CN1264291C - Polarization mould disporsion compensation system - Google Patents

Abstract

The invention discloses a polarization mode dispersion compensation system belonging to the field of optical fiber communication. The system includes a coupler, a polarization controller, an optical collimator, a variable optical delay line, a detector, a computer and a control system. The optical collimator and the variable optical delay line are placed on a straight line with the optical axis concentric; after the coupler, the detector, and the computer are connected in series, the computer is connected to the polarization controller and the variable optical delay line respectively to form a feedback control circuit. Its variable optical delay line adopts several sections of birefringent crystals and PRs arranged and fixed at intervals, so the structure is simple, the performance is stable, and it is less affected by the external environment. The feedback control circuit composed of computer, detector, etc. makes the polarization controller generate the polarization state opposite to the PMD direction of the optical fiber communication system, and cancel each other to achieve the purpose of polarization mode dispersion compensation. The device is simple to assemble, convenient to adjust, easy to manufacture, and convenient to carry and construct, thereby reducing its price and having broad application space.

Description

Polarization Mode Dispersion Compensation System

technical field

The invention belongs to the field of optical fiber communication, in particular to a polarization mode dispersion compensation system.

Background technique

Polarization mode dispersion is an important cause of signal broadening in high-speed and large-capacity optical fiber communication systems. In an ideal optical fiber transmission system, the optical fiber has a circular symmetric structure. However, in the actual system, the optical fiber is not an ideal circular symmetric structure due to manufacturing reasons and the influence of temperature, vibration, stress, etc., which causes random birefringence. When a beam of optical signal is transmitted in such an optical fiber, the optical signal will be split, thereby causing pulse broadening. This phenomenon of pulse broadening is called polarization mode dispersion (PMD for short). When analyzing the PMD in the transmission system, the optical fiber is usually considered as a birefringent medium with two orthogonal polarization states, and the transmission speed of the optical signal in the two orthogonal polarization states is different, resulting in signal broadening. When characterizing the PMD of a system, the direction of the polarization state with the fastest propagation speed among the orthogonal polarization states is usually defined as the direction of the PMD, and the group delay difference of the signal transmitted on the two orthogonal polarization states is called the group delay difference (DGD for short). ). PMD seriously affects the quality of signal transmission, especially in systems where the transmission rate of the optical cable laid in the early stage is greater than 10Gbps, the signal broadening caused by PMD will make the signal indistinguishable, so it must be compensated.

Due to the unpredictability of the random birefringence of the fiber, it is determined that the PMD compensation must be a dynamic adaptive method. For this reason, people have designed many compensation methods and devices. In addition to the electrical compensation method, these methods are basically based on the method of using a variable or fixed optical delay line unit plus a polarization controller and a feedback control circuit. The PMD that is artificially generated is equal to and opposite to the PMD in the system, and the two interact with each other. Offset to achieve the purpose of reducing the system PMD. However, in literature (1). D. Sobiski, D. Pikula, et al, "Fast first-order PMD compensation with low insertion loss for 10Gbit/s system", Electron. Lett., Vol.37, No.1, pp 46 ～48, ^4th Jan.2001; (2) Hok Yong Pua, et al, "Anadaptive first-order polarization-mode dispersion compensation systemaided by polarization scrambling: theory and demonstration", J.ofLightwave Technol., Vol.18, No. 6, pp832～841, Jun.2000; (3).FredHeismann, etal, "Automatic compensation of first-order polarization modedispersion in a 10Gb/s transmission system, in Proc.ECOC'98 (Madrid), pp529～5301 and patent In WO 02/35743, the method based on birefringent fiber and the method of interferometer proposed in WO 02/35743, 2002/5/2, the compensation system is either too long in length, or the device has high requirements on the environment. As in patent WO01/61385, 2001/8 Proposed in /21 adopts the polarization splitting prism and common self-focusing lens as the method of polarization controller and collimator, and there are too many optical-mechanical components, which is unfavorable for system integration. In document (4) S.Lee, etal, "Adjustable compensation of polarization mode dispersion using a high-birefringence nonlinearly chirped fiber Bragg grating", IEEE Photon. The grating method is complex and expensive. Literature (5) D.Sandel, etal, "10-Gb/s PMD compensation using deformed-helicalferroelectric liquid crystals", in Proc.ECOC'98 (Madrid), pp555~556 And the patent US 6,417,948 B1, 2002/7/9, proposed "the method of using liquid crystal as a polarization controller", but its reaction speed is limited due to the inherent properties of liquid crystal. Patent US 2002/0118455A1, 2002/8/29 "Compensation method using high birefringence material with equal length", when there is no PMD in the optical fiber, a certain amount of additional PMD is generated through the PMD compensator, while WO 01 In /10811 A2, 2002/2/7, although the state of PMD is zero is produced by adding electric birefringent crystal or liquid crystal, but usually the driving voltage is relatively high or the environmental requirements are relatively high. In order to make the PMD compensator truly applicable to the optical fiber communication system, it is necessary to design a PMD compensation that is simple in structure, stable in performance, small in size, easy to couple with the system, fast in response, low in power consumption, easy to implement in engineering and capable of dynamic control. system.

Contents of the invention

The purpose of the present invention is to provide a polarization mode dispersion compensation system that is easy to be coupled with the system, has rapid response, low power consumption, is easy to implement in engineering, and can be dynamically controlled. The system includes a coupler, a polarization controller, a pair of optical A collimator, a variable optical delay line and a feedback control circuit are characterized in that: the optical collimator and the variable optical delay line are concentrically placed on a straight line with the optical axis, and the variable optical delay line is located at Between the pair of optical collimators; the detector 5 and the computer 6 of the feedback control circuit are connected in series with the coupler 1, and the computer 6 is connected with the polarization controller 2 and the variable optical delay line 4 respectively, and from the polarization mode The beam emitted by the optical fiber of the optical fiber communication system used in the dispersion compensation system first enters the coupler 1, and the beam emitted by the coupler 1 is divided into two beams, one of which enters the detector 5 as a control signal, and is processed by the computer 6. Obtain the size and the direction of described optical fiber communication system PMD, another bundle enters polarization controller 2, and the light beam that emerges from polarization controller 2 enters optical collimator 3; Computer 6 feeds back the direction information of described optical fiber communication system PMD to The control circuit of polarization controller 2 makes polarization controller 2 produce the polarization state opposite to the optical fiber communication system PMD direction; Computer 6 feeds back the size information of said optical fiber communication system PMD to the control circuit of variable optical delay line 4, so that The DGD value of the variable optical delay line 4 is equal to the DGD in the optical fiber communication system, so as to achieve the purpose of compensating the PMD of the optical fiber communication system.

The coupler, the polarization controller and the optical collimator are all devices with pigtails, which are beneficial to connect with the optical fiber system; the polarization controller is controlled by a DC servo motor, which is convenient for fast and automatic control of the polarization state.

The variable optical delay line includes n+1 birefringent crystals and n polarization rotators PR based on the magneto-optic effect. The crystals and PRs are arranged at intervals in the order of the longest crystal from front to back, PR , the second-longest crystal, PR, until the shortest crystal; its n+1 segment birefringent crystals include n crystals whose length doubles and an additional crystal with the shortest length among the n birefringent crystals; the PR consists of electrified The coil and its internal magneto-optical crystal are composed; by controlling the magnitude and direction of the current in the energized coil, the magnitude and direction of the magnetic field passing through the magneto-optic crystal can be controlled, so that the fast axis or slow axis of the front and rear crystals in the variable optical delay line Overlap, thereby controlling the addition and subtraction of the DGD values of the two crystals; where n is a natural number.

The birefringent crystal is YVO4, the angle between the fast axis and the slow axis between two adjacent crystals is 45 degrees, and PR can rotate the polarization state of linearly polarized light by ±45 degrees.

Said n is determined according to the maximum DGD value required to be compensated by the optical fiber communication system and the allowable residual DGD value of the optical fiber communication system, where the maximum DGD=x of the optical fiber communication system is set, and the maximum DGD=y of the optical fiber communication system allows the residual, then the compensation system is required to distinguish The rate is less than or equal to 2y, so the DGD value produced by the smallest crystal in the optical delay line is required to be less than or equal to y, so the minimum value of n is determined by the calculation formula 2 ^n-1 +1≥x/2y, thus determining the material of the birefringent crystal and length.

Beneficial effects of the present invention: 1. The variable optical delay line adopts several segments of crystals and PRs arranged and fixed in a specific way, and there are no moving parts in the operation process, so the structure is simple, the performance is stable, and is less affected by the external environment; 2. The coupler, Both the polarization controller and optical collimator are devices with pigtails, which are beneficial to connect with the optical fiber system; the polarization controller is controlled by a DC servo motor, which is convenient for fast and automatic control of the polarization state. 3. The computer feeds back the direction information of the PMD of the optical fiber communication system to the polarization controller control circuit, so that the polarization controller 2 generates a polarization state opposite to the direction of the PMD of the optical fiber communication system. 4. It is easy to realize the dynamic control of PMD compensation in engineering. 5. When the system has no PMD, no additional PMD will be generated.

Description of drawings

Figure 1 is a schematic diagram of a polarization mode dispersion compensation system.

Detailed ways

The invention provides a polarization mode dispersion compensation system which is easy to be coupled with the system, has quick response, low power consumption, is easy to implement in engineering and can be dynamically controlled. The optical collimator 3 of the system and the variable optical delay line 4 composed of n+1 birefringent crystals and n polarization rotators PR based on the magneto-optical effect are placed concentrically on a straight line with the optical axis (n It is a certain natural number determined according to the maximum DGD value compensated by the optical fiber communication system and the allowable residual DGD value of the system). The above-mentioned crystals and PRs are spaced in such a way that from front to back they are the longest crystal, PR, the second longest crystal, PR, and finally the shortest crystal. The feedback control circuit comprises a detector 5 and a computer 6 connected in series with the coupler 1, and the computer is connected with the polarization controller 2 and the variable optical delay line 4 respectively (as shown in Figure 1), for feeding back the direction information of the PMD to the polarization The controller 2 controls the circuit so that the polarization controller 2 produces the polarization state opposite to the optical fiber communication system PMD direction; the size information of the optical fiber communication system PMD is fed back to the control circuit of the variable optical delay line 4, so that the variable optical delay The DGD value of line 4 is equal to the DGD in the optical fiber communication system. The above-mentioned coupler 1, polarization controller 2, and optical collimator 3 are all devices with pigtails, which are conducive to connection with the optical fiber system; the polarization controller 2 is controlled by a DC servo motor, which is convenient for rapid and automatic control of the polarization state. The present invention can be further described by the following operating principles:

1) The beam emitted from the optical fiber of the optical fiber communication system used in the polarization mode dispersion compensation system first enters the coupler 1, and the beam emitted by the coupler 1 is divided into two beams, one of which enters the detector 5 as a control signal, and passes through The computer 6 performs processing to obtain the size and direction of the PMD of the optical fiber communication system. 2) The computer feeds back the direction information of the PMD of the optical fiber communication system to the control circuit of the polarization controller 2, so that the polarization controller 2 generates a polarization state opposite to that of the PMD of the optical fiber communication system. 3) The light beam enters the collimator 3 from the polarization controller 2, so that the light beam becomes a collimated light beam and exits. The working distance of the collimator 3 should be greater than the length of the variable optical delay line 4 . 4) After the light beam is emitted from the collimator 3, it enters the variable optical delay line 4, and the computer 6 feeds back the size information of the optical fiber communication system PMD to the control circuit of the variable optical delay line 4, so that the variable optical delay line 4 The DGD value is equal to the DGD in the optical fiber communication system. 5) The variable optical delay line 4 is composed of n+1 birefringent crystals and n polarization rotators based on the magneto-optic effect. The angle between the fast axis or the slow axis between two adjacent crystals is 45 degrees. PR can rotate the polarization state of linearly polarized light by ±45 degrees. When PR rotates the polarization state of linearly polarized light by +45 degrees, the fast axis of the front crystal and the rear The slow axis of the crystal coincides, and the DGD value of the optical delay line is subtracted from the DGD value of the two separate crystals. On the contrary, when PR rotates the polarization state of linearly polarized light by -45 degrees, the fast axis of the front crystal and the fast axis of the back crystal Coincidentally, the DGD value of the optical delay line is the sum of the DGD values of the two crystals. By controlling the length of the crystal or selecting an appropriate birefringent crystal material, optical delay lines with different resolutions can be produced. By controlling the number of segments of the birefringent crystal, sufficient DGD states and large enough DGD values can be generated, so that the The requirements of different occasions. n is a certain natural number determined according to the maximum DGD value in the optical fiber communication system and the allowed residual DGD value of the system, and is determined by the allowed residual DGD value of the system. The calculation formula is as follows: Assuming that the maximum DGD=x in a certain optical fiber communication system, and the system allows the maximum DGD=y, then the resolution of the compensation system is required to be less than or equal to 2y (according to the principle of 4 rounding to 5, half of the resolution of the compensator is the allowable value of the communication system Residual DGD), so it is required that the DGD value produced by the smallest crystal in the optical delay line is less than or equal to y, so as to determine the material and length of the birefringent crystal; the compensation system needs to produce N≥x/2y kinds of DGD values, that is, 2 ^n-1 +1≥x/2y, thus determining the minimum value of n. 6) The light beam exits from the optical delay line 4 and enters the collimator 3 with a pigtail, and is connected to the optical fiber communication system. Since the invention only adopts several sections of birefringent crystals and PRs arranged and fixed in a specific way, there are no moving parts in the operation process, so the structure is simple, the performance is stable, and it is less affected by the external environment. Since a high birefringence crystal can produce a large DGD value in a very short length, its volume is relatively small under the condition of producing the same DGD. For example, a delay line that can produce a maximum DGD of 45 ps The length is only 150mm. Because the coupler, polarization controller 2, and collimator all use devices with pigtails, there are few optical and mechanical parts, less affected by the external environment, and easy to couple with the optical fiber communication system. The method of controlling the variation of the polarization rotation angle of the magneto-optic crystal in the delay line to obtain different DGD values is faster and more stable than the method of adjusting the DGD value by means of mechanical or liquid crystal. In the delay line unit, one minimum birefringence material in the general delay line is increased to two, and a PR is added between the two minimum birefringence materials, thus solving the problem that there is no DGD in the compensation system based on the general birefringence material = 0 state, so it will not increase the PMD in the system. In the electronic control part, the driving voltage of the polarization controller 2 is 12V, the current is 0.65A, the driving current of each PR is 50mA, and the driving voltage is 2.5V at 2KHz, so the power consumption is small. The whole system automatically compensates the PMD in the system through the feedback signal, so it is also self-adaptive and dynamic.

Give an example of calculation: it is assumed that the optical fiber communication system whose compensation DGD value is within 45ps, the allowable DGD of the system is 0.75ps. It is calculated that in order to meet the requirements, the DGD produced by the minimum length birefringent crystal is required to be ≤0.75ps, and there must be at least 30 DGD states. According to 2 ^n-1 +1≥30, n≥5.85 is obtained, and n=6, that is, double There are 7 segments of refracting crystals and 6 polarization rotators. The PMD value of this system is relatively large, and the resolution requirements are not high. Select high birefringent crystals, such as YVO4. The DGD value produced by 1mmYVO4 is 0.72ps, so its resolution can reach 1.44ps. The calculation results show that from front to back according to Optical delay lines arranged in 32mmYVO4, PR, 16mmYVO4, PR, 8mm YVO4, PR, 4mm YVO4, PR, 2mm YVO4, PR, 1mm YVO4, PR, 1mm YVO4 meet the system requirements and can produce 0, 1.44, 2.88, 4.32,… …, 33 states in 46.08ps, residual DGD≤0.72ps. Magneto-optic crystals use magneto-optic films with fast response speed and small saturation magnetic field, and coils use enameled wires or other materials with low inductance to increase system response speed and reduce power consumption. The coupler adopts a coupler with an appropriate splitting ratio to minimize the loss of the optical signal and it is advisable that the signal received by the detector will not be obliterated by noise.

Claims (4)

1. A polarization mode dispersion compensation system comprising a coupler, a polarization controller, a pair of optical collimators, a section of variable optical delay line and a feedback control circuit, characterized in that: the pair of optical collimators The collimator and the variable optical delay line are concentrically placed on a straight line with the optical axis, and the variable optical delay line is located between the pair of optical collimators; the detector (5) and The computer (6) is connected in series with the coupler (1), and the computer (6) is connected to the polarization controller (2) and the variable optical delay line (4) respectively, and the optical fiber of the optical fiber communication system applied in the polarization mode dispersion compensation system The outgoing light beam first enters the coupler (1), and the light beam emitted by the coupler (1) is divided into two beams, one of which enters the detector (5) as a control signal, and is processed by a computer (6) to obtain the described The size and the direction of the PMD of the optical fiber communication system, another beam enters the polarization controller (2), and the light beam that goes out from the polarization controller (2) enters the optical collimator; The direction information is fed back to the control circuit of the polarization controller (2), so that the polarization controller (2) produces the polarization state opposite to the PMD direction of the optical fiber communication system; the computer (6) sends the size information of the PMD of the optical fiber communication system Feedback to the variable optical delay line (4), so that the DGD value of the variable optical delay line (4) is equal to the DGD in the optical fiber communication system, thereby achieving the purpose of compensating the PMD of the optical fiber communication system. 2. A kind of polarization mode dispersion compensation system according to claim 1, characterized in that: said variable optical delay line comprises n+1 sections of birefringent crystals and n polarization rotators PR based on magneto-optical effect, so The above-mentioned crystals and PR are arranged at intervals, and the arrangement is from front to back: the longest crystal, PR, the second-longest crystal, PR, and the shortest crystal; its n+1 birefringent crystals include n lengths that grow exponentially The crystal and the crystal with the shortest length among the n birefringent crystals; the PR is composed of a energized coil and a magneto-optic crystal inside it; by controlling the magnitude and direction of the current in the energized coil, the magnetic field passing through the magneto-optic crystal can be controlled The size and direction of the variable optical delay line make the fast axis or slow axis of the front and rear crystals coincide, so as to control the addition and subtraction of the DGD value of the two crystals; where n is a natural number. 3. A kind of polarization mode dispersion compensation system according to claim 2, characterized in that: the birefringent crystal is YVO4, the fast axis or slow axis angle between two adjacent crystals is 45 degrees, PR can make linear polarization The polarization state of light is rotated by ±45 degrees, and the polarization mode dispersion compensation system can generate a sufficiently large DGD value by controlling the number of birefringent crystal segments. 4. A kind of polarization mode dispersion compensation system according to claim 2, characterized in that: said n is determined according to the maximum DGD value that the optical fiber communication system requires compensation and the residual DGD value that the optical fiber communication system allows, wherein the optical fiber communication system The maximum DGD=x, the optical fiber communication system allows the maximum residual DGD=y, then the resolution of the compensation system is required to be less than or equal to 2y, so the DGD value produced by the smallest crystal in the optical delay line is required to be less than or equal to y, so the minimum value of n is calculated by The formula 2 ^n-1 +1≥x/2y is determined, thereby determining the material and length of the birefringent crystal.

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