KR101098588B1 - Optical signal-to-noise ratio monitoring method for polarization-multiplexed optical signals - Google Patents

Abstract

A method of monitoring an optical signal-to-noise ratio of an optical signal using a polarization multiplexing method is disclosed. In order to monitor the optical signal-to-noise ratio of a polarized multiplexed optical signal, the method of the present invention receives a polarized multiplexed optical signal through polarization diversity coherent mixing, and samples sampled using an asynchronous delay tap sampling technique. The optical signal-to-noise ratio is monitored using the magnitude quality index obtained from the magnitude histogram. When the polarization mode dispersion and the polarization dependence loss present in the optical link cannot be ignored, coherent crosstalk may occur. The influence of the crosstalk can be effectively eliminated using a unitary matrix transformation method.

Description

Optical signal-to-noise ratio monitoring method for polarization-multiplexed optical signals

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for monitoring the optical signal-to-noise ratio of a polarized multiplexed optical signal. The present invention relates to a unitary matrix transformation method for compensating the effects of crosstalk and an optical signal-to-noise ratio monitoring method using a constant modulus algorithm (CMA).

Recently, researches on polarization division multiplexing (PDM), which multiplexes two perpendicular polarization axes in order to increase transmission capacity and spectral efficiency of optical transmission systems, have been actively conducted. However, when the polarization dependent loss (PDL) cannot be ignored in the optical link, the polarization dependency loss degrades the performance of the polarized multiplexed optical signal. The angle of one polarization axis of the polarization multiplexed optical signal and one polarization axis of the polarization dependent loss device is defined as an incidence angle in order to examine the effect of the incident angle when the polarized multiplexed optical signal is incident on the device having the polarization dependent loss. First, when the polarized multiplexed optical signal is incident on the polarization dependent loss element at an angle of incidence of 0 degrees, the power of the optical signals existing on the two polarization axes is reduced by the loss of each polarization axis of the polarization dependent loss element. In this case, although the polarization orthogonality of the polarized multiplexed optical signal is maintained, the optical signal-to-noise ratio (OSNR) of each polarization axis is different because the powers of the signals of the two polarization axes are different. They will be different. In addition, when the polarization multiplexed optical signal is incident on the polarization dependent loss element at an angle of incidence of 45 degrees, the power of each polarization axis of the polarization multiplexed optical signal is the same but polarization orthogonality of the optical signal is no longer maintained. Therefore, upon receiving an optical signal, each polarization component acts as a coherent crosstalk with respect to signals of different polarization axes. Such coherent crosstalk can be compensated by using a coherent receiver, but when the optical signal-to-noise ratio of each polarization axis of the polarization multiplexed optical signal is changed by the polarization dependent loss, the coherent crosstalk cannot be compensated. The performance of the optical signal is different with respect to the axis. In addition, when monitoring the optical signal-to-noise ratio of the polarization multiplexed optical signal using the conventional optical signal-to-noise ratio monitoring method has a problem that can be predicted better than the actual optical signal performance.

Accordingly, an object of the present invention is to provide a method for monitoring each optical signal-to-noise ratio with respect to two polarization axes of an optical signal using a polarization multiplexing scheme.

The optical signal-to-noise ratio monitoring method of the present invention for achieving the above technical problem is a method for monitoring the optical signal-to-noise ratio of the polarization multiplexed optical signal in the optical link of the optical communication system using a polarization multiplexing method, the polarization of the optical signal Receiving via diversity coherent mixing; Monitoring the optical signal-to-noise ratio using a magnitude quality index derived from the magnitude histogram of the samples sampled using an asynchronous delay tap sampling technique whose sampling frequency is significantly lower than the bit rate of the optical signal to be monitored. Characterized in having a.

Here, it is preferable to apply a unitary matrix transformation in order to remove the influence of coherent crosstalk generated by polarization mode dispersion or the like between two polarization axes of the polarized multiplexed optical signal.

In addition, the size histogram is composed only when the sample sampled among the samples is the same size as the sample sampled after a certain delay time, and the size quality index is obtained by the ratio of the mean value and the standard deviation of the samples in the size histogram. It is desirable to lose.

Meanwhile, the polarization diversity coherent mixing is performed by a dual polarization optical hybrid, a photodetector, and a local oscillator, and a frequency difference between the input optical signal and the local oscillator is homodyne or intradyne. It is preferable to make.

When unitary matrix transformation is applied to remove the effects of coherent crosstalk, if the optical signal is a PDM-QSPK (Polarization Division Multiplexing-Quadrature Phase Shift Keying) optical signal, the unitary matrix transformation method may include:

And initial conditions of the PDM-QPSK optical signal

It is preferable to separate the signal components of the two polarization axes using.

Here, (E _{in , x} , E _{in , y} ) and (E _x , E _y ) are two polarization components of the PDM-QPSK signal incident on the optical link and two polarization components received using a monitoring technique, and T Is a Jones matrix representing the polarization mode dispersion and the degree of polarization rotation present in the optical link, and α and δ are optical power splitting and phase delay differences between two polarization axes, respectively. Indicates.

According to the present invention described above, even if there are deterioration factors causing polarization crosstalk or breaking polarization orthogonality of an optical signal such as polarization dependence loss and polarization mode dispersion in the optical link, The information in the signal can be recovered to monitor the respective optical signal-to-noise ratios for both polarization axes.

1 is a block diagram of a monitoring apparatus for implementing an optical signal-to-noise ratio monitoring method of a polarization multiplexed optical signal according to an embodiment of the present invention;
2 is a graph showing that the effect of coherent crosstalk due to polarization mode dispersion of an optical link can be effectively eliminated using a unitary matrix transformation method and CMA;
3 is a graph measuring optical signal to noise ratio of each polarization axis of a PDM-QPSK signal according to an embodiment of the present invention;
4 is a graph evaluating the influence of polarization mode dispersion on the performance of the optical signal-to-noise ratio monitoring method according to an embodiment of the present invention;
5 is a graph evaluating the influence of the polarization dependent loss on the performance of the optical signal to noise ratio monitoring method according to an embodiment of the present invention; And
6 is a graph evaluating the effects of the polarization mode dispersion and the polarization dependent loss on the PDM-QPSK signal when the optical signal-to-noise ratio monitoring method according to the embodiment of the present invention is present.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are merely provided to understand the contents of the present invention, and those skilled in the art will be able to make many modifications within the technical scope of the present invention. Therefore, the scope of the present invention should not be construed as limited to these examples.

The present invention relates to an optical signal-to-noise ratio monitoring technique capable of operating on a polarization multiplexed optical signal using a multilevel phase modulation method. The optical signal to be monitored for performance evaluation of the proposed monitoring method is an optical communication system. In PDM-QPSK (quadrature phase shift keying) modulation method is most commonly used in the present invention, the scope of the present invention is not necessarily limited thereto.

1 is a block diagram of a monitoring apparatus for implementing an optical signal-to-noise ratio monitoring method of a polarization multiplexed optical signal according to an embodiment of the present invention.

Referring to FIG. 1, the monitoring apparatus 100 may include a polarization diversity coherent mixer 110, a delay tap sampling unit 120, and an A / D for polarization diversity coherent mixing. Converter 130 and a digital processor 140 for coherent crosstalk compensation and noise distribution measurement and analysis. This monitoring apparatus 100 is operated as follows. First, a signal present in two polarization axes of the PDM-QPSK signal is divided into a dual-polarization optical hybrid 114, four photo detectors 116, and a local oscillator 112. It is received using the polarization diversity coherent mixer 110 configured. The received signal is sampled as a digital signal by the asynchronous delay tap sampling unit 120 and the A / D converter 130 which are composed of an RF power divider, an RF delay line, and the like. The sampling frequency of this sampling technique need not be synchronized with the frequency of the bit rate of the optical signal to be monitored. Therefore, by using this sampling technique, only samples located at the center of the bits of the optical signal can be obtained without extracting the clock of the optical signal to be monitored. In addition, the sampling frequency of the asynchronous delay tap sampling technique may use a frequency much lower than the bit rate of the optical signal to be monitored, and the delay time should be smaller than the symbol period of the optical signal to be monitored.

If polarization mode dispersion or polarization dependent loss is present in the optical link of the optical communication system, this breaks the polarization orthogonality of the PDM-QPSK signal, resulting in coherent crosstalk. In general, in a digital coherent receiver, such coherent crosstalk can be compensated for using a CMA. However, since the present invention uses an asynchronous sampling technique, since there is no information between adjacent symbols, CMA cannot be directly applied to compensate for crosstalk. Because of this, coherent crosstalk causes errors in the proposed surveillance technique of the present invention. To solve this problem, we use unitary matrix transformation to compensate for coherent crosstalk, and then apply each CMA to each polarization axis of the optical signal to be monitored by applying CMA. Isolate. The unitary matrix conversion method is a digital signal processing process for the purpose of removing coherent crosstalk components included in a received optical signal. If the two polarization components of the PDM-QPSK signal incident on the optical link and the two polarization components of the received PDM-QPSK signal are (E _{in , x} , E _{in , y} ) and (E _x , E _y ), Equation 1 can be obtained.

Here, T is a Jones matrix representing the polarization mode dispersion and the degree of polarization rotation present in the optical link, and α and δ represent optical power distribution and phase delay differences between two polarization axes, respectively. From the characteristics of the PDM-QPSK signal incident on the optical link, an initial condition as shown in Equation 2 can be obtained.

Therefore, coherent crosstalk can be effectively removed by obtaining the inverse of T satisfying these two conditions from Equation 1. After applying the unitary matrix transformation method, CMA is applied to demultiplex the two polarizations of the PDM signal while simultaneously removing extra coherent crosstalk. In the present invention, since the received optical signal is asynchronously sampled and there is no information between adjacent symbols, a large value convergence coefficient is used, and the CMA is operated so that the coefficients of each tap of the CMA are recursively adapted. Such digital signal processing is performed in the digital processor 140.

FIG. 2 is a graph illustrating that the effects of coherent crosstalk due to polarization mode dispersion of an optical link can be effectively removed using a unitary matrix transformation method and CMA. When present, it represents the trajectory of the two polarization axes of the PDM-QPSK optical signal. At this time, the incident angle of the PDM-QPSK signal is 45 degrees. In FIG. 2, up and down means QPSK optical signals present in respective polarization axes of the PDM signal. 2 shows the PDM-QPSK received using the monitoring method proposed in the present invention. It can be confirmed that coherent crosstalk has occurred in the optical signal of each polarization axis by polarization mode dispersion. 2 shows the QPSK signal after the coherent crosstalk is removed using a unitary matrix transformation method. 2 is a QPSK signal after applying CMA. Referring to FIG. 2, it can be seen that the unitary matrix transformation method and the CMA can effectively remove the effects of coherent crosstalk.

After removing coherent crosstalk, a quality index is calculated from the distribution of noise using histograms for the magnitude components present in the two polarization axes of the PDM-QPSK signal. The quality index is defined here as the ratio of the mean and standard deviation values in the magnitude histogram. Since this quality index is highly correlated with the optical signal-to-noise ratio, the quality index can be used to independently monitor the optical signal-to-noise ratio for the two polarization axes of the PDM-QPSK optical signal.

3 is a graph measuring the optical signal to noise ratio of each polarization axis of the PDM-QPSK signal according to an embodiment of the present invention, specifically 42.7 Gbps PDM-QPSK measured using the optical signal to noise ratio monitoring method of the present invention Represent each optical signal-to-noise ratio for the two polarization axes of the signal. Referring to FIG. 3, it can be seen that the optical signal-to-noise ratio monitoring technique of the present invention can accurately monitor the optical signal-to-noise ratio of 12 to 28 dB with respect to the PDM-QPSK signal.

In addition, in order to confirm the influence of the polarization mode dispersion and the polarization dependence loss of the optical link on the performance of the monitoring method of the present invention, the optical signal-to-noise ratio of the optical signal is fixed at 20 dB, and the polarization mode dispersion is applied to the optical signal. And optical signal-to-noise ratios of the two polarization axes were measured by varying the magnitude of the polarization dependent loss.

4 is a graph evaluating the influence of polarization mode dispersion on the performance of the optical signal-to-noise ratio monitoring method according to an embodiment of the present invention. Referring to Figure 4, it can be seen that the monitoring method of the present invention can effectively eliminate the effects of the polarization mode dispersion of up to 20ps. For the polarization mode dispersion above this range, the monitoring performance deteriorates because the error in sampling the center of the bit is increased by using the asynchronous delay tap sampling technique.

5 is a graph evaluating the influence of the polarization dependent loss on the performance of the optical signal-to-noise ratio monitoring method according to an embodiment of the present invention. In the case where the optical signal-to-noise ratios are different from each other, the results of evaluating the performance of the monitoring method of the present invention are shown. To this end, the loss of the two optical paths of the polarization multiplexer for generating the optical signal is intentionally different. Therefore, the loss difference in the optical path means polarization dependent loss. After passing through the polarization multiplexer, the optical amplifier was used so that the total optical signal-to-noise ratio of the two polarization axes was always 20 dB. In Fig. 5, the square symbol represents the optical signal-to-noise ratio of each polarization axis measured using the monitoring method of the present invention, and the straight line represents the theoretically calculated optical signal-to-noise ratio value. Referring to FIG. 5, if only the entire optical signal-to-noise ratio is monitored without separating signals of each polarization axis using conventional optical signal-to-noise ratio monitoring technology, the performance of the signal of one polarization axis becomes overestimation and the rest The performance of the signal on the axis represents an underestimation. However, even if the optical signal-to-noise ratio of each polarization axis is changed by the polarization dependent loss, the monitoring method of the present invention reflects this well, and it can be seen that each optical signal-to-noise ratio can be independently monitored. Furthermore, when the polarization mode dispersion and the polarization dependent loss exist simultaneously, it was confirmed that the monitoring method of the present invention is effective.

6 is a graph evaluating the influence of the polarization mode dispersion and the polarization dependent loss on the PDM-QPSK signal when the optical signal-to-noise ratio monitoring method according to the embodiment of the present invention is present at the same time, that is, polarization When the mode variance varies from 5 to 25 ps and the polarization dependent loss varies from 0 to 6 dB, it represents the error of the optical signal to noise ratio measured using the monitoring method of the present invention. Referring to FIG. 6, it can be seen that the monitoring method of the present invention can accurately monitor the optical signal-to-noise ratio of each polarization axis even when polarization mode dispersion and polarization dependent loss exist simultaneously.

100: optical signal to noise ratio monitoring device
110: polarization diversity coherent mixer
112: Local Oscillator (LO)
114: dual-polarization optical hybrid
116: photo detector (PD)
120: delay tap sampling unit
130: A / D converter (ADC)
140: digital processing unit

Claims (6)

In monitoring the optical signal-to-noise ratio of the polarized multiplexed optical signal in the optical link of the optical communication system using the polarization multiplexing method,
Receiving the optical signal through polarization diversity coherent mixing;
Monitoring the optical signal-to-noise ratio using a magnitude quality index derived from the magnitude histogram of the samples sampled using an asynchronous delay tap sampling technique whose sampling frequency is significantly lower than the bit rate of the optical signal to be monitored. Provided with
And a unitary matrix transformation is applied to eliminate the effects of coherent crosstalk generated by polarization mode dispersion between two polarization axes of the polarization multiplexed optical signal. delete The method of claim 1, wherein the size histogram is formed only when the first sampled sample is the same size as the sample sampled after a predetermined delay time, and the size quality index is the average value and the standard of the samples in the size histogram. An optical signal-to-noise ratio monitoring method, characterized by a ratio of deviations. The method of claim 1, wherein the polarization diversity coherent mixing is performed by a dual polarization optical hybrid, a photodetector, and a local oscillator, and the frequency difference between the input optical signal and the local oscillator is homodyne or intradyne. optical signal-to-noise ratio monitoring method comprising: (intradyne). The optical signal of claim 1, wherein the optical signal is a PDM-QSPK (Polarization Division Multiplexing-Quadrature Phase Shift Keying) optical signal.
The unitary matrix transformation method,

And initial conditions of the PDM-QPSK optical signal

Optical signal to noise ratio monitoring method, characterized in that for separating the signal components of the two polarization axes using
Here, (E _{in, x} , E _{in, y} ) and (E _x , E _y ) are two polarization components of the PDM-QPSK signal incident on the optical link and two polarization components received using a monitoring technique, T Is a Jones matrix representing the polarization mode dispersion and the degree of polarization rotation present in the optical link, and α and δ represent optical power distribution and phase delay differences between two polarization axes, respectively. 2. The method of claim 1 wherein the asynchronous delay tap sampling technique is accomplished by an RF power divider or an RF delay line.

Images