KR100964084B1 - Birefringence generator including multi-hole hollow and PMD compensator using the optical fiber grating - Google Patents

Abstract

The present invention discloses an optical fiber grating element having a multiconjugated concentrate fiber and a polarization mode dispersion (PMD) compensator using the same.

The multiconjugated concentrate fiber of the present invention has a plurality of hollows arranged in a lattice from the center of the optical fiber at the core portion of the optical fiber and restricts the size of each hollow so that the injected liquid crystal molecules can be aligned in the axial direction. By improving the structure, the desired birefringence can be generated from " 0 " to any value by controlling the alignment direction of the liquid crystal molecules in the multipores uniformly according to the electric field applied to the multiconjugated concentrate fiber. In addition, by performing PMD compensation using the multiconjugated concentrate fiber, it is possible to implement a PMD compensation device having low light loss, easy manufacturing, and stable operation.

Description

Birefringence generator including multi-hole hollow and PMD compensator using the optical fiber grating}

1 is a diagram illustrating a birefringence phenomenon of an optical fiber.

2 is a diagram illustrating a principle in which polarization mode dispersion occurs.

3 is a waveform diagram showing an output waveform of an optical pulse that has traveled through an optical fiber in FIG. 2;

4 is a view showing the basic principle of the polarization mode dispersion compensation device.

5 is a view showing a birefringence change of the optical fiber with a change in time.

6 is a cross-sectional view showing a state of a conventional optical fiber injected with liquid crystal.

7 is a block diagram showing the configuration of a polarization mode dispersion compensation device according to the present invention.

8 is a cross-sectional view showing a state in which the liquid crystal is injected into the multiconjugated concentrate fiber according to the present invention.

9 is a view showing an embodiment in which a liquid crystal is injected into a multiconjugated concentrate fiber having no lattice structure.

10 is a view showing the alignment of the liquid crystal when the electric field is not applied to the multiconjugated concentrate fiber according to the present invention.

11 is a view showing the alignment of the liquid crystal when an electric field is applied to the multiconjugated concentrate fiber according to the present invention.

The present invention relates to a PMD compensation device for compensating Polarization Mode Dispersion (PMD) of a light path, and more particularly, has a plurality of hollows in which liquid crystals are injected in a line in a core part. A birefringence generator capable of variably compensating a PMD by adjusting a voltage applied to a multi-hole hollow fiber, and a PMD compensator using the same.

When the optical pulse is transmitted through the optical fiber, each optical pulse generates a signal distortion in which the shape of the optical pulse changes on the time axis due to the physical characteristics of the optical fiber as a transmission medium. The causes of such signal distortion can be largely classified into attenuation characteristics, dispersion characteristics, and nonlinear characteristics of an optical fiber. A large problem that emerges in recent high-speed, large-capacity, long-distance long-range dense wavelength division multiplexing (DWDM) and time division multiplexing (TDM) optical communication is a dispersion characteristic. This can be called. Dispersion that increases the width of the optical pulse after transmission occurs in three forms: mode dispersion, color dispersion, and polarization mode dispersion.

Among these, the polarization mode dispersion (PMD) is due to the birefringence of the optical fiber. When an optical pulse having a finite line width enters an optical fiber having a birefringence, each spectral component in the optical pulse experiences different birefringence. It refers to a phenomenon in which the width of an optical pulse becomes wider after passing through an optical fiber. In this case, since the polarization modes of the optical pulse signals guiding the optical fiber experience different birefringence, a group delay difference occurs, and thus the width of the optical pulses increases.

The causes of the birefringence difference between each polarization mode include intrinsic causes such as non-circularity of the optical fiber and external force or asymmetric internal stress or strain acting on each part of the optical fiber in the optical cable.

This PMD was a physical quantity that was not a big problem when the optical transmission system of 10Gbit / s or less, which was a problem of color dispersion and optical loss of light path, was applied to communication, but dispersion shifted fiber and chromatic 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 in which the PMD coefficient value is 0.5 to 2ps / km ^½ . Therefore, in the case of 10Gbit / s optical transmission system, the communication distance is 100km to the maximum due to PMD. It will be reduced to 25 km. Considering the urgent need for a high speed inter-network network, which is fueled by the recent explosive demand of subscribers' multimedia services over the Internet, the compensation problem of PMD in high-speed long-distance optical transmission networks is an urgent task of various communication providers.

On the other hand, the spun optical fiber, which is newly produced due to the recent development of production technology, is very small, with a PMD value of about 0.1 ps / km ^½ , but considering the increasing trend of optical subscriber network construction due to multimedia services, As a result, the speed of the inter-station network will be accelerated. PMD compensation problem will ultimately be solved because PMD will be the main cause of transmission restriction in high-speed transmission networks of 40Gbit / s or more even if the fabricated fiber is installed recently. can do.

1 is a diagram showing the birefringence of the optical fiber, the birefringence is not the original circular core of the optical fiber through which the optical signal proceeds due to the limitations of the manufacturing technology of the optical fiber or the environmental change such as the temperature or stress around the optical cable is installed. It is an elliptical shape close to, and it is a phenomenon that occurs as the refractive indices are different in the long axis and short axis direction of the ellipse core.

The mode of single-mode fiber (SMF) is actually degenerate two mutually orthogonal polarization modes. When the core of the optical fiber is ideally circular, the propagation speed of the two degenerate polarization modes is the same, but the core In the case of an ellipse, since the refractive indices in the major and minor 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, thereby widening the pulse width at the output terminal. It is a signal error at the receiver. 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.

The actual optical fiber has a shape in which the birefringence axis of the core randomly intersects at a certain distance in the manufacturing process, and this PMD has a statistical and complicated shape. 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. In this case, the time delay difference between two output pulses is referred to as a differential group delay (DGD), which corresponds to a primary PMD.

4 illustrates the basic principle of a general 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 terminal 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 intrinsic mode of the polarization maintaining optical fiber in the compensator through the polarization controller, and then the optical pulses having a large time delay are less than the refractive index of the polarization maintaining optical fiber. Light pulses having a small time delay are incident on the slow axes and fast axes having a large refractive index.

As a result, the principle is to reduce the time delay difference of the optical signal after passing through the short-length polarization maintaining optical fiber. 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 value.

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. In other words, there is a need for a dynamic compensation device that compensates in real time as the output PSP changes with time.

Recently, attempts have been made to implement polarization mode dispersion apparatuses using birefringence of liquid crystals.

FIG. 6 is a view illustrating a state in which a large amount of liquid crystals are injected into a hollow hollow fiber. FIG.

This method is based on the principle that the rotation of the liquid crystal molecules having the electric dipole properties according to the applied electric field, thereby generating a variable birefringence. Accordingly, variable birefringence can be generated by injecting a large amount of liquid crystal molecules into hollow hollow fiber and arranging liquid crystal molecules by applying a variable electric field around the optical fiber. Since this method is simple in structure, the variable birefringence of the PMD compensator can be obtained at low cost and can be easily obtained. Therefore, it is possible to provide adaptive feedback and have high reliability.

However, in the case of a polarization mode dispersion (PMD) compensation device using an optical fiber as shown in FIG. 6, the diameter of the core corresponding to the hollow of the hollow optical fiber into which the liquid crystal is inserted is limited to about 8 μm.

In this case, several layers of liquid crystals are stacked in the hollow optical fiber as shown in FIG. 6. Since each liquid crystal molecule itself has a property of a permanent dipole, electrical interactions between the liquid crystal molecules occur, so that the alignment direction of the liquid crystal molecules is random at room temperature. In this case, the liquid crystal optical fiber injected with the liquid crystal has a specific birefringence value of a specific value at room temperature, and it becomes difficult to control the birefringence value below the average birefringence value by applying an electric field, which acts as a significant obstacle to PMD compensation. In addition, there is a difficulty in uniform alignment of molecules due to the interaction of the liquid Vander Walls force between the liquid crystal molecules, so that the contribution of the non-uniformly aligned molecules acts as birefringence noise. It is difficult to obtain.

Accordingly, an object of the present invention for solving the above problems is to improve the alignment of the liquid crystal injected into the hollow by using a multi-hollow fiber (Multi hollow fiber) having a plurality of hollow (hollow) of the optical fiber.

Birefringence generator of the present invention for achieving the above object is a plurality of hollow (hollow) is formed in the core (multi-conjugated concentrate fiber in which the liquid crystal having the refractive index of the core is injected in a row in each hollow; And a voltage applying unit for selectively applying an electric field to the multiconjugated concentrate fiber to selectively change the alignment of the liquid crystals, wherein each hollow is larger than one diameter 2r of the liquid crystal and 2 of the diameters 2r. It is characterized by having a smaller diameter than the pear.

PMD compensator of the present invention includes a PMD monitoring unit for monitoring the delay value (PMD) of the optical pulse signal distorted by the polarization mode dispersion and the basic state (PSP) of the output polarization; A polarization controller for controlling and outputting a polarization direction of the distorted optical pulses according to a first control signal; A plurality of hollows are formed in the core, and each hollow is injected with a line of liquid crystals having the refractive index of the core, and the birefringence of the optical pulse signal output from the polarization controller is variable according to the magnitude of the electric field applied from the outside. Multiconjugated concentrate fibers to be output; A voltage applying unit applying an electric field to the multiconjugated concentrate fiber according to a second control signal; A DOP measuring unit outputting a polarization degree (DOP) by converting a basic state (PSP) of the output polarization of the optical signal output from the multiconjugated concentrate fiber into a polarization intrinsic mode of the multiconjugated concentrate fiber; And a control unit configured to output the first control signal and the second control signal by determining the polarization direction of the polarization controller and the magnitude of the electric field of the voltage applying unit using the monitoring result of the PMD monitoring unit and the polarization degree of the DOP measuring unit. do.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

7 is a configuration diagram showing the configuration of a PMD compensation device according to the present invention.

The PMD compensator of the present invention includes a PMD monitoring unit 10, a PMD compensator 20, a control unit 30, a DOP (polarization degree) measuring unit 40, and a receiver 50.

The PMD monitoring unit 10 extracts a signal for calculating the primary PMD value and the output PSP from the distorted optical pulse signal through the single mode optical path.

The PMD compensator 20 restores the optical pulses distorted by the PMD of the optical path.                     

The PMD compensator 20 includes a polarization controller 21, a multiconjugated concentrate fiber 22, and a voltage applying unit 23.

The polarization controller 21 inputs the distorted optical pulse signal passing through the single mode optical path to the multiconjugated concentrate fiber 22 in a specific direction. In the multiconjugated concentrate fiber 22, a plurality of hollows are provided in a portion corresponding to the core of the optical fiber in the axial direction, and liquid crystal is injected into each hollow. The voltage applying unit 23 is installed outside the multiconjugated concentrate fiber 22 to apply a voltage to the multiconjugated concentrate fiber 22 according to a control signal provided from the controller 30. A plurality of such voltage applying unit 23 may be installed outside the multiconjugated concentrate fiber 22 to apply different voltages to the multiconjugated concentrate fiber 22 for each part.

A feature of the present invention is the use of the multiconjugate concentrate fiber 22 in the PMD compensator 20, which is described in detail below.

The control unit 30 controls the polarization controller 21 and the voltage applying unit 23 by using the signals applied from the PMD monitoring unit 10 and the DOP measuring unit 40 to the PMD compensator 20. Feedback to. That is, the controller 30 calculates the first PMD value of the optical pulse signal received using the signal extracted by the PMD monitoring unit 10 to determine the voltage to be applied to the multiconjugated concentrate fiber 22 to apply the voltage. Transfer to 20. Then, the output PSP and the multiconjugate concentrate fiber are output by using the output PSP of the optical path and the DOP value of the output terminal of the multiconjugate concentrate fiber 22 received from the DOP measurement unit 40 using the signal applied from the PMD monitoring unit 10. Adjust the polarization controller 21 so that the polarization eigenmode of (22) coincides.                     

The DOP measuring unit 40 converts the output PSP of the optical pulse signal compensated by the PMD compensator 20 to the receiver 50 to the polarization eigenmode of the multiconjugated concentrate fiber 22 to control the DOP value. Transfer to 30.

8 is a view showing a cross section of a multiconjugated concentrate fiber according to the present invention.

In the multiconjugated concentrate fiber 22 of the present invention, a plurality of hollows are provided in a portion corresponding to the core of the optical fiber in the axial direction, and the liquid crystal having a refractive index similar to that of the core of the single mode optical fiber for communication at room temperature is It has an inserted structure.

In this case, the size and number of hollows in the multi-hole optical fiber guided by the light is determined by the size of the liquid crystal molecules used. That is, the diameter of the hollow is larger than the size of one liquid crystal molecule but smaller than twice. By limiting the size of the hollows as described above, the liquid crystal molecules injected in each hollow are axially aligned only in one layer without stacking in multiple layers as shown in FIG. 6.

In addition, the plurality of hollows in the multiconjugated concentrate fiber 22 are arranged radially as shown in FIG. 8 with respect to the center of the optical fiber so as to cancel each other's Vanderwald interaction between liquid crystal molecules injected into the hollow. .

If a plurality of hollows are randomly arranged as shown in FIG. 9, the Vanderwald interactions between the liquid crystal molecules in the multiconjugated concentrate fiber 22 do not cancel each other to have a specific birefringence value of a specific value at room temperature. This results in birefringence below the average birefringence value and birefringence noise when an electric field is applied to the multiconjugated concentrate fiber 22.                     

FIG. 10 is a cross-sectional view illustrating a state in which the multiconjugated concentrate fiber 22 of FIG. 8 is cut in the longitudinal direction, and the liquid crystal molecules in the multiconjugated concentrate fiber when the electric conjugate is not applied to the multiconjugate concentrate fiber 22 according to the present invention. Indicates the alignment state.

In this case, the birefringence value at room temperature is "0". In addition, since a single row of liquid crystal molecules is injected into each hollow in the multi-optic fiber 22, random alignment of the liquid crystal molecules due to interaction between the liquid crystal molecules does not occur. Therefore, birefringence noise can be greatly reduced.

The operation of the PMD compensation device of the present invention having the above-described configuration will be described below.

After the high-speed pulse modulation at 10 Gbit / s to pass through the long-distance optical path, the two optical pulse signal is pulsed apart as shown in Figure 3 by the PMD by the optical coupler 11 PMD monitoring unit 10 and the polarization controller ( 21). The optical pulses distorted by the PMD have polarizations in two output PSP states that are orthogonal to each other when considering only the primary PMD, and are divided into two optical pulses in which the group delay time is separated by the amount of the primary PMD as shown in FIG. 3.

The PMD monitoring unit 10 extracts a signal for calculating a primary PMD value and an output PSP from the applied optical pulse signal. This function is similar to the PMD monitoring function conventionally used. To briefly describe the monitoring operation, the PMD monitoring unit 10 amplifies a predetermined amount of the applied optical pulse signal, converts the signal into an electrical signal, and Fourier transforms the electrical signal to obtain an RF spectrum. Here, the RF spectrum is composed of 10 GHz high frequency components, and as the PMD value of the optical line increases, the interval between two pulses increases, the size of the first high frequency component of the RF spectrum decreases and the size of the second high frequency component increases. In this case, when an electrical signal passes through a RFBPF having a center frequency of 10 GHz, a change in PMD value that increases or decreases may be represented as a change in size of a 10 GHz radio frequency component. The PMD monitoring unit 10 may monitor the PMD value by measuring the magnitude change and simultaneously measuring the DOP in the applied optical pulse signal.

The optical pulse signal applied to the polarization controller 21 by the optical coupler 11 is converted to match the intrinsic polarization mode of the multiconjugated concentrate fiber 22 by the polarization controller 21 and is incident on the multiconjugated concentrate fiber 22. do.

)이 인가되어 있으므로 전기장의 세기에 해당하는 복굴절량을 가지게 된다.The multiconjugated crystal optical fiber 22 has an electric field by the voltage applying unit 23.

) Is applied, it has a birefringence corresponding to the strength of the electric field.

)을 인가하여 액정분자들의 정렬을 제어하는 모습을 보여주는 도면이다.11 shows an electric field in the multiconjugated concentrate fiber 22

Is a view showing a state of controlling alignment of liquid crystal molecules by applying a).

)을 가할 수 있는 캐패시터(capacitor)가 사용될 수 있으며, 다중공액정광섬유(22)는 그 캐패시터 내에 위치하게 된다. 이때, 다중공액정광섬유(22)는 광섬유 허용 구부림 반경으로 감겨져 캐패시터 내에 위치한다.In the embodiment of the voltage applying unit 23, the multiconjugated concentrate fiber 22 is embedded to provide a direct current or alternating current electric field (

A capacitor capable of applying a) may be used, and the multiconjugated concentrate fiber 22 is located in the capacitor. At this time, the multiconjugated concentrate fiber 22 is wound in the optical fiber allowable bending radius and positioned in the capacitor.

)이 가해지면 다중공액정광섬유(22) 내의 액정분자들은 전기장의 방향으로 늘어서게 된다. 이때, 캐패시터에 가하는 전기장(

)의 세기에 따라 액정분자들이 늘어서는 각도가 달라지거나 늘어서는 분자들의 개수가 달라지게 되어 이에 해당되는 분자 분극(molecular polarization)이 형성된다.The electric field (see FIG. 11) is applied to the multiconjugated concentrate fiber 22 by the voltage applying unit 23.

) Is applied, the liquid crystal molecules in the multiconjugated concentrate fiber 22 are lined up in the direction of the electric field. At this time, the electric field applied to the capacitor (

According to the intensity of the liquid crystal molecules, the angle at which the liquid crystal molecules line up or the number of molecules lined up is changed, thereby forming a molecular polarization corresponding thereto.

In particular, since the intensity of molecular polarization can be adjusted by changing the intensity of the electric field, a variable birefringence can be obtained by adjusting the intensity of the electric field applied to the multiconjugated concentrate fiber 22.

The length of the multiconjugate concentrate fiber 22 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 polarization controller 21 outputs from the optical path and the optical pulse signal having the smallest group delay among the two pulses divided on the time axis is the slow axis of the birefringence axis of the multiconjugated concentrate fiber 22, and the large group delay. The optical pulse signal is incident on the fast axes.

The operation of the polarizer 21 and the voltage applying unit 23 is controlled by the controller 30. That is, the controller 30 calculates a PMD value from the signal of the PMD monitoring unit 10, determines a voltage to be applied to the multiconjugated concentrate fiber 22, and transmits a control signal to the voltage applying unit 23. Then, the DOP measurement unit 40 receives the DOP value and adjusts the polarization controller 21 to maximize it.

The interval between the two pulses of the output PSP state passing through the optical path is reduced while passing through the multiconjugated concentrate fiber 22, so that the output terminal of the multiconjugated concentrate fiber 22 coincides with each other.

When the PMD value of the line changes due to environmental changes such as temperature and vibration around the light beam, the amount of PMD to be compensated by adjusting the intensity of the electric field applied to the multiconjugated concentrate fiber 22 can be determined.

When the PMD value changes over time, the controller 30 determines the intensity of the electric field to be applied to the multiconjugated concentrate fiber 22 based on the PMD value obtained from the PMD monitoring unit 10. The optical pulses passing through the multiconjugated concentrate fiber 22 are applied to the DOP measuring unit 40 and the receiver 50 by the optical coupler 41.

The DOP measuring unit 40 converts the output PSP of the optical pulse signal passing through the optical path into the polarization eigenmode of the multiconjugated concentrate fiber 22. If the output PSP of the optical path and the intrinsic polarization mode of the multiconjugated optical fiber 22 do not coincide, the total amount of PMD after passing through the PMD compensator 20 is greater than the amount of PMD of the optical path, so that The DOP at the output stage is greatly reduced.

Therefore, the control unit 30 adjusts the polarization controller 21 to maximize the DOP at the output terminal of the multiconjugated concentrate fiber 22, so that the output PSP of the optical pulse signal passing through the optical path and the multiconjugated concentrate fiber 22 Match the polarization eigenmodes.

As described above, the multiconjugated concentrate fiber of the present invention has a plurality of hollows in the core portion of the optical fiber and is injected into each hollow so that the liquid crystal molecules can be aligned in a row, thereby multiplying according to the electric field applied to the multiconjugate concentrate fiber. The alignment direction of the liquid crystal molecules in the hole can be controlled uniformly and can generate the desired birefringence from "0" to any value.

Claims (6)

delete A multiconjugated concentrate fiber in which a plurality of hollows are formed in a core and each hollow is injected with a liquid crystal having a refractive index of the core in a row; And And a voltage applying unit for selectively changing an alignment of the liquid crystals by variably applying an electric field to the multiconjugated concentrate fiber, Each hollow is, And a birefringence generator having a diameter greater than one diameter (2r) of said liquid crystal and less than twice the diameter (2r). The method of claim 2, Each hollow is, And a birefringence generator disposed radially with respect to the core center. A PMD monitoring unit for monitoring a delay value PMD of the optical pulse signal distorted by the polarization mode dispersion and a basic state PSP of the output polarization; A polarization controller for controlling and outputting a polarization direction of the distorted optical pulses according to a first control signal; A plurality of hollows are formed in the core, and each hollow is injected with a line of liquid crystals having the refractive index of the core, and the birefringence of the optical pulse signal output from the polarization controller is variable according to the magnitude of the electric field applied from the outside. Multiconjugated concentrate fibers to be output; A voltage applying unit applying an electric field to the multiconjugated concentrate fiber according to a second control signal; A DOP measuring unit outputting a polarization degree (DOP) by converting a basic state (PSP) of the output polarization of the optical signal output from the multiconjugated concentrate fiber into a polarization intrinsic mode of the multiconjugated concentrate fiber; And A control unit for outputting the first control signal and the second control signal by determining the polarization direction of the polarization controller and the magnitude of the electric field of the voltage applying unit using the monitoring result of the PMD monitoring unit and the polarization degree of the DOP measuring unit; Polarization mode dispersion compensation device comprising a. The method of claim 4, wherein Each hollow is, And a polarization mode dispersion compensating device having a diameter greater than one diameter of the liquid crystal and less than twice the diameter of the diameter 2r. The method according to claim 4 or 5, Each hollow is, Polarization mode dispersion compensation device, characterized in that arranged radially with respect to the center of the core.

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