CN1214263C - Polarization mould chromatic dispersion simulator and compensator and compensating system - Google Patents

Abstract

A polarization mode dispersion simulator, compensator and compensation system thereof, at least including a polarization controller, an optical circulator, a polarization beam splitter, an optical fiber ring and a rotating device, the optical circulator has at least three ports, from the first One port receives the optical signal output by the polarization controller, and outputs it to the polarization beam splitter from the second port, and then decomposes it into two orthogonal polarization components perpendicular to each other through the polarization beam splitter, and outputs it to the fiber ring. The optical fiber ring propagates in clockwise and counterclockwise directions respectively. Under the control of the external signal, by rotating the optical fiber ring, the two orthogonal polarization components of the light entering the optical fiber ring produce a certain value with the rotation of the optical fiber ring. The time delay difference is re-coupled into the input end of the polarization beam splitter after being transmitted in different directions, and then output through the third port of the optical circulator, thereby compensating the polarization mode dispersion of the signal, and the structure is simple, the response speed is fast, The algorithm is simple and the cost is low.

Description

Polarization Mode Dispersion Simulator and Compensator and Compensation System

technical field

The invention relates to a polarization mode dispersion simulator, a compensator and a compensation system, in particular to a polarization mode dispersion simulator, a compensator and a compensation system based on the Sagnac effect in an optical communication system.

Background technique

PMD (polarization mode dispersion, polarization mode dispersion) refers to the arrival time difference of two mutually orthogonal polarization modes LP ₀₁ ^X and LP ₀₁ ^Y transmitted in a single-mode fiber after a certain distance transmission in the fiber. The unit of measure for PMD is Ps. In an ideal situation, when the optical fiber is collimated, the cross section is a standard circular shape, and the refractive index distribution is uniform, symmetrical and isotropic everywhere, the transmission constants βx and βy of LP ₀₁ ^X and LP ₀₁ ^Y are equal, and the two The polarization modes are completely second degree merged (same propagation constant) and do not affect each other during transmission.

However, in actual situations, in the process of optical fiber production, cabling, laying, and changes in its surrounding environment, it is inevitable that the refractive index of the optical fiber will change in different directions, that is, the birefringence effect will appear. Figure 1 is the formation diagram of polarization mode dispersion. It can be seen from Figure 1 that after a certain period of transmission, a certain PMD will be generated in the transmission direction due to the different transmission constants βx and βy of LP ₀₁ ^X and LP ₀₁ ^Y. In addition, when an optical signal passes through some optical communication devices such as isolators, couplers, and filters, birefringence can also be caused due to the incompleteness of the device structure and material itself. And this birefringence effect will directly cause the two orthogonal polarization modes to have different phase and group velocities, thus losing merger and producing PMD.

Birefringence is the source of polarization mode dispersion. Birefringence includes intrinsic birefringence and induced birefringence. Among them, intrinsic birefringence mainly refers to the birefringence caused by materials and manufacturing processes in the manufacturing process of optical fibers, and induced birefringence refers to the birefringence caused by external force through the optical fiber medium. Compared with the birefringence caused by the photoelastic effect, the induced birefringence is more random.

Intrinsic birefringence (including: geometric birefringence, stress birefringence) and induced birefringence (including: bending, lateral force, twist, applied electric field and applied magnetic field) are considered comprehensively, and the interference between them is considered to be considered are uncorrelated, then the total birefringence can be expressed as:

Δβ＝Δβ _G +Δβ _S +Δβ _BF +Δβ _f +Δβ _C +Δβ _E +Δβ _h -----(1)

In the absence of mode coupling, the corresponding PMD per unit length can be simply expressed as:

However, when the propagation speed difference between the two polarization modes is very small, external influences can easily cause energy exchange between the two polarization modes, that is, mode coupling occurs. Generally speaking, the external influence is random, so this mode coupling also has the characteristics of randomness, which has a very important influence on the performance of PMD.

PMD has the following relationship with the average total birefringence Δτ of the fiber and the average polarization mode coupling length h:

In the above formula, l is the length of the optical fiber.

When l<<h, the coupling between the two polarization modes can be ignored, then:

In the time domain, the PMD effect is manifested as the delay difference between the optical pulse components transmitted along the fast and slow axes respectively. This delay difference makes the total optical pulse broaden after a period of light transmission, thus limiting the optical communication system. transmission rate. For short optical fibers, the value of PMD increases linearly with the transmission length, and the unit is ps/km ^1/2 .

When l>>h, the value in the brackets on the right side of formula (3) is about 2l/h, which is:

When an optical pulse is transmitted along a long optical fiber, due to changes in external factors, such as changes in temperature, mode coupling, that is, energy exchange between fast and slow modes, will be induced. Due to the randomness of external changes, mode coupling also occurs randomly. From the above formula, we can see that for long optical fibers, PMD increases with the square root of the transmission length. Unit is

Mode coupling not only simply determines the relationship between PMD and fiber length, but also the reason why PMD is very sensitive to factors such as temperature, vibration, and slight jitter of light source wavelength. Under the same conditions, stronger mode coupling corresponds to smaller polarization mode dispersion. For example, C.D. Poole proved the sensitivity of PMD to temperature changes in the experiment in 1991. Not only the value of PMD changes with the change of temperature, but also its rate of change depends on the speed of temperature change. When the temperature is constant, there is almost no obvious change in PMD, and when the temperature increases rapidly, the fluctuation of PMD also increases significantly.

PMD causes pulse broadening in a digital system, leading to an increase in the bit error rate and limiting the bandwidth of the system; it causes signal distortion in an analog system and limits the number of channels. Until a few years ago, PMD had insignificant impact on single-mode fiber systems when data rates were low and distances were relatively short in both digital and analog systems. With the growth of bandwidth demand, especially in 10Gb/s, 40Gb/s and higher rate systems, PMD begins to become an important factor limiting system performance. Because it will cause excessive pulse broadening or cause too low signal-to-noise ratio (Signalto noise ratio, SNR).

The maximum transmission distance of the system limited by PMD, that is, the maximum transmission distance recommended by ITU-T with 1dB power penalty as a reference, can be theoretically obtained by the following formula:

According to the above formula, the maximum transmission distance limited by PMD can be listed in the following table, which shows the relationship between transmission distance to PMD and bit rate

Due to the statistical characteristics of PMD, the PMD index of a single optical fiber (or cabled optical fiber) is not suitable as an index of system capacity. Instead, link values (ie connected fiber segments) are often used. Since each fiber segment is a random quantity, the link value is also a random quantity, which has a smaller variance due to averaging effects. The PMD link value is expressed by the following formula:

${\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; =</span>\sqrt{\frac{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}}}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; I</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1,2</span>}<span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 6</span>}<span\; class="notranslate">\; )</span>$

Among them, is the PMD value of the serial fiber link, Xi is the PMD of a single fiber, Li is the length of the serial optical segment, and M is the number of serial optical fibers. The link value of PMD reflects the system PMD value more accurately and effectively, and can make full use of the true potential of the optical fiber.

Due to the existence of PMD, it is necessary to compensate for PMD in high-speed optical communication systems. Generally speaking, the current PMD compensation technology mainly uses the polarization mode dispersion effect experienced by the optical signal propagated through the transmission fiber to be compensated by the birefringent compensator at the receiving end, in which the compensator automatically and adaptively generates a differential time delay amount, Roughly equal to the differential time delay experienced by the optical signal, but with the opposite sign, essentially canceling the undesired delay. For example, when an optical signal travels a certain distance through an optical fiber, the two main polarization states (PSP) of the optical pulse produce a delay of 10 ps (refer to Figure 1). In order to try to offset this delay, we need to make it go faster One of the main polarization states passes through the PMD compensator to produce a delay of 10 ps, while the other main polarization state does not delay, and then the two polarization states are coupled together, which cancels the time delay between the two main polarization states, and finally To achieve the effect of compensating PMD.

The polarization transformation that occurs in the fiber can be described by a 2×2 Jones matrix U:

${\mathrm{<span\; class="notranslate">\; u</span>}}^{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; =</span>\left[\begin{array}{cc}{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; )</span>& {\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; )</span>\\ <span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; )</span>& {\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; )</span>\end{array}\right]<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 7</span>}<span\; class="notranslate">\; )</span>$

Among them, u1 and u2 are complex functions that depend on the frequency of the optical signal and other physical quantities that affect the mode coupling in the fiber. Generally speaking, if the optical signal propagating through the optical fiber is the polarized light of one of the two PSPs, the optical signal will not have a large differential time delay. Therefore, at any optical frequency ω=ω ₀ , the matrix U can be "diagonalized" as follows

U(ω)＝W(ω ₀ )D(ω)·V(ω ₀ ) ^-1 (8)

where V and W correspond to the input and output principal polarization states. D(W ₀ ) is a diagonal matrix, and at least around W ₀ from a sufficiently small frequency range Δw, the matrix D can be expressed as:

$\mathrm{<span\; class="notranslate">\; D.</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; D.</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&Center\; Dot;</span>\left[\begin{array}{cc}{\mathrm{<span\; class="notranslate">\; e</span>}}^{\mathrm{<span\; class="notranslate">\; j</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&tau;</span>}}_{\mathrm{<span\; class="notranslate">\; f</span>}}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span>}& \mathrm{<span\; class="notranslate">\; 0</span>}\\ \mathrm{<span\; class="notranslate">\; 0</span>}& {\mathrm{<span\; class="notranslate">\; e</span>}}^{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; j</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&tau;</span>}}_{\mathrm{<span\; class="notranslate">\; f</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&CenterDot;</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span>}\end{array}\right]<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 9</span>}<span\; class="notranslate">\; )</span>$

here, ${\mathrm{\&tau;}}_f=2\sqrt{{\left(\frac{d}{\mathrm{d\&omega;}}{u}_1\right)}^2+{\left(\frac{d}{\mathrm{d\&omega;}}{u}_2\right)}^2}$ is the differential delay, which induces a differential time delay in an optical signal not transmitted in one of the two principal states of polarization. Since the differential time delay experienced by the optical signal propagating through the optical fiber can be compensated by introducing an opposite but equal differential time delay, τ _c = -τ _f , at the output end of the optical fiber. This is readily achieved using optical elements with the following polarization-dependent transfer functions:

${\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; comp</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; -</span>\left[\begin{array}{cc}{\mathrm{<span\; class="notranslate">\; e</span>}}^{\mathrm{<span\; class="notranslate">\; j</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&tau;</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&CenterDot;</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span>}& \mathrm{<span\; class="notranslate">\; 0</span>}\\ \mathrm{<span\; class="notranslate">\; 0</span>}& {\mathrm{<span\; class="notranslate">\; e</span>}}^{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; j</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&tau;</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&CenterDot;</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span>}\end{array}\right]<span\; class="notranslate">\; \&Center\; Dot;</span>{\mathrm{<span\; class="notranslate">\; W</span>}}^{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; V</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 10</span>}<span\; class="notranslate">\; )</span>$

Currently published US Patent No. 98119194 discloses two schemes of using a polarization maintaining fiber as a delay compensator and using a variable delay line as a compensator. Fig. 2 is a schematic diagram of a PMD compensation system using a variable delay line as a PMD compensator, and Fig. 3 is a schematic diagram of a PMD compensation system using a polarization controller plus a polarization-maintaining fiber as a PMD compensator. It can be seen from the two different schemes that no matter which scheme, the entire PMD compensation system generally includes four main parts, one is the polarization controller, which is used for the alignment of the main shaft; the other is the PMD compensator, which is used for Offset the PMD amount produced by the system; the third is a PMD detector, which is used to test the size of the PMD value and generate a monitoring signal; the fourth is a feedback controller, which is used for the generation of the feedback signal and the polarization controller or variable delay line. control. It can be seen from the schematic diagrams of the PMD compensation system shown in Figure 2 and Figure 3 that the difference between the two systems is the use of different PMD compensators (variable delay units between polarization modes).

It can be seen from the above two compensation systems that the advantage of the technical solution described in Figure 2 is that there are fewer parameters to be controlled (three parameters of the polarization controller and one parameter of the variable delay line), so the algorithm is simple and relatively easy to implement. But the disadvantage is that the feedback speed is slow, mainly because the horizontal movement of the lens is used to realize the time delay of one path in the variable delay line, so the reaction rate is limited to a certain extent, and the collimation between the lens and the optical fiber is very demanding. High, otherwise a large attenuation will occur.

The advantage of the technical solution shown in Figure 3 is that the structure is relatively simple and the reaction rate is high, but the disadvantage is that the calculation is complicated (two polarization controllers have six parameters in total), the software design cost is high, and the complexity of the algorithm is likely to sacrifice part of the hardware. response rate. At the same time, the dynamic range of the compensation is also limited.

It can be seen that in optical fiber communication systems, with the increase of single-channel transmission rate and the increase of analog signal transmission bandwidth, in addition to the limiting factors such as dispersion and nonlinearity, Polarization Mode Dispersion (Polarization Mode Dispersion, PMD) problem has become very prominent recently, especially for the long-distance transmission of the transmission system above 40Gbit/s, PMD is considered to be the ultimate limiting factor. Polarization mode dispersion will cause pulse broadening in digital communication systems and increase the bit error rate; in analog communication systems, it will produce high-order distortion effects and distort signals. Therefore, it is necessary to provide a compensation system for compensating polarization mode dispersion in a high-speed optical communication system.

Overview of the invention

The purpose of the present invention is to provide a polarization mode dispersion compensator based on the Sagnac effect, which can compensate PMD according to different compensation accuracy and compensation range according to the beat length of the polarization maintaining fiber.

Another object of the present invention is to provide a polarization mode dispersion compensation simulator based on the Sagnac effect, which can obtain different PMDs according to the rotation speed.

Another object of the present invention is to provide a kind of polarization mode dispersion compensation system based on Sagnac effect, effectively control the rotating speed of rotating device according to photodetector and feedback control unit, effectively reduce and eliminate the impact of PMD on transmission system, and The structure is simple, the response speed is fast, the algorithm is simple, and the cost is low.

In order to achieve the purpose of the present invention, we propose a polarization mode dispersion compensator, including a polarization controller, used to receive incident light, and change the polarization direction of the input optical signal to align it with the main axis of the optical fiber, wherein the polarization mode The dispersion compensator also includes: a circulator, having at least three ports, receiving the optical signal of the polarization controller from the first port, and outputting the optical signal from the second port; The optical signal is then decomposed into two orthogonal polarization components perpendicular to each other and output from the output end respectively; the fiber ring has two input ends, which are respectively coupled to the two beam splitting arms of the polarization beam splitter, receiving Two orthogonal polarization components from the output end of the polarization beam splitter, the polarization main state of the two orthogonal polarization components described by the polarization controller, so that the original polarization main state that is ahead in time enters the generation of the fiber ring One arm of the extension, the transmission direction of the signal on this arm is consistent with the rotation direction of the fiber ring; while the other polarization main state enters the other arm, on this arm, the transmission direction of the signal is opposite to the rotation direction of the fiber ring; The rotating device, under the control of an external signal, rotates the fiber ring so that the two orthogonally polarized components of the light entering the fiber ring produce a certain amount of time delay difference with the rotation of the fiber ring, and then transmit again in different directions. Re-coupled into the input end of the polarization beam splitter, and then output through the third port of the circulator, thereby compensating the polarization mode dispersion of the signal.

The polarization mode dispersion compensator described above, wherein the polarization beam splitter and the circulator are connected through an optical lens.

Said polarization mode dispersion compensator, wherein said rotating device is a disc driven by a stepping motor.

In the polarization mode dispersion compensator, the optical fiber ring is a polarization-maintaining optical fiber, and its beat length can be selected according to different compensation accuracy and compensation range requirements.

According to another aspect of the present invention, we provide a polarization-mode dispersion simulator, wherein the polarization-mode dispersion simulator includes: a circulator with at least three ports, receiving an optical signal from the first port, and receiving an optical signal from the second port The port outputs the optical signal; the polarization beam splitter, the input end receives the optical signal from the circulator, and then decomposes it into two orthogonal polarization components that are perpendicular to each other and outputs them respectively from the output end; the optical fiber ring, the optical fiber ring has two input end, respectively coupled to the two beam-splitting arms of the polarization beam splitter, receiving two orthogonal polarization components from the output end of the polarization beam splitter, and propagating in the clockwise and counterclockwise directions in the fiber ring; the rotating device, in Under the control of an external signal, by rotating the fiber ring, the two orthogonal polarization components of the light entering the fiber ring will produce a certain amount of time delay difference with the rotation of the fiber ring, and then recouple into the polarization component after being transmitted in different directions. The input end of the beamer, and then output through the third port of the circulator.

According to another aspect of the present invention, we provide a polarization mode dispersion compensation system, which is located between an optical transmitter and an optical receiver, including a polarization mode dispersion compensator, a polarization mode dispersion detector and a feedback control unit, the polarization mode dispersion The mode dispersion compensator includes a polarization controller, which is used to receive incident light, and change the polarization direction of the input optical signal to align it with the main axis of the optical fiber, wherein the polarization mode dispersion compensator also includes: a circulator with at least three The first port receives the optical signal of the polarization controller, and outputs the optical signal from the second port, and the polarization beam splitter, the input end receives the optical signal from the circulator, and then decomposes it into two mutually perpendicular Orthogonal polarization components are output from the output end respectively, and the fiber ring has two input ends, which are respectively coupled to the two beam splitting arms of the polarization beam splitter, and receive two orthogonal polarizations from the output end of the polarization beam splitter Component, the polarization main state of the two orthogonal polarization components described by the polarization controller, so that the original polarization main state that is ahead in time enters an arm of the fiber ring that generates time delay, and the transmission of the signal on this arm The direction is consistent with the rotation direction of the fiber ring; while the other polarization main state enters another arm, on this arm, the transmission direction of the signal is opposite to the rotation direction of the fiber ring, and the rotating device, under the control of an external signal, rotates The fiber ring makes the two orthogonal polarization components of the light entering the fiber ring produce a certain amount of time delay difference with the rotation of the fiber ring, and after being transmitted in different directions, it is recoupled into the input end of the polarization beam splitter, and then passes through the The third port output of the circulator, thereby compensated the polarization mode dispersion of the signal; The polarization mode dispersion detector includes an optical beam splitter and a polarization mode dispersion detection unit, and the optical beam splitter receives the signal of the third port of the circulator Outputting light, dividing the optical signal into two paths, one path is output to the optical receiver, and the other path is output to the polarization mode dispersion monitoring unit to detect the remaining polarization mode dispersion value in the system, and output a control signal; the feedback control unit The device receives the control signal of the polarization mode dispersion detector, and controls the polarization controller and the rotation device respectively, so that by adjusting the output polarization state of the polarization controller and the rotation rate of the rotation device, better compensation in the system resulting in polarization mode dispersion.

It can be seen that the Sagnac effect-based polarization mode dispersion compensator provided by the present invention can compensate PMD according to different compensation precisions and compensation ranges according to the beat length of the polarization-maintaining fiber. Another polarization mode dispersion compensation simulator based on the Sagnac effect provided by the present invention can obtain different PMDs according to the rotation speed. Simultaneously, use the compensation system of polarization mode dispersion compensator according to the present invention, effectively control the rotating speed of rotating device according to photodetector and feedback control unit, effectively reduce and eliminate the impact of PMD on transmission system, and simple in structure, responsive The speed is fast, the algorithm is simple, and the cost is low.

Description of drawings

Fig. 1 is the formation diagram of polarization mode dispersion;

Fig. 2 is the PMD compensation system schematic diagram of making PMD compensator with variable delay line;

Fig. 3 is the schematic diagram of the PMD compensation system that the polarization controller adds the polarization maintaining fiber as the PMD compensator;

Fig. 4 is a schematic diagram of the Sagnac effect;

Figure 5 is the time delay difference of the two polarization main states corresponding to different rotational speeds in the Sagnac effect;

Fig. 6 is the block diagram based on the PMD simulator of Sagnac effect;

Fig. 7 is the block diagram of the PMD compensator based on Sagnac effect;

Fig. 8 is a schematic diagram of the application of the PMD compensator of the present invention in an optical communication system.

Detailed ways

The Sagnac effect was first discovered and experimentally confirmed by the Frenchman Sagnac in 1913. It reveals the analytical relationship between the optical path difference of two counter-propagating lights in the same optical path and their rotational speed. Figure 4 shows a schematic diagram of the Sagnac effect. As shown in Figure 4, we consider a circular optical track, in which the optical path is composed of N turns of optical fiber. The clockwise and counterclockwise two paths of light travel at the same speed and rejoin at BS after the same distance 2πNa (a is the radius of the circular orbit). If the system is stationary, the two beams travel the exact same path and, therefore, have the same phase. However, if the circular orbit rotates clockwise with an angular velocity Ω, the time for the two paths of light to reach the meeting point (note that point A has turned to point A' at this time) is different. For clockwise light, we call it CW light, and its arrival time T _CW can be expressed as:

${\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; cw</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;Na</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; a\&Omega;</span>}{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; cw</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; /</span><span\; class="notranslate">\; [</span>\frac{\mathrm{<span\; class="notranslate">\; c</span>}}{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; a\&Omega;</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; no</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 11</span>}<span\; class="notranslate">\; )</span>$

The denominator in the above formula is the propagation speed of light in the moving medium. Similarly, the arrival time T _CCW of counterclockwise light, which we call CCW light, can be obtained as:

${\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; ccw</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;Na</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; a\&Omega;</span>}{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; ccw</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; /</span><span\; class="notranslate">\; [</span>\frac{\mathrm{<span\; class="notranslate">\; c</span>}}{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; a\&Omega;</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; no</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 12</span>}<span\; class="notranslate">\; )</span>$

According to formulas (11) and (12), the arrival time difference Δt of the two paths of light can be obtained as:

$\mathrm{<span\; class="notranslate">\; \&Delta;t</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; cw</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; ccw</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 4</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="Na"\; filenames="C0214174800212.png"\; state="\{\{state\}\}">Na</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; \&Omega;</span>}}{{\mathrm{<span\; class="notranslate">\; c</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 13</span>}<span\; class="notranslate">\; )</span>$

Equation (13) is deduced under the condition of an ideal single-mode fiber. When the polarization-maintaining fiber is used to wind the fiber ring, the n _x ≠ n _y of the two orthogonal polarization axes perpendicular to each other, after deduction, we can get:

$\mathrm{<span\; class="notranslate">\; \&Delta;t</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;Na</span>}<span\; class="notranslate">\; [</span>\frac{{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; x</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; a\&Omega;</span>}}{{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; the\; y</span>}}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; the\; y</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; a\&Omega;</span>}}{{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; x</span>}}}<span\; class="notranslate">\; )</span>}{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; a\&Omega;</span>}}{{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; x</span>}}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; a\&Omega;</span>}}{{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; the\; y</span>}}}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 14</span>}<span\; class="notranslate">\; )</span>$

After simplification, we can get:

$\mathrm{<span\; class="notranslate">\; \&Delta;t</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;Na</span>}<span\; class="notranslate">\; [</span>\frac{\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; x</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; the\; y</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; a\&Omega;</span>}}{{\mathrm{<span\; class="notranslate">\; c</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; a</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; \&Omega;</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; x</span>}}{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; the\; y</span>}}}}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 15</span>}<span\; class="notranslate">\; )</span>$

If it is further considered that the actual rotational speed Ω generally does not exceed the order of thousands of revolutions per second, formula (15) can be further simplified as:

$\mathrm{<span\; class="notranslate">\; \&Delta;t</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;Na</span>}<span\; class="notranslate">\; [</span>\frac{{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; x</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; the\; y</span>}}}{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; a\&Omega;</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 16</span>}<span\; class="notranslate">\; )</span>$

We can clearly see that when fixed parameters are selected, such as the number of turns N of the fiber ring, the radius a, and a specific polarization-maintaining fiber, the time delay Δt has a linear one-to-one correspondence with the angular rate Ω. According to the different requirements of different systems, we can choose different parameters to achieve different delay differences. Figure 5 shows that when N=5000, n _x -n _y =1.4×10 ^-6 , the radius is 0.1m respectively , 0.12m, 0.14m, 0.16m, 0.18m, 0.2m, the time delay difference of the two polarization main states corresponding to different rotational speeds.

The present invention is based on the principle of the Sagnac effect. As mentioned above, under the condition that the rotating speed of the optical fiber ring is less than several thousand revolutions, by rotating the optical fiber ring, the two arms of the optical fiber ring can generate different sizes that are linearly corresponding to the rotational speed Ω. The delay difference Δt.

1. PMD simulator

Figure 6 is a block diagram of a PMD simulator based on the Sagnac effect. As shown in Figure 6, the simulator includes a three-port optical circulator, a polarization beam splitter, an optical fiber loop and a rotating device. When used as a PMD simulator, its main function is to artificially introduce the required time delay difference between two polarization main states without a time delay difference, so as to provide experimental applications. The implementation process of the invention is that one input optical signal passes through port 1 to port 2 of the circulator, and then passes through the input port 4 of the polarization beam splitter, decomposes into two orthogonal polarization components perpendicular to each other and couples them into two optical fiber rings. There are two splitting arms 5 and 6, the two splitted light pulses propagate in the clockwise and counterclockwise directions respectively, and a certain amount of time delay difference is generated with the rotation of the fiber ring, after the two beams of light are transmitted in different directions It is coupled into the input end 4 of the polarization beam splitter again, and finally output through the port 3 of the circulator. The rotating device can be a disc driven by a stepping motor, and its rotating speed is controlled by the voltage provided by the control signal. In the actual production process, a separate optical element can be used between port 2 and port 4, such as The optical lens can not only meet the requirement of rotation, but also can meet the coupling of the optical signal between port 4 and port 2.

2. PMD compensator

Figure 7 is a block diagram of a PMD compensator based on the Sagnac effect. As shown in Figure 7, the compensator includes a polarization controller, a three-port optical circulator, a polarization beam splitter, a fiber loop formed by a polarization-maintaining fiber and a rotating platform. When applied as a PMD compensator, the function of the polarization controller is to adjust the polarization main state of the incident light, so that two orthogonal polarization main states are coupled to the two arms of the fiber ring through the input port 4 of the polarization beam splitter In , it is worth noting that the polarization controller needs to control the two orthogonal polarization main state, so that the main state of polarization that was "advanced" in time enters an arm that generates time delay, and the transmission direction of the signal on this arm is consistent with the rotation direction of the optical fiber loop. The other main state of polarization passes through the other arm, where the signal travels in the opposite direction to the rotation of the fiber loop. In this way, the PMD existing in the system can be compensated by controlling the optical fiber loop to rotate at an appropriate rate.

At the same time, by selecting fiber loops with different parameters (number of turns, radius, and beat length of polarization-maintaining fiber) to match different rotation rates, PMD simulators for different precision requirements and different compensation range requirements for different rate optical transmission systems can be realized. Or compensator; the so-called beat length is the distance that a certain polarization state returns to the same polarization state after being transmitted through the optical fiber. It is related to the birefringence coefficient of the optical fiber and reflects the size of the birefringence coefficient. Due to the use of polarization maintaining fiber, even if the fiber loop does not rotate, it also has a certain PMD compensation effect. We can preset the length of the polarization maintaining fiber according to the PMD link value of the actual system to calculate the average PMD value of the link. compensation, precise dynamic compensation can then be implemented through the rotation of the fiber optic ring.

For example, for the 10G optical transmission system whose total PMD value (average DGD value) of a system is 40ps, to implement PMD compensation to it by the present invention, we can first select the bigger polarization-maintaining fiber of birefringence index, make the whole The average DGD of the fiber ring at 0 rotation speed is 40 ps. In this way, even if the fiber ring does not rotate, the polarization controller can be used to adjust the polarization state of the optical signal entering the fiber to implement PMD compensation for the system. When the environment of the system changes, as we introduced earlier, the DGD value of the system also changes accordingly. For example, the system has a DGD change within 20ps on the basis of the inherent 40ps due to environmental changes. At this time The optical fiber ring rotates at a certain rate under the command of the central control unit to achieve an additional +20ps to -20ps DGD value to compensate for changes in system DGD caused by environmental changes. This realizes the dynamic compensation within 20-60 ps for the PMD characteristic of the system itself.

Fig. 8 is the schematic diagram that PMD compensator of the present invention is applied in the optical communication system, can find out from Fig. 8 book, and optical beam splitter is divided into two paths through the optical signal of PMD compensator, one goes to receiver, the other way to the PMD monitoring unit. The PMD monitoring unit mainly monitors the remaining PMD value in the system after being compensated by the PMD compensator. The representative quantity of monitoring can be the degree of polarization (DOP, Degree of polarization) or the optical power of a specific frequency component as described in other foreign patents. It can also be other physical parameters that can monotonically correspond to the size of the PMD. The PMD monitoring unit feeds back the detected information to the feedback control unit to generate a feedback control signal to control the polarization controller and the rotation device respectively. The ultimate goal of controlling the polarization controller and the rotation device is to make it adjust the polarization The output polarization state of the controller and the rotation rate of the rotating device are used to better compensate the PMD generated in the system.

The description of the invention, the detailed description and the above-mentioned drawings are not intended to limit the invention. For those of ordinary skill in the art, various corresponding modifications can be made under the teaching of the present invention without departing from the spirit and scope of the present invention, but this change should be included in the claims of the present invention and its equivalent scope within.

Claims (8)

1. A polarization-mode dispersion compensator, comprising a polarization controller, used to receive incident light, and change the polarization direction of the input optical signal so that it is aligned with the main axis of the optical fiber, characterized in that the polarization-mode dispersion compensator also includes: The circulator has at least three ports, receives the optical signal of the polarization controller from the first port, and outputs the optical signal from the second port; Polarization beam splitter, the input end receives the optical signal from the circulator, and then decomposes it into two orthogonal polarization components perpendicular to each other and outputs them respectively from the output end; An optical fiber ring, the optical fiber ring has two input ends, respectively coupled to the two beam splitting arms of the polarization beam splitter, and receives two orthogonal polarization components from the output end of the polarization beam splitter, the two polarization components described in the polarization controller The polarization main state of two orthogonal polarization components, so that the original polarization main state that is ahead in time enters an arm of the fiber ring that generates time delay, and the transmission direction of the signal on this arm is consistent with the rotation direction of the fiber ring; One polarization main state enters the other arm, where the transmission direction of the signal is opposite to the rotation direction of the fiber ring; The rotating device, under the control of an external signal, rotates the fiber ring so that the two orthogonally polarized components of the light entering the fiber ring produce a certain amount of time delay difference with the rotation of the fiber ring, and then transmit again in different directions. Re-coupled into the input end of the polarization beam splitter, and then output through the third port of the circulator, thereby compensating the polarization mode dispersion of the signal. 2. The polarization mode dispersion compensator according to claim 1, characterized in that the polarization beam splitter and the circulator are connected by an optical lens. 3. The polarization mode dispersion compensator according to claim 1, characterized in that the rotating device is a disc driven by a stepping motor. 4. The polarization mode dispersion compensator according to claim 1, wherein the optical fiber ring is a polarization-maintaining optical fiber, and its beat length can be selected according to different compensation accuracy and compensation range requirements. 5. A polarization mode dispersion simulator is characterized in that the polarization mode dispersion simulator comprises: a circulator having at least three ports, receiving an optical signal from a first port and outputting the optical signal from a second port; Polarization beam splitter, the input end receives the optical signal from the circulator, and then decomposes it into two orthogonal polarization components perpendicular to each other and outputs them from the output end respectively; The optical fiber ring has two input ends, which are respectively coupled to the two beam splitting arms of the polarization beam splitter, and receive two orthogonal polarization components from the output end of the polarization beam splitter, respectively clockwise and Propagate in counterclockwise direction; The rotating device, under the control of an external signal, rotates the fiber ring so that the two orthogonally polarized components of the light entering the fiber ring produce a certain amount of time delay difference with the rotation of the fiber ring, and then re-transmit through different directions. Coupled into the input end of the polarization beam splitter, and then output through the third port of the circulator. 6. The polarization mode dispersion simulator according to claim 5, characterized in that the polarization beam splitter and the circulator are connected by an optical lens. 7. A polarization-mode dispersion compensation system, which is located between an optical transmitter and an optical receiver, comprises a polarization-mode dispersion compensator, a polarization-mode dispersion detector and a feedback control unit, and the polarization-mode dispersion compensator comprises a polarization controller , which is used to receive the incident light and change the polarization direction of the input optical signal to align it with the main axis of the optical fiber, characterized in that, The polarization mode dispersion compensator also includes: a circulator having at least three ports, receiving the optical signal of the polarization controller from a first port and outputting the optical signal from a second port, Polarization beam splitter, the input end receives the optical signal from the circulator, and then decomposes it into two orthogonal polarization components perpendicular to each other and outputs them from the output end respectively, An optical fiber ring, the optical fiber ring has two input ends, respectively coupled to the two beam splitting arms of the polarization beam splitter, and receives two orthogonal polarization components from the output end of the polarization beam splitter, the two polarization components described in the polarization controller The polarization main state of two orthogonal polarization components, so that the original polarization main state that is ahead in time enters an arm of the fiber ring that generates time delay, and the transmission direction of the signal on this arm is consistent with the rotation direction of the fiber ring; One polarization main state enters the other arm, where the transmission direction of the signal is opposite to the rotation direction of the fiber ring, The rotating device, under the control of an external signal, rotates the fiber ring so that the two orthogonally polarized components of the light entering the fiber ring produce a certain amount of time delay difference with the rotation of the fiber ring, and then re-transmit through different directions. Coupled into the input end of the polarization beam splitter, and then output through the third port of the circulator, thereby compensating the polarization mode dispersion of the signal; The polarization mode dispersion detector includes an optical beam splitter and a polarization mode dispersion detection unit, the optical beam splitter receives the output light of the third port of the circulator, divides the optical signal into two paths, and outputs one path to the optical receiver, and the other path One output to the polarization mode dispersion monitoring unit to detect the remaining polarization mode dispersion value in the system, and output a control signal; The feedback control unit receives the control signal of the polarization mode dispersion detector, controls the polarization controller and the rotation device respectively, so that by adjusting the output polarization state of the polarization controller and the rotation rate of the rotation device, more Polarization mode dispersion occurs in a well compensated system. 8. The polarization mode dispersion compensation system according to claim 7, wherein the optical fiber ring is a polarization-maintaining optical fiber, and its beat length can be selected according to different compensation accuracy and compensation range requirements.

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