JP2008085612A - Optical transmission system and optical transmission method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize polarized wave scramble actually effective for deterioration reduction by PMD. <P>SOLUTION: A polarized wave scrambler executes a first polarized wave modulation step of modulating a polarized wave of signal light so as to draw a circle on a Poincare sphere and a second polarized wave modulation step of modulating a polarized wave of signal light on a Poincare sphere so as to draw a circle inclined by an angle of either Kπ+0.20π or more and Kπ+0.37π or less or Kπ-0.37π or more and Kπ-0.20π or less with respect to the circle drawn on the Poincare sphere by the first polarized wave modulation step. Modulation speeds of the first and second polarized wave modulation steps are different from each other. Alternatively, the modulation speeds of the first and second polarized wave modulation steps are different from each other but are synchronized causing a polarization state to change periodically. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to optical communication, and more specifically, to an optical transmission system that reduces degradation (penalty) caused by polarization mode dispersion (PMD) in an optical communication system.

  As the speed of optical transmission systems increases, signal quality degradation due to PMD has become a problem. PMD is generated when an optical transmission signal propagates through an optical fiber having birefringence or an optical component having birefringence. The two main polarization states (Principle State of Polarization: PSP) is caused by propagation of two orthogonal polarization components of the optical signal at different speeds. As a result, waveform distortion caused by PMD occurs in the optical signal reaching the receiver.

  The delay time difference between two orthogonal polarization components is called DGD (Differential Group Delay), and the amount is usually expressed in ps. When the optical fiber transmission line from the transmitter to the receiver is regarded as one transmission medium, the amount of PMD normally generated is not a problem with a low-speed signal, but with a high-speed signal exceeding 10 Gbit / s, Since the bit interval is shortened, the proportion of waveform distortion increases, and a large PMD causes a code error. Therefore, a technique for compensating for PMD is required for a transmission line having a large PMD.

  In addition, the polarization state of the optical fiber transmission line has a statistical property, and the polarization state changes with time due to environmental changes, and the change can be as fast as 1 millisecond or less. However, since the same polarization state may continue, a practical technique for compensating PMD is difficult.

  The ratio of the power of one main polarization component to the sum of the powers of the two main polarization components of the optical transmission medium is called the polarization splitting ratio (Splitting Ratio) and is represented by the symbol γ. I know it depends. When the powers of the two orthogonal main polarization components are equal, that is, when γ = 0.5, the deterioration is severe, and when the power is biased to either, that is, when γ = 0 or γ = 1, the deterioration occurs. small.

  As described above, the polarization state of the optical fiber transmission line has a statistical property, and γ of the optical signal incident on the transmission line cannot be fixed to a specific state. Although the state of γ = 1 is desirable, it can be a state of γ = 0.5. Therefore, it is necessary to design a transmission system considering that the worst state of γ = 0.5 continues.

  Therefore, by modulating the polarization direction, the state of γ = 0.5 is not continued, and by uniformizing the value of γ, the deterioration is averaged and reduced from the worst value to improve the transmission performance. can do. Forward error correction (FEC) is an effective technique for increasing the system margin, but its effect decreases when there are many errors in the FEC block. Therefore, by polarization scrambling, errors are averaged by averaging the polarization state over the length of the FEC block, the effect of FEC is improved, and the influence of polarization mode dispersion in the transmission of FEC-coded signals is affected. A method of reducing is described in Patent Document 1.

U.S. Pat. No. 6,437,892 US Pat. No. 5,361,270 US Pat. No. 6,914,707 US Pat. No. 7,046,416 F. Heismann, “Compact Electro-Optical Polarization Scramblers for Optically Amplified Lightwave Systems”, Journal of Lightwave Technology, Vol. 18-19, Journal of Light Technology.

  Patent Document 1 shows a Pockels cell as a polarization modulator, but a variable retarder like a Pockels cell is used in a normal manner, that is, changes between linear polarization and circular polarization. When the polarization modulator to be used is used as a polarization scrambler, if the principal axis of the change in polarization incident on the transmission line coincides with the principal polarization (PSP) axis of the transmission medium, that is, the PMD vector, the polarization Even if it changes, the splitting ratio remains unchanged at γ = 0.5, and deterioration due to PMD is not improved.

  Also, even if the polarization state is set such that the splitting ratio is first modulated, the main polarization direction of the transmission path changes with time due to environmental changes, and may become an invalid polarization direction. . Further, in this modulator, since the input polarization to the modulator must be a linear polarization inclined by 45 degrees with respect to the main axis of the modulator, it is necessary to fix or control the input polarization to the modulator. There is also a problem. Therefore, this type of polarization modulator has a practical problem.

  As a measuring apparatus for measuring polarization dependent loss, a polarization scrambler that generates all polarization states in a pseudo-random manner is commercially available. A polarization scrambler that generates a polarization state in a pseudo-random manner may be used to average the polarization-dependent loss of the optical transmission line and stabilize the transmission characteristics. These polarization scramblers are for generating all polarization states in a pseudo-random manner, and various devices have been made so that as many polarizations as possible can be generated uniformly.

  Moreover, the change rate of the polarization state is also pseudo-random and is not constant but distributed over a wide range. If such a polarization scrambler is used for the purpose of reducing degradation due to PMD, a large number of polarization states are required for the averaging to be effective, and the averaging is effective in the time of the FEC block period of about several microseconds. In some cases, the effect of FEC is reduced.

  That is, since the polarization state occurs pseudo-randomly, a bad polarization direction may continue. To avoid this, if the polarization change speed is increased so that many polarization states are generated so that averaging is sufficiently effective in the time of the FEC block period, the speed of the jitter caused by PMD is increased. Therefore, there is a problem that the clock extraction circuit of the receiver does not follow and causes signal degradation due to high-speed jitter.

  Therefore, in a transmission system using FEC, if a polarization scrambler having a structure that generates a polarization state in a pseudo-random manner is used, there is a problem that the effect of polarization scramble cannot be exhibited sufficiently.

  Heismann describes in Non-Patent Document 1 that the polarization of an optical signal is depolarized, that is, the degree of polarization (DOP) is zero, for the purpose of preventing polarization hole burning in an erbium-doped fiber amplifier. A summary of the scrambler (so-called Monochromatic Depolarizer) is reported. It is also described in Patent Document 2.

  However, in order to reduce the deterioration due to PMD, it is not necessary that the DOP becomes zero, and it is unclear whether the polarization scrambling method that makes the DOP zero is effective in reducing the deterioration due to PMD.

  In addition, Noe et al. In JP-A-2001-228707 output polarization as a polarization scrambler used in a PMD detection device, and as a polarization scrambler that draws a three-dimensional figure on a Poincare sphere used in a PMD detection device. A polarization scrambler is described in which the correlation matrix for the normalized Stokes vector of the wave is approximately 1/3 of the 3 × 3 unit matrix.

  However, in order to reduce degradation due to PMD, the correlation matrix does not need to be 1/3 of the 3 × 3 unit matrix, and the polarization is such that the correlation matrix is 1/3 of the 3 × 3 unit matrix. Whether the scrambling method is effective in reducing degradation due to PMD is unknown.

  As described above, although the patent document indicates that polarization scrambling is effective for reducing waveform deterioration due to PMD, there is a polarization scrambler that is practically effective for reducing deterioration due to PMD. There wasn't.

  An object of the present invention is to provide an optical transmission system and method capable of realizing polarization scrambling which is practically effective for reducing degradation due to PMD.

  In order to solve the above-described problems, the present invention provides an optical transmission system including a polarization scrambler that scrambles an input polarization to a transmission line. The feature of the present invention is the polarization scrambler. Includes a first polarization modulation means for modulating the polarization of the signal light so as to draw a circle on the Poincare sphere, and Kπ + 0... With respect to the circle drawn on the Poincare sphere by the first polarization modulation means. Second to modulate the polarization of the signal light on the Poincare sphere so as to draw a circle inclined by an angle of 20π to Kπ + 0.37π or Kπ−0.37π to Kπ−0.20π (where K is an integer). The first and second polarization modulation means have different modulation speeds (first invention).

  Alternatively, the present invention is an optical transmission system that includes a polarization scrambler that scrambles the input polarization to the transmission line, and averages the signal over the length of the FEC block. The polarization scrambler includes first polarization modulation means for modulating the polarization of signal light so as to draw a circle on the Poincare sphere, and a circle drawn on the Poincare sphere by the first polarization modulation means. The second polarization that modulates the polarization of the signal light on the Poincare sphere so as to draw a circle inclined by an angle of Kπ + 0.20π or more and Kπ + 0.37π or less or Kπ−0.37π or more and Kπ−0.20π or less. Wave modulation means, and the modulation speeds of the first and second polarization modulation means are different from each other. Alternatively, the first and second polarization modulation means have different modulation speeds but are synchronized with each other, and the change in polarization state is periodic (second invention).

  Further, in the first or second invention, the circle drawn on the Poincare sphere by the second polarization modulation means is different from the circle drawn on the Poincare sphere by the first polarization modulation means. There is a deviation between the first polarization modulation means and the second polarization modulation means so as to be inclined by an angle of Kπ + 0.20π or more and Kπ + 0.37π or less or Kπ−0.37π or more and Kπ−0.20π or less. A wave conversion means can also be provided (third invention).

  In the case of the third invention, a circle drawn on the Poincare sphere is observed only by the first polarization modulation means, and a normal vector of a plane including the circle is defined as a, and the second polarization When the circle drawn on the Poincare sphere is observed only by the wave modulation means and the normal vector of the plane including the circle is b, the angle formed by a and b is Kπ + 0.20π or more and Kπ + 0.37π or less or It is desirable to adjust the polarization conversion means so that Kπ−0.37π or more and Kπ−0.20π or less (fourth invention).

  Alternatively, in the case of the third invention, the first and second polarization modulation means are rotation half-wave plates, the polarization conversion means is a phase shifter, and the two rotation 1 / Two-wave plates have the same rotation axis direction, and frequencies N × f [Hz] and M × f [Hz] (N and M are natural numbers such that N ≠ M and N ≠ 2M and 2N ≠ M, and f is a frequency. ) And the phase of the phase shifter can be changed by Kπ + 0.20π or more and Kπ + 0.37π or less, or Kπ−0.37π or more and Kπ−0.20π or less (fifth invention).

  Further, in the case of the fifth invention, it is desirable that one of the two half-wave plates has a frequency f [Hz] and the other 3 f [Hz] (the sixth invention). ).

  Alternatively, the present invention provides a polarization scrambler that scrambles the polarization input to the transmission line and averages the polarization state of the signal, or averages the polarization state of the signal over the length of the FEC block. An optical transmission system provided with the present invention is characterized in that the polarization scrambler comprises a polarization controller comprising a rotation half-wave plate, a phase shifter, and a rotation quarter-wave plate. The rotation half-wave plate and the rotation quarter-wave plate have the same rotation axis direction, and the frequencies P × f [Hz] and Q × f [Hz] (P and Q are P ≠ Q and P ≠ 2Q is a natural number, f is a frequency) and the phase shifter changes the phase by Kπ + 0.20π to Kπ + 0.37π or Kπ−0.37π to Kπ−0.20π. (Seventh invention).

  Alternatively, the present invention provides a polarization scrambler that scrambles the polarization input to the transmission line and averages the polarization state of the signal, or averages the polarization state of the signal over the length of the FEC block. An optical transmission system provided with the present invention is characterized in that the polarization scrambler includes first and second rotated quarter-wave plates and the first and second rotated quarter wavelengths. The rotation half-wave plate comprises a polarization controller comprising a rotation half-wave plate provided between the plates, and the initial rotation angle of the first rotation quarter-wave plate is k × π / 3. The initial rotation angle of the second rotation quarter wave plate is 0, and the initial rotation angle of the second rotation quarter wave plate is 0 (where k is an integer other than a multiple of 3), or the first rotation quarter wave plate The initial rotation angle of the rotating half-wave plate is set to 0. k × π / 6, the initial rotation angle of the second rotation quarter-wave plate is 0, or the initial rotation angle of the first rotation quarter-wave plate is 0, and the rotation 1 / The initial rotation angle of the two-wave plate is 0, the initial rotation angle of the second rotation quarter-wave plate is k × π / 3, and the first rotation quarter-wave plate is frequency n × f [Hz]. ], The rotated half-wave plate is rotated at a frequency (n + m) / 2 × f [Hz], and the second rotated quarter-wave plate is rotated at a frequency m × f [Hz] ( Where n and m are non-zero integers such that | n | ≠ | m | and | n | ≠ | 2m | and | 2n | ≠ | m |, and the signs of n and m represent the direction of rotation, for example, positive is right If it is determined to be round, negative is counterclockwise, f is frequency) (eighth invention).

  In the case of the third invention, the first and second polarization modulation means are variable phase shifters, and the polarization conversion means is a fixed half-wave plate, and the directions of the main axes are equal. The maximum polarization phase displacement of each of the two variable phase shifters is set to N × 2π and M × 2π, and is changed synchronously with a period of frequency f [Hz], and the fixed half-wave plate is the variable It can also be installed by being rotated by an angle of Kπ / 4 + 0.05π or more and Kπ / 4 + 0.093π or Kπ / 4-0.093π or more and Kπ / 4-0.05π or less with respect to the principal axis direction of the phase shifter ( Ninth invention).

  In the case of the first or second invention, the first polarization modulation means is the first variable phase shifter, and the second polarization modulation means is the first variable phase shifter. A second variable phase shifter tilted by a principal axis direction Kπ / 2 + 0.10 π or more and Kπ / 2 + 0.185π or Kπ−0.185π or more and Kπ−0.10π or less. The maximum polarization phase displacement of each of the variable phase shifters can be set to N × 2π and M × 2π, and can be changed synchronously with a period of frequency f [Hz] (tenth invention).

  Furthermore, in the ninth or tenth invention, it is desirable that the time waveform of the phase change of the variable phase shifter is a triangular wave having a frequency f [Hz] (eleventh invention).

  In the case of the second invention, it is desirable that the repetition frequency of the polarization state coincides with a frequency corresponding to the length of the FEC block or is a natural number multiple (twelfth invention). ).

  In the case of the second aspect of the invention, the repetition frequency of the polarization state is higher than about ½ times the frequency corresponding to the length of the FEC block, and the maximum frequency of polarization fluctuation is the transmission line. It can also be a frequency lower than the allowable jitter frequency of the receiver of the signal output from (13th invention).

  Alternatively, in the case of the second invention, the repetition frequency of the polarization state is higher than about 1/2 times the frequency corresponding to the length of the FEC block, and the maximum frequency of the polarization fluctuation is the transmission. It is a frequency that is lower than the allowable jitter frequency of the receiver of the signal output from the path, and this frequency can be changed with time (fourteenth invention).

  In the case of any one of the first to seventh or ninth to fourteenth inventions, the modulation speeds of the two polarization modulation means are substantially synchronized, but from the frequency to be synchronized. Deviation within a range of approximately 10% can be allowed (fifteenth invention).

  In the case of any one of the first to fifteenth inventions, the polarization scrambler is further provided with a polarization modulator on the input side, the transmission line side, or both sides thereof, and is synchronized with the modulation frequency. It is possible to perform polarization modulation at a frequency lower than the modulation frequency (the sixteenth invention).

  When the present invention is viewed from the viewpoint of a polarization scrambler, the present invention includes first polarization modulation means for modulating the polarization of signal light so as to draw a circle on the Poincare sphere, and the first polarization. On the Poincare sphere so as to draw a circle inclined by an angle of Kπ + 0.20π to Kπ + 0.37π or Kπ−0.37π to Kπ−0.20π with respect to the circle drawn on the Poincare sphere by the wave modulation means A second polarization modulation means for modulating the polarization of the signal light, and the modulation speeds of the first and second polarization modulation means are different from each other but synchronized so that the change of the polarization state is periodic. A polarization scrambler (seventeenth invention).

  Further, when the present invention is viewed from the viewpoint of an optical transmission method, the present invention is an optical transmission method applied to an optical transmission system including a polarization scrambler that scrambles an input polarization to a transmission path, The polarization scrambler has a first polarization modulation step for modulating the polarization of the signal light so as to draw a circle on the Poincare sphere, and a circle drawn on the Poincare sphere by the first polarization modulation step. A second polarization that modulates the polarization of the signal light on the Poincare sphere so as to draw a circle inclined by an angle of Kπ + 0.20π or more and Kπ + 0.37π or less or Kπ−0.37π or more and Kπ−0.20π or less. The modulation step is performed, and the modulation speeds of the first and second polarization modulation steps are different from each other (eighteenth invention).

  Alternatively, the present invention is an optical transmission method that is applied to an optical transmission system that includes a polarization scrambler that scrambles an input polarization to a transmission path and averages a signal over the length of an FEC block. The wave scrambler includes a first polarization modulation step for modulating the polarization of the signal light so as to draw a circle on the Poincare sphere, and a circle drawn on the Poincare sphere by the first polarization modulation step. Second polarization modulation that modulates the polarization of the signal light on the Poincare sphere so as to draw a circle inclined by an angle between Kπ + 0.20π and Kπ + 0.37π or Kπ−0.37π and Kπ−0.20π And the modulation speeds of the first and second polarization modulation steps are different from each other. Alternatively, the first and second polarization modulation steps have different modulation speeds but are synchronized with each other, and the polarization state changes periodically (19th invention).

  Alternatively, the present invention provides a polarization scrambler that scrambles the polarization input to the transmission line and averages the polarization state of the signal, or averages the polarization state of the signal over the length of the FEC block. An optical transmission method applied to an optical transmission system provided with a polarization controller comprising a rotation half-wave plate, a phase shifter, and a rotation quarter-wave plate, and the rotation half-wave plate And the rotation quarter-wave plate, the rotation half-wave plate and the rotation quarter-wave plate in a polarization scrambler having the same direction of the rotation axis are frequency P × f [Hz], Q × f [ Hz], and the phase change in the phase shifter is Kπ + 0.20π or more and Kπ + 0.37π or less, or Kπ−0.37π or more and Kπ−0.20π or less. invention).

  Alternatively, the present invention provides a polarization scrambler that scrambles the polarization input to the transmission line and averages the polarization state of the signal, or averages the polarization state of the signal over the length of the FEC block. An optical transmission method applied to an optical transmission system provided with the first and second rotated quarter-wave plates and a rotation 1/2 provided between the first and second rotated quarter-wave plates. In the polarization scrambler configured by a polarization controller including a wave plate, the initial rotation angle of the first rotation quarter wave plate is k × π / 3, and the initial rotation angle of the rotation half wave plate Is set to 0 and the initial rotation angle of the second rotating quarter-wave plate is set to 0, or the initial rotation angle of the first rotating quarter-wave plate is set to 0 and the rotated half-wave plate The initial rotation angle is k × π / 6, and the second rotation 1/4 wave The initial rotation angle of the plate is 0, or the initial rotation angle of the first rotation quarter wave plate is 0, the initial rotation angle of the rotation 1/2 wave plate is 0, and the second rotation The initial rotation angle of the quarter-wave plate is k × π / 3, the first rotated quarter-wave plate is rotated at a frequency n × f [Hz], and the rotated half-wave plate is frequency (n + m). ) / 2 × f [Hz], and the second rotated quarter-wave plate is rotated at a frequency m × f [Hz] (21st invention).

  According to the present invention, the optical transmission signal is polarization-scrambled regardless of the conditions of the optical fiber, and the polarization is scrambled at a speed suitable for optical communication. A simple polarization scrambler can be realized.

  An optical transmission system using a polarization scrambler according to an embodiment of the present invention is shown in FIG. On the transmission side, the modulator 2 modulates the optical signal emitted from the light source 3 by the data encoded with FEC by the FEC encoder 1, and this optical signal is wavelength-multiplexed by the wavelength multiplexing device 5 and is polarized scrambler. 4, the polarization branching ratio is modulated in a lump and then sent to an optical transmission line 8 including an optical fiber 6 and an optical amplifier 7.

  The transmitted signal is wavelength-separated by the wavelength demultiplexer 9 on the receiving side, then received by the receiver 10, error-corrected by FEC by the FEC decoder 11, and data is obtained. Here, there is an effect of reducing the PMD penalty regardless of the change period of the polarization branching ratio modulated by the polarization scrambler 4, but the PMD penalty is particularly reduced when it is less than or equal to the period of the FEC block. Has the effect of reducing.

  FIG. 2 shows a first example of a polarization scrambler used in the optical transmission system of FIG. 1 and used in this embodiment. The polarization scrambler in FIG. 2 includes a polarization controller including a rotation half-wave plate 20-1, a phase shifter 21, and a rotation half-wave plate 20-2. -1 and 20-2 have the same rotation axis direction, and are rotated at frequencies of N × f [Hz] and M × f [Hz]. The phase shifter 21 gives a phase difference between the main polarizations by k × π / 3 (where k is an integer excluding multiples of 3), and changes the polarization state by k × π / 3 on the Poincare sphere. .

  Here, the phase shifter gives a delay difference (= phase difference) to two orthogonal polarizations. In this specification, the term phase difference is used to mean a phase difference between two orthogonally polarized waves.

  The rotation direction of the rotation half-wave plates 20-1 and 20-2 may be either clockwise or counterclockwise with respect to the light traveling direction.

The rotating half-wave plates 20-1 and 20-2 may be actual wave plates, but by using a LiNbO ₃ polarization controller that operates equivalent to the wave plate by utilizing the electro-optic effect. It can be electrically controlled and enables high-speed polarization rotation.

  Further, a polarization controller of PLZT or other material using the electro-optic effect may be used, or a magneto-optic effect or other optical effect may be used.

  As the phase shifter 21, a crystal material or device having birefringence, for example, a wave plate or a polarization controller combining the same is used.

Since the LiNbO ₃ polarization controller can perform two types of operations, that is, the operation of the variable phase shifter and the rotation wavelength plate, depending on the voltage application method, the LiNbO ₃ polarization controller is a LiNbO ₃ polarization controller having _three modulation units. The rotation half-wave plate 20-1, the phase shifter 21 (π / 3 phase shifter), and the rotation half-wave plate 20-2 are equivalently configured, and one rotation half-wave plate 20-1 is formed. A periodic electrical signal having a frequency of N × f [Hz] in a modulation unit equivalent to the above and a periodic electrical signal having a frequency of M × f [Hz] in a modulation unit equivalent to the other rotating half-wave plate 20-2. The first example can be realized by adding The wave plate rotates at a rotational speed equal to the frequency of the electrical signal.

  The operation of the polarization scrambler in FIG. 2 will be described with reference to FIG. FIG. 3A shows the operation of the rotating half-wave plate 20-1. FIG. 3B shows the operation of the phase shifter 21. FIG. 3C shows the operation of the rotating half-wave plate 20-2. It is the figure which looked at the Poincare sphere from the direction perpendicular to the S2 and S3 planes, and the thick solid line shows the movement of the polarization. First, the polarization of the signal light is modulated so as to draw a circle parallel to the S1 and S2 planes on the Poincare sphere by rotating the rotating half-wave plate 20-1.

  The rotational angular velocity of the circle on the Poincare sphere is twice the rotational angular velocity of the rotating half-wave plate 20-1, that is, when the rotating half-wave plate 20-1 is rotated once, the circle on the Poincare sphere is 2 Note that it rotates. In FIG. 3A, the movement of the polarization is indicated by a straight line parallel to the S2 axis.

  Next, using the phase shifter 21, the origin on the S1 and S2 planes is set so that the center axis of the circle on the Poincare sphere before and after the movement forms an angle of k × π / 3. Rotate and move on the Poincare sphere with the passing straight line as the axis. FIG. 3B shows a case where the S1 axis is rotated by π / 3 with the rotation axis. Furthermore, by rotating the first half-wave plate 20-1 and the half-wave plate 20-2 whose rotation axis is in the same direction, the first half-wave plate 20-1 is deflected to draw a circle on the Poincare sphere with the same axis as the first. Wave modulation.

  In this operation, first, the polarization of the signal light is modulated so as to draw a circle on the Poincare sphere, and then an axis that forms an angle of k × π / 3 with respect to the rotation axis of the circle is used as the rotation axis. Furthermore, it is equivalent to modulating in a circle.

  Actually, the output polarization of the polarization scrambler is measured by a polarization analyzer, and the circle drawn only by the first polarization modulation means (rotated half-wave plate 20-1 in the example of FIG. 2). And the normal vector of the plane including the circle is a, and the circle drawn only by the second polarization modulation means (rotated half-wave plate 20-2 in the example of FIG. 2) is observed. When the normal vector of the plane including the circle is b, the polarization conversion means (the phase shifter 21 in the example of FIG. 2) is adjusted so that the angle formed by a and b is k × π / 3. Thus, the angle of the central axis of the circle can be set to k × π / 3.

  By polarization scrambling as described above, the following excellent effects can be obtained as a polarization scrambling method for reducing the PMD penalty.

  The effect of the transmission scheme that reduces the PMD penalty (degradation) using polarization scrambling is required not to depend on the direction of polarization incident on the polarization scrambler and the direction of coupling of polarization on the PMD medium.

  As a result showing the effect of this scrambler, FIG. 4 shows the maximum value of PMD penalty in any polarization coupling direction when the incident polarization direction is (θ, φ), that is, the incidence to the scrambler of the maximum penalty. Polarization dependence (FIG. 4 (a)), and the maximum value of PMD penalty in any polarization incident direction when the coupling polarization direction to the PMD medium is (θ, φ), that is, transmission of the maximum penalty The calculation result of the coupling polarization dependence (FIG. 4 (b)) of a road is shown. The calculation was performed by setting the inclination of two circles to π / 3.

  It can be seen that any polarization is improved to 2.0 dB or less under the condition that degradation of 2.5 dB occurs at γ = 0.5. It can also be seen that the dependence on the incident polarization and the dependence on the coupling polarization direction to the PMD medium are small.

  In a polarization modulator that changes between linearly polarized waves and circularly polarized waves, such as a Pockels cell, there is a coupling polarization direction (a direction in which the polarization modulation direction and the coupling direction match) with no improvement effect. This is a clear advantage. In particular, in the WDM transmission system, since it is effective for polarization in any incident direction, there is an effect that collective scrambling is possible.

  The effect of the present invention is shown in FIG. When the angle formed by the circle drawn by the first polarization modulation means and the circle drawn by the second polarization modulation means is ψ, ψ (horizontal axis) and the maximum penalty (maximum penalty for all input / output polarizations) ) (Vertical axis) shows the numerical calculation results. According to this calculation result, it is understood that the maximum penalty is minimum when the angle is 0.30π or 0.70π, and the highest effect is obtained when the inclination of the circle is 0.30π or 0.70π. Considering the periodicity, Kπ ± 0.30π.

  Therefore, the highest effect can be obtained when the angle of inclination of the circle described in the specifications so far is replaced by k × π / 3 and Kπ ± 0.30π.

  From FIG. 5, if the deterioration of 0.1 dB is allowed from the state where the maximum penalty is minimum, there is a sufficient effect when the angle is 0.20π to 0.37π. In consideration of the periodicity, the angle of the inclination of the circle described in the specification so far is not k × π / 3, but Kπ + 0.20π or more and Kπ + 0.37π or less, or Kπ−0.37π or more and Kπ−0.20π or less. When you do, there is a sufficient effect. Further, from this figure, if the degradation of 0.05 dB is allowed from the state where the maximum penalty is minimized, the angle is sufficiently effective when the angle is 0.23π to 0.35π (0.29π ± 0.06π).

  Considering the periodicity, the angle of the inclination of the circle described in the specification so far is not k × π / 3, but Kπ + 0.23π or more and Kπ + 0.35π or less, or Kπ−0.35π or more and Kπ−0.23π or less. When you do, there is a sufficient effect.

  As described in the conventional example, if the polarization change speed is increased, the speed of jitter caused by PMD increases, the clock extraction circuit of the receiver does not follow, and high-speed jitter causes signal degradation. Since there are problems, it is desirable that the polarization change rate is low.

  From this point, it is particularly effective when the rotated half-wave plates 20-1 and 20-2 having the lowest order are rotated at the frequencies f and 3f. First, the rotating half-wave plate 20-1 is rotated at a frequency f, then a phase difference of π / 3 is given by the phase shifter 21, and the rotating half-wave plate 20-2 is rotated at a period of 3f. It can be realized by rotating with. The frequency that is the greatest common divisor of the rotation frequency is f, and the wave plate rotates repeatedly with a period of 1 / f. However, since polarization modulation repeats the same change twice during 1 / f, The wave repeats the same change with a period of 0.5 / f. Here, in this specification, the reciprocal of the repetition cycle of the polarization state is referred to as the polarization state repetition frequency.

  The time-varying characteristics of the polarization split ratio vary depending on the incident polarization direction to the scrambler, the coupling polarization direction to the PMD medium, and the polarization scrambler control method. FIG. 6 shows the operation of this polarization scrambler. To illustrate, the rotating half-wave plate 20-1 is rotated at the frequency f, then a phase difference of π / 3 is given by the phase shifter 21, and the rotating half-wave plate 20-2 is An example of the time change characteristic of the branching ratio when rotated at a period of 3f is shown. As shown in FIG. 6, the polarization split ratio repeatedly fluctuates at a repetition frequency of 0.5 f in the polarization state due to polarization scrambling.

  In this example, the branching ratio greatly fluctuates four times during one period. However, when a detailed examination is conducted, when the rotating half-wave plate is rotated at the lowest order of the frequencies f and 3f, 1 is obtained. It was found that the fluctuation of the branching ratio in the cycle was 4 times at the maximum. That is, the maximum frequency of polarization fluctuation is 4 times or less the repetition frequency of the polarization state. Therefore, by using a receiver having a jitter tolerance of at least four times the repetition frequency of the polarization state, it is possible to receive without signal deterioration due to high-speed jitter.

  Further, in this scrambler, the fluctuation of the polarization branching ratio is not pseudo-random, and is repeated with a period of 0.5 / f. Therefore, the repetition period of the polarization state of the polarization scramble is made to coincide with the period of the FEC block. Thus, in the FEC block, the degradation within the polarization scramble period is completely averaged, and unlike the case of pseudo-random, the penalty is the same for any FEC block.

  Further, when the polarization scrambling period is made coincident with the FEC block period, as shown in the FEC block period example 1 or the FEC block period example 2 in FIG. Even if starts, there is an effect that the effect is equal. Therefore, there is no need to synchronize the phase of the polarization scrambling and the cycle of the FEC block by outputting a synchronization signal. It is also clear that the polarization scrambler period may be set so that the FEC block period is a natural number multiple of the polarization scramble period.

  In addition, when the polarization fluctuation period and the FEC block period do not match, the error rate slightly varies for each FEC block, and the effect of FEC is reduced compared to the case where complete averaging is performed. However, since it is excluded that the state of γ = 0.5 continues unlike the case of pseudo random, it is clear that it is more effective in reducing the PMD penalty than the pseudo random scrambler.

  Since the polarization fluctuation period and the FEC block period are not synchronized, the penalty varies with the beat frequency. The repetition of the polarization state in which the repetition frequency of the polarization state is higher than about 1/2 times the frequency corresponding to the length of the FEC block and the maximum frequency of the polarization fluctuation is lower than the allowable jitter frequency of the receiver. In terms of frequency, it is more effective in reducing the PMD penalty than a pseudo-random scrambler.

  Also, a polarization whose repetition frequency in the polarization state is higher than about 1/2 times the frequency corresponding to the length of the FEC block and whose maximum frequency of polarization fluctuation is lower than the allowable jitter frequency of the receiver. The repetition frequency of the state may be changed with time. This can be realized by FM-modulating the repetition frequency of the polarization state.

  In the above description, the modulation speeds of the two modulation means are synchronized. That is, it has been assumed that there is a relationship of frequencies N × f [Hz] and M × f [Hz]. By the way, the present invention does not require the initial phases of the two modulations to be 0, and there may be an arbitrary phase difference between the initial phases. Therefore, it is not always necessary that the phase difference between the two modulation means is equal in perfect synchronization. The phase difference of the modulation may change little by little.

  That is, the modulation speed may be slightly shifted. Even if the modulation periods of the two modulation means are deviated by about 10%, the effect of reducing PMD degradation does not change so much. Therefore, the modulation speeds of the two modulation means of the present invention are almost synchronized, but the frequency to be synchronized is the same. May deviate by approximately 10%. In this case, the deviation from the synchronization frequency becomes the beat frequency, and the phase difference changes periodically. Since the effect of reducing PMD degradation also depends on the phase difference of the modulation, the phase difference dependency can be eliminated by shifting the modulation frequency slightly and performing time averaging.

  A second example of the polarization scrambler used in FIG. 1 and used in the present invention is shown in FIG. The polarization scrambler shown in FIG. 7 includes a polarization controller including a rotation half-wave plate 30, a phase shifter 31, and a rotation quarter-wave plate 32. The direction of the rotation axis of the four-wave plate 32 is the same, and the rotation half-wave plate 30 and the rotation quarter-wave plate 32 are rotated at a frequency of P × f [Hz] and Q × f [Hz].

The phase shifter 31 gives a phase difference between the main polarizations, and changes the polarization by Kπ + 0.20π or more and Kπ + 0 · 37π or less, or Kπ−0.37π or more and Kπ−0.20π or less on the Poincare sphere. This is realized by, for example, a LiNbO ₃ polarization controller having _three modulation units. The repetition period of the polarization branching ratio is 1 / f. In the case of this configuration, according to the result of the same calculation as in FIG. 4, the penalty after the polarization scramble is slightly larger than the first example of the polarization scrambler, but it is sufficient. effective.

  FIG. 8 shows a third example of the polarization scrambler used in FIG. 1 and used in the present invention. The polarization scrambler shown in FIG. 8 is composed of a polarization controller including a rotation quarter-wave plate 40-1, a rotation half-wave plate 41, and a rotation quarter-wave plate 40-2. The initial phase, that is, the inclination (rotation angle) of the main axis is set to k × π / 3, 0, 0 or 0, k × π / 6, 0 or 0, 0, k × π / 3, and each wave plate is set to frequency n ×. Rotate at f [Hz], (n + m) / 2 × f [Hz], m × f [Hz].

  The wave plate may be rotated in any direction as long as the relationship between the rotation frequencies of the wave plates is satisfied. The polarization scrambler in this example is physically equivalent to the first example as the polarization scramble operation, and the effect of reducing the PMD penalty is exactly the same as that in FIG. The change in the branching ratio is repeated at a period of 1 / f.

  FIG. 9 shows a fourth example of the polarization scrambler used in FIG. 1 and used in the present invention. The polarization scrambler includes a polarization controller including a first variable phase shifter 50-1, a fixed half-wave plate 51, and a second variable phase shifter 50-2. The phases of the variable phase shifters 50-1 and 50-2 having the same principal axis direction are changed in synchronization by N × 2π and M × 2π, respectively, during time 1 / f.

  The direction of the main axis is Kπ / 4 + 0.05π or more and Kπ / 4 + 0.093π or less, or Kπ / 4-0.093π or more and Kπ / 4-0.05π or less with respect to the main axis direction of the variable phase shifters 50-1 and 50-2. The fixed half-wave plate 51 rotated by this angle converts the polarization state on the Poincare sphere by Kπ + 0.20π or more and Kπ + 0.37π or less, or Kπ−0.37π or more and Kπ−0.20π or less.

As the variable phase shifter 51, a dielectric crystal such as LiNbO ₃ or PLZT, a semiconductor crystal such as GaAs or InP, or other material using an electro-optic effect or an optical waveguide is used. A magneto-optical effect or other optical effects may be used.

  In the case of electrical control, the voltage of the control electrical waveform is set so that the maximum phase change is N × 2π or M × 2π, and the phase is changed in synchronization. If the phase is changed by changing the voltage, the state returns to the polarization state of phase 0 in multiples of phase 2π, but the voltage is not equal to the state of phase 0, and cannot be repeated as it is. So, for example, if the control electric waveform is a repetitive waveform of frequency f and reaches the maximum phase at time 1 / (2f), then the voltage is turned back, the change reverse to the first half is made and the first voltage is changed. Go back and repeat.

  Therefore, the repetition cycle is 1 / f, and the change in the branching ratio is repeated at the cycle 1 / f. Further, the polarization scrambler of this example is physically equivalent to the first example as the polarization scramble operation, and the effect of reducing the PMD penalty is exactly the same.

  In a variable phase shifter in which the phase change is proportional to the applied voltage, when the control electrical waveform is a triangular wave, the time waveform of the phase change is also a triangular wave, and the phase change is linear with respect to time. Since the control electrical waveform is easier to generate when it is a sine wave, it may be a sine wave. Since the polarization direction dependency is smaller when modulated with a triangular wave than with a sine wave, the time waveform of the phase change is more preferably a triangular wave.

  In addition, the direction of the principal axis of the second variable phase shifter 50-2 is changed from the first variable phase shifter 50-1 to Kπ / 2 + 0.10π or more and Kπ / 2 + 0.185π or less, or Kπ−0.185π or more and Kπ-0. When tilted by 10π or less, the rotation axis of the second variable phase shifter 50-2 on the Poincare sphere is Kπ + 0.20π or more and Kπ + 0.37π or less with respect to the rotation axis of the first variable phase shifter 50-1. Alternatively, a fifth example in which the fixed half-wave plate 51 is not necessary because the angle is Kπ−0.37π or more and Kπ−0.20π or less is also conceivable.

  Further, even if the orientation of the main axis of the second variable phase shifter 50-2 is arbitrarily tilted from the first variable phase shifter 50-1, the Poincare sphere by the two variable phase shifters 50-1 and 50-2 A fourth aspect is that the tilt of the fixed half-wave plate 51 is adjusted so that the axis of polarization rotation is an angle between Kπ + 0.20π and Kπ + 0.37π or Kπ−0.37π and Kπ−0.20π. Variations of the example are also conceivable.

  Further, the polarization rotation by the fixed half-wave plate 51 corresponds to the rotation about the S3 axis on the Poincare sphere, and the variable phase shifters 50-1 and 50-2 are S1- on the Poincare sphere. This corresponds to rotation about a straight line passing through an arbitrary origin included in the S2 plane.

  Therefore, a sixth example of the polarization scrambler used in FIG. 1 and used in the present invention is shown in FIG. A phase shifter having an appropriate phase amount and inclination is sandwiched between two polarization rotations, and the two polarization rotation axes are Kπ + 0.20π or more and Kπ + 0.37π or less, or Kπ−0.37π or more and Kπ−0.20π. As shown in FIG. 10, the rotation angle ½ wavelength plate 60, the phase shifter 61, and the variable phase shifter 62 (the rotation ½ wavelength plate 60 and the variable phase shifter are formed by forming the following angles. The polarization scrambler of the present invention can also be realized by a combination of the order of 62).

  Further, in any of the above polarization scramblers, a polarization modulator composed of a rotating wavelength plate or a variable phase shifter is provided on the input side, transmission line side, or both sides, and the modulation frequency is not synchronized with the modulation frequency. When the polarization modulation is performed at a lower frequency, the incident polarization dependency of FIG. 4 is averaged when provided on the input side, and the coupling polarization dependency of FIG. 4 is averaged when provided on the transmission line side. As a result, the polarization direction dependency is further improved.

  According to the present invention, since a polarization scrambler that is practically effective for reducing degradation due to PMD can be realized, high-quality optical communication can be realized.

The figure which shows the structural example of the optical transmission system using the polarization scrambler which is an Example of this invention. The figure which shows the 1st example of the polarization scrambler of this invention. The figure explaining operation | movement of the polarization scrambler of this invention. The figure explaining the effect of this invention. The figure explaining the effect of this invention. The figure explaining operation | movement of this invention. The figure which shows the 2nd example of the polarization scrambler of this invention. The figure which shows the 3rd example of the polarization scrambler of this invention. The figure which shows the 4th example of the polarization scrambler of this invention. The figure which shows the 6th example of the polarization scrambler of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 FEC encoder 2 Modulator 3 Light source 4 Polarization scrambler 5 Wavelength multiplexing apparatus 6 Optical fiber 7 Optical amplifier 8 Optical transmission line 9 Wavelength separation apparatus 10 Receiver 11 FEC decoders 20-1, 20-2, 30, 41, 60 Rotating half-wave plates 21, 31, 61 Phase shifters 32, 40-1, 40-2 Rotating quarter-wave plates 50-1, 50-2, 62 Variable phase shifters 51 Fixed half-wave plates

Claims (23)

In an optical transmission system equipped with a polarization scrambler that scrambles the input polarization to the transmission line,
The polarization scrambler is
First polarization modulation means for modulating the polarization of the signal light so as to draw a circle on the Poincare sphere;
An angle between Kπ + 0.20π and Kπ + 0.37π or Kπ−0.37π and Kπ−0.20π (where K is an integer) with respect to the circle drawn on the Poincare sphere by the first polarization modulation means And a second polarization modulation means for modulating the polarization of the signal light on the Poincare sphere so as to draw a tilted circle,
The optical transmission system, wherein the first and second polarization modulation means have different modulation rates. It has a polarization scrambler that scrambles the input polarization to the transmission line.
Error Correction) In an optical transmission system that averages the signal over the length of the block,
The polarization scrambler is
First polarization modulation means for modulating the polarization of the signal light so as to draw a circle on the Poincare sphere;
A circle tilted by an angle of Kπ + 0.20π or more and Kπ + 0.37π or Kπ−0.37π or more and Kπ−0.20π or less with respect to the circle drawn on the Poincare sphere by the first polarization modulation means. And a second polarization modulation means for modulating the polarization of the signal light on the Poincare sphere,
The optical transmission system, wherein the first and second polarization modulation means have different modulation rates. In an optical transmission system comprising a polarization scrambler that scrambles the input polarization to the transmission line, and averaging the signal over the length of the FEC block,
The polarization scrambler is
First polarization modulation means for modulating the polarization of the signal light so as to draw a circle on the Poincare sphere;
A circle tilted by an angle of Kπ + 0.20π or more and Kπ + 0.37π or Kπ−0.37π or more and Kπ−0.20π or less with respect to the circle drawn on the Poincare sphere by the first polarization modulation means. And a second polarization modulation means for modulating the polarization of the signal light on the Poincare sphere,
The optical transmission system, wherein the first and second polarization modulation means have different modulation speeds but are synchronized with each other, and the polarization state changes periodically.   The circle drawn on the Poincare sphere by the second polarization modulation means is Kπ + 0.20π or more and Kπ + 0.37π or less or Kπ-0 with respect to the circle drawn on the Poincare sphere by the first polarization modulation means. 4. Polarization conversion means is provided between the first polarization modulation means and the second polarization modulation means so as to be inclined by an angle of 37π or more and Kπ−0.20π or less. The optical transmission system according to any one of the above.   A circle drawn on the Poincare sphere only by the first polarization modulation means is observed, and a normal vector of a plane including the circle is defined as a, and the circle drawn on the Poincare sphere only by the second polarization modulation means. The angle formed by a and b is Kπ + 0.20π or more and Kπ + 0.37π or less, or Kπ−0.37π or more and Kπ−0. The optical transmission system according to claim 4, wherein the polarization conversion unit is adjusted so as to be 20π or less.   The first and second polarization modulation means are rotation half-wave plates, the polarization conversion means is a phase shifter, and the two rotation half-wave plates have the same rotation axis direction. , Frequency N × f [Hz], M × f [Hz] (N, M are natural numbers where N ≠ M and N ≠ 2M and 2N ≠ M, and f is a frequency), and the phase shifter is The optical transmission system according to claim 4, wherein the phase is changed by Kπ + 0.20π or more and Kπ + 0.37π or less, or Kπ−0.37π or more and Kπ−0.20π or less.   The optical transmission system according to claim 6, wherein one of the rotation frequencies of the two rotation half-wave plates is a frequency f [Hz] and the other is 3 f [Hz]. In an optical transmission system having a polarization scrambler that scrambles the polarization input to the transmission line and averages the polarization state of the signal or averages the polarization state of the signal over the length of the FEC block ,
The polarization scrambler is
It consists of a polarization controller consisting of a rotating half-wave plate, a phase shifter, and a rotating quarter-wave plate,
The rotation half-wave plate and the rotation quarter-wave plate have the same rotation axis, and the frequencies P × f [Hz] and Q × f [Hz] (P and Q are P ≠ Q and P ≠ 2Q, respectively). And the phase shifter changes the phase by Kπ + 0.20π or more and Kπ + 0.37π or less, or Kπ−0.37π or more and Kπ−0.20π or less. Optical transmission system. In an optical transmission system having a polarization scrambler that scrambles the polarization input to the transmission line and averages the polarization state of the signal, or averages the polarization state of the signal over the length of the FEC block. ,
The polarization scrambler is
A polarization controller comprising a first and second rotating quarter-wave plate and a rotating half-wave plate provided between the first and second rotating quarter-wave plates;
The initial rotation angle of the first rotation quarter-wave plate is k × π / 3, the initial rotation angle of the rotation half-wave plate is 0, and the initial rotation of the second rotation quarter-wave plate is 0 The angle is 0 (where k is an integer excluding multiples of 3), or
The initial rotation angle of the first rotation quarter-wave plate is 0, the initial rotation angle of the rotation half-wave plate is k × π / 6, and the initial rotation of the second rotation quarter-wave plate is Set the angle to 0, or
The initial rotation angle of the first rotation quarter-wave plate is 0, the initial rotation angle of the rotation half-wave plate is 0, and the initial rotation angle of the second rotation quarter-wave plate is k ×. π / 3,
The first rotation quarter-wave plate is rotated at a frequency n × f [Hz], the rotation half-wave plate is rotated at a frequency (n + m) / 2 × f [Hz], and the second rotation is performed. Rotate quarter wave plate at frequency m × f [Hz] (where n and m are non-zero integers such that | n | ≠ | m | and | n | ≠ | 2m | and | 2n | ≠ | m |) , N, and m represent rotation directions, for example, if positive is determined to be clockwise, negative is counterclockwise (f is frequency)
An optical transmission system characterized by that.   The first and second polarization modulation means are variable phase shifters, the polarization conversion means is a fixed half-wave plate, and the maximum polarization of each of the two variable phase shifters having the same principal axis direction. The phase displacement is set to N × 2π and M × 2π and is changed synchronously with a period of frequency f [Hz], and the fixed half-wave plate is Kπ / 4 + 0 with respect to the main axis direction of the variable phase shifter. 5. The optical transmission system according to claim 4, wherein the optical transmission system is installed by being rotated by an angle of 0.05π or more and Kπ / 4 + 0.093π or less, or Kπ / 4−0.093π or more and Kπ / 4−0.05π or less.   The first polarization modulation means is a first variable phase shifter, and the second polarization modulation means is a principal axis direction Kπ / 2 + 0.10π or more with respect to the first variable phase shifter Kπ / 2 + 0. A second variable phase shifter tilted by 185π or less or Kπ−0.185π or more and Kπ−0.10π or less, wherein the maximum polarization phase displacement of each of the first and second variable phase shifters is expressed as N × 4. The optical transmission system according to claim 1, wherein the optical transmission system is set to 2 [pi] and M * 2 [pi] and is changed synchronously with a period of a frequency f [Hz].   The optical transmission system according to claim 10 or 11, wherein a time waveform of a phase change of the variable phase shifter is a triangular wave having a frequency f [Hz].   The optical transmission system according to claim 3, wherein the repetition frequency of the polarization state coincides with a frequency corresponding to the length of the FEC block or is a natural number multiple.   The repetition frequency of the polarization state is higher than about 1/2 times the frequency corresponding to the length of the FEC block, and the maximum frequency of polarization fluctuation is the allowable jitter frequency of the receiver of the signal output from the transmission path. The optical transmission system according to claim 3, wherein the frequency is lower.   The repetition frequency of the polarization state is higher than approximately ½ times the frequency corresponding to the length of the FEC block, and the maximum frequency of the polarization fluctuation is a jitter tolerance of the receiver of the signal output from the transmission line. The optical transmission system according to claim 3, wherein the frequency is lower than the frequency, and the frequency is changed with time.   16. The modulation speeds of the two polarization modulation means are substantially synchronized, but allow a deviation within a range of approximately 10% from the frequency to be synchronized. Optical transmission system.   17. The polarization scrambler is further provided with a polarization modulator on an input side, a transmission line side, or both sides thereof, and performs polarization modulation at a frequency lower than the modulation frequency not synchronized with the modulation frequency. An optical transmission system according to any one of the above. First polarization modulation means for modulating the polarization of the signal light so as to draw a circle on the Poincare sphere;
A circle tilted by an angle of Kπ + 0.20π or more and Kπ + 0.37π or Kπ−0.37π or more and Kπ−0.20π or less with respect to the circle drawn on the Poincare sphere by the first polarization modulation means. And a second polarization modulation means for modulating the polarization of the signal light on the Poincare sphere,
A polarization scrambler in which the modulation speeds of the first and second polarization modulation means are different from each other but synchronized and the change in polarization state is periodic. In an optical transmission method applied to an optical transmission system including a polarization scrambler that scrambles an input polarization to a transmission line,
The polarization scrambler is
A first polarization modulation step for modulating the polarization of the signal light so as to draw a circle on the Poincare sphere;
A circle tilted by an angle of Kπ + 0.20π or more and Kπ + 0.37π or Kπ−0.37π or more and Kπ−0.20π or less with respect to the circle drawn on the Poincare sphere by the first polarization modulation step. And a second polarization modulation step for modulating the polarization of the signal light on the Poincare sphere,
The optical transmission method characterized in that modulation rates of the first and second polarization modulation steps are different from each other. In an optical transmission method applied to an optical transmission system that includes a polarization scrambler that scrambles an input polarization to a transmission line and averages signals over the length of an FEC block,
The polarization scrambler is
A first polarization modulation step for modulating the polarization of the signal light so as to draw a circle on the Poincare sphere;
A circle tilted by an angle of Kπ + 0.20π or more and Kπ + 0.37π or Kπ−0.37π or more and Kπ−0.20π or less with respect to the circle drawn on the Poincare sphere by the first polarization modulation step. And a second polarization modulation step for modulating the polarization of the signal light on the Poincare sphere,
The optical transmission method characterized in that modulation rates of the first and second polarization modulation steps are different from each other. In an optical transmission method applied to an optical transmission system that includes a polarization scrambler that scrambles an input polarization to a transmission line and averages signals over the length of an FEC block,
The polarization scrambler is
A first polarization modulation step for modulating the polarization of the signal light so as to draw a circle on the Poincare sphere;
A circle tilted by an angle of Kπ + 0.20π or more and Kπ + 0.37π or Kπ−0.37π or more and Kπ−0.20π or less with respect to the circle drawn on the Poincare sphere by the first polarization modulation step. And a second polarization modulation step for modulating the polarization of the signal light on the Poincare sphere,
The optical transmission method characterized in that the modulation speeds of the first and second polarization modulation steps are different from each other but are synchronized and the change in polarization state is periodic. An optical transmission system having a polarization scrambler that scrambles the polarization input to the transmission line and averages the polarization state of the signal or averages the polarization state of the signal over the length of the FEC block. In the applied optical transmission method,
It is composed of a polarization controller comprising a rotating half-wave plate, a phase shifter, and a rotating quarter-wave plate, and the rotating half-wave plate and the rotating quarter-wave plate have the same rotation axis direction. In the polarization scrambler, the rotating half-wave plate and the rotating quarter-wave plate are rotated at a frequency of P × f [Hz] and Q × f [Hz],
A phase change in the phase shifter is Kπ + 0.20π or more and Kπ + 0.37π or less, or Kπ−0.37π or more and Kπ−0.20π or less. An optical transmission system having a polarization scrambler that scrambles the polarization input to the transmission line and averages the polarization state of the signal or averages the polarization state of the signal over the length of the FEC block. In the applied optical transmission method,
A polarization scrambler comprising a polarization controller comprising a first and second rotating quarter-wave plate and a rotating half-wave plate provided between the first and second rotating quarter-wave plates. The initial rotation angle of the first rotation quarter-wave plate is k × π / 3, the initial rotation angle of the rotation half-wave plate is 0, and the initial rotation angle of the second rotation quarter-wave plate is Set the rotation angle to 0, or
The initial rotation angle of the first rotation quarter-wave plate is 0, the initial rotation angle of the rotation half-wave plate is k × π / 6, and the initial rotation of the second rotation quarter-wave plate is Set the angle to 0, or
The initial rotation angle of the first rotation quarter-wave plate is 0, the initial rotation angle of the rotation half-wave plate is 0, and the initial rotation angle of the second rotation quarter-wave plate is k ×. π / 3,
The first rotation quarter-wave plate is rotated at a frequency of n × f [Hz], the rotation half-wave plate is rotated at a frequency (n + m) / 2 × f [Hz], and the second rotation is performed. An optical transmission method comprising rotating a quarter-wave plate at a frequency of m × f [Hz].

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