JP4935718B2 - Setting method of optical transmission system - Google Patents

Description

  The present invention relates to a method for setting an optical transmission system using an optical fiber transmission line.

  In an optical transmission system using an optical fiber as a transmission path, if the transmission speed is increased or the transmission distance is increased, the amount of polarization mode dispersion (PMD) with respect to one bit interval of the optical transmission signal increases. Therefore, the influence of signal characteristic deterioration cannot be ignored. As a result, PMD makes it difficult to increase the transmission speed and increase the transmission distance.

  PMD is dispersion that occurs because the group velocity in an optical fiber depends on the polarization direction. Specifically, this is a phenomenon in which the optical signal waveform deteriorates due to the different propagation speeds of the orthogonal polarization components of the optical signal. The polarization dependence of an optical fiber is mainly due to the deviation from the perfect circle of the optical fiber, the distortion of internal stress that occurs during the production of the optical fiber, and the distortion of non-uniform external stress that occurs during and after the installation of the optical fiber. caused by. PMD is a phenomenon in which the generation amount fluctuates on the time axis and the wavelength axis, and as a result, the optical waveform fluctuates. When the transmission speed is increased and the time width between adjacent pulses is shortened, It can be one of the major deterioration factors.

  In the section where an optical fiber transmission line is laid like an electric wire passed to a telephone pole (called “aerial section”), the polarization state of the signal in the optical fiber (State of It is known that Polarization: SOP) fluctuates greatly and rapidly (Non-Patent Documents 1-3).

  When there is an influence of polarization mode dispersion, the TE component and the TM component that are orthogonal to each other on the time axis are caused by the group velocity difference in the optical fiber depending on the polarization direction, so-called differential group delay (DGD). Separate with.

  As a technique for suppressing the influence of DGD on the signal characteristics, a PMD suppression method for adjusting the SOP of the optical transmitter output signal light so as to match the principal state of polarization (PSP) has already been proposed. (Non-Patent Document 3).

Patent Document 1 describes an optical transmission system that controls the polarization direction on the transmission side when the error rate or PMD on the reception side exceeds a predetermined value.
JP 2006-033213 A D. Waddy, P. Lu, L. Chen, and X. Bao, "Fast state of polarization changes in aerial fiber under different climatic conditions", IEEE Photonics Technology Letters, Vol. 13, no. 9, pp.1035-1037 , 2001. H. Kogelnik, RM Jopson and LE Nelson, "Optical fiber telecommunications IVB systems and impairments", Chapter 15, Academic Press, 2002. I. Kaminow and TL Koch, "Optical Fiber Telecommunications III", Academic Press, 1997.

  The method described in Non-Patent Document 3 cannot appropriately cope with PMD that varies with time.

  Further, in the configuration described in Patent Document 1, it is necessary to provide a feedback control system, which increases the cost.

  It is an object of the present invention to provide an optical transmission system setting method that can appropriately cope with long-term PMD fluctuations in an optical fiber transmission line without complicated control.

The optical transmission system setting method according to the present invention includes a measurement preparation step of connecting a polarization degree measurement optical transmitter to the incident side of the optical fiber transmission line and connecting a polarization degree measurement optical receiver to the output side; predetermined period variations in the degree of polarization of the optical fiber transmission line in the polarizing measuring optical transmission apparatus and the degree of polarization measuring light receiving device, a step of measuring the autocorrelation Stokes parameters from the measurement result of the measuring step An autocorrelation calculation step for calculating a function, a determination step for determining an average polarization direction according to the calculation result of the autocorrelation calculation step, and the incidence on the optical fiber transmission line in the average polarization direction Connecting an optical transmission device to the optical fiber transmission line, and connecting an optical reception device corresponding to the optical transmission device to the output side of the optical fiber transmission line. The features.

  According to the present invention, good transmission characteristics can be obtained in the long term with respect to fluctuations in the polarization state of an optical fiber transmission line without complicated control.

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

  FIG. 1 shows a schematic configuration diagram of a configuration for carrying out an embodiment of the present invention, and FIG. 2 shows a setting procedure of the present embodiment.

  Part or all of the optical fiber transmission line 10 is an aerial section. A polarization measuring optical transmitter 12 is arranged on the transmission side of the optical fiber transmission line 10, and a polarization measuring optical receiver 14 corresponding to the polarization measuring optical transmitter 12 is arranged on the receiving side. Then, the Stokes parameters s1, s2, and s3 of the polarization degree are measured over a long period (for example, about 24 hours) using the polarization degree measurement optical transmitter 12 and the polarization degree measurement optical receiver 14 (S1). . An example of a method for measuring the Stokes parameters is the Jones matrix method.

  FIG. 3 shows an example of temporal changes of the Stokes parameters s1, s2, and s3 measured by the polarization measuring optical transmitter 12 and the polarization measuring optical receiver 14. The horizontal axis indicates an arbitrary period within the measurement time, and the vertical axis indicates the magnitude of the Stokes parameters s1, s2, and s3. Here, 1376 points per minute were continuously measured with an optical signal having a signal wavelength of 1558 nm. FIG. 3 shows part of the results measured over 23 hours. It can be seen that it fluctuates violently and randomly in the range of −1 to +1.

自己相関関数Ａ（τ）を求めた。 The autocorrelation function of the Stokes parameter measured in this way is calculated (S2). That is, the autocorrelation function ∫s (t) · s (t + τ) of the measurement result shown in FIG. 2 is obtained. FIG. 3 shows an autocorrelation function for the measurement result shown in FIG. Actually, by the finite integral shown in the following formula,

An autocorrelation function A (τ) was obtained.

  It can be seen from FIG. 3 that the correlation coefficient is about 0.4 despite having an imaginary section. That is, the degree of polarization does not change at random, but is biased with a certain amount of weight.

  In this embodiment, according to this measurement result, an average SOP is determined over the long term, and the polarization direction of the optical signal incident on the optical fiber transmission line 10 is adjusted to the SOP.

  That is, the optical transmission device 20 and the polarization control device 22 are arranged instead of the polarization degree measuring optical transmission device 12, and the output signal light of the optical transmission device 20 is transmitted through the polarization control device 22 to the optical fiber transmission line 10. (S3). Then, the polarization state of the input signal light to the optical fiber transmission line 10 is controlled by polarization while monitoring the reception result of the polarization measuring device 14 so that the polarization state of the Stokes parameter generated most frequently is obtained. Adjustment is performed using the device 22 (S4). Of course, without using the polarization controller 22, the positional relationship between the optical transmitter 20 and the incident end of the optical fiber transmission line 10 is adjusted, and similarly, the polarization of the signal light incident on the optical fiber transmission line 10 is adjusted. The wave direction may be adjusted.

  Finally, the optical receiver 24 is connected to the output end of the optical fiber transmission line 10 instead of the polarization measuring optical receiver 14 (S5).

  Thus, by matching the polarization direction of the signal light incident on the optical fiber transmission line 10 with the SOP viewed from the long-term viewpoint of the optical fiber transmission line 10, good performance can be obtained on average. Can be achieved.

  Although the invention has been described with reference to specific illustrative embodiments, various modifications and alterations may be made to the above-described embodiments without departing from the scope of the invention as defined in the claims. This is obvious to an engineer in the field to which the present invention belongs, and such changes and modifications are also included in the technical scope of the present invention.

It is a schematic block diagram showing the system configuration of the present embodiment. It is a flowchart of the method which concerns on a present Example. It is a measurement result of long-term fluctuation of the degree of polarization. It is an autocorrelation function of the measurement result shown in FIG.

Explanation of symbols

10: optical fiber transmission line 12: polarization measuring optical transmitter 14: polarization measuring optical receiver 20: optical transmitter 22: polarization controller 24: optical receiver

Claims (1)

A measurement preparation step of connecting the polarization measuring optical transmitter (12) to the incident side of the optical fiber transmission line and connecting the polarization measuring optical receiver (14) to the output side;
Predetermined period variations in the degree of polarization of the optical fiber transmission line in the polarizing measuring optical transmission device (12) and the polarization degree measurement optical receiver (14), a step of measuring,
An autocorrelation calculation step for calculating the autocorrelation function of the Stokes parameter from the measurement result of the measurement step;
In accordance with the calculation result of the autocorrelation calculation step, a determination step for determining an average polarization direction;
Connecting an optical transmission device to the incident side of the optical fiber transmission line in the average polarization direction;
And a step of connecting an optical receiver corresponding to the optical transmitter to the emission side of the optical fiber transmission line.

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