KR100394562B1 - A Polarization Mode Dispersion Compensation System - Google Patents

Abstract

PMD monitoring device that performs the function of extracting the polarization mode dispersion value from the distorted optical pulse signal through the optical transmission fiber in real time, and the optical pulse distorted by the PMD of the optical transmission fiber And a main controller for compensating for the PMD by generating a feedback control signal to the PMD compensator by using a signal provided from the PMD monitoring device. PMD compensation system.

Description

Polarization Mode Dispersion Compensation System

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a compensation system for polarization mode dispersion (PMD) of a light path, and more particularly, to compensate for a PMD that can generate a compensation PMD by adjusting a voltage applied to an optical fiber into which a liquid crystal is inserted. A compensation system provided with a device.

When an optical pulse is transmitted through an optical fiber, each optical pulse is deformed such that its width is increased so that the pulses overlap each other. This phenomenon increases the error of the signal detected at the receiver, thereby limiting the information transmission capacity of the optical fiber transmission path, which is called dispersion. In general, dispersion occurs in two forms of propagation mode and color. Mode dispersion refers to a phenomenon in which signal light pulses occur due to a difference in arrival time due to different transmission paths depending on a propagation mode through an optical fiber. Color dispersion refers to a phenomenon in which the refractive index of glass, a fiber optic material, is caused by a difference depending on the wavelength of light, and thus, a signal light pulse is caused by a difference in arrival time.

Polarization mode dispersion (PMD) occurs as a result of randomly polarized connections by external forces acting on the optical fiber in the optical fiber or small amounts of excess birefringence entering the core of the fiber by asymmetric internal stresses or strains. That is, when the optical pulses having a finite line width enter the optical fiber having the birefringence due to the birefringence of the optical fiber, the width of the optical pulse after passing through the optical fiber is wide because each spectral component in the optical pulse experiences different birefringence. It is a phenomenon of loss. As a result, PMDs can severely impair signal transmission within the fiber optic network. Such PMD was a physical quantity that was not a big problem when the optical transmission system of 10bit / s or less, which was a problem of color dispersion and optical loss of optical path, was applied to communication, but it was dispersion shifted fiber and chromatic dispersion compensation. With the development of dispersion compensation technology, it is now one of the most important communication obstacles in high speed optical transmission systems of 10 Gbit / s or more. In particular, the existing optical cable already installed in the inter-station transmission network has a relatively large value of 0.5-2 ps / km ^½ of PMD. Therefore, in the case of 10 Gbit / s optical transmission system, the communication distance is 100 km due to PMD. Up to 25 km. Considering the urgent need for a high-speed inter-network network, which is fueled by the recent explosive demand for multimedia services by subscribers over the Internet, the compensation problem of PMD in the high-speed backbone network is an urgent task of various carriers.

On the other hand, the PMD value is about 0.1ps / km ^½ which is newly produced due to the recent development of production technology. However, considering the increasing trend of the construction of the optical subscriber network according to the multimedia service, The speed of the inter-station network will be accelerated, and even if the fabricated fiber is installed in recent years, the PMD compensation problem will be inevitable in any case since PMD will be the main cause of transmission restriction again in the high-speed transmission network over 40Gbit / s. It can be called a task.

1 illustrates the birefringence of an optical fiber. The birefringence is not an ideal circular circle in which a core of an optical signal progresses due to limitations in the manufacturing technology of the optical fiber or environmental changes such as temperature or stress around the optical cable. Since the elliptical core is close to each other, the refractive indexes in the major and minor axes of the ellipse core are different from each other. In the mode of a single mode optical fiber, two mutually orthogonal polarization modes are degenerate. When the core of an optical fiber is ideally circular, the propagation speed of the two degenerate polarization modes is the same, but the core is an ellipse. Since the refractive indices of the long and short axes are different, the propagation speeds of the two polarization modes are different.

Therefore, in the optical optical communication system, as the optical pulse signal travels inside the optical fiber having the birefringence, as shown in FIG. 2, the two polarization modes in the optical pulses that have been degenerate from each other are separated from each other, and the width of the pulse is widened at the output terminal. It causes signal error. In this case, the delay difference between the two optical pulses separated from each other at the output terminal is referred to as a primary PMD. Since the actual optical fiber has a shape in which the birefringence axis of the core randomly intersects a certain distance from the manufacturing process, this PMD has a statistical and complicated aspect. However, recent research has shown that when an optical signal with a narrow linewidth enters an optical fiber having such random birefringence, the output optical pulses are orthogonal to each other, called the Principal States of Polarization (output PSP). It was found that the light pulse is divided into, which is shown in FIG. At this time, the time delay difference between the two output pulses is called DGD (Differential Group Delay), and corresponds to the first PMD.

4 illustrates the basic principle of a PMD compensator. Basically, a polarization maintaining fiber (Polarization Maintaining Fiber) forcibly applying a large birefringence value to a short length is connected to an output end of an optical path having an arbitrary birefringence value, and a polarization controller (PC) is positioned therebetween. At this time, the two optical pulses divided into the output PSP state after passing through the optical path are converted to the eigen mode of the polarization maintaining optical fiber in the compensator through the polarization controller, and then the optical pulse having a large time delay is low speed with low refractive index of the polarization maintaining optical fiber. The principle is to reduce the time delay difference of the optical signal after passing through the short-length polarization maintaining optical fiber by injecting the light pulse with a small time delay into the fast axes with a large refractive index. have. At this time, since the primary PMD of the optical path is the group delay difference between the two optical pulse signals, the length of the polarization maintaining optical fiber used for the PMD compensator is determined by the time delay difference between the two optical pulses, that is, the primary PMD amount.

However, the actual PMD value of the optical fiber is changed with the birefringence of the optical fiber with time due to the temperature change around the optical path and the vibration caused by the traffic, and the output PSP also changes with it. In order to compensate, it is necessary to continuously change the length of the polarization maintaining optical fiber in the compensator. 5 illustrates this situation. That is, as the output PSP changes with time, a dynamic compensation device is required.

PMD compensators developed to date can be largely divided into three types: all optical, all electrical, and hybrid. The dual all-optical PMD compensator is to process the restoration of the signal distorted by the PMD as an optical signal without any electrical processing. Some examples are as follows.

FIG. 6 illustrates a case in which an all-optical PMD compensator includes a polarization controller (PC), a pair of polarization beam splitters (PBS), and a group delay using bulk optics. The optical pulse signal divided into two output PSPs by the PMD through the optical path is input to the birefringence axis of the PBS by the polarization controller, and then the two optical pulse signals are distorted by passing one optical pulse through an adjustable group delay. Restore In order to extract the PMD value of the changing optical path and feed it back to the PMD compensator, the polarization state of the optical signal distorted by the optical path is measured with a Febry-Perot type scanning filter. Extract it. This method uses a group delay made of bulk optics to compensate for the PMD value of the changing optical path, which is disadvantageous in that the optical loss is large, large, difficult to fabricate, and difficult to use.

FIG. 7 illustrates a PMD compensator using several strands of polarization-maintaining fiber fragments having a different group delay, a single mode fiber, and an optical switch, and having a different PMD value connected to a distortion signal of an optical path output by an optical switch. By switching between the polarization-maintaining fiber membrane and the single mode optical fiber whose PMD value is negligibly small, compensation is made in response to the PMD value of the changing optical path. This has the advantage that the compensation speed is high, but the optical switch is used, so the loss is large, the birefringence axis alignment of the polarization maintaining optical fibers is virtually impossible, and the PMD extraction portion for the control feedback is practically impossible.

In addition, a method of mechanically twisting a specific portion of a single polarization maintaining optical fiber in a variable PMD compensator as shown in FIG. 8 has been proposed, but has a disadvantage in that a control method is difficult and complete primary PMD compensation is difficult.

The electric PMD compensator is a method of compensating the signal distorted by the PMD by processing the PMD compensation as an electrical signal using a change in the electrical spectrum of the RF signal at the rear end of the receiver. The amount of possible PMD is small, and the high cost of the electronic device is expensive.

The hybrid method is a combination of the above two methods, which uses expensive high-speed electronic devices, and it is difficult to implement as the bit rate increases, and has a disadvantage in that it depends heavily on the transmission method.

An object of the present invention is to provide a polarization mode dispersion (PMD) compensation system for eliminating the influence of polarization mode dispersion of an optical fiber.

Another object of the present invention is to provide a polarization mode dispersion (PMD) compensation system to which a PMD compensation device with low light loss is applied.

Still another object of the present invention is to provide a polarization mode dispersion (PMD) compensation system to which a PMD compensation device is easily manufactured.

The PMD compensation system according to the present invention is characterized by a polarization controller for redirecting the polarization of a specific component of an optical signal received through an optical transmission fiber, and a liquid crystal having a refractive index similar to that of a core of a single mode optical fiber for communication at room temperature. The liquid crystal optical fiber composed of the inserted hollow core optical fiber, and the liquid crystal molecules constituting the core of the liquid crystal optical fiber by varying the electric field provided to the outside of the liquid crystal optical fiber in accordance with the control signal provided from the main controller It is provided with a PMD compensation device including a voltage applying device for generating a compensation PMD by adjusting the molecular polarization intensity.

Another feature of the PMD compensation system according to the present invention is that a plurality of voltage applying devices for controlling the magnitude of the electric field applied to the outer side of the liquid crystal optical fiber are installed.

Another characteristic of the PMD compensation system according to the present invention is a tapping optical coupler (optical) that is located at the output end of the optical path and guides the primary input value transmitted from the single mode optical path to the polarization controller and extracts the distorted signal. a tap, an optical amplifier for amplifying the signal extracted by the tapping optical coupler, a 50:50 optical coupler for separating the output signal of the optical amplifier into two optical signals, and A high speed optical receiver connected to one output terminal of a 50:50 optical coupler to convert an optical pulse signal into an electrical signal, and a first harmonic of a radio frequency signal connected to a rear end of the high speed optical receiver corresponding to a communication data ratio A radio frequency bandpass filter for measuring the intensity of the component and transmitting it to the main controller, an automatic polarization controller connected to another output terminal of the 50:50 optocoupler, and the ruler Is that having been made in the linear polarizer attached to the rear end of the polarization controller (linear polarizer), by detecting the output signal after passed through the linear polarizers including a DC optical detector to pass to the main controller monitoring device PMD.

1 is an exemplary diagram depicting birefringence of an optical fiber,

2 is an exemplary view showing a principle in which polarization mode dispersion occurs;

3 is an exemplary diagram of an output PSP represented by optical pulses orthogonal to each other;

4 is an exemplary diagram illustrating a basic principle of a PMD compensator;

5 is an exemplary diagram of a change in birefringence of an optical fiber with a change in time;

6 is an exemplary view showing an example of an all-optical PMD compensator composed of a group polarizer using a polarization controller (PC), a pair of polarizing beam splitters (PBS), and a bulk optic,

7 is an exemplary diagram of a PMD compensator implemented by using multiple strands of polarization-maintaining fiber fragments having a different group delay, a single mode optical fiber (SMF), and an optical switch;

8 is an exemplary view illustrating a method of performing PMD compensation by mechanically twisting several specific portions of a single polarization maintaining optical fiber;

9 is a block diagram showing the configuration of a PMD compensation system according to an embodiment of the present invention;

10 is a block diagram showing the configuration of a PMD compensation system according to another embodiment of the present invention;

11A to 11B are exemplary views illustrating an embodiment according to a modification of FIG. 10;

12 is a block diagram showing the configuration of a PMD compensation system according to another embodiment of the present invention;

13 is a view illustrating an output signal of the PMD monitoring apparatus according to the present invention;

14 is a spherical angular view showing a polarization state of an optical signal output with time;

15 is an exemplary configuration diagram of a PMD compensator according to the present invention;

16 shows an electric field in the liquid crystal optical fiber ( Is an example of the case where

Hereinafter, the configuration of the PMD compensation system and its operation will be described with reference to the accompanying drawings.

FIG. 9 illustrates a PMD compensation system according to an exemplary embodiment of the present invention, wherein a PMD monitoring apparatus performs a function of extracting a signal to calculate a primary PMD value and an output PSP from a distorted optical pulse signal through an optical path. (PMD monitoring unit) 10 and the primary PMD value and output from the signals obtained from the PMD compensation device 20 and the PMD monitoring device 10 that performs the function of restoring the optical pulses distorted by the PMD of the optical path Calculate the PSP and generate a feedback control signal to the PMD compensation device 20 using the same, and perform a function of transmitting the feedback signal to the receiver 50 through the main board 30 and the PMD compensation device 20. And a DOP measuring device 40 for converting the output PSP of the optical pulse signal transmitted to the polarization eigenmode of the PMD compensator 20.

9 shows the most preferred configuration of the present invention, in which the PMD monitoring apparatus has a conventional configuration and the PMD compensator 20 of FIG. 9 is applied, or the PMD of FIG. 9 is provided with a conventional PMD compensation apparatus. The change in the case where only the monitoring apparatus 10 is applied is possible.

The PMD monitoring device 10 is located at the output end of the optical path and guides the primary input value transmitted from the single mode optical path to the polarization controller, and extracts the distorted signal. ), An optical amplifier 12 for amplifying the signal extracted by the tapping optical coupler 11, and a 50:50 light for separating the output signal of the optical amplifier 12 into two optical signals. A coupler 13, a high speed optical receiver 14 connected to one output terminal of the 50:50 optocoupler 13 to convert an optical pulse signal into an electrical signal, and a rear end of the high speed optical receiver 14 A radio frequency band pass filter (RFBPF) 15 which measures the intensity of the first harmonic component of the radio frequency signal corresponding to the communication data ratio and transmits the intensity to the main controller 30; Connected to the other output of the 50:50 optocoupler 13 The automatic polarizer 16, the linear polarizer 17 connected to the rear end of the automatic polarizer 16, and the output signal after passing through the linear polarizer 17 are sensed and transmitted to the main controller. Consisting of a DC light sensor 18.

The PMD compensator 20 includes one polarization controller 21, one strand of liquid crystal optical fiber 22, and the liquid crystal optical fiber 22 according to a control signal provided from the main controller 30. It consists of a voltage application device 23 for applying a voltage to the.

The DOP measuring device 40 is a tapping coupler 41 for converting the output PSP of the optical pulse signal passing through the optical path into the polarization eigenmode of the liquid crystal optical fiber 22, and the light provided through the tapping coupler 41 Automatic polarizer 42 for converting the polarization of the signal, a linear polarizer (43) connected to the rear end of the automatic polarizer (42), and detects the output signal after passing through the linear polarizer (43) It consists of a DC light sensor 44 to deliver to the main controller (30).

FIG. 10 illustrates a PMD compensation system according to another embodiment of the present invention, and unlike the example of FIG. 9, it can be seen that a plurality of voltage supply devices 23 are installed outside the liquid crystal optical fiber 22. . In this case, the intensity of the electric field provided to the outer side of the liquid crystal optical fiber 22 by providing different control values to each voltage applying device 23 in the main controller 30 ( Are applied differently.

FIG. 11A is a perspective view of a liquid crystal optical fiber 22 and a voltage applying device 23 in accordance with an embodiment of the modification of FIG. 10, unlike in the case of FIG. 10. ) Is provided at different angles to the liquid crystal molecules in the liquid crystal optical fiber. At this time, the control signal applied from the main controller 30 to the voltage application device 23 will exhibit the same value. 11B is an exemplary view when FIG. 11A is viewed from the side.

FIG. 12 illustrates a configuration of a PMD compensation system according to another embodiment of the present invention, and may be applied to each case of FIGS. 9 to 11. That is, unlike the above description example, the polarization controller 21 is formed on a part of the liquid crystal optical fiber 22 and can distinguish the polarization control unit 21 based on the A point. It is also possible to control the polarization by the voltage applied to the outer side of the liquid crystal optical fiber region by the control signal provided from the main controller 30.

The operation of the PMD compensation system according to the present invention will now be described based on common points of the embodiments. First, the operation of the PMD monitoring apparatus 10 will be described. After high speed pulse modulation at 10 Gbit / s and passing through a long-distance optical fiber, the optical pulse signal whose pulse width is widened by the PMD is passed through the tapping port of the tapping optical coupler 11 and then by the optical amplifier 12. It is amplified by a certain amount. Thereafter, it is divided into two optical signals through the 50:50 optical coupler (13). One end of the 50:50 optical coupler 13 output is converted into an electrical signal by a high speed optical receiver 14 having a sensing speed of 10 Hz or more, wherein the electrical signal of the rear end of the high speed optical receiver 14 and its radio frequency It can be seen that the spectrum (Radio Frequency: hereinafter referred to as "RF") changes as shown in FIG. 13A to FIG. 13B showing the case where the PMD is "0".

The optical pulse signal of two pulses having the same magnitude by the first PMD is converted into an electrical signal by the high speed optical receiver 14, and then Fourier transformed to obtain an RF spectrum, and then the first harmonic component of the RF spectrum (10). Variation in magnitude is shown for changes in PMD values. As such, when the magnitude ratio of the two pulses is determined, the PMD value may be monitored by measuring the intensity of the electrical signal at the output terminal of the RFBPF 15.

As shown, the RF spectrum of the rear end of the optical receiver 14 is composed of 10 Hz harmonic components, and the magnitude of the first harmonic component of the RF spectrum is increased as the PMD value of the optical line increases. It can be seen that the decrease and the size of the second harmonic component increase. At this time, when the electrical signal passing through the high speed optical receiver 14 passes through a Radio Frequency Band Pass Filter (RFBPF) 15 having a center frequency of 10 kHz, the PMD value increases (or decreases). The change can be expressed as a change in magnitude of the 10 kHz radio frequency component.

As mentioned above, in order to accurately obtain the first-order PMD value in real time, it is necessary to additionally know the magnitude difference between the two pulses caused by the PMD. As is well known, the primary PMD of the optical path represents the polarization state of the optical pulse signal at the optical path output stage on the poangare sphere as shown in FIG. 14, and the output optical pulse signal after the PMD is As the wavelength changes, the circle's trajectory is plotted on the spherical surface of the Poangcurry centered on the output PSP by the spectral width of the optical pulse signal, where the rotation rate of the circle is the size of the primary PMD. At this time, the magnitude ratio of the two output light pulses determines the position of the starting point of rotation of the Poangcurry sphere according to the wavelength change. Therefore, when the two pulses are the same, the point is located on the equator around the output PSP. As the size decreases, the starting point of rotation moves toward the pole about the output PSP, so the size ratio between the two pulses can be known by measuring the DOP of the output optical pulse after passing the optical path.

If this is actually implemented as a device, the optical pulse signal passing through the remaining output terminal of the 50:50 optocoupler 13 is incident on the DC light sensor 18 via the auto polarizer 16 and the linear polarizer 17, where The DOP can be measured by scanning the polarization controller and reading the output of the DC light sensor 18 after passing through the linear polarizer 17. In summary, the real-time PMD monitoring apparatus can be implemented. If the bit rate of the optical communication signal is high, the center frequency of the RF bandpass filter at the rear end of the high speed optical receiver 14 may be adjusted accordingly, so this method is applied to a high speed communication signal of 10G bit / s or more. This has the advantage of being possible. The tapping optical coupler 11 is preferably used as long as it does not significantly change the power of the optical pulse signal input to the receiver of the optical communication system, preferably 90:10 or 70:30, and is tapped for extraction. Since the signal is small in size, the use of an optical amplifier 12 with an ASE noise canceling filter in front of the 50:50 optocoupler 13 is preferred.

Another characteristic of the PMD compensation system according to the present invention is that the configuration of the PMD compensation device 20 is one polarization controller 21, one strand of liquid crystal optical fiber 22 and the voltage of the liquid crystal optical fiber 22 The point is composed of a voltage applying device 23 for applying a.

As shown in FIG. 15, the liquid crystal optical fiber 22 in the PMD compensator 20 is basically a hollow fiber having an empty portion corresponding to the core, and has a refractive index similar to that of the core of the communication single mode optical fiber at room temperature. It has a structure in which a liquid crystal having an is inserted. DC or AC electric fields after being Is located between the capacitor (C) for applying ().

When no electric field is applied to the liquid crystal optical fiber at room temperature, the direction of the liquid crystal molecules constituting the core of the liquid crystal optical fiber is random or aligned in the axial direction. If the capacitor containing the liquid crystal optical fiber (capacitor) as shown in FIG. ), The liquid crystal molecules in the liquid crystal optical fiber are lined up in the direction of the electric field. At this time, the electric field applied to the capacitor (C) According to the intensity of the liquid crystal molecules, the lined angle of the liquid crystal molecules changes or the number of molecules varies, thereby forming a corresponding molecular polarization, and the intensity of the molecular polarization can be adjusted by changing the intensity of the electric field. Therefore, a variable birefringence can be obtained by adjusting the intensity of the electric field applied to the liquid crystal optical fiber. The length of the liquid crystal optical fiber can be obtained by calculating the birefringence amount corresponding to the maximum value of the change value of the PMD to be compensated and then calculating the maximum polarization group delay (PMD).

The optical pulses distorted by the PMD through the optical path have polarizations of two output PSP states that are orthogonal to each other when considering only the primary PMD, and the group delay time is divided into two optical pulses separated by the amount of the primary PMD. The output pulse is converted to match the intrinsic polarization mode of the liquid crystal optical fiber through the polarization controller 21 in the PMD compensator 20 and then incident to the input terminal of the liquid crystal optical fiber 22. The liquid crystal fiber has an electric field ( ) Is applied, it has a birefringence corresponding to the electric field strength. At this time, the optical pulse signal having the smallest group delay among the two pulses output from the optical path and divided on the time axis is the low axis among the birefringence axes of the liquid crystal fiber fiber, and the optical pulse signal having the large group delay is used as the high speed axis ( Adjust using a polarization controller to be incident to Fast Axes).

The interval between the two pulses of the output PSP state passing through the optical path is reduced while passing through the liquid crystal optical fiber in the PMD compensator so that they coincide with each other at the output terminal of the liquid crystal optical fiber. When the PMD value of the line changes due to environmental changes such as temperature and vibration around the light beam, it is possible to determine the amount of PMD to be compensated by adjusting the intensity of the electric field applied to the liquid crystal optical fiber. Therefore, the maximum possible amount of PMD compensation should be determined based on the length of the liquid crystal optical fiber because the liquid crystal molecules in the liquid crystal fiber are lined up in line with the direction of the electric field or all the liquid crystal molecules are aligned in the direction of the electric field.

When the PMD value changes over time, the intensity of the electric field to be applied to the liquid crystal optical fiber in the PMD compensator is determined based on the PMD value obtained from the above-mentioned PMD monitoring device. The DOP measuring device 40 after the PMD compensator 20 converts the output PSP of the optical pulse signal passing through the optical path into the polarization eigenmode of the liquid crystal optical fiber. If the intrinsic polarization mode of the optical fiber output PSP and the liquid crystal optical fiber in the compensator does not match, the total amount of PMD after passing through the PMD compensator is larger than the amount of PMD in the line, so that the DOP at the output terminal of the optical fiber is greatly reduced. Therefore, by adjusting the polarization controller 21 of the PMD compensator 20 to maximize the DOP at the output terminal of the liquid crystal optical fiber, the output PSP of the optical pulse signal passing through the optical path and the polarization intrinsic mode of the liquid crystal optical fiber can be matched.

Finally, the operation of the main controller 30 will be described. The main controller is a kind of computer that calculates a PMD value from the signal of the PMD monitoring device 10, determines a voltage to be applied to the liquid crystal optical fiber 22 in the PMD compensator 20, and controls the signal to control the PMD. It is sent to the voltage application device 23 in (20). In addition, the polarization controller 21 located at the front end of the liquid crystal optical fiber 22 is adjusted to receive the DOP value from the DOP measuring device 40 at the rear end of the liquid crystal optical fiber.

As described above, the PMD compensation system according to the present invention monitors the PMD value, calculates a control value through the main controller, and adjusts the intensity of the electric field applied to the liquid crystal optical fiber to provide low compensation, easy manufacturing, and stable compensation. Has the effect of performing.

Claims (8)

PMD monitoring device that performs the function of extracting Polarization Mode Dispersion value from the distorted optical pulse signal through the optical transmission fiber, and restores the optical pulse distorted by the PMD of the optical transmission fiber. A PMD compensation system comprising a PMD compensation device to perform and a main controller for compensating the PMD by generating a feedback control signal to the PMD compensation device using the signal provided from the PMD monitoring device; The PMD compensation device, A polarization controller which redirects the polarization of specific components of the optical signal received through the optical transmission fiber, A liquid crystal optical fiber composed of a hollow core optical fiber inserted with a liquid crystal having a refractive index similar to that of a core of a communication single mode optical fiber at room temperature; And a voltage applying device for generating a compensating PMD by controlling the molecular polarization intensity of the liquid crystal molecules constituting the core of the liquid crystal optical fiber by varying an electric field in accordance with a control signal provided from the main controller. PMD compensation system, characterized in that made. The method of claim 1; PMD compensation system, characterized in that a plurality of voltage applying device is installed on the outer periphery of the liquid crystal optical fiber to align the liquid crystal in the liquid crystal optical fiber. The method of claim 2; PMD compensation system, characterized in that the intensity of the electric field applied to the liquid crystal optical fiber outside in a plurality of voltage applying device installed on the outside. The method of claim 2; In a plurality of voltage applying device installed on the outside, the PMD compensation system, characterized in that the electric field of the same intensity is applied to the outside of the liquid crystal optical fiber at a different angle with respect to the liquid crystal molecules in the liquid crystal optical fiber. The method according to any one of claims 1 to 4; The polarization controller is composed of a liquid crystal optical fiber PMD compensation system, characterized in that for controlling the polarization by the voltage applied to the outside of the control signal of the main controller. PMD monitoring device that performs the function of extracting Polarization Mode Dispersion value from the distorted optical pulse signal through the optical transmission fiber, and restores the optical pulse distorted by the PMD of the optical transmission fiber. A PMD compensation system comprising a PMD compensation device to perform and a main controller for compensating the PMD by generating a feedback control signal to the PMD compensation device using the signal provided from the PMD monitoring device; The PMD monitoring device, A tapping optical coupler positioned at the output of the optical path and guiding a primary input value transmitted from the single mode optical path to the polarization controller and extracting the distorted signal; An optical amplifier for amplifying the signal extracted by the tapping optical coupler, A 50:50 optical coupler for separating the output signal of the optical amplifier into two optical signals; A high speed optical receiver connected to one output terminal of the 50:50 optical coupler and converting an optical pulse signal into an electrical signal; A radio frequency band pass filter connected to a rear end of the high speed optical receiver and measuring an intensity of a first harmonic component of a radio frequency signal corresponding to a communication data ratio and transmitting the measured intensity to a main controller; An automatic polarization controller connected to the other output terminal of the 50:50 optocoupler, A linear polarizer connected to a rear end of the automatic polarizer, And a DC light sensor for sensing an output signal after passing through the linear polarizer and transmitting the signal to the main controller. The method of claim 1 or 6; A DOP measuring device for converting the output PSP of the optical pulse signal transmitted to the receiver via the PMD compensator into the polarization eigenmode of the PMD compensator is added between the rear end of the PMD compensator and the main controller. PMD compensation system. The method of claim 7; The DOP measuring device, A tapping optical coupler for separating an optical signal transmitted from the PMD compensator to a receiver; An automatic polarization controller for polarizing the output optical signal of the PMD compensation device separated by the tapping optical coupler; A linear polarizer connected to a rear end of the automatic polarizer, And a DC light sensor for sensing an output signal after passing through the linear polarizer and transmitting the signal to the main controller.