CN1333285C - Polarization controller and use thereof - Google Patents

Abstract

A polarization controller and its application belong to the field of optical fiber communication. In order to realize the polarization control in the receiving end, the demultiplexing end and the polarization mode dispersion compensation device of the optical fiber communication system, and at the same time realize the polarization state random perturbation function in the experiment related to the random characteristics of polarization mode dispersion, the invention discloses a polarization control The device includes an optical part and an electronic control part composed of a computer and a drive circuit. The optical part is composed of a Faraday rotator made of three magneto-optic crystals and two quarter-wave plates cascaded alternately along the direction of the optical path. The two quarter-wave plates are fixedly placed, the three Faraday rotators are controlled by the electronic control part, and the electronic control rotation angle ranges are respectively ±45°, ±90°, ±45°, In order to realize the conversion from any determined input polarization state to any determined output polarization state. The invention has the advantages of simple structure, precise control, low cost and easy system integration.

Description

A Polarization Controller and Its Application

technical field

The invention belongs to the field of optical fiber communication.

Background technique

In the optical fiber communication system, the polarization state of the optical signal often has obvious fluctuations in the process of propagating along the optical fiber and various optical devices, and the receiving end requires an optical signal with stable polarization characteristics, especially the polarization multiplexing system requires demultiplexing. To accurately identify the polarization state at the receiving end, it is necessary to accurately control the polarization state of the optical signal at the receiving end. In addition, with the continuous improvement of the single channel rate of the optical fiber communication system and the wide application of the dense wavelength division multiplexing system, the polarization mode dispersion effect is considered to be the ultimate obstacle to the development of the optical communication system to a higher rate, large capacity, and long distance. In the polarization mode dispersion compensation scheme, a device that can convert any input polarization state to any output polarization state is essential. In the process of polarization mode dispersion related research, it is also necessary to use polarization state perturbation devices to simulate actual fiber links. The stochastic law of the polarization characteristics of optical signals in China. The above functions of controlling the polarization state of the optical signal can all be realized by a polarization controller.

Existing polarization controllers mainly include fiber extrusion/winding type, wave plate rotation type and liquid crystal/electro-optic modulation type. Among them, the optical fiber extrusion/winding type polarization controller mainly uses piezoelectric ceramics and other devices to apply external forces in different directions to the optical fiber, thereby generating a stress birefringence effect to change the polarization state. The advantages are that the structure is relatively simple and the cost is relatively low. However, due to factors such as physical fatigue, its performance is unstable and precise control cannot be achieved; the wave plate rotation polarization controller is cascaded through more than three half-wave plates and quarter-wave plates, and is manually or electronically controlled Rotate the main axis direction of the wave plate to introduce birefringence in different directions, so as to achieve the purpose of polarization control. This method is more accurate than the fiber extrusion/winding type, but it affects the control speed due to the mechanical rotation of the wave plate. ; The liquid crystal/electro-optic modulation type polarization controller uses the electro-optic effect of liquid crystal or LiNO ₃ and other crystals to introduce birefringence in different directions to control the polarization state. This type of method has high precision and fast speed, but the structure is complex and the cost is high.

Contents of the invention

The purpose of the present invention is to realize a polarization controller that can be applied to the receiving end, demultiplexing end and polarization mode dispersion compensation device of the optical fiber communication system, and can realize the random disturbance of the polarization state applied to the correlation research of polarization mode dispersion at the same time Function. Compared with the polarization controller of the liquid crystal/electro-optic modulation type, the polarization controller has a simpler structure and lower cost. Compared with the polarization controller of the fiber extrusion/winding type and the wave plate rotation type, it avoids the mechanical movement of the components, making Controls are more precise and faster.

The present invention proposes a polarization controller, including an optical part, and an electric control part composed of a computer and a drive circuit, characterized in that: the optical part of the polarization controller is a Faraday rotator made of three magneto-optical crystals and two quarter-wave plates alternately cascaded along the optical path direction, the two quarter-wave plates are fixedly placed, and the three Faraday rotators are controlled by the electronic control part, The ranges of electronically controlled rotation angles are ±45°, ±90°, and ±45° respectively, so as to realize the conversion from arbitrarily determined input polarization state to arbitrarily determined output polarization state.

The application of the above-mentioned polarization controller in the research experiment of the random characteristics of polarization mode dispersion is characterized in that: the Faraday rotator made of three magneto-optic crystals is randomly controlled by the electric control part of the polarization controller to realize the random disturbance of the output polarization state, so that the output The polarization states are spread throughout the ganged sphere.

The advantage of the polarization controller of the present invention is that the wave plate is fixedly placed, and the polarization state is rotated by the magneto-optic effect, avoiding mechanical rotation, and can ensure control accuracy, so that it has the advantages of simple structure, precise control, and low cost. Low cost and easy system integration.

Description of drawings

Fig. 1 is a structural schematic diagram of an embodiment of the polarization controller of the present invention, in which the slow axes of the two quarter-wave plates are fixedly placed at 0° to the x-axis of Jones space.

Fig. 2 is a structural schematic diagram of another embodiment of the polarization controller of the present invention, in which the slow axes of two quarter-wave plates are fixedly placed at 0° and 45° to the x-axis of Jones space, respectively.

3a to 3d are schematic diagrams of the polarization control process realized by using the polarization controller of the present invention.

Fig. 4 is a schematic diagram of the process of realizing random disturbance of the polarization state by using the polarization controller of the present invention.

Detailed ways

1. The principle analysis of Faraday rotator realized by magneto-optic crystal.

The magneto-optic effect, also known as the Faraday effect, refers to the phenomenon of circular birefringence in the material when a magnetic field is applied to the magneto-optic material, that is, right-handed polarized light and left-handed polarized light experience different refractive indices in the medium. The magneto-optic effect makes the polarization state of the light propagating in the magneto-optic medium rotate along the _S3 axis of the Stokes space, and the rotation angle is related to the strength of the magneto-optic effect, which can be changed by changing the magnetic field applied to the magneto-optic material Intensity is controlled. This rotation effect on the polarization state can be represented by the following Jones matrix J and Stokes matrix S:

$\mathrm{<span\; class="notranslate">\; J</span>}<span\; class="notranslate">\; =</span>\left[\begin{array}{cc}\mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; \&alpha;</span>}& <span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; sin</span>}\mathrm{<span\; class="notranslate">\; \&alpha;</span>}\\ \mathrm{<span\; class="notranslate">\; sin</span>}\mathrm{<span\; class="notranslate">\; \&alpha;</span>}& \mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; \&alpha;</span>}\end{array}\right]<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

$\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; =</span>\left[\begin{array}{ccc}\mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&alpha;</span>}& <span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; sin</span>}\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&alpha;</span>}& \mathrm{<span\; class="notranslate">\; 0</span>}\\ \mathrm{<span\; class="notranslate">\; sin</span>}\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&alpha;</span>}& \mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&alpha;</span>}& \mathrm{<span\; class="notranslate">\; 0</span>}\\ \mathrm{<span\; class="notranslate">\; 0</span>}& \mathrm{<span\; class="notranslate">\; 0</span>}& \mathrm{<span\; class="notranslate">\; 1</span>}\end{array}\right]<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

where α represents the angle of rotation.

The Faraday rotator rotates the polarization state of light based on the above-mentioned magneto-optic effect. A variety of magneto-optic crystal materials are available, for example, bismuth-substituted rare earth iron garnet single crystal has the characteristics of low insertion loss and obvious Faraday effect. The electric control part composed of computer and driving circuit drives the coil to generate a magnetic field with controllable strength and direction and applies it to the magneto-optical crystal, so that the rotation angle of the polarization state can be controlled within the range of ±45° or ±90°. In order to reduce the loss fluctuation of the magneto-optic crystal, two variable magnetic fields can be applied from the transverse direction and the longitudinal direction respectively, so that the total controllable magnetic field can reach saturation.

2. Analysis and examples of the parameter control principle of the polarization controller.

The present invention utilizes magneto-optic crystals and a quarter-wave plate to implement a polarization controller, the polarization controller includes an optical part, and an electric control part composed of a computer and a drive circuit, the structures of which are shown in Figures 1 and 2. The optical part is composed of three Faraday rotators made of magneto-optic crystals and two quarter-wave plates cascaded alternately, and the two quarter-wave plates can be selected from the center wavelength of 1550nm suitable for optical fiber communication. plate, or other wave plates suitable for other central wavelengths in other fields, the two quarter-wave plates are fixedly placed, and the slow axis of the quarter-wave plate can form any angle with the x-axis of Jones space, preferably 0° (See Figure 1) or 45° (See Figure 2). The electronically controlled rotation angle ranges of the three Faraday rotators are respectively ±45°, ±90°, and ±45°, which are controlled by an electronically controlled part composed of a computer and a drive circuit. The computer control algorithm and drive circuit design and manufacture in the electronic control part can be realized by conventional technology.

The working principle of the polarization controller is as follows: the first Faraday rotator 1 rotates the polarization state of any input optical signal around the _S3 axis (latitude direction) on the Ganga sphere by an appropriate angle; The wave plate 10 rotates the rotated polarization state by 90° around the _S1 axis, so that the optical signal becomes a linearly polarized state; then the second Faraday rotator 2 rotates the polarization state around the _S3 axis along the equator of the added sphere by an appropriate angle; The second quarter-wave plate 20 rotates the linear polarization state around the S _axis by 90° to make it into elliptically polarized light or circularly polarized light of a certain ellipsoid; finally, the third Faraday rotator 3 converts the optical signal to Rotate about the _S3 axis to the specified output polarization state.

Taking the first embodiment of the polarization controller shown in FIG. 1 as an example, the angles between the slow axes of the two quarter-wave plates and the x-axis of the Jones space are 0°. Suppose the polarization state of the input optical signal is [a, b, c]', and the specified output polarization state is [x, y, z]', then the electronically controlled rotation angles of the three Faraday rotators  ₁ ,  ₂ ,  ₃ are respectively for:

Among them, fix is a rounding function towards 0,

$\mathrm{<span\; class="notranslate">\; \&Delta;\&theta;</span>}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; sgn</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; a</span>}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; arctan</span>}\frac{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; c</span>}}{\mathrm{<span\; class="notranslate">\; sgn</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; a</span>}<span\; class="notranslate">\; )</span>\sqrt{{\mathrm{<span\; class="notranslate">\; a</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; b</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; sgn</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; arctan</span>}\frac{\mathrm{<span\; class="notranslate">\; z</span>}}{\mathrm{<span\; class="notranslate">\; sgn</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; )</span>\sqrt{{\mathrm{<span\; class="notranslate">\; x</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; the\; y</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 6</span>}<span\; class="notranslate">\; )</span>$

Among them, sgn is a symbolic function.

Write a computer control program according to the above control algorithm, and control the driving circuit to drive the coil, and change the magnetic field applied to the Faraday rotator in the optical part to realize the rotation control of the polarization state of the light.

If the slow axis of the two quarter-wave plates is fixed at other angles such as 45° to the x-axis of the Jones space, the above control algorithm needs to be modified accordingly.

Figures 3a to 3d show an example process of controlling the conversion of an arbitrarily determined input polarization state to an arbitrarily determined output polarization state. The entire transformation process is expressed on the help sphere, where the input polarization state is [0.6000, 0.3000, -0.7416]', and the required output polarization state is [-0.5000, -0.5000, 0.7071]'. The first Faraday rotator 1 rotates the polarization state of the input optical signal around the _S3 axis (latitude direction) on the side-adding sphere-26.5651 °, and the polarization state changes to [0.6708, 0,-0.7416]'; the first quarter A wave plate 10 rotates the rotated polarization state by 90° around the _S1 axis, so that the optical signal becomes a linearly polarized state [0.6708, 0.7416, 0]'; the second Faraday rotator 2 rotates the polarization state around the _S3 axis along the The equator of the added sphere is rotated by 87.1304°, and the polarization state changes to [-0.7071, 0.7071, 0]'; the second quarter-wave plate 20 rotates the linear polarization state by 90° around the S ₁ axis to make it elliptical polarization Light [-0.7071, 0, 0.7071]'; Finally, the third Faraday rotator 3 rotates the optical signal by 45.0000° around the _S3 axis to the specified output polarization state [-0.5000, -0.5000, 0.7071]'.

Since the polarization controller of the present invention can realize the output of any polarization state, it can be applied in the research experiment of the random characteristic of polarization mode dispersion to realize the function of random disturbance of the polarization state. When any input polarization state is specified, if the electronic control part is used to control the three Faraday rotators to randomly take values within the range of their respective angles, the output polarization state will appear randomly on the entire helper sphere, as shown in Figure 4 for 5000 The distribution of the output polarization state of the sub-independent sampling, it can be seen that the output polarization state spreads all over the added sphere. On the other hand, if the input polarization state is a fluctuating unknown value, and it is necessary to control the output polarization state to be constant at a certain value, a polarizer can be used at the output end to detect the output polarization state, and an optimization algorithm is used to feed back and control the three polarization controllers. A control parameter, so as to achieve the effect of stabilizing the output polarization state.

Claims (2)

1. Polarization Controller, comprise opticator, and the automatically controlled part of forming by computing machine and driving circuit, it is characterized in that: the opticator of described Polarization Controller is to be formed along the alternate cascade of optical path direction by Faraday rotator and two quarter-wave plates that three magneto-optical crystals are made, described two quarter-wave plate fixed placement, described three Faraday rotators are controlled by described automatically controlled part, automatically controlled rotation angle range is respectively ± and 45 °, ± 90 °, ± 45 °, to realize determining that arbitrarily input polarization is to the conversion of determining the output polarization attitude arbitrarily.

2. Polarization Controller according to claim 1 is characterized in that: the slow axis of described quarter-wave plate becomes 0 ° or 45 ° with the x axle in Jones space.