector (V) ₁ to V _M (M≥N), then the relationship between (P) and (V) is as follows:

(V)=[T](P) (2)

where [T] is a (M x N) transmission matrix. The transmission matrix [T] is generally dependent on the relative alignment between the spectral channel and the underlying channel sensing element, as well as the intrinsic properties (such as photoresponse properties) of the electro-conductive diode array used. The transmission matrix [T] is typically band-diagonal unless one or more channel sensing elements each receive multiple spectral channels. Those skilled in the art will appreciate that equation (2) above also applies to the embodiment of FIG. 6B , where N=M, and the transmission matrix [T] is true diagonal.

Based on equation (2), there is the following equation:

(P)＝[C](V) (3)

where [C] is a (N x M) transformation matrix and can be obtained from the transmission matrix [T] in equation (2). In order to determine the transmission matrix [T], a calibration can be performed (e.g. at the factory) in which a calibration optical signal is characterized by having substantially the same wavelength as the spectral channel to be probed and such that a known power level is coupled into Fig. 6A The input port 610 of , thus passing through the same optical path as that to be experienced by the spectrum channel. (For example, the calibration light signal may be provided by a tunable laser). The output voltage signal of photodiode array 640B in response to the incident calibration light signal was then measured. By substituting the measured voltage signal and the known power level of the calibration optical signal into equation (2), the transmission matrix [T] can be calculated. The transfer matrix [C] in equation (3) can be further derived from this transfer matrix [T] by means of suitable matrix algorithms known in the art. The transformation matrix [C] thus obtained may be stored in a system memory, for example in the signal processor which is part of the processing element 660-2 in Fig. 6A. Then during operation, the conversion matrix [C] remains substantially unchanged, as long as the spectral channels remain substantially at the same locations where the calibration optical signal is incident on the photodiode array 640B. The necessary alignment may be maintained, for example, by the servo-based alignment compensation unit 660 in FIG. 6A described above. This allows the signal processor to easily calculate the power level of the spectral channel incident on photodiode array 640B from the voltage signal thus generated in a manner according to equation (3). Those skilled in the art will recognize that the background contribution (e.g. due to "dark current" of the photodiode array and/or due to "stray light" from the environment can be determined independently if this is desired in a practical application). ”), and then take it into account during the calibration and operation described above.

In the embodiment of FIG. 2C , the incident position of the reference spectral component λ _c can be monitored by using an appropriate normalized differential detection scheme to measure the voltage signals V ₁ , V ₂ output from the non-shaded regions 640-1, 640-2, respectively. , for example by detecting the position error signal (V ₁ −V ₂ )/(V ₁ +V ₂ ) in the manner described for FIG. 6B . Likewise, the embodiment of FIG. 2C may alternatively be implemented in FIG. 6A to implement a light sensing array 640 .

Reference is made back to the embodiment of FIG. 6A. The driving device 660-1 may be a stepper motor, a solenoid actuator, a piezoelectric actuator, a voice coil actuator, or any type of driving device known in the art. Processing element 660-2 may contain circuitry, controllers, and signal processing algorithms for processing output signals received from reference position sensing element 640-C (such as the voltage signal V output from light sensing array 640A in FIG. 6B ). ₁ , V ₂ ) and obtain the real-time incidence position of the reference spectral component λ _c through the detected signal. The processing element 660-2 accordingly generates appropriate control signals to be applied to the driving device 660-1 to adjust the alignment of the reference spectral component _λc and the spectral channels _λ1 to _λN in such a way that the reference spectral The component λ _c remains at a predetermined position x _o . Electronic circuits and associated signal processing algorithms/software for processing elements in servo control systems are well known in the fields of electronic engineering and servo control systems.

Those skilled in the art will understand that, in addition to (or in combination with) moving the light sensing array 640 as described above, the converging lens 630 in FIG. The converging lens is translated or rotated, thereby controlling the incident position of the diffracted beam and performing a similar alignment function. As described above, translation/rotation of the converging lens 630 can be achieved by coupling the converging lens to a suitable drive device. In some cases, the alignment adjustment can also be achieved (or compensated) by appropriately changing the angle of incidence of the input multi-wavelength optical signal (and reference signal) on the diffraction grating 620, such as by rotating the grating or by rotating the input port. A dynamically adjustable mirror is placed between 610 and diffraction grating 620, as long as such an adjustment does not substantially change the pitch of the spectral array formed by the diffracted beams. As can be appreciated from this detailed description, those skilled in the art will know how to design appropriate alignment adjustment elements and corresponding processing elements for a servo-based alignment compensation unit according to the present invention so as to best suit the a given application.

Fig. 7A depicts a second embodiment of a spectrum power monitoring device according to the present invention. By way of example, the spectrum power monitoring apparatus 700 may utilize the structure and various elements used in the embodiment of FIG. 6A, such as those indicated by the same numerals. Note that there is no "moving" of the alignment adjustment device in this system. Alternatively, a software-based alignment compensation unit 760 is implemented, which may be a signal processor in communication with the light sensing array 640 .

FIG. 7B shows in further detail how the light sensing array 640 in the embodiment of FIG. 7A is configured. By way of example, the photodiode array 640C of FIG. 7B may be substantially similar in configuration and manner of operation to the photodiode array 640A described in FIG. 6B, and thus these elements are labeled with the same numerals. In this case, two or more adjacent channel sensing elements occupying an adjacent segment of photodiode array 640C (referred to herein as the "reference segment") can be utilized to monitor the incident reference spectral component _λc . Location. For example, if the reference spectral component _λc is located within two adjacent channel sensing elements, such as those associated with the unshaded regions 640-1, 640-2 as depicted in FIG. 7B, then The voltage signals V ₁ , V ₂ output from the non-shaded regions 640-1, 640-2, respectively, can be measured by using an appropriate normalized differential detection scheme, for example by detecting the position error signal (V ₁ - V ₂ )/(V ₁ +V ₂ ). In cases where the reference spectral component λ _c may experience a shift in alignment, or "walk-off" extending beyond the two channel sensing elements (as during calibration or operation as described below), the reference segment may contain multiple channel sensing elements, and their respective output voltage signals are detected. Accordingly, a series of position error signals can be correspondingly generated, each position error signal being related in the manner described above to the voltage signals output from two adjacent channel sensing elements in the segment, from which the reference The incident position of the spectral component λ _c . Those skilled in the art can know how to design an appropriate signal detection and processing scheme for effectively monitoring the real-time incidence position of the reference spectral component _λc .

In FIG. 7B, spectral channels _λ1 through _λN may be incident on multiple channel sensing elements located within a segment (referred to herein as a "channel segment") of photodiode array 640C that is correlated with reference spectral component λ _c is completely separated by the designated reference segment, so that any voltage signal output from the reference segment does not contain contributions from the spectral channel, or vice versa. The correspondence between the spectral channels and the channel sensing elements located therebelow may be as described in the embodiment of Fig. 6B or 6C. In either of the two schemes, the power level of the spectral channel incident on the photodiode array 640C can be made by a predetermined conversion matrix [C] in the manner as indicated in equation (3) above. Associated with the voltage signal thus generated by the channel segments of the photodiode array 640C.

The spectrum power monitoring device 700 in the embodiment of FIG. 7A can work in the following manner. During initial (or factory) calibration, an optical signal with the same wavelength as the spectral channel to be probed and a known power level is coupled into the input port in a way that "mimics" the spectral channel of interest 610. (For example, the calibration light signal may be provided by a tunable laser.) The calibration light signal and reference signal _λc emerge from input port 610, are spatially separated by diffraction grating 620 and converging lens 630 and are then incident on the photodiode array 640 (such as the photodiode array 640C of FIG. 7B). Then, the incident position of the reference spectral component λ _c is changed incrementally (eg, along the x-direction as shown in Fig. This is achieved by translating the array 640 . Then, as described above, the position x of the reference spectral component λ _c is determined by the voltage signal (such as the voltage signal V ₁ , V ₂ ) output from the specified reference segment. At each position x of the reference spectral component _λc , the voltage signals generated by the calibration optical signal are also measured, and then substituting these voltage signals and the known power level of the calibration optical signal into equation (2) above, by This calculation results in the corresponding transmission matrix [T(x)]. In turn, the transfer matrix [C(x)] in equation (3) can be derived from this transfer matrix [T(x)] by using an appropriate matrix algorithm known in the art. Thus, the calibration process builds a matrix calibration table containing [C(x)] as a function of x, which can be stored in the alignment compensation unit 760 . Then during operation, the alignment compensation unit 760 monitors the real-time incident position x of the reference spectral component _λc , and measures the voltage signal generated by the spectral channel incident on the channel segment of the photodiode array 640C at the corresponding position x. Then, the alignment compensation unit 760 looks up the corresponding conversion matrix [C(x)] from a predetermined matrix calibration table, thereby obtaining the power level of the incident spectrum channel from the measured voltage signal by using the above equation (3). Likewise, the spectral power unit 700 effectively compensates for alignment shifts that may occur during operation through software control, without involving any "moving" drive units. Using such a software-based alignment compensation unit also relaxes manufacturing tolerances and requirements for precision during initial assembly, which leads to a simpler and more robust construction of the spectral power monitoring device of the present invention.

It should be understood that exemplary photoresponse characteristics of photodiode array 640A as depicted in FIG. 6B are provided by way of example to illustrate the basic principles of the present invention. Those skilled in the art will understand that in the spectral power monitoring device of the present invention, other optical power sensor (or photodiode) arrays with different optical response characteristics can be alternatively implemented for the same way (such as Described by equations (1)-(3) above) provide essentially the same function. For example, the optical power sensor arrays of the present invention need not have a continuous overall optical response function (eg, there can be one or more "dead zones" between optical sensing regions). As can be appreciated by the teachings of the present invention, one skilled in the art will know to design an appropriate spectral power monitoring device to best suit a given application.

In the embodiment of FIG. 6A or 7A, a multi-wavelength optical signal comprising wavelengths _λ1 to _λN may be provided by input fiber 601 coupled to a fiber collimator used as input port 610. The reference signal _λc may be provided by a reference light source 602, which may be a distributed feedback (DFB) laser, a Fabry-Perot (FP) laser (in combination with an appropriate modulation/control system that suppresses mode hopping and stabilizes the output signal ), or any other light source known in the art that can provide a suitable reference signal with a well-defined and stable wavelength. An optical multiplexer 603 such as a fused fiber coupler can be used to couple the reference light source 602 to the input fiber 601 , effectively coupling the multi-wavelength optical signal and the reference signal into the input port 610 . In this way, the spectral power monitoring device has an independent, internal reference light source. Alternatively, the input multi-wavelength optical signal may itself contain spectral components (such as traffic channels in optical networking applications) that can be used as reference signals, as in WDM optical networking applications. In such a scheme, it is not necessary to implement an internal reference light source 602 and fiber coupler 603 . The input fiber 601 can be a single mode fiber, a multimode fiber or a polarization maintaining fiber.

Furthermore, diffraction grating 620 may be a ruled diffraction grating, a holographic diffraction grating, or an echelle grating, all of which are commonly used in the art to separate multi-wavelength signals by wavelength. The wavelength disperser in the spectrum power monitoring device of the present invention can be realized by using other types of wavelength separation devices known in the art, such as transmission diffraction gratings or dispersion prisms. The beam concentrator 630 may also be a collection of converging lenses, or any other suitable beam converging device known in the art. The converging function can also be provided by using a curved diffraction grating that performs the dual functions of wavelength separation and beam converging. It should be noted that in applications where the spectral channel and reference spectral components are well separated, a beam converger such as converging lens 630 in Figures 6A or 7A may not be used.

It is well known that the diffraction efficiency of a diffraction grating can be polarization dependent. For example, a grating in a standard mounting configuration diffracts more efficiently for p (or TM) polarization than for s (or TE) polarization, or vice versa, and where p polarization is perpendicular to the grooves on the grating and s polarization Orthogonal to p polarization. In order to reduce this polarization sensitive influence, can adopt a kind of suitable polarization sensitive element (such as leakage beam splitter) in the spectral power monitoring device of the present invention, be used for input multi-wavelength optical signal (and one or more reference Signals) before being incident on the diffraction grating, one polarization state (such as p-polarization) component of these signals is attenuated according to a pre-proportioned ratio relative to the other polarization state (such as s-polarization) component, thereby compensating for the polarization dependence caused by the diffraction grating . This can be achieved, for example, by placing a suitable weak polarizer (eg a leaky beam splitter) on the optical path between the input port 610 and the diffraction grating 620 in the embodiment of Figure 6A or 7A. Alternatively, a suitable polarization diversity scheme may be implemented as discussed in Section III below.

As can be understood from the description of the present invention, those skilled in the art can know how to design a spectral power monitoring device by using an appropriate alignment compensation unit and an appropriate polarization diversity scheme, thus best suited for a given specific application. For example, by using an InGaAs-based photodiode array (which is particularly sensitive in the wavelength range of 1-1.7 μm) as the photo-sensing array in the above-described embodiments, the present invention provides a series of photodiodes capable of active alignment. Compensated new spectral power monitors, these monitors will be particularly suitable for DWDM optical networking applications.

III. Polarization Diversity Scheme

Fig. 8 shows an exemplary embodiment of the optical spectrum power monitoring device of the present invention. By way of example, showing the principle and overall structure of the present invention, the optical spectrum power monitoring device 800 includes an input port 810 for multi-wavelength optical signals in the form of a fiber collimator; a polarization separation element 870, one of which The form may be a polarization beam splitter; a polarization rotation element 880, which may be a half-wave plate; a wavelength disperser 820, which may be a diffraction grating in one form; a beam concentrator 830, which may be a converging lens; and an optical power sensor Array 840 (herein referred to as "light sensing array").

The principle operation of the optical spectral power monitoring device 800 of FIG. 8 is as follows. Input port 810 transmits a multi-wavelength optical signal (eg, it may contain wavelengths λ ₁ through λ _N ). The polarization separation element 870 decomposes the multi-wavelength optical signal into a p (or TM) polarization component (perpendicular to the concave lines on the grating) and an s polarization (or TE) polarization component (positive to the p polarization component) with respect to the diffraction grating 820. pay). (The p-polarization and s-polarization components may also be referred to as "first and second polarization components.") As an example, assume that p-polarization is the "preferred direction" for diffraction grating 820 (i.e., the diffraction efficiency for p-polarization is greater than that for s polarization high), the polarization rotation element 880 then rotates the s polarization component (or the second polarization component) by 90 degrees, whereby the optical signals incident on the diffraction grating 820 all have p polarization. Diffraction grating 820 angularly separates the incident optical signal by wavelength into first and second sets of beams (eg, where each set contains beams having wavelengths _λ1 to _λN ). Converging lens 830 may then converge the diffracted beams to corresponding convergence points so that the first and second beams with respect to the same wavelength (eg, λ _i ) are incident on photosensor array 840 at substantially the same location (or at within the same optical power sensor). (It should be understood that in this detailed description and appended claims, the polarization rotation produced by a polarization rotation element (such as polarization rotation element 880) can be understood as having a slight difference for a specified angle (such as 90 degrees), which means that is caused by existing defects in the actual system. However, this difference will not significantly affect the overall performance of the present invention.)

The aforementioned overlapping of the first and second light beams (with the same polarization direction and the same wavelength) leads to coherent interference producing unwanted intensity fringes. To avoid this, an auxiliary polarization rotation element 890 may be implemented in the embodiment of FIG. direction. An auxiliary polarization rotation element may be implemented between the diffraction grating 820 and the light sensing array 840 and used to rotate either of the first and second sets of light beams by 90 degrees before being incident on the light sensing array 840 . In FIG. 8, by way of example, an auxiliary polarization-selective element 890 is placed between the diffraction grating 820 and the converging lens 830, so that the polarization direction of the first set of light beams undergoes a 90° degree rotation. It should be understood that, alternatively, the auxiliary polarization rotation element 890 may be placed between the diffraction grating 820 and the converging lens 830 in such a way that the polarization The orientation undergoes a 90 degree rotation. In either of these two schemes, the first and second light beams associated with the same wavelength (eg, λ _i ) become to have two mutually orthogonal polarization directions before being incident on the light sensing array 840, Any coherent intensity interference is thereby eliminated.

Likewise, the polarization sensitivity of diffraction grating 820 becomes insignificant in optical spectral power monitoring device 800 by advantageously employing the polarization diversity scheme described above. This allows the device of the present invention to increase spectral resolution (eg by utilizing highly dispersive diffraction gratings commonly available in the art) while simultaneously improving the accuracy of optical spectral power detection in a simple and cost-effective construction.

Fig. 9 depicts another embodiment of an optical spectrum power monitoring device using a polarization diversity scheme according to the present invention. By way of example, optical spectral power monitoring apparatus 900 may utilize the general structure and various elements used in the embodiment of FIG. 8, as indicated by those elements labeled with the same numerals. In such a case, modulation component 985 may be implemented and configured such that the first and second sets of light beams are incident on light sensing array 840 in a time-division multiplexed (eg, alternating) manner. By way of example, adjustment assembly 985 is shown as having first and second shutter elements 981, 982 and control unit 983 positioned between polarization separating element 870 and polarization rotating element 880 and diffraction grating 820. , thereby controlling the first and second polarization components, respectively. Any one of the first and second shutter elements 981, 982 may be configured so that under the action of an appropriate control signal (such as a signal provided by the control unit 983) it allows optical signals to pass; and in the absence of any In the absence of a control signal, it remains closed to the input optical signal. Thus, by utilizing the control unit 983 to operate the first and second shutter elements 981, 982 in an alternating manner according to an appropriate control scheme, the first and second sets of beams are time-division multiplexed in sequence is incident on the light sensing array 840, as indicated by the solid and dashed lines in the figure. This allows the first and second sets of light beams to be detected separately, whereby the optical power spectrum associated with each polarization component of the input multi-wavelength optical signal can be obtained independently. Those skilled in the art can understand that the first and second shutter elements 981, 982 (and the control unit 983) can alternatively be implemented between the diffraction grating 820 and the light sensor array 840, thereby controlling the first The first and second sets thus provide substantially similar functionality.

In the embodiments described above, the modulation assembly 985 may alternatively be provided by a chopper (and associated control unit), such as an opaque rotating disk equipped with at least one aperture or any other suitable means known in the art, They allow two incident light signals to pass in an alternating fashion. The chopper may be implemented between the polarization separating element 870 and the polarization rotating element 880 and the diffraction grating 820, or between the diffraction grating 820 and the light sensing array 840, thus providing substantially similar function.

In the embodiment of FIG. 9, each of the first and second sets of light beams may have a predetermined alignment with the light sensing array 840 located therebelow. Alternatively, the first and second light beams associated with the same wavelength (eg, λ _i ) may be incident on light sensing array 840 at substantially the same location (albeit at different times). Light sensing array 840 can be substantially identical to array 240 or 640, and can comprise a photodiode array (such as a photodiode array produced by Sensors Unlimited, Inc., Princeton, New Jersey), or other suitable photodiodes known in the art. power sensing device. Those skilled in the art will know how to implement an appropriate light sensing array and design an appropriate detection scheme that is best suited for a given application.

Similar to the embodiment of FIG. 8, the optical spectral power monitoring device 900 is polarization-insensitive to the diffraction grating 820, and thus can provide accurate detection of multi-wavelength optical signals with improved spectral resolution. An additional benefit of the optical spectral power monitoring device 900 is that by having the first and second sets of light beams incident on the optical sensing array in a time-division multiplexed manner, it is possible to independently determine The optical power spectrum of each polarization component in , which is useful in optical networking applications. For example, polarization multiplexing (encoding a data stream onto two mutually orthogonal polarization components of a single wavelength channel) has emerged as another method for increasing the information capacity of optical fibers. Therefore, it would be desirable to have a device that can separately detect two mutually orthogonal polarization components of a single wavelength channel.

Those skilled in the art will recognize that the above-described functionality of modulation component 985 can be generalized to modulate the first and second sets of light beams in a frequency division multiplexed manner, whereby these sets can be separately identified on light sensing array 840 . Applicable to this situation, Figure 10 shows another embodiment of an optical spectral power monitoring device. By way of example, optical spectral power monitoring device 1000 may employ the general structure and various elements used in FIG. 8, as indicated by elements labeled with the same numerals. In this case, a modulation component 1085 may be placed on the polarization separation element 870 and on the optical path between the polarization rotation element 880 and the diffraction grating 820 for modulating the first and second polarization components, respectively. The modulating component 1085 may be in the form of first and second modulating elements 1081, 1082, which may be electro-optic intensity modulators (liquid crystal based intensity modulator). The first and second modulating elements 1081, 1082 may operate under the control of two different alternating (or "dithering") control signals (such as provided by the control unit 1083), one form of which may be A sinusoidal function of time with two different frequencies (such as a first and a second "dither frequency"). Either of the modulating elements 1081, 1082 may be configured so as to introduce a "dither modulation signal" into the optical power level of its corresponding light beam, wherein the optical power level contains a linear response to the control signal, and the The power level is controlled by this control signal. Likewise, when emerging from the first and first beam modulating elements 1081, 1082, the first and second polarization components may respectively carry first and second dither modulation signals (e.g., at first and second high The vibration frequency is characteristic). Thus, the first and second sets of light beams diffracted by diffraction grating 820 may also carry respective dither modulation signals, and the first and second sets are incident on light sensing array 840 . The electrical signal thus generated by the light sensing array 840 in a similar manner contains the same characteristic dither modulation signal and can be detected by the synchronization detection unit 1090 in communication with the light sensing array 840 . As those skilled in the art can understand, if necessary in practical applications, the synchronization detection unit 1090 can also communicate with the control unit 1083 .

In the present invention, a "spectral channel" is characterized by a unique center wavelength and associated bandwidth, and may carry a unique information signal as in WDM optical networking applications. A "dither modulation signal" refers to any modulation in the optical power level of an optical signal produced by a modulation component, as compared to an "intrinsic" modulation signal (eg, an information signal) carried by an input multi-wavelength optical signal. Correspondingly, the dither modulation signal is allocated within a spectral range that is sufficiently separated from the frequencies of other "intrinsic" modulation signals carried by the spectral channel.

As in the case of FIG. 9, the first and second sets of light beams in the embodiment of FIG. 10 may both have a predetermined alignment with the light sensing array 840 located therebelow. Alternatively, the first and second light beams associated with the same wavelength (eg, λ _i ) may be incident on the light sensing array 840 at substantially the same location (or within the same optical power sensor). In either of these two schemes, the different dither modulation signals carried by the two sets of beams allow the two sets of beams to be detected separately, eg using the simultaneous detection unit 1090 . In order to correlate the measurements provided by the synchronous detection unit 1090 with the corresponding optical power levels in the input multi-wavelength optical signal, a calibration process can be performed whereby the light relative to each polarization component in the input multi-wavelength optical signal can be obtained power spectrum. From the description of the present invention, one skilled in the art will know how to implement an appropriate light sensing array and design an appropriate detection scheme for a given application.

Modulation assembly 1085 may also be provided by a chopper (and associated control unit), such as an opaque rotating disk equipped with two sets of holes. Each set of apertures effectively "chops" its corresponding beam (eg, first or second polarization component) at a frequency determined by the spatial arrangement of its constituent apertures. By arranging the two sets of apertures in the desired scheme, the first and second light beams reaching the light sensing array 840 are characterized by different modulations, thereby allowing them to be detected separately. It should be appreciated that modulating component 1085 (eg, first and second modulating elements 1081, 1082) may alternatively be implemented between diffraction grating 820 and light sensing array 840 to modulate the first and second sets of light beams, respectively. As can be understood from the description of the invention, those skilled in the art will know how to implement appropriate modulation components in an optical spectral power monitoring device according to the invention to best suit a given application.

In the above embodiments, the polarization splitting element 870 may be a polarizing beam splitter, a birefringent beam displacer, or other types of polarization splitting devices known in the art. The polarization rotation element 880 or the auxiliary polarization rotation element 890 can be a half-wave plate, a Faraday rotator, a liquid crystal rotator, or any other known in the art that can rotate the polarization direction of a light beam by a specified angle (such as 90 degrees). Polarization Rotator. Either of the first and second shutter elements 981, 982 may be a liquid crystal-based shutter element, such as one that rotates the polarization of an incident light beam by 90 degrees without any control signal and upon activation of an appropriate control signal. A liquid crystal rotator plate that acts to keep the polarization direction constant, and a polarizer whose polarization axis is perpendicular to the rotated polarization direction produced by the liquid crystal rotator plate. Any one of the first or second shutter element 981, 982 may also be a MEMS (micro-electromechanical system) based element that acts as a shutter, or any other similar shutter element known in the art. It is opened or kept closed for the incident beam by suitable drive means. The control unit 983 may contain circuits and signal processing algorithms known in the art for controlling the first or second shutter elements 981, 982 according to the desired scheme.

Furthermore, either of the first and second modulating elements 1081, 1082 may be an electro-optical intensity modulator, such as a liquid crystal intensity modulator, or any other suitable modulating means known in the art. Those skilled in the art will know how to design an appropriate control unit 1083 so that the desired dither modulation signal is generated by the first and second modulation elements 1081 , 1082 . The synchronous detection unit 1090 generally contains circuitry and signal processing algorithms designed to perform synchronous detection of the dither modulated signals generated in the first and second sets of light beams, respectively.

In the present invention, the wavelength disperser (eg, diffraction grating) 820 can be a ruled diffraction grating, a holographic diffraction grating, or an echelle grating, all of which are commonly used in the art to separate multi-wavelength signals by wavelength. Generally, other types of wavelength separation devices known in the art can be used to implement the wavelength disperser 820 in the optical spectrum power monitoring device of the present invention, such as a transmission diffraction grating or a dispersion prism. The beam concentrator 830 may also be a collection of converging lenses, or any other suitable beam converging device known in the art. The converging function can also be provided by using a curved diffraction grating that performs the dual functions of wavelength separation and beam converging. The fiber optic collimator used as input port 810 may come in the form of a collimating lens (eg, a GRIN lens) and ferrule-mounted optical fiber enclosed together in a mechanically rigid stainless steel (or glass) tube.

It should be understood that the polarization diversity compensation scheme described above also uses active alignment compensation in conjunction with the spectrum monitor described in Sections I and II. For example and without limitation, by integrating the polarization splitting element 870, the polarization rotator elements 880 and/or 890, and the modulation components 985, 1085 into corresponding locations within the spectrum monitor (such as at the input port and wavelength disperser and/or beam between convergers), polarization diversity schemes can be used within the spectrum monitor.

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the principle and scope of the invention. Therefore, the scope of the present invention should be determined by the following claims and their legal equivalents.

Claims (17)

1. An optical device comprising: an input port providing a multi-wavelength optical signal and at least one reference signal; a wavelength disperser, separating the multi-wavelength optical signal and the at least one reference signal into a plurality of spectral channels and at least one reference spectral component having a predetermined relative arrangement according to wavelength; a beam receiving array comprising at least one reference wavelength sensing element and a plurality of beam receiving elements positioned to respectively receive said at least one reference spectral component and said spectrum channel; a first alignment adjustment element that adjusts the alignment between the spectral array and the beam receiving array so that the at least one reference spectral component can be on the at least one reference wavelength sensing element Aligned at the predetermined position of ; and a beam converger for converging the spectral channels and the at least one reference spectral component in the predetermined relative arrangement to corresponding convergence points in the spectral array, and wherein the first alignment adjustment element comprises a coupled to The driving device of the beam converger is used to move the beam converger. 2. The optical device of claim 1, wherein the at least one reference signal comprises first and second reference signals having first and second reference spectral components, respectively, and wherein the The first alignment adjustment element is adapted to enable alignment of the first reference spectral component at a first predetermined position on the first reference wavelength sensing element, and enable alignment of the second reference spectral component at the The second reference wavelength is aligned at a second predetermined location on the sensing element. 3. The optical device of claim 1, further comprising a servo control unit comprising a processing element, wherein said processing element monitors said at least one reference spectral component incident on said at least one reference wavelength transmission position of incidence on the sensing element, and accordingly provides control over the first alignment adjustment element, thereby ensuring that the at least one reference spectral component is at the predetermined position on the at least one reference wavelength sensing element Stay aligned. 4. The optical device as claimed in claim 3, wherein said servo control unit further comprises a second alignment adjustment element for adjusting the pitch of said spectrum array, and said second alignment adjustment element is connected to said processing element to communicate. 5. The optical device of claim 4, wherein said second alignment adjustment element comprises a control mirror in optical communication with said input port and with said wavelength disperser and for adjusting said Alignment between the spectral array and the beam receiving array. 6. The optical device of claim 4, wherein the second alignment adjustment element includes a drive device coupled to the wavelength disperser for rotating the wavelength disperser. 7. The optical device of claim 1, wherein the at least one reference wavelength sensing element comprises an element selected from the group consisting of a position sensitive detector, a split detector and a quadrant detector. 8. The optical device of claim 1, wherein the wavelength disperser comprises an element selected from the group consisting of a ruled diffraction grating, a holographic grating, an echelle grating, a curved diffraction grating, a transmission grating, and a dispersion prism. 9. The optical device of claim 1, wherein said input port comprises a fiber collimator coupled to an input optical fiber, wherein said optical device further comprises at least one optical device for coupling at least one reference light source to said input An optical multiplexer for optical fibers, and wherein the input optical fiber transmits the multi-wavelength optical signal, and the at least one reference light source provides the at least one reference signal. 10. The optical device of claim 1, wherein the beam receiving element comprises an optical power sensor. 11. The optical device of claim 1, wherein the beam receiving element comprises a micromirror. 12. The optical device of claim 1, wherein the beam receiving element comprises an optical fiber. 13. The optical device of claim 1 , further comprising a polarization separating element and a polarization rotating element in optical communication with the input port and with the wavelength disperser, wherein the polarization separating element divides the multi-wavelength The optical signal and the reference signal are decomposed into first and second polarization components, and the polarization rotation element rotates the polarization of the second polarization component by 90 degrees. 14. The optical device of claim 13, wherein the polarization splitting element comprises an element selected from the group consisting of a polarization beam splitter and a birefringent beam shifter. 15. The optical device of claim 13, wherein the polarization rotator element comprises an element selected from the group consisting of a half-wave plate, a liquid crystal rotator plate, and a Faraday rotator plate. 16. The optical device of claim 13 , further comprising an auxiliary polarization rotation element in optical communication between said wavelength disperser and said beam receiving array such that each of said first polarization components The polarization of a dispersed beam undergoes a 90 degree rotation. 17. The optical device of claim 16, wherein the auxiliary polarization rotating element comprises an element selected from the group consisting of a half-wave plate, a Faraday rotator plate, and a liquid crystal rotator plate.

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