KR100975589B1 - High speed polarization scrambler and actuating method thereof - Google Patents

Abstract

The present invention relates to a fast polarization scrambler, and more particularly, to two or more piezoelectric element-type optical fiber birefringence modulators modulated at different frequencies, but providing an input method of the driving parameters to output light in all polarization states. The present invention relates to a high speed polarization scrambler which will be used as an alternative to solve the signal error problem due to polarization mode dispersion (PMD) in a high speed transmission communication system of 10G to 40G by making a high speed scramble.

Polarization mode dispersion (PMD), fast polarization scrambler, forward error correction (FEC).

Description

High speed polarized scrambler and its driving method {HIGH SPEED POLARIZATION SCRAMBLER AND ACTUATING METHOD THEREOF}

The present invention relates to a fast polarization scrambler, and more particularly, to two or more piezoelectric element-type optical fiber birefringence modulators modulated at different frequencies, but providing an input method of the driving parameters to output light in all polarization states. The present invention relates to a high speed polarization scrambler which will be used as an alternative to solve the signal error problem due to polarization mode dispersion (PMD) in a high speed transmission communication system of 10G to 40G by making a high speed scramble.

The recent increase in Internet demand has led to a surge in communications information, which is expected to accelerate further. In order to accommodate a large amount of information, a wide bandwidth optical transmission system is required, and studies to simultaneously implement a time division multiplexing (TDM) and a wavelength division multiplexing (WDM) simultaneously have been extensively implemented. It's going on. In time-division multiplexing, the problem that becomes particularly important as the bit rate increases is polarization mode dispersion (PMD).

Polarization Mode Dispersion (PMD) is an arbitrary birefringence of optical fiber on a transmission path due to an asymmetric fiber structure, internal factors such as stress deformation, and external factors such as temperature changes, vibration changes, and pressure changes around the transmission path. This is a waveform distortion that is induced and consequently undergoes an optical signal traveling through it. This waveform distortion occurs due to the differential time delay of two perpendicular polarization components called Principal State of Polarization (PSP) during transmission.

In general, polarization mode dispersion (PMD) is one of the major obstacles in increasing the transmission speed in a long wavelength WDM transmission system. In order to minimize deterioration of the system due to the polarization mode dispersion (PMD), a study for compensating the polarization mode dispersion (PMD) is being conducted.

Most methods for compensating the polarization mode dispersion (PMD) are complicated by monitoring the optical signal at the receiver to adjust the amount of polarization mode delay or input polarization to be compensated. This is because the polarization mode dispersion (PMD) of the optical path not only varies irregularly with environmental factors and time, but also does not completely eliminate the effects of the second polarization mode dispersion (PMD).

The technology of polarizing scrambler using optical fiber is based on Korean Patent No. 1999-0070552, which uses a circular piezoelectric element with an improved optical fiber birefringence modulator with a modulation frequency of several hundreds of kilohertz to 1 MHz, which has low loss and stable performance regardless of input polarization. Provided are a scrambler and a driving parameter input method thereof.

The modulation frequency is determined according to the resonance frequency of the piezoelectric element. The resonance frequency of the circular piezoelectric element is generally dependent on the material system of the piezoelectric element, but is generally expressed as Equation 1 below.

(Equation 1)

Where f is the resonance frequency T is the wall thickness. Therefore, by using a hollow cylindrical piezoelectric element having a wall thickness of about 2 mm to 5 mm, a resonance frequency between 400 kHz and 1 MHz can be utilized as a birefringent modulation frequency. To increase the resonance frequency, the wall thickness must be thin, but it is difficult to manufacture piezoelectric elements stably. This limitation requires a different approach to modulating birefringence at high speeds above 5 MHz.

In particular, since a signal error problem due to polarization mode dispersion (PMD) has to be solved in a high speed communication system of 10G to 40G, an implementation of a fast polarization scrambler is required.

The present invention implements an optical fiber birefringence modulator that is actually driven at a high speed of 5 MHz or more by setting an appropriate drive parameter by using a cylindrical piezoelectric element having a resonance frequency of 1 MHz as an alternative to the above-described prior art. It is an object of the present invention to provide a high speed polarization scrambler for using a modulator to scramble output light in all polarization states at high speed.

In addition, the present invention is to compensate for the self-polarization mode dispersion by using a polarization maintaining optical fiber to reduce the polarization mode dispersion that occurs while constructing a birefringent modulator, and to be used in conjunction with the FEC coding technique in a long-distance high-speed communication system It is an object of the present invention to alleviate signal degradation caused by PMD).

In order to achieve the above object, a high speed polarization scrambler according to the present invention is a polarization scrambler having a birefringence modulator using an optical fiber and an alternating voltage source for inducing birefringence modulation by applying an alternating voltage to the birefringence modulator, wherein the optical fiber birefringence modulator Is wound around the outer wall of the piezoelectric element of the hollow-cylindrical cylinder shape, but wound without an twist in the axial direction of the optical fiber to prevent the induction of circular birefringence, thereby increasing the modulation efficiency of the AC voltage of the AC voltage source Inputs by increasing the length of the optical fiber causing the interaction with or increasing the amplitude of the AC voltage source, or simultaneously increasing the length of the optical fiber and the increased amplitude input of the AC voltage source, wherein the optical fiber birefringence modulator has two or three Dogs in series, shearing Located to align to the input to FIG. (N is an integer) 45 ± 90n to the birefringent axis of the optical fiber birefringence modulator is characterized in that to increase the speed of the polarization scrambling.

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In the present invention, comprising a polarization maintaining optical fiber configured to be connected between each of the optical fiber birefringence modulator, and to be aligned perpendicular to the birefringence axis of the optical fiber birefringence modulator to compensate the polarization mode dispersion caused by the winding of the optical fiber birefringence modulator It is characterized by.

In the present invention, the optical fiber of the optical fiber birefringence modulator and the polarization-maintaining optical fiber is melt-bonded and inserted into the inside of the ferrol and then fixed by twisting around the axial direction, it is characterized in that it further comprises a rotating body to align the axis by rotation. .

In the present invention, the amplitude of the AC voltage source of the birefringence modulation is set to be any one selected from among the values that are the solutions of the first Bessel function in order to maintain the polarization degree of the output light at zero.

In the present invention, in order to generate the output light of all the polarization state using only the two optical fiber birefringent modulator, the optical fiber input to the first optical fiber birefringent modulator is connected to the polarization maintaining optical fiber and incident to the input light in the linear polarization state parallel to the optical axis The linearly polarized light is adjusted to be input at 45 ± 90n degrees (n is an integer) with respect to the birefringence axis direction of the first optical fiber birefringence modulator.

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The high speed polarization scrambler according to the present invention is installed in the transmitting end to solve the signal error caused by polarization mode dispersion (PMD) in a high speed transmission communication system of 10G or more. By generating output light to correct signal distortion due to polarization mode dispersion and having the receiver process the signal with forward error correction (FEC), signal loss due to polarization mode dispersion (PMD) can be improved.

The present invention has an effect of compensating for Polarization Mode Dispersion (PMD) using polarization maintaining fibers (PM) to reduce polarization mode dispersion generated while constructing a birefringent modulator.

The high speed polarization scrambler according to the present invention has an effect of enabling high speed polarization scrambling more than five times the modulation frequency actually driven than the previous piezoelectric element type fiber birefringence modulator.

Hereinafter, a preferred embodiment of a fast polarization scrambler according to the present invention will be described in detail with reference to the accompanying drawings.

Hereinafter, an example of a birefringent modulator configured using an optical fiber will be described. However, the present invention may be similarly applied to a birefringent modulator having an optical waveguide.

1 is a diagram of an optical fiber birefringence modulator that is a key element of a fast polarization scrambler according to the present invention.

Referring to FIG. 1, in the optical fiber birefringence modulator 100, a single mode optical fiber 120 is wound around an outer wall of a piezoelectric element 110 having a hollow cylindrical shape, and a voltage signal by the AC voltage source 140 is a sine wave. It is configured to be authorized.

In this case, the piezoelectric element 110 is preferably composed of a piezoelectric transducer (PZT) or the like, and the optical fiber 120 is wound without twisting in the axial direction of the optical fiber 120 to prevent the induction of circular birefringence. This is preferred.

On the other hand, when a voltage signal having a specific amplitude is applied to the piezoelectric element 110 from the AC voltage source 140 in a sinusoidal waveform, not only phase modulation but also birefringence modulation is induced to light traveling through the optical fiber 120. At this time, the birefringence shaft 160 is a direction parallel to the cylindrical axis of the piezoelectric element 110 and a direction perpendicular to the cylindrical surface.

The magnitude of such birefringence modulation is represented by Equation 2,

 (Equation 2)

Here, V _m and ω _m correspond to the amplitude and angular frequency of the voltage signal applied to the piezoelectric element, respectively, and α is the birefringence modulation coefficient of the piezoelectric element with respect to the optical fiber. Not only depends on the type of optical fiber 120, but particularly on the frequency of the applied voltage signal.

The optical fiber birefringence modulator 100 is vertically connected to the birefringence axis 160 of the optical fiber birefringence modulator 100 to maintain polarization to compensate polarization mode dispersion generated by the winding of the optical fiber birefringence modulator 100. The optical fiber 130 is provided. In this case, the polarization maintaining optical fiber 130 is preferably connected by melt bonding to the optical fiber 120.

In addition, the rotating body to adjust the optical axis of the polarization maintaining optical fiber 130 is connected to the optical fiber 120 of the optical fiber birefringence modulator 100 to 45 degrees or 90 degrees to the vertical horizontal axis of the optical fiber birefringence modulator 100 ( It is preferable to further comprise 150).

At this time, the rotating body 150 is the optical fiber 120 and the polarization maintaining optical fiber 130 of the optical fiber birefringent modulator 100 is melt-bonded and inserted into the inside of the ferrol (not shown) to twist the axis around the axis direction It is preferable to configure to be able to rotate, and by this rotation the axis can be aligned.

2 is a diagram illustrating an embodiment of a fast polarization scrambler according to the present invention.

2, the fast polarization scrambler according to the present invention is coupled to at least two or three or more optical fiber birefringent transformation early (100, 101, 102), to the optical fiber birefringent modulator (100, 101, 102) In order to compensate for the influence on the polarization mode dispersion by the wound optical fiber 120, a polarization maintaining optical fiber 130 having a constant length is connected between the optical fiber birefringence modulators 100, 101, and 102.

In this case, the polarization maintaining optical fiber 130 is preferably aligned perpendicular to each other with the birefringence axis 160 of the optical fiber birefringent modulator 100 is configured by melt bonding to the optical fiber 120.

In addition, the optical fiber fast polarization scrambler for polarization mode dispersion mitigation according to the present invention is aligned with the birefringence axis 160 of the optical fiber birefringence modulator 100 perpendicular to each other to adjust the main axis to compensate for the polarization mode dispersion It can further comprise 150.

Here, the rotor 150 is not shown in the drawings, but the optical fiber 120 and the polarization-retaining optical fiber 130 is melt-bonded and inserted into and fixed inside the ferrol (not shown), and then around the axial direction. Preferably, it is inserted into and fixed to the rotating means for twisting the shaft to be aligned, wherein the optical fiber birefringence modulator located at the front end and the axis alignment of the polarization maintaining optical fiber or between the polarization maintaining optical fiber and the optical fiber birefringence modulator located at the rear end The axial alignment of allows to use the same rotor as that used in the all-optical inline polarization controller.

In addition, the rotating body 151 located between the polarization maintaining optical fiber 130 and the optical fiber birefringence modulator 101 connected to the rear side is adjusted to be incident in a 45 degree direction with respect to the main axis direction of the optical fiber birefringence modulator 100 located in front. . Of course, the rear rotor 153 is also the same.

On the other hand, when two optical fiber birefringent modulators (101, 102) are to be used to generate output light in all polarization states, instead of deleting the first optical fiber birefringent modulator (100) indicated by dotted lines, the second optical fiber birefringent modulator ( The linearly polarized state input light is incident on the optical axis of the polarization-maintaining optical fiber 130 input to the polarization maintaining optical fiber 130, and the linearly polarized light is controlled to be incident in the 45 degree direction with respect to the birefringence axis 160a of the optical fiber birefringence modulator 101. It is possible. Here, if the polarization state of the input optical fiber cannot be fixed, three or more optical fiber birefringent modulators may be used to generate output light of all polarization states for an input polarization state.

Since the optical axes of the polarization maintaining optical fibers 103, 131, and 132 are perpendicular to the respective optical fiber birefringence modulators connected to the front end by the above configurations, the main axis direction of the second or third optical fiber birefringence modulator is 45 ±. 90n degrees (n is an integer) is input.

In addition, the driving parameter input method of the fast polarization scrambler of the present invention is as follows.

First, the AC voltage for inducing birefringence modulation applied to each optical fiber birefringence modulator is to be a sine wave voltage of different frequencies. Here, the direction perpendicular to the cylindrical surface becomes the birefringence axis, and the magnitude of the birefringence modulation is expressed by Equation 2 above.

The induction of such birefringence modulation is proportional to the square of the frequency of the applied voltage signal. The method for increasing the birefringence modulation coefficient is to select the resonance frequency of the piezoelectric element and increase the number of windings of the optical fiber.

Referring to the configuration of the polarization scrambler using the optical fiber of the Republic of Korea Patent No. 1999-0070552 to the amplitude of the AC voltage for the drive parameter input method of the high speed polarization scrambler of the present invention, the light from the light source is input polarization regulator, optical fiber polarization scrambler (In this case, it means the configuration shown in Figure 2 of the present invention.), And then through the polarizer, which is an optical element with a large loss due to the polarization of the output polarization regulator after analyzing the output optical signal detected by the photodetector polarization modulation size Sets the value to be a specific value.

Here, as an example, a case in which a birefringence modulation of 5 Mhz is described will be given. A specific value of the polarization modulation magnitude is 5 pi radians (5π rad), and it is preferable to set the value of the amplitude of the AC voltage so that this specific value is obtained. Do.

On the other hand, if the polarization state of the input light is made 45 degrees linearly polarized with respect to the main axis of the optical fiber birefringence modulator with the input polarization controller, the electric field of the same intensity is incident on the two perpendicular intrinsic polarization axis is most effectively occurs.

Next, the output polarization controller is adjusted so that the output light generated by the polarization modulation in the optical fiber birefringence modulator interferes with each other at the rear end of the polarizer so as to be 45 degrees with respect to the direction of the polarizer. Then, an output waveform of the form as shown in Equation 3 is obtained.

             (Equation 3)

Here, when the polarization modulation magnitude (V _m α) is 5 pi, the same effect as rotating the Poincare sphere 5 times within the ω _m modulation period enables polarization scrambling 5 times faster than the modulation frequency. That is, it is preferable to increase the birefringence modulation size so that the polarization scrambling rate is a multiple of the modulation frequency for high speed polarization scrambling.

In addition, in order to add a function of maintaining the polarization degree of the output light to zero, each birefringence modulation amplitude should satisfy the following equation (4).

                    (Equation 4)

Where J ₀ is the first Bessel function, the birefringence modulation amplitudes V _m1 α ₁ , V _m2 α ₂ and V _m3 α ₃ should be the solution of the first Bessel function, which is 2.405, 5.52, 8.654, 11.792 , 14.931, 18.071 ..., etc., which can vary according to the required fast polarization modulation size. For example, if 5Mhz is required, the polarization modulation size is set to be 14.931 or 18.071 which is close to 5 pi. Since the degree of polarization of light is close to zero, effects due to polarization, such as polarization dependent gain of an Erbium Doped Fiber Amplifier in optical transmission, can also be eliminated.

On the other hand, if the number of times the optical fiber is wound to increase the birefringence modulation coefficient increases the polarization mode dispersion of the birefringent modulator itself can not be ignored, it should be compensated for this.

Accordingly, the polarization-maintaining optical fiber having a length corresponding to the polarization mode dispersion of the optical fiber birefringence modulator is melt-bonded and connected to the optical fiber of the optical fiber birefringence modulator. In detail, the second birefringence axis passes through the slow birefringence axis of the polarization maintaining optical fiber, and the polarization mode dispersion cancels each other after being inserted and fixed in the secondary to the rotation means of the rotating body provided to twist around the axial direction. In other words, the angles are fixed so that the optical axes are 90 degrees to each other.

3 is a schematic diagram of a device in which a high speed polarization scrambler according to the present invention is used in a high speed transmission communication system.

Referring to FIG. 3, the high speed polarization scrambler of the present invention is used with a transponder arranged at intervals of several hundred kilometers at the transmitting end of a 10G or higher speed transmission communication system, so that all polarization states within a given unit time (in FEC frame unit time) Output light is generated to correct signal distortion due to polarization mode dispersion and at the same time, the signal is processed by FEC (forward error correction) on the receiving end to effect the influence of polarization mode dispersion (PMD) on the optical fiber line. It can be used for the purpose of preventing signal loss caused by PMD).

Here, the fast polarization scrambler required to achieve polarization mode dispersion (PMD) compensation with FEC should be able to generate all polarization states within the FEC frame time. In other words, if the bad polarization state (45 degrees tilted to PSP) and the good polarization state (parallel to PSP) can be statistically evenly distributed within the FEC frame time, the FEC technique can detect the bit error in the bad polarization state. It can be removed by a coding technique. Since the frame unit time of the FEC is required from 0.5us to 0.1us (2MHz ~ 10MHz), it is necessary to be able to perform birefringence modulation to a frequency of 5MHz or more in order to generate enough polarized output light within this period.

The bit error rate (BER) can be improved by increasing the polarization scrambling rate to intentionally cause and correct bit errors caused by the influence of the polarization mode dispersion (PMD) within the FEC frame unit time.

FIG. 4 is a diagram of a polarization modulation signal calculated when the birefringence modulation size is 1 pi (1π) at a frequency of 1 MHz in Equation 3, and FIG. 5 is a birefringence by increasing the amplitude of a voltage signal at a frequency of 1 MHz. The figure shows the polarization modulation signal calculated when the modulation size is 5 pi (5π).

Comparing FIG. 4 and FIG. 5, it can be seen that as the birefringence modulation size is increased five times at the same frequency, higher order frequency components appear. In other words, the speed of scrambling is increased by five times.

Meanwhile, in the present invention as described above, it is obvious that the object of the present invention can be achieved by configuring the birefringent modulator using the optical waveguide in addition to the optical fiber birefringent modulator.

That is, the present invention increases the modulation efficiency by increasing the length of the optical fiber or optical waveguide that interacts with the AC voltage of the AC voltage source or increases the amplitude of the AC voltage source so that the speed of polarization scrambling is increased. Optical scrambling can be achieved.

The present invention described above is limited to the above-described embodiments and the accompanying drawings as various substitutional modifications and changes are possible within a range without departing from the technical spirit of the present invention for those skilled in the art. It doesn't happen.

1 is a diagram of an optical fiber birefringence modulator as a key element of a fast polarization scrambler according to the present invention;

2 is a diagram illustrating an embodiment of a fast polarization scrambler according to the present invention;

3 is a schematic diagram of a device in which a high speed polarization scrambler according to the present invention is used in a high speed transmission communication system,

4 is a diagram of a polarization modulation signal calculated when the birefringence modulation size is 1 pi (1π) at a frequency of 1 MHz in Equation 3,

FIG. 5 is a diagram illustrating a polarization modulation signal calculated when the birefringence modulation size is 5 pi (5π) by increasing the amplitude of the voltage signal at a frequency of 1 MHz.

*** Explanation of symbols for the main parts of the drawing ***

100 to 102: optical fiber birefringence modulator

110: piezoelectric element

120: optical fiber

121 to 122: optical fiber cross section

130 to 132: polarization maintaining optical fiber

140: AC voltage source

150 to 154: rotating body

160 to 162: birefringence axis

Claims (7)

A polarization scrambler having a birefringent modulator using an optical fiber and an alternating voltage source for inducing birefringence modulation by applying an alternating voltage to the birefringent modulator, The optical fiber birefringence modulator is wound around the outer wall of the piezoelectric element of the hollow-cylindrical cylindrical shape by winding the optical fiber without twisting the axial direction of the optical fiber to prevent the induction of circular birefringence, thereby increasing the modulation efficiency Inputting by increasing the length of the optical fiber that interacts with the alternating voltage of the voltage source or by increasing the amplitude of the alternating voltage source, or simultaneously implementing the increase in the length of the optical fiber and the increased amplitude input of the alternating voltage source, the optical fiber birefringence modulator Connect two or three in succession, aligning so as to be input at 45 ± 90n degrees (n is an integer) with respect to the birefringence axis direction of the optical fiber birefringence modulator located in the front end to increase the speed of polarization scrambling High speed polarized scrambler. delete The method of claim 1, And a polarization maintaining optical fiber configured to be connected between each of the optical fiber birefringent modulators and to be aligned perpendicular to the birefringence axis of the optical fiber birefringence modulator to compensate for the polarization mode dispersion generated by the winding of the optical fiber birefringence modulator. Polarized scrambler. The method of claim 3, wherein And a rotating body for fusion-bonding the optical fiber of the optical fiber birefringence modulator with the polarization-maintaining optical fiber and inserting and fixing the inside of the ferrole, twisting around the axial direction to rotate and align the axis. The method of claim 4, wherein And the amplitude of the AC voltage source of the birefringence modulation is set to be any one selected from the values that are the solutions of the first Bessel function in order to maintain the polarization degree of the output light at zero. The method of claim 1, In order to generate output light in all polarization states using only the two optical fiber birefringence modulators, the optical fiber inputted to the first optical fiber birefringence modulator is connected to the polarization maintaining optical fiber and is incident to the input light in the linear polarization state parallel to the optical axis, and the linear polarized light is the first optical fiber. A high speed polarization scrambler, characterized in that it is adjusted to be input at 45 ± 90n degrees (n is an integer) with respect to the birefringence axis direction of the birefringence modulator. delete

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