JP6783743B2 - Polarized multiplex optical signal transmission system - Google Patents

Description

The present invention relates to a technique for estimating or compensating for polarization-dependent loss in transmission of an optical signal using a polarization multiplex optical transmission method.

In an optical transmission system that realizes high-speed and large-capacity communication, a polarization multiplex optical transmission method may be adopted in order to realize high frequency utilization efficiency. This method is a method in which two optical signals having the same wavelength are multiplexed as two polarized waves orthogonal to each other and simultaneously sent from an optical transmitter to a transmission line. According to an optical transmission system adopting this method (hereinafter referred to as "polarized multiplex optical transmission system"), the frequency utilization efficiency can be improved by about twice as compared with an optical transmission system using a single polarized wave. It will be possible.

However, in a polarization multiplex optical transmission system, polarization-dependent loss (PDL: Polarization Dependent Loss) possessed by an optical transmission line or a component of an optical transmitter becomes a problem. Due to this PDL, a phenomenon may occur in which the light intensity of one polarized light becomes smaller than the light intensity of the other polarized light after transmission of both polarized waves. Therefore, in the receiver, an undesired situation may occur in which the signal quality of one demodulated polarized wave is high and the signal quality of the other polarized wave is low.

Therefore, it is important to make the PDL of the optical transmission line or the optical transmitter as small as possible and to construct an optical transmission system so that the same optical loss is obtained in any polarization state. However, PDL is often accompanied by wavelength dependence and temperature dependence, and it is extremely difficult to completely suppress PDL over all wavelengths and all temperatures under the operating environment of a polarized multiplex optical transmission system.

Against this background, control methods have been studied to continuously monitor the PDL during the operation period of the optical transmission system and dynamically compensate for the increase in the PDL when it is detected. For example, in Patent Document 1, a tone-modulated signal having a different frequency is superimposed on two polarization-multiplexed optical signals in an optical transmitter, and these different tone-modulated signals are detected in a relay node. Techniques for monitoring and compensating for PDL have been disclosed. Further, for example, in Patent Document 2, two types of training signals are embedded in each of two polarization-multiplexed optical signals, and the intensities of the two types of frequency bands corresponding to the periodic patterns of each training signal are compared. Discloses a technique for monitoring PDL.

FIG. 7 is a schematic view showing the configuration of the polarization multiplex optical signal transmission system 900 according to the prior art. The CW (Continuous Wave) light output from the CW light source 911 in the polarization multiplex optical signal transmitter 910 is branched into two by the polarization retention coupler 912, and each of the branched lights is the first light modulator 913. It is input to -1 and the second optical modulator 913-2, respectively. The first light modulator 913-1 is driven by the first modulator drive unit 914-1, and the second light modulator 913-2 is driven by the second modulator drive unit 914-2. The first light modulator 913-1 and the second light modulator 913-2 generate two types of optical signals by performing different modulations on CW light.

At this time, two types of dither signals (also referred to as "tone modulation signals") having different frequencies are superimposed on the output signals of the first modulator drive unit 914-1 and the second modulator drive unit 914-2. .. Specifically, the output signal of the first modulator drive unit 914-1 is superposed with the dither signal of the first frequency generated by the first dither signal output unit 915-1, and the second modulator drive unit A second frequency dither signal generated by the second dither signal output unit 915-2 is superimposed on the output signal of 914-2. The dithering frequency produced by these dither signals is generally much lower than the baud rate, and the amount of variation is much shallower than the modulation depth of the main signal.

Each of the two types of generated optical signals constitutes a single polarization, and each polarization is multiplexed so as to be orthogonal to each other by the polarization multiplexing unit 916. The polarization-multiplexed optical signal generated by this multiplexing is output from the polarization-multiplexed optical signal transmitter 910 and sent to the optical transmission line 920.

The polarization multiplex optical signal sent to the optical transmission line 920 is input to the polarization level adjusting unit 931 at the relay node 930. The polarization multiplex optical signal output from the polarization level adjustment unit 931 is branched by the tap 932 and sent to the dither signal detection unit 933. The dither signal detection unit 933 uses dither signals of two types of frequencies superimposed on the two polarizations of the signal light (hereinafter, one is referred to as "X polarization" and the other is referred to as "Y polarization"). To detect.

Here, when the light intensity of X-polarized light is larger than the light intensity of Y-polarized light due to the PDL provided by the optical transmission path 920 and the polarized light multiplex optical signal transmitter 910, the dither signal corresponding to the X-polarized light Is detected as a signal having a higher intensity than the dither signal corresponding to the Y polarization. The dither signal detection unit 933 outputs this detection result to the PDL estimation unit 934. The PDL estimation unit 934 calculates the size of the PDL based on the output of the dither signal detection unit 933, and feeds it back to the polarization level adjustment unit 931. The polarization level adjusting unit 931 compensates for the PDL of the received signal based on the magnitude of the feedback PLD.

Japanese Patent No. 5827379 Japanese Patent No. 5635923

However, the above-mentioned conventional technique has the following problems. First, in order to detect two types of dither signals, two detection circuits corresponding to the respective frequencies are required, which increases the circuit scale. Further, in this case, since it is necessary to block the frequency of the dither signal superimposed on the polarization that is not the detection target, these two detection circuits must have a filtering function. Further, when not only the fundamental wave but also the harmonics are generated due to the superimposition of the dither signal (tone modulation), there is also a problem that the requirements regarding the frequency interval and the filtering characteristics become strict in the two types of tone modulation. is there. When two types of training signals are used instead of the two types of dither signals (tone modulation signals), it is necessary to demodulate the training signals after performing polarization separation, which causes an increase in the circuit scale.

Further, if the input / output characteristics of the dither signal detection unit 933 are not ideal, there is a problem that an error is likely to occur in the estimation of PDL. As described above, in the polarization multiplex optical signal transmission system, when the light intensity of X polarization is larger than the light intensity of Y polarization due to PDL, the intensity of the dither signal corresponding to X polarization is high. It is greater than the intensity of the dither signal corresponding to the Y polarization. However, when the non-linearity of the input / output characteristics of the photodetector (PD: Photo Detector) that detects the dither signal and the electronic circuit around the PD cannot be ignored, or even if the linearity is high, the proportionality coefficient largely depends on the frequency of the dither signal. In that case, a complicated conversion formula is required when determining the size of the PDL. Therefore, in such a case, it becomes difficult to "compare the amplitudes of dither signals of two different frequencies". Even when two types of training signals are used instead of the two types of dither signals (tone modulation signals), the same problem as described above may occur if the input / output characteristics of the power monitor are not ideal.

In view of the above circumstances, an object of the present invention is to provide a technique capable of accurately estimating the PDL that can occur in the transmission of an optical signal using the polarization multiplex optical transmission method with a simpler configuration. There is.

One aspect of the present invention transmits polarized multiplex signal light generated by multiplexing a first optical signal and a second optical signal having the same wavelength so that their polarizations are orthogonal to each other. A polarization multiplex optical signal transmission system, the first optical modulator that generates the first optical signal, the second optical modulator that generates the second optical signal, and the first light. A first optical modulator drive unit that outputs a first drive signal that is an electric signal for driving the modulator, and a second drive signal that is an electric signal for driving the second optical modulator. A second optical modulator drive unit that outputs the above, a first dither signal superimposing unit that superimposes the first dither signal on the first optical signal, and a second dither signal superimposed on the second optical signal. A dither signal that detects the sum of the second dither signal superimposing unit to be generated, the first dither signal superimposed on the first optical signal, and the second dither signal superimposed on the second optical signal. PDL (Polarization Dependent Loss) caused by a part or all of the polarization multiplex optical signal transmission system is estimated based on the detection unit and the sum of the detected first dither signal and the second dither signal. The first dither signal superimposing unit and the second dither signal superimposing unit have the same repetition frequency f and amplitude, and are of the first optical signal and the second optical signal. This is a polarization multiplex optical signal transmission system in which the dither signal is superimposed by adjusting the phase so that the intensity of one of the optical signals reaches the maximum value and the intensity of the other optical signal becomes the minimum value.

One aspect of the present invention is the polarization multiplex optical signal transmission system, wherein the first dither signal superimposition unit and the second dither signal superimposition unit are the first light modulator and the second light. The voltage value of the DC voltage applied to the bias terminal of the modulator, the amplitude of the first and second drive signals, or the center value of the amplitude of the first and second drive signals is periodically minute. By changing, the first and second dither signals are superimposed on the first optical signal and the second optical signal.

One aspect of the present invention is the polarization multiplex optical signal transmission system, wherein a part of the polarization multiplex signal light is branched and input, and a part of the input polarization multiplex signal light is electrified. A photoelectric conversion circuit that converts the signal into a signal and outputs the signal is further provided, and the dither signal detection unit detects a frequency component of the frequency f superimposed on the electric signal output by the photoelectric conversion circuit and shows the detection result. The detection signal is output to the PDL estimation unit, and the PDL estimation unit acquires the sum of the first dither signal and the second dither signal based on the detection signal output from the dither signal detection unit. To do.

One aspect of the present invention is the polarization multiplex optical signal transmission system, wherein a part of the first optical signal output from the first optical modulator is input and a part of the input is input. A first photoelectric conversion circuit that converts the first optical signal into an electric signal and outputs the signal, and a part of the second optical signal output from the second optical modulator are input and input. A second photoelectric conversion circuit that converts a part of the second optical signal into an electric signal and outputs it, an electric signal output from the first photoelectric conversion circuit, and an output from the second photoelectric conversion circuit. The dither signal detection unit further includes an adder circuit that adds and outputs the electric signal to be output, and the dither signal detection unit detects a frequency component of the frequency f superimposed on the electric signal output from the adder circuit and outputs the same. A detection signal indicating a detection result is output to the PDL estimation unit, and the PDL estimation unit outputs the first dither signal and the second dither signal based on the detection signal output from the dither signal detection unit. To get the sum of.

One aspect of the present invention is the polarization multiplex optical signal transmission system, wherein the PDL estimation unit includes an addition signal indicating the sum of the detected first dither signal and the second dither signal. , The RF (Radio Frequency) power signal obtained by the squared average square root of the added signal, and the synchronous detection signal obtained by synchronously detecting the added signal or the RF power signal at the frequency of the dither signal. The PDL is estimated based on one signal.

One aspect of the present invention is the polarization multiplex optical signal transmission system, wherein the first attenuator that attenuates the intensity of the first optical signal with the first attenuation factor and the second optical signal. By changing the second attenuator that attenuates the intensity with the second attenuation rate, the first attenuation rate, and the second attenuation rate, the intensity of the first optical signal and the second optical signal is changed. The PDL control unit further includes a PDL control unit that controls the ratio, and the PDL control unit sets the first attenuation rate or the second attenuation rate so that the PDL value estimated by the PDL estimation unit approaches 0 dB. change.

One aspect of the present invention is the above-mentioned polarized multiplex optical signal transmission system, in which the PDL control unit converges the PDL estimated by the PDL estimation unit to 0 dB within a predetermined error range. The first attenuation factor and the first attenuation factor so that the total light intensity of the polarized multiplex signal light obtained by multiplexing the first optical signal and the second optical signal is within a predetermined range. Change the second attenuation factor.

One aspect of the present invention is the polarization multiplex optical signal transmission system, wherein the PDL estimation unit has a PDL value of 0 dB when the value of the synchronous detection signal is equal to a predetermined value. Is determined to be.

According to the present invention, PDL can be estimated with high accuracy with a simpler configuration.

It is a figure which shows the structural example of the polarization multiplex optical signal transmitter 1 of 1st Embodiment. It is a figure which shows the specific example of the action of the dither signal superposed on each polarization in the polarization multiplex optical signal transmitter 1 of 1st Embodiment. It is a figure which shows the specific example of the action of the dither signal superposed on each polarization in the polarization multiplex optical signal transmitter 1 of 1st Embodiment. It is a figure explaining the feedback control of each attenuator 17 in the polarization multiplex optical signal transmitter 1 of 1st Embodiment. It is a figure which shows the configuration example of the polarization multiplex optical signal transmission system 100 of 2nd Embodiment. It is a figure which shows the structural example of the polarization multiplex optical signal transmitter 1b of 3rd Embodiment. It is the schematic which shows the structure of the polarization multiplex optical signal transmission system 900 by the prior art.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following embodiment, the light intensity is measured in a time sufficiently longer than the reciprocal of the baud rate of the optical signal generated by the transmitter and sufficiently shorter than the dithering cycle.

[First Embodiment]
FIG. 1 is a diagram showing a configuration example of the polarized multiplex optical signal transmitter 1 of the first embodiment. In the present embodiment, the PDL (Polarization Dependent Loss) estimation means is installed inside the polarized multiplex optical signal transmitter 1. The PDL estimation means estimates only the PDL derived from the component of the polarization multiplex optical signal transmitter 1. When the polarization multiplex optical signal transmitter 1 determines that the PDL is generated, the polarization multiplex optical signal transmitter 1 corrects the PDL to 0 [dB]. Therefore, in the present embodiment, the polarization multiplex optical signal transmitter 1 does not estimate the PDL including the optical transmission line and compensate for it.

The polarization multiplex optical signal transmitter 1 includes a CPU (Central Processing Unit), a memory, an auxiliary storage device, and the like connected by a bus, and executes a program. The polarization multiplex optical signal transmitter 1 includes a CW light source 11, a polarization retention coupler 12, a first optical modulator 13-1, a second optical modulator 13-2, and a first dither signal output unit 14-by executing a program. 1. 2nd dither signal output unit 14-2, 1st adder 15-1, 2nd adder 15-2, 1st modulator drive unit 16-1, 2nd modulator drive unit 16-2, 1st It functions as a device including an attenuator 17-1, a second attenuator 17-2, a polarization multiplexing unit 18, and a feedback circuit 19. All or part of each function of the polarization multiplex optical signal transmitter 1 is realized by using hardware such as ASIC (Application Specific Integrated Circuit), PLD (Programmable Logic Device), and FPGA (Field Programmable Gate Array). May be done. The program may be recorded on a computer-readable recording medium. The computer-readable recording medium is, for example, a flexible disk, a magneto-optical disk, a portable medium such as a ROM or a CD-ROM, or a storage device such as a hard disk built in a computer system. The program may be transmitted over a telecommunication line.

The CW light source 11 is a light source that emits CW (Continuous Wave) light. The CW light emitted from the CW light source 11 is output to the polarization holding coupler 12.

The polarization holding coupler 12 splits the CW light output from the CW light source 11 into two. Each branched CW light is output to the first light modulator 13-1 and the second light modulator 13-2. The CW light output to the first modulator 13-1 is an example of the first optical signal in the present invention, and the CW light output to the second modulator 13-2 is the second optical signal in the present invention. This is an example.

The first light modulator 13-1 and the second light modulator 13-2 perform different modulations on the input CW light. Two types of optical signals are generated by the first optical modulator 13-1 and the second optical modulator 13-2 performing different modulations on the same CW light branched into two. Hereinafter, the optical signal modulated by the first light modulator 13-1 is referred to as a first optical signal, and the optical signal modulated by the second light modulator 13-2 is referred to as a second optical signal.

The first dither signal output unit 14-1 and the second dither signal output unit 14-2 output a dither signal. The dither signal is a signal superimposed on the optical signal. The first dither signal output unit 14-1 generates a dither signal (an example of the first dither signal) superimposed on the first optical signal, and outputs the generated dither signal to the first adder 15-1. .. Similarly, the second dither signal output unit 14-2 generates a dither signal (an example of the second dither signal) superimposed on the second optical signal, and the generated dither signal is used as the second adder 15-2. Output to.

The first modulator drive unit 16-1 generates an electric signal (hereinafter referred to as “drive signal”) for driving the first optical modulator 13-1, and the generated drive signal is used as the first adder 15-. Output to 1. The drive signal (first drive signal) output from the first modulator drive unit 16-1 is combined with the dither signal for the first optical signal in the first adder 15-1. The first adder 15-1 outputs a drive signal in which the dither signal is synthesized to the first optical modulator 13-1. When such a drive signal is input to the first optical modulator 13-1, the first optical modulator 13-1 superimposes the dither signal on the first optical signal, and the dither signal is superposed on the first optical signal. The optical signal of 1 is output to the first attenuator (attenuator) 17-1 in the subsequent stage.

Similarly, the second modulator drive unit 16-2 generates a drive signal for driving the second optical modulator 13-2, and outputs the generated drive signal to the second adder 15-2. The drive signal (second drive signal) output from the second modulator drive unit 16-2 is combined with the dither signal for the second optical signal in the second adder 15-2. The second adder 15-2 outputs a drive signal in which the dither signal is synthesized to the second optical modulator 13-2. When such a drive signal is input to the second optical modulator 13-2, the second optical modulator 13-2 superimposes the dither signal on the second optical signal, and the dither signal is superposed on the second optical modulator. The optical signal of 2 is output to the second attenuator 17-2 in the subsequent stage.

The first attenuator 17-1 (an example of a first attenuator) and the second attenuator 17-2 (an example of a second attenuator) are attenuators that attenuate an input optical signal. The first attenuator 17-1 attenuates the first optical signal output from the first optical modulator 13-1 and outputs it to the polarization multiplexing unit 18. Similarly, the second attenuator 17-2 attenuates the second optical signal output from the second optical modulator 13-2 and outputs it to the polarization multiplexing unit 18. The attenuation rate of the first attenuator 17-1 is an example of the first attenuation rate in the present invention, and the attenuation rate of the second attenuator 17-2 is an example of the second attenuation rate in the present invention.

In the polarization multiplexing unit 18, the polarization of the first optical signal output from the first attenuator 17-1 and the polarization of the second optical signal output from the second attenuator 17-2 are orthogonal to each other. Multiplex as follows. Hereinafter, the polarization of the first optical signal at the output of the polarization multiplexing unit 18 is defined as X polarization, and the polarization of the second optical signal is defined as Y polarization. The polarization multiplexing unit 18 transmits the multiplexed signal light (hereinafter referred to as “polarization multiplex signal light”) to the optical waveguide 2. The polarized multiplex signal light output to the optical waveguide 2 is branched at the tap 21 on the transmission path and output to the feedback circuit 19. The feedback circuit 19 includes a monitor PD (Photo Detector) 191, a synchronous detection circuit 192, a PDL estimation unit 193, and a PDL control unit 194. The polarized multiplex signal light branched at the tap 21 is first input to the monitor PD191.

The monitor PD191 converts an optical signal into an electrical signal. The synchronous detection circuit 192 synchronously detects the dither signal superimposed on the electric signal output from the monitor PD191. The result of the synchronous detection is output to the PDL estimation unit 193.

The PDL estimation unit 193 estimates the PDL based on the result of the synchronous detection. The PDL control unit 194 determines the attenuation rate to be reflected in the first attenuator 17-1 and the second attenuator 17-2 based on the PDL value estimated by the PDL estimation unit 193. Specifically, the PDL control unit 194 determines the attenuation rate of each attenuator 17 so that the value of the PDL after reflection becomes 0 (zero) [dB]. The PDL control unit 194 reflects the attenuation factors determined for each of the first attenuator 17-1 and the second attenuator 17-2 in the first attenuator 17-1 and the second attenuator 17-2.

2 and 3 are diagrams showing a specific example of the action of the dither signal superimposed on each polarization in the polarization multiplex optical signal transmitter 1 of the first embodiment. FIG. 2 shows an example of operation when it is assumed that the polarization multiplex optical signal transmitter 1 is in an ideal state and no PDL is generated (that is, PDL = 1 = 0 [dB]). In the upper figure of FIG. 2, the intensity of the dither signal changes with respect to the X-polarized light signal and the Y-polarized light signal with a period T2-T0 (that is, frequency f = 1 / (T2-T0)) ( It is shown to bring about (change of envelope). In FIG. 2, for the sake of simplicity, the intensity change caused by the dither signal is shown as extremely large, but the actual dither signal has a small intensity so as not to affect the quality of the transmitted signal. Gives a change in (modulation depth).

Here, the dither signal corresponding to each polarization is output so that its phase satisfies the following conditions.
(Condition 1) The time when the light intensity of X-polarized light becomes maximum and the time when the light intensity of Y-polarized light becomes minimum are the same (for example, time T1 in the examples of FIGS. 2 and 3).
(Condition 2) The time when the light intensity of X-polarized light becomes the minimum and the time when the light intensity of Y-polarized light becomes maximum are the same (for example, times T0 and T2 in the examples of FIGS. 2 and 3).

Here, it is desirable that T2-T1 = T1-T0, but it is not always essential. The pattern of change in light intensity (change in envelope) caused by two dither signals having different phases is preferably a sine wave, but it does not necessarily have to be a sine wave as long as it has periodicity. 2 and 3 show an example in which a slightly distorted sine wave is assumed as a pattern of change in light intensity.

Further, here, it is assumed that the intensity of the light emitted by the CW light source 11 is P0, and the light loss rates of the first attenuator 17-1 and the second attenuator 17-2 are both VOA (0 <VOA <1). Further, the total optical loss rate of the drive loss rate of each light modulator 13 and the light loss rate of other optical parts is defined as LOSS, and LOSS fluctuates between the maximum value LOSSmin and the minimum value LOSSmax by applying a dither signal. It is assumed that (0 <LOSSmax <LOSS <LOSSmin <1).

In this case, since the X-polarized light and the Y-polarized light are orthogonal to each other and do not interfere with each other, the light intensity of the polarized light multiplex signal light input to the monitor PD191 is the light intensity of the X-polarized light and the Y-polarized light. It is represented by a simple linear sum with the light intensity. Here, since it is assumed that the pattern of intensity modulation by the dither signal is represented by a distorted sine wave, the output of the monitor PD191 is twice the frequency of the dither signal as shown in the lower figure of FIG. It fluctuates at 2f.

However, in the example of FIG. 2, since the PDL is assumed to be 0 [dB], the output of the monitor PD191 is the same at any of the times T0, T1, and T2 corresponding to the peaks and valleys of the dither signal waveform. It is proportional to the value of P0 × VO × (LOSSmin + LOSSmax). In other words, if the monitor PD191 outputs a constant value at these times, it can be determined that the PDL is 0 [dB]. This criterion does not change even when the input / output response of the monitor PD191 has non-linearity, and the light-electricity conversion efficiency of the monitor PD191 depends on whether the frequency is f or 2f. Even if they are different, they will not change.

Next, the operation of the dither signal when the polarization multiplex optical signal transmitter 1 is not in the ideal state will be described. For example, FIG. 3 shows an example of operation when PDL is generated in any component of the polarization multiplex optical signal transmitter 1 and the light intensity of Y polarization becomes smaller than the light intensity of X polarization. In this case, the light intensity of X-polarized light is the same as in FIG.
P0 x LOSSmin x VOA ~ P0 x LOSSmax x VO
The light intensity of Y-polarized light varies within the range of
It varies within the range of P0 × Rpdl × LOSSmin × VOA to P0 × Rpdl × LOSSmax × VOA.

Here, Rpdl is a value representing the size of PDL, and takes a value of 0 <Rpdl <1. In this case, the light intensity of the polarized multiplex signal light at time T1 is
P0 x LOSSmin x VOA + P0 x Rpdl x LOSSmax x VOA
= P0 x VOA x (LOSSmin + Rpdl x LOSSmax)
Therefore, the light intensity of the polarized multiplex signal light at time T0 and T2 is
P0 x Rpdl x LOSSmin x VOA + P0 x LOSSmax x VOA
= P0 × VOA × (Rpdl × LOSSmin + LOSSmax)
Will be.

here,
(LOSSmin + Rpdl x LOSSmax)-(Rpdl x LOSSmin + LOSSmax)
= (1-Rpdl) x LOSSmin + (Rpdl-1) x LOSSmax
= (1-Rpdl) × (LOSSmin-LOSSmax)> 0
Therefore, the output value of the monitor PD191 at the time T1 is larger than the output value of the monitor PD191 at the time T0 or the time T2. On the other hand, when the light loss of X polarization becomes larger than the light loss of Y polarization, 1 <Rpdl <∞, and the above equation becomes less than 0. From this, by comparing the magnitude of the output of the monitor PD191 at each time, it is possible to determine whether the light loss is larger in the X polarization or the Y polarization.

Here, when the electronic circuit of the monitor PD191 or its surroundings has non-linearity in the input / output response in the frequency domain of f = 1 / (T2-T0), and the output of the monitor PD191 is saturated at a predetermined light intensity or higher. think of. The lower figure of FIG. 3 shows a specific example of the output of the monitor PD191 obtained in such a case. For example, in the example of the lower figure of FIG. 3, the output of the monitor PD191 is saturated around the time T1 and shows almost no change. However, if the difference between the output of the monitor PD191 at time T1 and the output of the monitor PD191 at time T0 or time T2 is large enough to be determined, the above-mentioned determination criteria are used as they are even if there is saturation. , X polarization or Y polarization, which makes it possible to determine which of the light loss is larger. Further, the above-mentioned determination criteria do not change even if the response speed of the electronic circuit around the monitor PD191 is different depending on whether the frequency is f or 2f.

FIG. 4 is a diagram illustrating feedback control of each attenuator 17 in the polarized multiplex optical signal transmitter 1 of the first embodiment. Here, it is assumed that the output of the monitor PD191 is similar to that shown in the lower part of FIG. 3 (FIG. 4 (A)). The output of the monitor PD191 obtained as shown in FIG. 4A is input to the synchronous detection circuit 192. The synchronous detection circuit 192 performs synchronous detection using a clock synchronized with the first dither signal output unit 14-1 or the second dither signal output unit 14-2 (hereinafter referred to as “reference clock”). FIG. 4B shows a specific example of the reference clock. Here, it is assumed that the reference clock shown in FIG. 4B is synchronized with the first dither signal output unit 14-1.

Since the synchronous detection circuit 192 takes the product of the output of the monitor PD191 shown in FIG. 4 (A) and the reference clock shown in FIG. 4 (B), the synchronous detection circuit 192 of the synchronous detection is shown in the right half of FIG. 4 (C). The result is positive. However, when the light intensity of Y-polarized light is larger than the light intensity of X-polarized light, the phase of the output of the monitor PD191 is inverted, so that the result of synchronous detection is negative as shown in the left half of FIG. 4 (C). It becomes.

The result of the synchronous detection (the result of detecting the dither signal) performed by such a method is input to the PDL estimation unit 193 in the subsequent stage. The PDL estimation unit 193 estimates the PDL of the polarized multiplex signal light based on the result of the synchronous detection by the synchronous detection circuit 192. Specifically, the PDL estimation unit 193 monitors the sign of the synchronous detection signal indicating the synchronous detection result according to the judgment criteria explained by the above mathematical formula, so that the light loss occurs in either the X polarization or the Y polarization. Determine if it is getting bigger. Further, the PDL estimation unit 193 acquires the magnitude of the PDL by monitoring the absolute value (magnitude) of the intensity of the synchronous detection signal. The estimation result of the PDL estimation unit 193 is input to the PDL control unit 194.

Based on the estimation result of the PDL estimation unit 193, the PDL control unit 194 performs feedback control for each attenuator 17 so that the PDL of the polarized multiplex signal light is compensated. Specifically, the PDL control unit 194 determines the attenuation rate of the first attenuator 17-1 and the second attenuator 17-2 so that Rpdl = 1 = 0 [dB]. The PDL control unit 194 reflects the determined attenuation rate on the first attenuator 17-1 and the second attenuator 17-2.

Feedback control to the first attenuator 17-1 and the second attenuator 17-2 can be realized, for example, by transmitting a command with a digital signal. The calculation of the feedback gain and the generation of the command are performed by the PDL control unit 194. By stopping the feedback control when the output of the synchronous detection circuit 192 becomes 0, the PDL can be maintained at 0 [dB].

The polarization multiplex optical signal transmitter 1 of the first embodiment configured as described above is provided with a configuration in which two dither signals having different phases are synchronously detected by one synchronous detection circuit 192, whereby "two different ones". PDL monitoring and compensation thereof can be performed without performing the process of "comparing the amplitudes of frequency dither signals". As described above, the polarized multiplex optical signal transmitter 1 of the first embodiment can estimate the PDL with a simpler configuration with high accuracy.

The first dither signal output unit 14-1, the first adder 15-1, and the first modulator drive unit 16-1 are examples of the first dither signal superimposition unit in the present invention, and the second dither signal output unit. 14-2, the second adder 15-2, and the second modulator drive unit 16-2 are examples of the second dither signal superimposition unit in the present invention. The monitor PD191 is an example of the dither signal detection unit in the present invention.

[Second Embodiment]
FIG. 5 is a diagram showing a configuration example of the polarization multiplex optical signal transmission system 100 of the second embodiment. The polarization multiplex optical signal transmission system 100 includes a polarization multiplex optical signal transmitter 1a, a relay node 3, and an optical transmission line 4 connecting the polarization multiplex optical signal transmitter 1a and the relay node 3. The difference between the second embodiment and the first embodiment is that the functional unit related to the feedback circuit 19 provided in the polarization multiplex optical signal transmitter 1 in the first embodiment is biased via the optical transmission line 4. This is a point provided in the relay node 3 connected to the wave multiplexing optical signal transmitter 1a.

In the second embodiment, it is difficult for the relay node 3 to acquire the reference clock from the polarization multiplex optical signal transmitter 1a. Therefore, the relay node 3 includes a BPF (Band Pass Filter) 195 and an RF (Radio Frequency) power monitor 196 instead of the synchronous detection circuit 192, and a PDL estimation unit 193a instead of the PDL estimation unit 193. Further, the polarization multiplex optical signal transmission system 100 of the present embodiment does not have a means for correcting the estimated PDL.

Other configurations are the same as those of the polarized multiplex optical signal transmitter 1 of the first embodiment. Therefore, the same functional parts as those in the first embodiment will be designated by the same reference numerals as those in FIG. 1, and the description thereof will be omitted.

In this case, in the relay node 3, the BPF 195 extracts an RF signal (an example of an addition signal) which is a variable component of the frequency f = 1 / (T2-T0) from the polarized multiplex signal light, and RFs the extracted RF signal. Output to the power monitor 196.

The RF power monitor 196 measures the RF power of the extracted RF signal of the frequency f, and inputs the RF power signal indicating the measured value to the PDL estimation unit 193a. The RF power signal is obtained by the root mean square of the RF signal.

When the measured RF power is 0 or extremely small, the PDL estimation unit 193a can estimate that the PDL is 0, as in FIG. The PDL estimated here corresponds to the PDL of the entire system including the polarization multiplex optical signal transmitter 1a and the optical transmission line 4. When this RF power is not 0, it can be estimated that the PDL is not 0, as in FIG. The PDL estimation unit 193a can monitor the PDL magnitude of the entire system by converting the magnitude of the RF power into the amplitude of the dither signal and estimating Rpdl based on the converted value.

The polarization multiplex optical signal transmission system 100 of the second embodiment configured in this way includes a relay node 3 for synchronously detecting two dither signals having different phases with one RF power monitor 196. PDL monitoring can be performed without performing the process of "comparing the amplitudes of dither signals of two different frequencies". As described above, the polarized multiplex optical signal transmission system 100 of the second embodiment can estimate the PDL with a simpler configuration with high accuracy.

However, on the other hand, in the polarized light multiplex optical signal transmission system 100 of the present embodiment, it is not possible to determine which of the X-polarized light and the Y-polarized light has the higher light intensity. Therefore, in this respect, the functions are more limited than those of the prior art (see, for example, Japanese Patent No. 5827379 and Japanese Patent No. 5635923).

[Third Embodiment]
FIG. 6 is a diagram showing a configuration example of the polarized multiplex optical signal transmitter 1b according to the third embodiment. The polarization multiplex optical signal transmitter 1b includes a first light modulator 13b-1 instead of the first optical modulator 13-1, and a second optical modulator 13b instead of the second optical modulator 13-2. The point provided with -2, instead of monitoring the light intensity of the tapped polarization multiplex signal with one monitor PD, the monitor PD built into each of the first and second light modulators is used. It differs from the polarization multiplex optical signal transmitter 1 of the first embodiment in that the light intensity of each of the first and second optical signals is individually measured and that the feed forward circuit 19b is provided instead of the feedback circuit 19. Other configurations are the same as those of the polarized multiplex optical signal transmitter 1 of the first embodiment. Therefore, the same functional parts as those in the first embodiment are designated by the same reference numerals as those in FIG. 1, and the description thereof will be omitted.

The first light modulator 13b-1 includes a first built-in monitor PD131-1, and the second light modulator 13b-2 includes a second built-in monitor PD131-2. Each built-in monitor PD131 detects a dither signal from each optical signal by measuring the intensity of each of the X-polarized light and Y-polarized light signals instead of the monitor PD191 in the first embodiment. As described above, in the polarization multiplex optical signal transmitter 1b of the second embodiment, the polarization multiplex optical signal transmitter 1 of the first embodiment detects the dither signal by one monitor PD191. , It differs from the polarized light multiplex optical signal transmitter 1 of the first embodiment in that a dither signal is detected for each polarized light.

The feedforward circuit 19b differs from the feedback circuit 19 in the first embodiment in that it does not include the monitor PD191 and further includes an offset correction unit 197 and an addition circuit 198. The offset correction unit 197 outputs a predetermined offset value with respect to the synchronous detection signal of the synchronous detection circuit 192. The offset value is added to the synchronous detection signal of the synchronous detection circuit 192 by the addition circuit 198. A synchronous detection signal to which an offset value is added is output to the PDL estimation unit 193. The adder circuit 198 adds the optical signals output from the first built-in monitor PD131-1 and the second built-in monitor PD131-2 and outputs them to the synchronous detection circuit 192.

As described above, since the X-polarized light and the Y-polarized light are orthogonal to each other, the light intensity of the polarized multiple signal light is the linear sum of the light intensity of the X-polarized light signal and the light intensity of the Y-polarized light. It becomes. Therefore, by adding the outputs of the first built-in monitor PD131-1 and the second built-in monitor PD131-2 by the addition circuit 198 and inputting them to the synchronous detection circuit, the PDL estimation unit 193 similar to the first embodiment can be used. The PDL can be estimated.

However, the PDL that can be estimated by the PDL estimation unit 193 of the present embodiment is a PDL that is caused by some components of the polarization multiplex optical signal transmitter 1b. For example, the PDL generated in the polarization multiplexing unit 18 cannot be estimated. Further, since the polarization multiplex optical signal transmitter 1b of the present embodiment includes two monitor PD131s, the first built-in monitor PD131-1 and the second built-in monitor PD131-2, when the sensitivities of these two monitors PD131 are different. Or, when the PDL generated in the polarization multiplexing unit 18 is so large that it cannot be ignored, the PDL of the entire polarization multiplexing optical signal transmitter 1b is 0 even though the synchronous detection signal of the synchronous detection circuit 192 is 0. An undesired situation can occur in which it is not [dB]. Specifically, the graph shown in FIG. 4C is a synchronous detection signal shifted downward or upward.

In order to solve this problem, the polarization multiplex optical signal transmitter 1b of the present embodiment includes an offset correction unit 197 that corrects by adding a predetermined offset value to the output of the synchronous detection circuit 192. This correction value may be determined based on the difference in sensitivity between the two built-in monitors PD131 and the optical characteristics of the polarization multiplexing unit 18.

Further, by causing the first dither signal output unit 14-1 and the second dither signal output unit 14-2 to output the dither signal with slightly different amplitudes, the above problem can be obtained without using the offset correction unit 197. It is also possible to eliminate. For example, if the amplitude of the dither signal output from the first dither signal output unit 14-1 is slightly larger than the amplitude of the dither signal output from the second dither signal output unit 14-2, FIG. 4C shows. Since the graph shown shifts to the left, it is possible to compensate for the error caused by the difference in sensitivity between the two monitors PD131.

The polarized light multiplex optical signal transmitter 1b of the third embodiment configured in this way detects the dither signal for each of the X-polarized light and the Y-polarized light, thereby "amplitude of the dither signal of two different frequencies". PDL monitoring and compensation thereof can be performed without performing the "compare" process. As described above, the polarization multiplex optical signal transmitter 1b of the third embodiment can estimate the PDL with a simpler configuration with high accuracy.

Since the polarization multiplex optical signal transmitter 1b of the third embodiment uses two different monitors PD131, the manufacturing error of each monitor PD131 affects the estimation result of the PDL. Therefore, it is expected that the error of the PDL estimation result will be larger than that of the polarized multiplex optical signal transmitter of other embodiments. However, since most of the conventional general optical modulators have a built-in monitor PD, by utilizing this built-in monitor PD, the PDL can be estimated accurately while making the polarization multiplex optical signal transmitter smaller. It becomes possible.

[Fourth Embodiment]
In the above-described embodiment, the method of compensating the PDL of the optical signal by controlling the attenuation rate of the attenuator 17 so that Rpdl = 1 = 0 [dB] has been described, but the attenuation rate of the attenuator 17 is controlled. There are several variations on the method. In the fourth embodiment, an example of a method of controlling the attenuation rate of the attenuator 17 will be described.

When the light intensity of the X-polarized light exceeds the light intensity of the Y-polarized light, the PDL control unit 194 may increase the light intensity of the Y-polarized light by reducing the attenuation rate of the second attenuator. However, since the attenuation rate of any attenuator cannot be smaller than 0 [dB], there is a limit to compensating for PDL by this method.

On the contrary, when the light intensity of X-polarized light exceeds the light intensity of Y-polarized light, the PDL control unit 194 may reduce the light intensity of X-polarized light by increasing the attenuation rate of the first attenuator. .. However, always using this technique can lead to deterioration of the quality of the optical signal. As an example, if the magnitude relationship of the light intensity on the X-polarized side and the Y-polarized side continues to change as a result of repeated rises and falls in the environmental temperature, the loss of each attenuator 17 alternates. Since it continues to increase but does not decrease, the light intensity of both polarizations eventually becomes extremely small, and the quality of the optical signal deteriorates. In order to solve such a problem, it is desirable that the PDL control unit 194 performs the following control.

First, the PDL control unit 194 sets the initial values of the attenuation rates of the first attenuator 17-1 and the second attenuator 17-2 at the time of starting the polarization multiplex optical signal transmitter 1 (or 1a or 1b, the same applies hereinafter). Set to VOAinit. VOAinit is a positive number less than 1 in linear notation (notation not logarithmic notation such as decibel notation) like VOA shown in FIG. The minimum value of the loss of each attenuator 17 is VOAmin (a positive number less than 1 in linear notation), and the maximum value of PDL caused by temperature change and wavelength change is Rpdl_max (linear notation similar to Rpdl shown in FIG. 3). However, whether to use X polarization or Y polarization as a reference is selected so that Rpdl_max is 1 or more).
(VOAmin / VOAinit) ² >Rpdl_max> 1
VOAinit is set so as to satisfy the above relationship.

Specific examples are shown below in decibel notation.
2log (VOAmin / VOAinit)> log (Rpdl_max)> 0
10log (VOAmin / VOAinit)
= 10log (VOAmin) -10log (VOAinit)
> 10log (Rpdl_max) / 2
However, it is assumed here that Rpdl_max, which is the maximum value of PDL, is 4 dB in decibel notation, and VOAmin, which is the minimum value of loss of each attenuator 17, is -0.5 dB in decibel notation. From the above formula
-0.5-10log (VOAinit)> 4/2
Therefore, the decibel notation (negative number) of the initial setting value VOAinit of the light loss is set to a loss larger than −2.5 dB.

The PDL control unit 194 compensates for the PDL by complementaryly changing the attenuation rate of each attenuator 17 initially set in this way. Specifically, when the PDL control unit 194 changes the attenuation rate of the first attenuator 17-1 to α times (changes only -β [dB] with 10 × logα = β), the attenuation rate of the second attenuator is changed. Change to 1 / α times (that is, change only + β [dB]). As a result, the light intensity ratio is changed to α ² (changes by 2β [dB]), and this amount of change is offset by the amount of change in PDL.

With such a control method, the attenuation rates of the first attenuator 17-1 and the second attenuator repeatedly increase and decrease in the long term, so that the intensity of the optical signal increases beyond the limit of the adjustment range. That, or the ever-decreasing situation, can be avoided.

More preferably, when the PDL compensation is completed, the PDL control unit 194 outputs the output of the monitor PD191 or the adder circuit 198 that receives the polarization multiplex signal light every period sufficiently longer than the period of the dither signal. If the transition of this average value shows a decreasing tendency, the attenuation rate of each attenuator 17 is lowered together, and conversely, the transition of this average value seems to take an increasing tendency. If there is, it may be configured to increase the attenuation rate of each attenuator 17 together. By such a control method, it is possible to further stabilize the light intensity of the polarized multiplex signal light after the PDL is compensated.

[Modification example]
In each of the above embodiments, the outputs of the first modulator drive unit 16-1 and the second modulator drive unit 16-2 are the first dither signal output unit 14-1 and the second dither signal output unit 14-2, respectively. Although the dither signal of the frequency f generated by is superposed, there are some variations in the specific superimposing method of the dither signal. Which superimposition method should be selected depends on the signal format of the optical signal.

For example, the first dither signal output unit 14-1 and the second dither signal output unit 14-2 adjust the output amplitudes of the first modulator drive unit 16-1 and the second modulator drive unit 16-2 by minute amounts. It may be configured to output a dither signal that changes periodically. In this case, the polarization multiplex optical signal transmitter 1 includes a first RF amplifier and a second RF amplifier that amplify the first drive signal and the second drive signal, and includes a first dither signal output unit 14-1 and a second dither. The signal output unit 14-2 is configured to fluctuate the gains of the first RF amplifier and the second RF amplifier with the frequency f.

This superimposition method is suitable when the modulation signal is a CS-RZ (Carrier-Suppressed Return to Zero) signal or a QAM (Quadrature Amplitude Modulation) signal. However, in this superimposition method, even if the amount of change in the output amplitude of the first modulator drive unit 16-1 and the second modulator drive unit 16-2 is kept the same, each optical modulator It should be noted that the modulation depth may not be the same for the X-polarized light and the Y-polarized light depending on the manufacturing error of 13. In such a case, the amplitude of the output of the first modulator drive unit 16-1 and the second modulator drive unit 16-2 so that the modulation depth is the same for the X polarization and the Y polarization. It is advisable to adjust so that the amount of (minor) change in is slightly different.

Further, the first dither signal output unit 14-1 and the second dither signal output unit 14-2 set the center value of the output amplitudes of the first modulator drive unit 16-1 and the second modulator drive unit 16-2. It may be configured to output a dither signal that changes periodically in small increments. In this case, the first adder 15-1 adds the first drive signal and the output of the first dither signal output unit 14-1 and outputs them to the first optical modulator 13-1, and the second adder. 15-2 adds the second drive signal and the output of the second dither signal output unit 14-2 and outputs the second drive signal to the second optical modulator 13-2. This superimposition method is suitable when the modulation signal is an NRZ (Non Return to Zero) signal or a PAM (Pulse Amplitude Modulation) signal.

In addition, many conventional general optical modulators have a bias terminal for controlling an operating point. In this case, the first dither signal output unit 14-1 and the second dither signal output unit 14-2 periodically change the direct current (DC) voltage applied to the bias terminal little by little. It may be configured in. This superposition method is applicable to almost all signal formats. However, the DC voltage value that gives the optimum operating point drifts with time, and in general, this drift is also subject to monitoring or control. Therefore, it is preferable that the control of the PDL and the control of the operating point are adjusted by time sharing so that they do not interfere with each other.

Further, the control of the light intensity of the X-polarized light and the Y-polarized light signals may be realized without using each attenuator 17. For example, control of the light intensity of the X-polarized light and Y-polarized light signals may be realized by changing the branch ratio of the light signal branched by the polarization holding coupler 12. Further, for example, control of the light intensity of the X-polarized light and Y-polarized light signals is realized by changing the output amplitudes of the first modulator drive unit 16-1 and the second modulator drive unit 16-2. May be done. These controls are terminated when the PDL is eliminated, and the amplitude of the output of each modulator drive unit 16 is not periodically changed by a minute amount by the dither signal as described above. This control method is suitable when the modulated signal is a CS-RZ signal or a QAM signal.

A part or all of the polarization multiplex optical signal transmitters 1, 1a, 1b and the polarization multiplex optical signal transmission system 100 in the above-described embodiment may be realized by a computer. In that case, the program for realizing this function may be recorded on a computer-readable recording medium, and the program recorded on the recording medium may be read by the computer system and executed. The term "computer system" as used herein includes hardware such as an OS and peripheral devices. Further, the "computer-readable recording medium" refers to a portable medium such as a flexible disk, a magneto-optical disk, a ROM, or a CD-ROM, or a storage device such as a hard disk built in a computer system. Further, a "computer-readable recording medium" is a communication line for transmitting a program via a network such as the Internet or a communication line such as a telephone line, and dynamically holds the program for a short period of time. It may also include a program that holds a program for a certain period of time, such as a volatile memory inside a computer system that serves as a server or a client in that case. Further, the above program may be for realizing a part of the above-mentioned functions, and may be further realized for realizing the above-mentioned functions in combination with a program already recorded in the computer system. It may be realized by using a programmable logic device such as FPGA (Field Programmable Gate Array).

Although the embodiments of the present invention have been described in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs and the like within a range that does not deviate from the gist of the present invention.

The present invention is applicable to a system for transmitting an optical signal using a polarization multiplex optical transmission method.

1,1a, 1b ... Polarization multiplex optical signal transmitter, 11 ... Light source, 12 ... Polarization retention coupler, 13 ... Optical modulator, 13-1, 13b-1 ... First optical modulator, 13-2, 13b -2, 2nd optical modulator, 14-1 ... 1st dither signal output section, 14-2 ... 1st dither signal output section, 15-1 ... 1st adder, 15-2 ... 2nd adder, 16 -1 ... 1st modulator drive unit, 16-2 ... 1st modulator drive unit, 17-1 ... 1st attenuator, 17-2 ... 2nd attenuator, 18 ... Polarization multiplex unit, 19, 19b ... Feedback circuit , 191 ... Monitor PD (Photo Detector), 192 ... Synchronous detection circuit, 193, 193a ... PDL (Polarization Dependent Loss) estimation unit, 194 ... PDL control unit, 195 ... BPF (Band Path Filter), 196 ... RF (Radio Frequency) ) Power monitor, 197 ... Offset correction unit, 198 ... Addition circuit, 2 ... Optical waveguide, 21 ... Tap, 3 ... Relay node, 4 ... Optical transmission path, 100 ... Polarized multiplex optical signal transmission system, 900 ... Polarized multiplex Optical signal transmission system, 910 ... Polarization multiplex optical signal transmitter, 911 ... Light source, 912 ... Polarization retention coupler, 913-1 ... First optical modulator, 913-2 ... Second optical modulator, 914-1 ... 1st modulator drive unit, 914-2 ... 2nd modulator drive unit, 915-1 ... 1st dither signal output unit, 915-2 ... 2nd dither signal output unit, 916 ... polarization multiplex unit, 920 ... optical Transmission line, 930 ... Relay node, 931 ... Polarization level adjustment unit, 932 ... Tap, 933 ... Diza signal detection unit, 934 ... PDL estimation unit

Claims (8)

A polarized multiplex optical signal transmission system that transmits polarized multiplex signal light generated by multiplexing a first optical signal and a second optical signal having the same wavelength so that their polarizations are orthogonal to each other. And
The first optical modulator that generates the first optical signal and
A second optical modulator that generates the second optical signal, and
A first optical modulator drive unit that outputs a first drive signal that is an electric signal for driving the first optical modulator, and a first optical modulator drive unit.
A second light modulator drive unit that outputs a second drive signal, which is an electric signal for driving the second light modulator, and a second light modulator drive unit.
A first dither signal superimposing unit that superimposes the first dither signal on the first optical signal,
A second dither signal superimposing unit that superimposes the second dither signal on the second optical signal,
A dither signal detection unit that detects the sum of the first dither signal superimposed on the first optical signal and the second dither signal superimposed on the second optical signal.
PDL estimation unit that estimates PDL (Polarization Dependent Loss) caused by a part or all of the polarized light multiplex optical signal transmission system based on the sum of the detected first dither signal and the second dither signal. When,
With
The first dither signal及beauty said second dither signal has the same repetition frequency f and amplitude,
The first dither signal superimposing unit and the second dither signal superimposing unit are of the other optical signal when the intensity of one of the first optical signal and the second optical signal reaches the maximum value. The phase is adjusted so that the intensity becomes the minimum value, and the first and second dither signals are superimposed .
When the times when the intensities of the first optical signal and the second optical signal are maximized are T1 and T2, respectively, the PDL estimation unit outputs the dither signal detection unit at time T1 and the dither signal detection unit at time T2. By comparing with the output of the dither signal detection unit, it is determined which of the first optical signal and the second optical signal receives the larger optical loss.
A polarized multiplex optical signal transmission system characterized by this . The first dither signal superimposing unit and the second dither signal superimposing unit are the voltage value of the DC voltage applied to the bias terminals of the first optical modulator and the second optical modulator, or the first one. By periodically changing the amplitude of the first and second drive signals or the center value of the amplitudes of the first and second drive signals by a minute amount, the first optical signal and the second optical signal can be converted into the first optical signal. Superimpose the first and second dither signals,
The polarized multiplex optical signal transmission system according to claim 1. A photoelectric conversion circuit is further provided in which a part of the polarization multiplex signal light is branched and input, and a part of the input polarization multiplex signal light is converted into an electric signal and output.
The dither signal detection unit detects the frequency component of the frequency f superimposed on the electric signal output by the photoelectric conversion circuit, and outputs a detection signal indicating the detection result to the PDL estimation unit.
The PDL estimation unit acquires the sum of the first dither signal and the second dither signal based on the detection signal output from the dither signal detection unit.
The polarized multiplex optical signal transmission system according to claim 1. The PDL estimation unit includes an addition signal indicating the sum of the detected first dither signal and the second dither signal, an RF (Radio Frequency) power signal obtained by the squared average square root of the addition signal, and the RF (Radio Frequency) power signal. The PDL is estimated based on one of the synchronous detection signal obtained by synchronously detecting the addition signal or the RF power signal at the frequency of the dither signal.
The polarized multiplex optical signal transmission system according to claim 1. When the value of the synchronous detection signal is equal to a predetermined value, the PDL estimation unit determines that the value of the PDL is 0 dB.
The polarized multiplex optical signal transmission system according to claim 4 . A first attenuator that attenuates the intensity of the first optical signal with the first attenuation factor,
A second attenuator that attenuates the intensity of the second optical signal with a second attenuation factor,
A PDL control unit that controls the intensity ratio of the first optical signal and the second optical signal by changing the first attenuation rate and the second attenuation rate.
With more
The PDL control unit changes the first attenuation factor or the second attenuation factor so that the PDL value estimated by the PDL estimation unit approaches 0 dB.
The polarized multiplex optical signal transmission system according to claim 1. In the PDL control unit, after the PDL estimated by the PDL estimation unit has converged to 0 dB within a predetermined error range, the first optical signal and the second optical signal are multiplexed. The first attenuation factor and the second attenuation factor are changed so that the total light intensity of the wave multiplexing signal light is within a predetermined range.
The polarized multiplex optical signal transmission system according to claim 6. A polarized multiplex optical signal transmission system that transmits polarized multiplex signal light generated by multiplexing a first optical signal and a second optical signal having the same wavelength so that their polarizations are orthogonal to each other. And
The first optical modulator that generates the first optical signal and
A second optical modulator that generates the second optical signal, and
A first optical modulator drive unit that outputs a first drive signal that is an electric signal for driving the first optical modulator, and a first optical modulator drive unit.
A second light modulator drive unit that outputs a second drive signal, which is an electric signal for driving the second light modulator, and a second light modulator drive unit.
A first dither signal superimposing unit that superimposes the first dither signal on the first optical signal,
A second dither signal superimposing unit that superimposes the second dither signal on the second optical signal,
A dither signal detection unit that detects the sum of the first dither signal superimposed on the first optical signal and the second dither signal superimposed on the second optical signal.
PDL estimation unit that estimates PDL (Polarization Dependent Loss) caused by a part or all of the polarized light multiplex optical signal transmission system based on the sum of the detected first dither signal and the second dither signal. When,
A first photoelectric conversion in which a part of the first optical signal output from the first optical modulator is input, and a part of the input first optical signal is converted into an electric signal and output. Circuit and
A second photoelectric conversion in which a part of the second optical signal output from the second optical modulator is input, and a part of the input second optical signal is converted into an electric signal and output. Circuit and
An adder circuit that adds and outputs an electric signal output from the first photoelectric conversion circuit and an electric signal output from the second photoelectric conversion circuit.
With
The first dither signal superimposition unit and the second dither signal superimposition unit have the same repetition frequency f and amplitude, and the intensity of one of the first optical signal and the second optical signal is maximum. When taking a value, the phase is adjusted so that the intensity of the other optical signal becomes the minimum value, and the dither signal is superimposed.
The dither signal detection unit detects the frequency component of the frequency f superimposed on the electric signal output from the addition circuit, and outputs a detection signal indicating the detection result to the PDL estimation unit.
The PDL estimation unit acquires the sum of the first dither signal and the second dither signal based on the detection signal output from the dither signal detection unit.
Polarized multiplex optical signal transmission system.

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