WO2003081324A1

WO2003081324A1 - Optical filter with dynamically controlled lineshape and method of operation

Info

Publication number: WO2003081324A1
Application number: PCT/US2003/008173
Authority: WO
Inventors: Tairan Wang; James S. Foresi; Jean-François VIENS; Attila Mekis
Original assignee: Clarendon Photonics
Current assignee: Clarendon Photonics
Priority date: 2002-03-18
Filing date: 2003-03-17
Publication date: 2003-10-02
Anticipated expiration: 2004-09-18

Abstract

A resonator-system (100) of a planar waveguide system includes two or more resonators (112-1,112-2) that are controlled using thermo-optic heaters (122-1,122-2). The resonator-system is coupled to an input waveguide (10) and an output waveguide (12). The heaters control the coupling and resonance frequencies of the resonators. By shifting pairs of resonance frequencies in opposite directions, ON/OFF switching functions are provided. By shifting pairs of resonance frequencies by different amounts and in different directions, variable splitting functions, such as variable optical attenuation and tapping, are provided.

Description

OPTICAL FILTER WITH DYNAMICALLY CONTROLLED LINESHAPE AND

METHOD OF OPERATION

PRIORITY INFORMATION This application claims priority from U.S. patent application Ser. No.

10/236,140 filed September 6, 2002 which claims priority to provisional application Ser. No. 60/365,649 filed March 18, 2002, both of which are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

In wavelength division multiplexing (WDM) optical communication systems, spectral filters are important components for processing optical signals. They can be used to transfer optical channels from one waveguide to another, such as for add and drop operations, or selectively accepting and rejecting portions of the spectrum, such as for variable optical attenuation or tapping.

In planar waveguide systems and other technologies, spectral filters employ resonator-systems comprised of resonators, such as ring resonators and grating resonators, to perform frequency specific operations. Resonators provide a spectral passband having a lineshape that is centered at a resonance frequency and spans a specific range of frequencies. By increasing the number of resonators, the lineshape of the filter can be made flatter on top with sharper sides.

A variety of methods have been developed to dynamically control the resonator- systems. Typically, the methods seek to affect the lineshape of the resonator-systems to provide switching and modulation operations by moving the passband or spoiling the resonators.

In one proposed method, a spectral filter uses a grating based resonator-system.

The resonances of all the resonators are tuned together. Tuning mechanisms such as thermo-optic refractive index control or electro-absorption are suggested. ON/OFF functionality is provided by controlling the resonances of all the resonators together so that the filter passband is moved out of the region of interest or the resonances are spoiled by absorption. Another proposed spectral filter has a resonator-system based on side-coupled ring resonators. The resonator-system is controlled using an electro-absorption effect. Each ring resonator is controlled via an electrode formed over the resonator. ON/OFF functionality is provided by inducing electro-absorption in the resonators to inhibit signal propagation in the resonator-system. Signal level modulation is provided by controlling the amount of absorption in the resonators so that a portion of an input signal propagates through the resonator-system.

However, these methods of resonator-system control do not offer flexible and efficient control of the lineshape. In the first filter, the grating resonators in the resonator-system are shifted together to move the lineshape outside the region of interest. In some applications it may not be possible to move the lineshape out of the region of interest or the lineshape may have to be moved a long way across other optical channels resulting in undesirable crosstalk. This method is also inflexible because it does not allow the shape of the lineshape to be controlled.

In the second filter, the ring-resonators in the resonator-system are controlled using an electro-absorption effect. The electro-absorption effect offers control of the level of the lineshape. However, control of the shape of the lineshape is limited. The resonator-system may also be sensitive to resonance frequency variations.

In many DWDM applications, a sharper lineshape and greater control of the shape of the lineshape are required. Moreover, in commercial planar waveguide fabrication, errors in lithographic patterning and material deposition processes can shift lineshape resonances from the nominal design, thereby reducing yields.

SUMMARY OF THE INVENTION

To address these problems, the invention is directed to a photonic filter having a dynamically controllable lineshape. The filter is comprised of a resonator-system, having multiple coupled resonators. The resonator-system itself is coupled to an input waveguide and an output waveguide. The resonators are differentially controlled to provide various lineshape functions, dynamically, by independent control over their resonant frequencies. The resonator-system includes multiple optical resonators, such as grating phase-shift resonators, Fabry-Perot cavities, and/or ring resonators. These resonators are coupled to each other (resonator-resonator coupling), coupled to an input waveguide

(resonator-waveguide coupling), and/or coupled to an output waveguide (resonator- waveguide coupling).

The resonance frequencies of the resonators and the resonator-resonator and resonator- waveguide coupling determine the lineshape spectra of the filter. The resonant frequencies and couplings of the resonators are influenced by the underlying dielectric properties of the waveguide material, such as the refractive index and absorption coefficient. Changing the dielectric properties of the material at the location of the resonators or at the coupling regions can directly change the resonant frequency and the resonator coupling and, thereby, the lineshape spectra of the filter.

According to an aspect of the invention, the dielectric properties are changed by tuning devices that are based on thermo-optic, electro-optic, acousto-optic, electro- refractive effects or any other effects that change the local dielectric property dynamically. Changes in the resonance frequencies and the resonator couplings will change the resulting filter lineshape. By controlling the relative frequencies of the resonator-system and the resonators' couplings, dynamic lineshape control is provided.

In one embodiment, a resonator-system includes multiple resonators, such as two, four, or more resonators that are controlled using tuning devices based on heaters.

This resonator-system is coupled to an input waveguide and an output waveguide. The heater tuning devices modulate or change the coupling and resonant frequencies of the resonators by changing the temperatures of the resonators and thus their refractive indices. By shifting pairs of resonance frequencies in opposite directions, ON/OFF switching functions are provided. By shifting pairs of resonance frequencies differentially, such as by different amounts and/or in different directions, variable splitting functions, such as variable optical attenuation and tapping, are provided.

In another embodiment, two resonator-systems are coupled to an input waveguide and an output waveguide, as in a directional-coupler assisted add/drop filter configuration, and controlled using thermo-optic heaters.

The invention also addresses the problem of lineshape control in multiple- channel spectral de-interleavers or other channel-specific resonant filters having multiple resonant frequencies and sharp add-drop filter characteristics.

The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention.

FIG. 1 is a block diagram of a filter for a planar waveguide device that comprises a resonator-system with two resonators according to the invention;

FIG. 2 is a plot showing the response of a two resonator Butterworth filter and its response in a detuned state;

FIG. 3 is a plot showing the response of a two resonator Butterworth filter and its response in a reduced coupling state; FIG. 4 is a schematic view of a planar waveguide inline grating resonator- system comprising two grating resonators;

FIG. 5 is schematic of a planar waveguide ring resonator-system comprising two ring resonators;

FIG. 6 is a schematic of a planar waveguide side-coupled grating resonator- system comprising two side-coupled grating resonators;

FIG. 7 is a block diagram of an alternative filter design for a planar waveguide device that comprises a resonator-system with two resonators according to the invention;

FIG. 8 is a plot showing the response of an alternative configuration two resonator filter and its detuned resonance response ;

FIG. 9 is a block diagram of a filter comprising a resonator-system with four resonators according to the invention; FIG. 10 is a plot of a Butterworth filter response for a resonator-system comprised of four resonators;

FIG. 11 is a block diagram showing a shifting scheme for ON/OFF switching of a resonator-system; FIG. 12 is a plot showing the OFF filter response for an embodiment of the invention where, in a resonator-system comprised of four resonators, the first and fourth resonators are shifted in the opposite direction to the second and third resonators by the same amount;

FIG. 13 is a block diagram showing another shifting scheme for ON/OFF switching of a resonator-system;

FIG. 14 is a plot showing the OFF filter response for an embodiment of the invention where, in a resonator-system comprised of four resonators, the first and third resonators are shifted in the opposite direction to the second and fourth resonators by the same amount; FIG. 15 is a block diagram showing another shifting scheme for ON/OFF switching;

FIG. 16 is a plot showing the OFF filter response for an embodiment of the invention having low side peaks where, in a resonator-system comprised of four resonators, the first and second resonators are shifted in the opposite direction to the third and fourth resonators;

FIG. 17 is a block diagram showing another shifting scheme for ON/OFF switching;

FIG. 18 is a plot showing the OFF filter response for an embodiment of the invention having a flat top where, in a resonator-system comprised of four resonators, the first and third resonators are shifted in the opposite direction to the second and fourth resonators;

FIG. 19 is a plot of the filter response for various resonant frequency shifts;

FIG. 20 A and 20B are plots of the spectral response illustrating phase and stitch error compensation; FIG. 21 is a block diagram of an alternate filter configuration comprising a resonator-system comprising four resonators according to the invention;

FIG. 22 is a schematic diagram of a directional-coupler assisted add/drop filter having two resonator-system sub-elements, each with four resonators according to the invention;

FIG. 23 is a schematic diagram of a side-coupled filter in which two identical resonator-systems are coupled together and the resonator couplings are dynamically changed using electrodes to induce electro-optic refractive index effects according to the invention;

FIG. 24 is a schematic diagram of a multi-channel add/drop filter in which two identical grating waveguides are coupled together and controlled using heaters according to the invention; FIG. 25 is a plot of the spectral response for a de-interleaver multi-channel add/drop filter according to the invention;

FIG. 26 is a plot showing the OFF spectral response of the de-interleaver multichannel add/drop filter according to the invention; and

FIG. 27 is a schematic view of a concatenation of filters with multiple controllers according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to an optical resonator-system filter where spectra of interest, including the filter lineshape, are dynamically controlled by changing the dielectric properties of the constituent resonators or the couplings of the resonators in the resonator-system. The filter uses a resonator-system as a coupling element between an input waveguide and one or more output waveguides or even one waveguide, when a circulator or directional coupler is used, allowing the single waveguide to function as an input and output port. The resonator-system preferably includes two or more coupled resonators, such as photonic crystal resonators, grating resonators, side- coupled grating resonators, side-coupled ring resonators, ring resonators, and/or Fabry- Perot resonators.

When isolated from other resonators and the waveguides, each resonator can support one or more resonant modes. The frequencies of the resonant modes are generally determined by the geometry of the resonator and the dielectric properties of the waveguide materials. The dielectric properties include the refractive index and the absorption coefficient of the material. In the resonator-systems, however, each resonator can be coupled to a waveguide, a resonator, or a resonator and a waveguide. Each resonator can also be coupled to multiple waveguides, multiple resonators, or multiple waveguides and multiple resonators. The coupling coefficients of the resonators are determined by the dielectric properties of the material in the coupling regions, such as the mutual coupling between two resonators or the waveguide coupling between a resonator and a waveguide. The coupling of the resonators modifies the spectral characteristics from the isolated resonator.

For any given configuration of the resonators, including the mutual couplings and the waveguide couplings, the spectra, or state, of the resonators can be expressed using a variety of methods. In the following description, the spectra are described using resonator-coupled-mode theory. The spectra are a function of the frequencies of the resonant modes, the coupling coefficients between the resonators, and the coupling coefficients between the resonators and the waveguides. Generally, the spectra of interest can be changed by: 1) changing the intrinsic resonant frequencies of the resonators; and/or 2) changing the coupling coefficients between the resonators and between the resonators and the waveguides.

These changes can be achieved using optical effects, such as thermo-optic effects, electro-optic effects, acousto-optic effects, charge-filling effects, and carrier injection effects. In most cases, these effects are used to enable modulation of the refractive index of the regions carrying the optical energy.

The resonance frequencies of the resonators can be changed, for example, by changing the material refractive index of a resonator by the thermo-optic effect. In thermo-optical operation, the refractive index of a resonator is changed by thermal heating from a heater near the location of the resonator.

Altering the resonance frequencies of the resonators, using any refractive index effect, changes the spectra of interest and the state of the resonator-system. When the resonance frequencies of the resonators are matched, the coupling coefficients can be designed such that for one or more desired frequency ranges, ω_N , or channels, the resonator-system will transmit the full power of the incoming signal, and for other frequency ranges, ω_M , or channels, the resonator- system will reflect the full power of the incoming signal. Generally, when the resonance frequencies of the resonators are detuned from each other or mismatched, the resonator-system will transmit a fractional power of ω_N and reflect the remaining power of ω_N while maintaining reflection of the full power of

This method can be applied to resonator-systems with isotropic or anisotropic dielectric properties.

Resonator-systems with two resonators

FIG. 1 shows a first embodiment of the invention, in which a resonator-system 100 is coupled to a first waveguide 10 and to a second waveguide 12. The resonator- system 100 includes a first resonator 112-1 and a second resonator 112-2, each supporting one resonant mode. The first waveguide 10 provides an input port II and an output port TI . The second waveguide 12 provides an input port 12 and an output port TZ. The first resonator 112-1 is coupled to the first waveguide 10 by coupling region

114 and the second resonator 112-2 is coupled to the second waveguide 12 by coupling region 116. The two resonators 112-1 , 112-2 are also coupled to each other by coupling region 118. In this embodiment, the coupling coefficient of coupling regions 114, 116 between each resonator 112-1 , 112-2 and its corresponding waveguide 10, 12 and the coupling coefficient of coupling region 118 between the two resonators preferably have the same magnitude, γ 1 ^ . The nominal intrinsic resonance frequencies of the resonators, ω₀ , are also the same, in one implementation. Therefore, in this embodiment, the flat-top power spectrum of the resonator-system 100 is known as a two-pole Butterworth response, where the T2 spectrum, normalized to an input at II, is

as shown by the solid line in FIG. 2 and FIG. 3. A similar TI response results for an input at 12 instead of II .

Resonance Frequency Control Shifting the resonant frequencies of resonators 112-1, 112-2 changes the response of the resonator-system 100 and allows the resonator-system to be dynamically switched between states. In this embodiment, the resonance frequencies of the two resonators 112-1 , 112-2 are shifted or tuned using a thermo-optic effect. By employing a heater 122-1, 122-2 as a tuning device at each resonator 112-1, 112-2, the resonance frequency of each resonator is dynamically controlled. Specifically, the local temperatures of the resonators 112-1, 112-2 can be individually controlled using controller 120. By controlling the temperatures of the resonators using the heaters 122- 1, 122-2, the refractive indices of the resonators are adjusted to shift (increase or decrease) the resonant frequencies of the resonators and thereby change the state of the filter. Generally, the controller 120 executes one of five resonant frequency control schemes for dynamically controlling the resonator-system 100: 1) none of the resonance frequencies are shifted; 2) the resonance frequencies of the resonators are shifted together; 3) the resonance frequency of the first resonator is shifted; 4) the resonance frequency of the second resonator is shifted; 5) the resonance frequencies of the resonators are shifted in opposite directions; and 6) the resonance frequencies of the resonators are shifted in the same direction but by different amounts.

In schemes 3, 4, 5, and 6, the resonance frequencies of the two resonators are differentially shifted, and the resonators are differentially controlled.

Dynamic control of the resonator-system 100 is implemented by switching between the different schemes. For example, when the nominal resonance frequencies of the two resonators are shifted in opposite directions by a frequency shift of Aω , the T2 spectrum of the resonator-system 100, normalized to the input at II, becomes

T2(ω) - ^r

(ω- ω₀γ - 2Aω² (ω - ω₀)² + (y² + Aω²)² as shown by the dashed line in FIG. 2. A similar TI response results for an input at 12 instead of II .

This resonator-system control is achieved by dynamically switching from an initial resonator-system state where there is no shift in the resonators 112-1, 112-2 or there is an initial calibrating shift in the resonators 112-1, 112-2 to a resonator-system state where the resonators 112-1, 112-2 are shifted opposite to each other using resonant frequency scheme (5). In this example, the resonance frequency of the first resonator 112-1 is changed by + Aω , and the resonant frequency of the second resonator 112-2 is changed by - Aω . Alternatively, the resonant frequency of the first resonator 112-1 can be changed by - Aω , and the resonant frequency of the second resonator 112-2 can be changed by + Aω . The resonant frequencies of resonators 112- 1, 112-2 can also be changed by different amounts, such as + Aω and - 2Δ-y , respectively, in other control schemes.

In more detail, in one specific example, the controller 120 drives heaters 122-1 and 122-2 to heat the corresponding resonators 112-1 and 112-2 to their nominal operating temperatures. Typically, this operating temperature is determined during a calibration operation of the system and corresponds to the nominal operational frequencies for the resonators. The controller 120 then executes frequency control scheme 5 above, for example, by decreasing the power to one of the heaters while increasing the power to the other heater. This differential heating relative to the nominal operating temperatures has the effect of differentially changing the resonant frequencies of the resonators 112-1 and 112-2 with respect to each other from their nominal operational frequencies, thereby yielding the switching functionality.

By varying the amount of frequency shift with the optical effect, analog control of the transmission signal magnitude is provided. As a result, a wide range of transmission signal magnitudes can be obtained for an optical channel by varying the optical effect.

Coupling Control

The coupling between the resonators 112-1, 112-2 and between the resonators and the waveguides 10, 12 are varied, in one embodiment, to change the resonator- system's lineshape and allow the resonator- system to be switched between states. In order to change the couplings between the resonators 112-1, 112-2, the dielectric properties of the material of coupling region 118 between the resonators 112-1, 112-2 and/or coupling regions 114, 116 are changed, preferably using the same optical effects as in the resonator shifting schemes described above. The optical effects are used to increase or decrease the coupling coefficients of the resonators from the nominal intrinsic coupling. In this embodiment, the couplings of the resonators are changed, by an electro-optic effect, using controller 120. By employing electrodes in the region of each coupling region 114, 116, 118, the coupling of each resonator 112-1 , 112-2 can be dynamically controlled.

The couplings can be individually controlled or controlled together, resulting in four categories of schemes for dynamically controlling the couplings of the resonator- system: (1) none of the couplings are changed; (2) one coupling is changed; (3) two couplings are changed; and (4) all three couplings are changed.

When two couplings are changed, the couplings can be changed in the same direction (e.g. both increased) or the couplings can be changed in opposite directions (e.g. one increased and one decreased). This results in six ways of changing two couplings at a time. Similar to the schemes for shifting resonance frequencies, described above, the couplings in schemes 2 and 3 are differentially controlled using controller 120 in one mode of operation.

The resonator-system 100 is also dynamically controlled by switching between resonator-system states using different coupling schemes. For example, when the nominal coupling coefficients of the first and third coupling regions 114, 116 are decreased by Aγl X , the T2 spectrum of the resonator-system, normalized to the input at II, becomes:

as shown by the dashed line in FIG. 3. A similar TI response results for an input at 12 instead of II . This resonator-system control is achieved by dynamically switching from a resonator-system state where the couplings are not changed to a state where two couplings are changed using coupling scheme (3). In this example, the coupling between the first waveguide and the first resonator is changed by - Aγl 2 , and the coupling between the second waveguide and the second resonator is changed by - Aγl . The couplings can also be changed by different amounts, such a

and - 2Δ/ /2 , respectively.

By varying the amount of coupling change with the optical effect, analog control of the transmission signal magnitude is provided. As a result, a wide range of transmission signal magnitudes can be obtained for an optical channel by varying the optical effect. The preceding description has generally used an in-line waveguide presentation and notation to describe the coupled resonators. However, the invention is applicable to other methods of coupling resonators, such as side-coupling.

In another embodiment, a resonator-system 100 is comprised of inline phase- shift gratings coupled to a first waveguide 10 and a second waveguide 12, as shown in FIG. 4.

In yet another embodiment, a resonator-system 100 is comprised of side-coupled ring resonators coupled to a first waveguide 10 and a second waveguide 12, as shown in FIG. 5. In a further embodiment of the invention, a resonator-system 100 is comprised of side-coupled phase- shift gratings coupled to a first waveguide 10 and a second waveguide 12, as shown in FIG. 6.

Alternate Two-Resonator Configuration The two resonators can also be configured so that one resonator is coupled to the input and output waveguides and the second resonator is only coupled to the first resonator. FIG. 7 shows another exemplary embodiment of the invention in which a resonator-system 100 is coupled to a first waveguide 10 and to a second waveguide 12. The resonator-system 100 comprises a first resonator 112-1 and a second resonator 112-2, each supporting one mode. The first waveguide 10 provides an input port II and an output port TI . The second waveguide 12 provides an input port 12 and an output port T2.

The first resonator 112-1 is coupled to the first waveguide 10 by coupling region 114 and to the second waveguide 12 by coupling region 116. The second resonator 112-1 is coupled to the first resonator 112-1 by coupling region 118. In this embodiment, the coupling coefficient of coupling regions 114, 116 between the first resonator 112-1 and the waveguides 10, 12 and the coupling coefficient of coupling region 118 between the two resonators preferably have the same magnitude, χl X2 . The nominal intrinsic resonance frequencies of the resonators, ω_0l and ω₀₂ , are not the same. The normalized T2 power spectrum of the resonator-system 100 is shown by the solid line in FIG. 8.

As in the previous embodiment, the resonance frequencies and the couplings of the two resonators 112-1, 112-2 are controlled using a thermo-optic effect. By employing heaters 122-1, 122-2 as tuning devices at each resonator 112-1, 112-2, the resonance frequency of each resonator is dynamically controlled. By employing an electrode at each coupling region 114, 116, 118, the couplings of the resonators are dynamically controlled. Specifically, the resonators 112-1, 112-2 can be individually controlled, as in the previous embodiment, using controller 120.

The controller 120 also executes similar control schemes for dynamically controlling the resonator-system 100: 1) none of the resonance frequencies are shifted; 2) the resonance frequencies of the resonators are shifted together; 3) the resonance frequency of the first resonator is shifted; 4) the resonance frequency of the second resonator is shifted; 5) the resonance frequencies of the resonators are shifted in opposite directions; and 6) the resonance frequencies of the resonators are shifted in the same direction but by different amounts.

In schemes 3, 4, 5, and 6, the resonance frequencies of the two resonators are differentially shifted, and the resonators are thereby differentially controlled.

Dynamic control of the resonator-system 100 is implemented by switching between resonator-system states using the different schemes. For example, when the nominal resonance frequency of the first resonator is shifted by a frequency shift of + Δ-y, and the nominal resonance frequency of the second resonator is shifted by a frequency shift of - Aω₂ , the transmission spectrum of the resonator- system 100 is shown by the dashed line in FIG. 8. The resonance frequencies of the resonators can also be changed by different amounts, such as +

and - 2Δ-y₂ , respectively, in other control schemes.

This resonator-system control is achieved by dynamically switching from a resonator-system state where the resonances are not shifted to a state where the resonances of the resonators 112-1, 112-2 are shifted opposite to each other using resonant frequency scheme (5).

By varying the amount of frequency shift and coupling change with the optical effect, analog control of the transmission signal magnitude is provided. As a result, a wide range of transmission signal magnitudes can be obtained for an optical channel by varying the optical effect. Four resonator-svstem

By increasing the number of resonators in the resonator-system, a greater degree of control over the lineshape is possible and a larger number of lineshapes are possible.

FIG. 9 shows a block diagram of another embodiment where a resonator- system 100 is composed of four resonators. Here, the resonator-system 100 is coupled to a first waveguide 10 and a second waveguide 12. The resonator-system has two input ports, II and 12, and two output ports, TI and T2. The resonator-system 100 is comprised of a first resonator 112-1, a second resonator 112-2, a third resonator 112-3, and a fourth resonator 112-4, each supporting one resonance mode, and being coupled to each other in series. A first coupling region 114 couples the first resonator 112-1 to the first waveguide 10 and a second coupling region 116 couples the fourth resonator 112-4 to the second waveguide 12. In this embodiment, the nominal intrinsic resonance frequencies, ω₀ , of the four resonators 112-1-112-4 are all substantially the same or calibrated to be the same. The coupling coefficients of coupling regions 118-1-118-3 between the resonators and of the coupling regions 114, 116 between the outer resonators 112-1, 112-4 and the waveguides 10, 12 are designed to achieve a variety of filter responses depending on the application, such as a flat-top Butterworth response or an equal-ripple Chebychev Type I response. In this embodiment, a flat-top power spectrum with a four-pole Butterworth response is used, where, for an input at II , the normalized T2 spectrum is

T2(ω) = ^r— .

(ω- ω₀ + γ*

The TI and the T2 spectra of this quiescent state are illustrated in FIG. 10. A similar TI response results for an input at 12 instead of II . In this embodiment, where loss is neglected, TI can be described as the difference between II and T2. Similar to the previous embodiment, the resonator frequencies and the couplings can be changed using an optical effect produced by heaters 122-1-122-4. However, because there are now four resonators, the number of control schemes executed by controller 120 is much larger. All of these schemes can be useful, but several particular schemes, where the resonance frequencies are differentially shifted and the resonators are differentially controlled, provide desirable responses for common filtering and DWDM functions, such as ON/OFF switching, variable optical attenuation, and tapping. Complex combinations can also be used for various applications, such as phase and stitch error compensation.

Several particular resonance frequency control schemes can be used to provide lineshape control for ON/OFF switching. For example, a scheme in which the resonance frequencies of the middle two resonators 112-2, 112-3 are shifted away from the resonant frequencies of the outer two resonators 112-1, 112-4 or a scheme in which the resonance frequencies of alternate resonators are shifted together can be used to obtain an ON/OFF switching function.

1. Middle Resonators

FIG. 11 shows the shifting of the resonant frequencies of the middle two resonators 112-2, 112-3 away from the outer two resonators 112-1, 112-4, by ± Aω , by control of the corresponding heaters 122-1-122-4 using controller 120. This yields the lineshape control and filter response, as illustrated in FIG. 12. The optical channels typically used in WDM optical communication systems are indicated by dotted lines in FIG. 12. In this scheme, the frequencies of the outer two resonators are shifted by + Aω and the frequencies of the middle two resonators are shifted by - Aω . Alternatively, the frequencies of the outer two resonators can be shifted by - Aω and the frequencies of the middle two resonators can be shifted by + Aω . The resulting TI spectrum, solid line, has a low hump and two narrow side peaks. The low hump and narrow side peaks can be positioned between channels to obtain a switching function. The resonator-system in this OFF state, directs a significant portion of the power of an II signal, in the optical channel of interest, to T2 without significantly affecting the neighboring channels. By switching between the unshifted resonator frequency state, as shown in FIG. 10, and this OFF state, an ON/OFF switching function is provided.

2. Alternate Resonators

FIG. 13 shows the shifting of the resonant frequencies of alternate resonators by ± Aω . This also produces the lineshape control and filter response illustrated in FIG.

14, which is applicable to ON/OFF switching. The optical channels typically used in WDM optical communication systems are indicated by dotted lines in FIG. 14. In this scheme, the frequencies of the first and third resonators 112-1, 112-3 are shifted by + Aω and the frequencies of the second and fourth resonators 112-2, 112-4 are shifted by - Aω . Alternatively, the frequencies of the first and third resonators 112-1, 112-3 are shifted by - Aω and the frequencies of the second and fourth resonators 112-2, 112-4 are shifted by + Aω .

The TI spectrum has two sharp peaks that are spaced apart. As in the previous scheme, the peaks can be positioned in between channels to obtain a switching function.

The resonator-system 100 in this OFF state, directs a significant portion of the power of the II signal, in the optical channel of interest, to T2 without significantly affecting the neighboring channels. By switching between the unshifted resonator frequency state, as shown in FIG. 10, and this OFF state, another ON/OFF switching function is provided.

3. Paired Resonators

FIG. 15 shows a control scheme in which the resonant frequencies of the first two resonators 112-1, 112-2 and the last two resonators 112-3, 112-4 are shifted in opposite directions. This yields the lineshape control and filter response shown in FIG. 16, which can be used for ON/OFF switching. The optical channels typically used in WDM optical commumcation systems are indicated by dotted lines. In this scheme, the frequencies of the first and second resonators 112-1, 112-2 are shifted in the same direction but by different amounts, and the frequencies of the third and fourth resonators 112-3, 112-4 are shifted in the same direction but by different amounts. The resonant frequencies of the first and second resonators 112-1, 112-2 are shifted in the opposite direction of the resonant frequencies of the third and fourth resonators 112-3,

112-4. In this scheme, the frequency of the first resonator 112-1 is shifted by + Aω_x , the frequency of the second resonator 112-2 is shifted by + Aω₂ , the frequency of the third resonator 112-3 is shifted by - Aω₂ , and the frequency of the fourth resonator 112-4 is shifted by + Aω , where |Δ-y,| > |Δ-y.| . Alternatively, as in the previously described schemes, the direction in which all of the frequencies are shifted can be reversed.

The TI spectrum has two low peaks, with side lumps, that are spaced apart. As in the previous scheme, the peaks can be positioned in between channels to obtain a switching function.

The resonator-system in this OFF state directs a significant portion of the power of the II signal, in the optical channel of interest, to T2 without significantly affecting the neighboring channels. By switching between the unshifted resonator frequency state and this OFF state, another ON/OFF switching function is provided.

Flat Top Response

Numerous other schemes can be used for achieving an ON/OFF switching function, as well. However, in DWDM systems, a flat top response across the optical channel of interest is desirable for intermediate TI levels between ON and OFF.

FIG. 17 shows the shifting of alternating resonator frequencies in the same direction, but by different amounts. A lineshape control and filter response obtained is shown in FIG. 18. The optical channels typically used in WDM optical commumcation systems are indicated by dotted lines. In this scheme, the frequency of the first resonator 112-1 is shifted by + Δ-y, , the frequency of the second resonator 112-2 is shifted by - Aω₂ , the frequency of the third resonator 112-3 is shifted by + Aω₂ , and the frequency of the fourth resonator 112-4 is shifted by - Aω , where

.

Alternatively, as in the previously described schemes, the direction in which all of the frequencies are shifted can be reversed.

The TI spectrum has two peaks that are spaced apart and separated by a flat, saddle region. As in the previous schemes, the peaks can be positioned between channels without significantly affecting the neighboring channels.

By varying the amounts of the frequency shifts, Aω_{ and Aω₂ , the TI spectrum level can be varied over a large range while maintaining flat response in the channel window, as shown in FIG. 19. This variable control can be used to provide a variable splitting function. Variable Splitting

In a variable splitting function, a desired portion of the II channel signal is directed to TI while the remaining portion of the signal is directed to T2. By dynamically adjusting the resonance frequency control, the level of the flat-top channel lineshape can be varied to achieve any desired T2 level, within the dynamic range of the filter, as illustrated in FIG. 19. This can be used to provide a variable optical attenuation (VOA) function for the TI channel signal. A VOA function is useful for gain flattening channel signals.

By splitting a small portion of the signal, such as < 5% , a tap function is provided. A tap function is useful for testing the level of a channel signal without significantly affecting the signal. Using the dynamic lineshape control, the tap can also be switched between ON and OFF states.

Phase and Stitching Error Compensation During planar waveguide manufacture, fabrication errors, such as phase errors or stitching errors, can occur. Phase errors are typically associated with line-width or index fluctuations that cause the resonant frequencies to deviate from their design frequency. Stitching errors are associated with lithography techniques that write large patterns using small fields that are "stitched" together. Stitching errors can affect both the design frequencies of the resonance frequencies and the coupling between resonators.

Figs. 20A and 20B show the correction of phase and stitch error in a four resonator-system using resonance frequency shifting. The random fabrication errors, such as stitch errors and phase errors, distort the lineshape of the filter, as shown in FIG. 20 A. This type and level of distortion of the lineshape is unacceptable for most applications, such as WDM filters.

By individually controlling the resonance frequency of each resonator, the resonator-system is adjusted to compensate for the phase and stitch errors that arise from the fabrication process. In this embodiment, the resonance of the first resonator is shifted by - Aω_λ , the resonance of the second resonator is shifted by + Aω₂ , the resonance of the third resonator is shifted by - Aω₃ , and the resonance of the fourth resonator is shifted by + Aω . The filter lineshape errors and central wavelength errors are corrected to a high degree of fidelity from the original design, as shown in FIG. 20B.

Once error compensation has been applied to the resonator-system, such that it is calibrated or tuned to its nominal operating frequency based on the design criteria, the lineshape control schemes previously described can be implemented to provide the desired functionality, such as ON/OFF switching, variable optical attenuation, and tapping by controlling the resonator frequencies and couplings with respect to the compensated system.

Pre-Biasing

While fabrication errors can result in unwanted resonance frequency shifting, in some cases it is advantageous to pre-bias the resonator-system for a desired function with no active, or dynamic control.

In another exemplary embodiment, the resonator-system of FIG. 13, for example, which is comprised of four resonators, is pre-biased to an OFF state so that the channel signal is not directed to T2. The intrinsic resonance frequencies of the resonators 112-1-112-4 are designed differently so that the resonant frequencies of the first and third resonators 112-1, 112-3 are at ω_γ and the resonance frequencies of the second and fourth resonators 112-2, 112-4 are at ω₂ , where ω = ω₀ + Aω and ω₂ - ω₀ - Aω . In order to m the resonator-system 100 ON, the resonant frequencies of the first and third resonators are shifted by - Aω and the resonant frequencies of the second and fourth resonators are shifted by + Aω by control of the respective heaters by controller 120. As a result, the resonant frequencies of all the resonators are the same, ω₀ , and the channel signal is directed to T2. This example of pre-biasing results in a filter with a nominal OFF state. It can be advantageous for power consumption optimization or in power loss conditions where an OFF state is maintained when the controller has lost power or is disabled.

Pre-biasing can also be used to design a resonator-system 100 for a particular function, which can then be dynamically changed. In another embodiment, a resonator- system comprised of four resonators is pre-biased to function as a tap. By dynamically controlling the resonance frequencies of the resonators, the resonator-system can be changed to function as a VOA and as an ON/OFF switch. The resonators can also be pre-biased in a partially ON or half-ON state to reduce overall power consumption.

Group Heater Control Tuning mechanisms, such as heaters, can also be used to control more than one resonator. FIG. 21 shows a block diagram of another embodiment where a resonator- system 100 is comprised of four resonators. The resonator-system 100 is coupled to a first waveguide 10 and a second waveguide 12. The resonator-system 100 is comprised of a first resonator 112-1, a second resonator 112-2, a third resonator 112-3, and a fourth resonator 112-4, each supporting one resonance mode, and being coupled to each other in series.

Similar to the previous embodiments, the resonator frequencies and the couplings can be changed using an optical effect produced by heaters 122-1-122-2. In this embodiment, each heater affects two resonators and is controlled using a controller 120.

The filters in the embodiments described above employ resonator-systems with two and four resonators, but can be easily extended to any number of resonators, greater than two, in the resonator-system, and in any configuration. The methods above can also be extended to systems where the coupling between resonators is changed using the controller 120. A number of specific control schemes have been described; however, the resonators can also be controlled to provide any desired control scheme.

Filters with two resonator-system sub-elements The lineshape control schemes described in the previous embodiments can also be used for two identical resonator-system sub-elements according to a side-coupled filter design, described in U.S. Patent 6,101,300, teachings of which are incorporated herein by this reference in their entirety, or a directional-coupler assisted add/drop filter design as described, for example, in U.S. Pat. Appl. Ser. No. 10/096,616, entitled "Directional-Coupler Assisted Add/Drop Filter With Induced On/Off Switching and Modulation" filed March 7, 2002 by common assignee, which is incorporated herein by this reference in its entirety. In another embodiment, a directional-coupler assisted add/drop filter uses two identical resonator-system sub-elements, such as 4 coupled resonators as shown in FIG. 9, coupled to an input waveguide and an output waveguide by directional couplers.

FIG. 22 shows this embodiment. The resonance frequencies of the resonators are shifted or controlled using a thermo-optical effect. Directional-couplers 150, 152 provide the filter 50 with two input ports, IN and ADD, and two output ports, DROP and TRANSMISSION. In this embodiment, each sub-element 100-1, 100-2 is comprised of four phase-shift grating resonators 112-1-112-4 and each resonator 112 has a resonant frequency that is tuned or shifted using a corresponding heater 122-1- 122-4.

The transmission and reflection spectra of the filter 50 are dynamically adjusted by controlling the resonance frequencies of the resonators 112 in the sub-elements 100- 1 , 100-2 to provide functions such as switching, tapping, VOA, and error compensation, as described in the previous embodiments. Both sub-elements 100-1, 100-2 in the filter can be controlled using the same resonance frequency shifting scheme, such as for switching, or each sub-element can be controlled using different frequency shifting schemes, such as in error compensation. For example, for the switching function, when the filter is ON, a desired channel is dropped to the DROP port and the remaining channels are transmitted to the TRANSMISSION port. When the filter is OFF, all the channels of interest are directed to the TRANSMISSION port.

In a VOA or tapping function a desired portion of the IN port channel signal is directed to the DROP port.

The coupling coefficients can also be varied to control the filter lineshape and achieve a switching function or for error compensation. FIG. 23 shows another embodiment. The coupling coefficients of two resonator-system sub-elements 100-1, 100-2, each comprised of four resonators 112-1 to 112-4, in a side-coupled filter 50 are varied using the coupling control schemes previously described. The side-coupled add/drop filter uses two identical sub-elements 100-1, 100-2, each with 4 coupled resonators as shown in FIG. 9. These sub-elements 100-1, 100-2 are side-coupled to an input waveguide and an output waveguide.

The couplings of the resonators are controlled in this embodiment using an electro-optical effect. In this embodiment, each sub-element 100-1 , 100-2 includes four phase-shift grating resonators 112-1 to 112-4 and the resonators' couplings are controlled using electrodes 160. The side-coupled filter has two input ports, IN and ADD, and two output ports, DROP and TRANSMISSION.

In this embodiment, the coupling region electrodes 160 are used to control the refractive index of the material in the coupling region between each resonator 112. Reducing the coupling between the resonators 112 by control of the coupling region electrodes 160 results in a collapse of the lineshape of the DROP port signal and ultimately stops any drop from occurring at the desired channel. This results in a switching function. The transmission and reflection spectra of the filter 50 are dynamically adjusted by controlling the coupling between the resonators 112 in the sub-elements 100-1 , 100- 2 to provide functions such as switching and error compensation, as described in the previous embodiments. Both sub-elements 100-1, 100-2 in the filter can be controlled using the same coupling control scheme, such as for switching, or each sub-element can be controlled using different coupling control schemes, such as in error compensation. For example, in the switching function, a desired channel is dropped to the DROP port and the remaining channels are transmitted to the TRANSMISSION port, when the filter is ON. When the filter is OFF, all the channels of interest are directed to the TRANSMISSION port. The lineshape control schemes described in the previous embodiments can also be used for a multi-channel add/drop filter, described, for example, in U.S. Pat. Appl. Ser. No. 10/098,577, entitled "Superstructure Photonic Band-Gap Grating Add-Drop Filter" filed March 14, 2002 by common assignee, which is incorporated herein by this reference in its entirety. In another embodiment, shown in FIG. 24, two identical waveguide arms, each having a resonator-system 100-1, 100-2, are coupled together by two directional couplers 150, 152 to yield a multi-channel add/drop filter. In this filter 50, the resonator-system 100-1, 100-2 of each grating waveguide arm is configured with identical superstructure photonic bandgap gratings each having six equally spaced grating sections 110-1-110-6 that form five coupled Fabry-Perot cavities, or five coupled multiple frequency resonators 112-1-112-5. Each resonator has an associated heater 122-1-122-5. The resonators 112-1-112-5 together produce a multiple resonant cavity Fabry-Perot interferometer with a periodic flat-top lineshape, as illustrated in FIG. 25. The filter has two input ports, IN and ADD, and two output ports, DROP and TRANSMISSION. In this embodiment, the filter performs a spectral de- interleaver operation, where every second channel is dropped to the DROP port and the other non-dropped channels are routed to the TRANSMISSION port. The center frequencies of the optical channels typically used in WDM optical commumcation systems are indicated by the dotted lines.

The dynamic lineshape control of the filter can be obtained by shifting the resonant frequencies of the resonators 112 and by changing the coupling between the resonators, as described in the previous embodiments. In this embodiment, the resonators 112 are controlled using their associated heaters 122. By controlling the resonators, the interleaver can be turned OFF so that all the channels are directed to the DROP port, as shown in FIG. 26. Also, by dynamically controlling the resonators, a dynamic ON/OFF function is provided for the filter.

Multipurpose Application

As described in the previous embodiments, the same filter design can be used for various functions by using different control schemes. The general design of the filter does not need to be changed. Moreover, because the same filter can provide different functions, defined by how the resonances and couplings are controlled, the same filter device can be used to replace a variety of different components by simply using different control schemes. For example, in a system that requires an add/drop filter, a VOA, and a tap, dynamically controlled lineshape filters can be used for each component, using the appropriate function, instead of several different components.

Controllers

In all of the previous embodiments, a programmable controller can also be used.

In one current implementation, a circuit board employing a signal processor, such as a

TMS32024x™ chip from Texas Instruments, is used to control the resonators. In another exemplary embodiment, multiple dynamic lineshape control filters are connected. A controller controls the resonators of each filter and thereby controls the function performed by each filter. By changing the resonance frequencies and couplings of the resonators via the controller, the dynamic lineshape filter can be used for a variety of static and dynamic filter systems. The filters can be statically programmed to provide a single function or multiple functions or can be dynamically reprogrammed during operation. The filters can also be controlled together, so that they have the same function, or independently, so that each filter can have a different function.

Although all of the above embodiments use one controller to control the resonators, multiple controllers can of course be used. In another embodiment, several dynamic lineshape filters 50-1, 50-2, 50-3, 50-4 are connected in succession, as shown in FIG. 27. Each filter has a sub-controller 120-1, 120-2, 120-3, 120-4 for controlling the resonators in the filter. The multiple sub-controllers are further controlled using a system controller 120-5. Sub-controllers can also be grouped with each group controlled by another controller.

Although the invention has been shown and described with respect to several exemplary embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.

What is claimed is:

Claims

CLAIMS An optical lineshape filter comprising: a resonator-system comprising at least two coupled resonators; and at least one controller, wherein said controller controls said resonator-system by differentially changing the refractive indices of at least two of said resonators.

2. A lineshape filter of claim 1 , wherein said controller controls said resonator- system by shifting resonance frequencies of said resonators.

3. A lineshape filter of claim 1. wherein said controller increases a resonance frequency of at least one resonator while decreasing a resonance frequency of at least one other resonator.

4. A lineshape filter of claim 1, wherein said controller shifts a resonance frequency of at least one resonator while substantially not shifting a resonance frequency of at least one other resonator.

5. A lineshape filter of claim 1 , wherein said controller shifts a resonance frequency of at least one resonator more than a resonance frequency of at least one other resonator.

6. A lineshape filter of claim 1 , wherein said resonators include grating resonators.

7. A lineshape filter of claim 1 further comprising an input waveguide that is coupled to said resonator-system.

8. A lineshape filter of claim 7, wherein said resonator-system is coupled to said input waveguide by at least one directional coupler.

9. A lineshape filter of claim 1 further comprising an output waveguide that is coupled to said resonator-system.

10. A lineshape filter of claim 9, wherein said resonator-system is coupled to said output waveguide by at least one directional coupler.

11. A lineshape filter of claim 1 , wherein said resonators include single mode 2 resonators.

1 12. A lineshape filter of claim 1 , wherein said controller controls at least two of said resonators together.

1 13. A lineshape filter of claim 1, wherein said resonators include phase-shift grating resonators.

1 14. A lineshape filter of claim 1 , wherein said controller controls said resonators for an ON/OFF switching function.

1 15. A lineshape filter of claim 1, wherein said controller controls said resonators for a variable splitting function.

1 16. A lineshape filter of claim 15, wherein said controller controls said resonators in said variable splitting function for signal attenuation.

1 17. A lineshape filter of claim 15, wherein said controller controls said

2 resonators in said variable splitting function for signal tapping.

1 .

18. A lineshape filter of claim 1 further comprising heaters for changing the

2 refractive indices of said resonators.

1 19. A lineshape filter of claim 1 further comprising a heater for each resonator

2 for changing the refractive indices of each resonator.

1 20. A lineshape filter of claim 1 , wherein said controller controls said

2 resonators for post-fabrication compensation.

1 21. A lineshape filter of claim 1, wherein said controller controls said

2 resonators for a flat top response.

1 22. A lineshape filter of claim 1, wherein said resonator-system comprises at

2 least 4 phase-shift grating resonators.

1 23. A lineshape filter of claim 1 , wherein said resonator-system comprises at

2 least 8 phase-shift grating resonators.

l

24. A lineshape filter of claim 1, wherein said resonator-system comprises at least 12 phase-shift grating resonators.

25. A lineshape filter of claim 6, wherein said grating resonators include superstructure photonic bandgap gratings.

26. A lineshape filter of claim 1 , wherein said resonators include Fabry-Perot resonators.

27. A lineshape filter of claim 7 , wherein said resonator-system is side-coupled to said input waveguide.

28. A lineshape filter of claim 9, wherein said resonator-system is side-coupled to said output waveguide.

29. A lineshape filter of claim 1, wherein said resonators include ring- resonators.

30. A lineshape filter of claim 1, wherein said controller controls couplings of said resonators.

31. A method for controlling a resonator-system comprising at least two resonators, said method comprising:

filtering an input signal with the resonator-system in a first state to apply a first filtering function to produce an output signal; and

switching the resonator-system to a second state by differentially changing the refractive indices of resonators of the resonator-system to apply a second filtering function to produce the output signal.

32. A method as claimed in claim 31, wherein the step of switching the resonator-system to the second state comprises changing temperatures of the resonators.

33. A method as claimed in claim 31, wherein the step of switching the resonator-system to the second state comprises changing temperatures of the resonators by different amounts.

34. A method as claimed in claim 31, wherein the step of switching the resonator-system to the second state comprises increasing temperatures of the resonators by different amounts.

35. A method as claimed in claim 31, wherein the step of switching the resonator-system to the second state comprises decreasing temperamres of the resonators by different amounts.

36. A method as claimed in claim 31, wherein the step of switching the resonator-system to the second state comprises increasing a temperature of one of the resonators while decreasing a temperature of another resonator.

37. A method as claimed in claim 31, wherein the step of switching the resonator-system to the second state comprises changing a temperature of one of the resonators while maintaining a temperature of another resonators.

38. A method as claimed in claim 31, wherein the step of filtering the input signal with the resonator-system in the first state comprises controlling temperatures of resonators in the resonator-system in response to a post-fabrication calibration.

39. A method as claimed in claim 31 , wherein the resonators include phase- shifted grating resonators.

40. A method for controlling a resonator-system of a planar waveguide system, the resonator-system comprising at least two resonators, said method comprising:

heating the resonators of the resonator-system to calibration temperatures so that the resonator-system functions according to a first state to apply a first filtering function to produce an output signal; and

changing the temperatures of the resonators from the calibration temperatures so that the resonator-system functions according to a second state to apply a second filtering function to produce the output signal.