Digital coherent receiver, optical reception system, and optical reception method

A digital coherent receiver, includes: an acquisition circuit configured to acquire a plurality of digital electrical signals obtained by sampling a plurality of analog electrical signals by using a sampling signal and digitally converting the plurality of analog electrical signals, the plurality of analog electrical signals being obtained by subjecting a plurality of optical signals to photoelectric conversion; a phase deviation detector configured to output a detection value corresponding to a phase deviation between the sampling signal and the optical signals by using the digital electrical signals; a determination circuit configured to determine whether or not a variation amount of the detection value is equal to or less than a first variation amount; and a compensation circuit configured to compensate wavelength dispersion of the digital electrical signals based on the detection value when the variation amount is equal to or less than the first variation amount.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2013-097152, filed on May 2, 2013, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to a digital coherent receivers, an optical reception system, and an optical reception method.

BACKGROUND

An increase in internet traffic has led to a demand for larger capacity of trunk line optical communication systems, and optical receivers/transmitters capable of transmitting signals at a rate of over 100 Gbit/s per wavelength are being researched and developed. As a bit rate per wavelength increases, a deterioration in signal quality occurs due to a degradation in the optical signal noise ratio (OSNR) tolerance, wavelength dispersion in a transmission path, or a waveform distortion caused by polarization mode dispersion or nonlinear effects.

Related arts are disclosed in Japanese Laid-open Patent Publication No. 2011-015013.

SUMMARY

According to one aspect of the embodiments, a digital coherent receiver, includes: an acquisition circuit configured to acquire a plurality of digital electrical signals obtained by sampling a plurality of analog electrical signals by using a sampling signal and digitally converting the plurality of analog electrical signals, the plurality of analog electrical signals being obtained by subjecting a plurality of optical signals to photoelectric conversion; a phase deviation detector configured to output a detection value corresponding to a phase deviation between the sampling signal and the optical signals by using the digital electrical signals; a determination circuit configured to determine whether or not a variation amount of the detection value is equal to or less than a first variation amount; and a compensation circuit configured to compensate wavelength dispersion of the digital electrical signals based on the detection value when the variation amount is equal to or less than the first variation amount.

DESCRIPTION OF EMBODIMENTS

For example, a digital coherent reception system having an OSNR tolerance and a tolerance for a waveform distortion in a transmission path may be employed. In the digital coherent reception system, a compensation amount of a dispersion compensation circuit is controlled based on a sensitivity for detecting a phase deviation between the frequency of a reception optical signal and a sampling signal in order to appropriately carry out dispersion compensation.

However, the sensitivity for detecting the phase deviation between the frequency of the reception optical signal and the sampling signal is also influenced by the optical signal intensity, and thus the accuracy of the dispersion compensation may be degraded depending on the state of the optical signal intensity.

FIG. 1Aillustrates an example of an optical reception device. An optical reception device100illustrated inFIG. 1Aincludes a local oscillation light source110, a 90-degree hybrid circuit120, photoelectric conversion circuits131to134, a sampling clock source140, analog/digital converters (ADCs)151to154, and a digital signal processing circuit160.

The optical reception device100may be an optical reception device of a coherent method, which employs the 90-degree hybrid circuit120. The local oscillation light source110generates local oscillation light and outputs the generated local oscillation light to the 90-degree hybrid circuit120. The 90-degree hybrid circuit120receives input of a reception optical signal and the local oscillation light output from the local oscillation light source110. The 90-degree hybrid circuit120extracts a real part signal and an imaginary part signal of two orthogonally polarized waves of the input reception optical signal by using the input local oscillation light.

For example, the 90-degree hybrid circuit120includes two polarizing beam splitters and two 90-degree hybrids. The 90-degree hybrid circuit120splits the reception optical signal and the local oscillation light into optical signals of two polarization directions (H axis and V axis) through the polarizing beam splitters and extracts, by using the 90-degree hybrids, a real part component (I component) and an imaginary part component (Q component) from the optical signals by using the local oscillation light of each of the polarization directions.

The 90-degree hybrid circuit120outputs, to the photoelectric conversion circuit131, an optical signal that corresponds to the amplitude and the phase of the I component of signal light polarized along the H axis (horizontal polarization). The 90-degree hybrid circuit120outputs, to the photoelectric conversion circuit132, an optical signal that corresponds to the amplitude and the phase of the Q component of the signal light polarized along the H axis.

The 90-degree hybrid circuit120outputs, to the photoelectric conversion circuit133, an optical signal that corresponds to the amplitude and the phase of the I component of signal light polarized along the V axis (vertical polarization). The 90-degree hybrid circuit120outputs, to the photoelectric conversion circuit134, an optical signal that corresponds to the amplitude and the phase of the Q component of the signal light polarized along the V axis.

The photoelectric conversion circuit131subjects the light output from the 90-degree hybrid circuit120to photoelectric conversion and outputs, to the ADC151, an electrical signal corresponding to the intensity of the received light. The photoelectric conversion circuit132subjects the light output from the 90-degree hybrid circuit120to photoelectric conversion and outputs, to the ADC152, an electrical signal corresponding to the intensity of the received light. The photoelectric conversion circuit133subjects the light output from the 90-degree hybrid circuit120to photoelectric conversion and outputs, to the ADC153, an electrical signal corresponding to the intensity of the received light. The photoelectric conversion circuit134subjects the light output from the 90-degree hybrid circuit120to photoelectric conversion and outputs, to the ADC154, an electrical signal corresponding to the intensity of the received light. Each of the photoelectric conversion circuits131to134may, for example, be a photodiode (PD).

The sampling clock source140generates a clock signal that serves to determine a sampling timing and outputs the clock signal to each of the ADCs151to154.

The ADC151digitally samples a signal output from the photoelectric conversion circuit131, in synchronization with the clock of the clock signal output from the sampling clock source140, and quantizes the sampled signal to carry out digital conversion. The ADC152digitally samples a signal output from the photoelectric conversion circuit132, in synchronization with the clock of the clock signal output from the sampling clock source140, and quantizes the sampled signal to carry out digital conversion.

The ADC153digitally samples a signal output from the photoelectric conversion circuit133, in synchronization with the clock of the clock signal outputted from the sampling clock source140, and quantizes the sampled signal to carry out digital conversion. The ADC154digitally samples a signal output from the photoelectric conversion circuit134, in synchronization with the clock of the clock signal output from the sampling clock source140, and quantizes the sampled signal to carry out digital conversion. Each of the ADCs151to154outputs the digitally converted signal to the digital signal processing circuit160.

The digital signal processing circuit160includes a wavelength dispersion compensation circuit161, a sampling phase deviation detection circuit162, a carrier wave frequency/phase synchronization circuit163, an adaptive equalization type waveform distortion compensation circuit164, an identification/demodulation circuit165, a sensitivity monitor circuit166, a wavelength dispersion compensation amount setting unit167, a sensitivity time variation memory168, and a sensitivity time variation monitor circuit169. The digital signal processing circuit160may correspond to a digital coherent receiver. The digital signal processing circuit160may be a digital circuit such as a digital signal processor (DSP). The digital signal processing circuit160may include a single DSP or may include a plurality of DSPs of different kinds.

The wavelength dispersion compensation circuit161compensates wavelength dispersion of signals output from the ADCs151to154. For example, the wavelength dispersion compensation circuit161subjects the signals output from the ADCs151to154to digital signal processing to compensate the wavelength dispersion of the reception optical signal. The wavelength dispersion compensation circuit161carries out wavelength dispersion compensation in accordance with a compensation amount set by the wavelength dispersion compensation amount setting unit167. The wavelength dispersion compensation circuit161outputs the signals of which the wavelength dispersion has been compensated to each of the sampling phase deviation detection circuit162and the sensitivity monitor circuit166.

The sampling phase deviation detection circuit162detects a deviation in the phase of each of the signals output from the wavelength dispersion compensation circuit161. For example, the sampling phase deviation detection circuit162outputs a detection value that corresponds to a phase deviation between the sampling signal used at the time of AD conversion and the analog electrical signals to be input to the ADCs151to154, or for example, the modulation frequency of the reception optical signal. The sampling phase deviation detection circuit162controls the sampling clock source140by using the detection value obtained through the phase deviation detection and controls the phase or the frequency of the sampling signal. Thus, the sampling signal and the modulation frequency of the reception optical signal synchronize with each other.

The sampling phase deviation detection circuit162controls a phase synchronization loop of the sampling signal to a closed loop or to an open loop. For example, the sampling phase deviation detection circuit162provides feedback on the sampling phase deviation to the sampling clock source140to set the phase synchronization loop of the sampling signal to the open loop and carries out a wavelength dispersion compensation operation.

The sampling phase deviation detection circuit162stops providing feedback on the sampling phase deviation to the sampling clock source140to set the phase synchronization loop of the sampling signal to the closed loop. In this case, the wavelength dispersion compensation operation may not be carried out. When the phase synchronization loop of the sampling signal is set to the open loop, the sampling phase deviation detection circuit162controls the sampling clock source140by using the detection value obtained through the phase deviation detection to control the phase or the frequency of the sampling signal. This control may be carried out continuously.

The carrier wave frequency/phase synchronization circuit163corrects a phase rotation that is based on a deviation between the frequency of the reception optical signal and the frequency or the phase of the local oscillation light by using the signal output from the sampling phase deviation detection circuit162and synchronizes the frequency of the reception optical signal with the frequency of the local oscillation light. The carrier wave frequency/phase synchronization circuit163outputs signals of which the frequencies have been synchronized to the adaptive equalization type waveform distortion compensation circuit164.

The adaptive equalization type waveform distortion compensation circuit164subjects each of the signals output from the carrier wave frequency/phase synchronization circuit163to waveform distortion compensation of an adaptive equalization type. For example, the adaptive equalization type waveform distortion compensation circuit164compensates a wavefront distortion component that has been generated in a transmission path and that varies at high speed. The adaptive equalization type waveform distortion compensation circuit164outputs signals of which the wavefront distortion components have been compensated to the identification/demodulation circuit165. The identification/demodulation circuit165demodulates the signal output from the adaptive equalization type waveform distortion compensation circuit164. The identification/demodulation circuit165outputs the demodulated data to the outside.

The sensitivity monitor circuit166monitors the sensitivity for sampling phase deviation detection. For example, the sensitivity monitor circuit166detects the sensitivity, to the phase deviation, of the detection value output by the sampling phase deviation detection circuit162. The sensitivity monitor circuit166monitors the sensitivity for sampling phase deviation detection when the phase synchronization loop of the sampling signal is set to the open loop by the sampling phase deviation detection circuit162. The sensitivity monitor circuit166outputs the monitored sensitivity (monitor value) for the sampling phase deviation detection to the wavelength dispersion compensation amount setting unit167and the sensitivity time variation memory168.

The sensitivity time variation memory168stores the sensitivity for the sampling phase deviation detection output from the sensitivity monitor circuit166. The sensitivity time variation monitor circuit169may refer to a value stored in the sensitivity time variation memory168to determine that a variation amount of the sensitivity has reached or fallen below a certain variation amount in a case in which a disparity among a plurality of sensitivities detected by the sensitivity monitor circuit166within a period has reached or fallen below a threshold value.

For example, the sensitivity time variation monitor circuit169may determine that a variation amount of the sensitivity has reached or fallen below a certain variation amount in a case in which a disparity among a plurality of sensitivities detected by the sensitivity monitor circuit166within a certain period has reached or fallen below a threshold value. The disparity may correspond to a difference or a ratio. For example, the sensitivity time variation monitor circuit169may determine that the variation amount of the sensitivity has reached the certain variation amount in a case in which a state in which a disparity among the sensitivities detected successively by the sensitivity monitor circuit166is equal to or less than the threshold value continues for a certain period or longer.

The sensitivity time variation monitor circuit169may determine whether or not the variation amount of the detection value output from the sampling phase deviation detection circuit162has reached or fallen below a certain variation amount. In this case, the sensitivity time variation monitor circuit169may determine that the variation amount of the detection value has reached or fallen below the certain variation amount in a case in which a disparity among a plurality of detection values output by the sampling phase deviation detection circuit162within a certain period has reached or fallen below a threshold value. The sensitivity time variation monitor circuit169may determine that the variation amount of the detection value has reached or fallen below a predetermined variation amount in a case in which a state in which a disparity among a plurality of detection values output by the sampling phase deviation detection circuit162within a certain period is equal to or less than a threshold value continues for a certain period or longer.

The wavelength dispersion compensation amount setting unit167sets the compensation amount of the wavelength dispersion compensation circuit161. The wavelength dispersion compensation amount setting unit167compensates wavelength dispersion of a digital electrical signal based on the detection value output by the sampling phase deviation detection circuit162. For example, the wavelength dispersion compensation amount setting unit167may set the compensation amount of the wavelength dispersion compensation circuit161such that the wavelength dispersion compensation circuit161does not compensate the wavelength dispersion until the sensitivity time variation monitor circuit169determines that the variation amount of the sensitivity has reached or fallen below the certain variation amount. For example, the wavelength dispersion compensation amount setting unit167sets, in the wavelength dispersion compensation circuit161, a compensation amount that does not vary relative to the wavelength.

The wavelength dispersion compensation amount setting unit167may cause the wavelength dispersion compensation circuit161to compensate the wavelength dispersion upon the sensitivity time variation monitor circuit169determining that the variation amount of the sensitivity has reached or fallen below the certain variation amount. For example, the wavelength dispersion compensation amount setting unit167sets the compensation amount of the wavelength dispersion compensation circuit161based on the sensitivity for the sampling phase deviation detection output from the sensitivity monitor circuit166to cause the wavelength dispersion compensation circuit161to compensate the wavelength dispersion. The wavelength dispersion compensation amount setting unit167sets the compensation amount such that the sensitivity for the sampling phase deviation detection output from the sensitivity monitor circuit166becomes maximum.

The wavelength dispersion compensation amount setting unit167may cause the wavelength dispersion compensation circuit161to compensate the wavelength dispersion of a digital electrical signal based on the detection value in a case in which it is determined that the variation amount of the detection value output by the sampling phase deviation detection circuit162has reached or fallen below the certain variation amount.

The local oscillation light source110, the 90-degree hybrid circuit120, the photoelectric conversion circuits131to134, the sampling clock source140, and the ADCs151to154acquire a digital electrical signal from a received optical signal. In the acquisition processing, a plurality of analog electrical signals that have been obtained by subjecting a plurality of optical signals indicating the intensity information and the phase information of the received optical signal to photoelectric conversion are sampled by using a sampling signal, and thus a plurality of digitally converted digital electrical signals are acquired.

The 90-degree hybrid circuit120combines the received optical signal and the local oscillation light to obtain the plurality of optical signals indicating the intensity information and the phase information of the optical signal. The photoelectric conversion circuits131to134subject the plurality of optical signals, which have been obtained by the 90-degree hybrid circuit120, to photoelectric conversion. The ADCs151to154sample the plurality of analog electrical signals, which have been converted by the photoelectric conversion circuits131to134, by using the sampling signal to carry out the digital conversion.

The sampling phase deviation detection circuit162outputs a detection value corresponding to a phase deviation between the sampling signal and the optical signal by using the digital electrical signals acquired from the ADCs151to154. The sensitivity monitor circuit166detects the sensitivity, to the phase deviation, of the detection value output by the sampling phase deviation detection circuit162. The sensitivity time variation monitor circuit169determines whether or not the variation amount of the sensitivity detected by the sensitivity monitor circuit166has reached or fallen below a certain variation amount.

The wavelength dispersion compensation circuit161and the wavelength dispersion compensation amount setting unit167start the wavelength dispersion compensation of the digital electrical signals. In the compensation processing, the wavelength dispersion compensation of the digital electrical signals, which uses the sensitivity detected by the sensitivity monitor circuit166, is started in a case in which the sensitivity time variation monitor circuit169determines that the variation amount of the sensitivity has reached or fallen below the certain variation amount.

FIG. 1Billustrates an example of an optical transmission system. An optical transmission system101illustrated inFIG. 1Bincludes the optical reception device100, an optical transmission device102, an optical transmission path103, and amplifiers104and105. The optical transmission device102transmits an optical signal to the optical reception device100through the optical transmission path103. The amplifiers104and105are provided in the optical transmission path103to amplify the optical signal. For example, the amplifiers104and105may control the optical signal to have a gentle rising slope so that an optical surge does not occur at the rise of the optical signal. Thus, it may take time until the optical signal intensity stabilizes in the optical transmission system101.

FIG. 2illustrates an example of phase detection through Gardner's method. The sampling phase deviation detection circuit162may include a phase detector that is compatible with the quadrature phase shift keying (QPSK) modulation method used in coherent transmission. For example, such a phase detector may be a phase detector of the Gardner's method (F. M. Gardner, “A BPSK/QPSK timing-error detector for sampled receivers”,IEEE Trans. Commun., vol. COM-34, pp. 423-429, May 1986).

The sampling phase deviation detection circuit162accepts input of an I component and a Q component of the H axis or the V axis which have been two-times oversampled and calculates, for each of the I component and the Q component, a product of an intra-signal difference by one symbol and a signal with a phase offset by a ½ symbol. The sampling phase deviation detection circuit162carries out processing of adding the calculated products at a symbol rate (=½ downsampling) to detect a phase signal in which the signal with a phase offset by a ½ symbol is a zero cross point.

For example, the sampling phase deviation detection circuit162may determine that the timing is ahead in a case in which {y(k)−y(k−1)×y(k−0.5)}<0. The sampling phase deviation detection circuit162may determine that the timing is behind in a case in which {y(k)−y(k−1)×y(k−0.5)}>0. The sampling phase deviation detection circuit162may carry out control so that the value of {y(k)−y(k−1)×y(k−0.5)} approaches 0.

FIG. 3illustrates an example of a sampling phase deviation detection circuit of Gardner's method. The sampling phase deviation detection circuit162illustrated inFIG. 3may be a phase detector of the Gardner's method. As illustrated inFIG. 3, the sampling phase deviation detection circuit162includes a delay element311, a delay element312, an addition unit313, a multiplication unit314, a delay element321, a delay element322, an addition unit323, a multiplication unit324, and an addition unit330. The sampling phase deviation detection circuit162may receive input of, for example, a two-times oversampled signal.

An I channel component (H_i or V_i) of signal light inputted to the sampling phase deviation detection circuit162is input to the delay element311and the addition unit313. The delay element311delays the input signal by an amount equivalent to a ½ symbol and outputs the delayed signal to the delay element312and the multiplication unit314. The delay element312delays the signal output from the delay element311by an amount equivalent to a ½ symbol and outputs the delayed signal to the addition unit313.

The addition unit313adds the signal output from the delay element312and a signal of a negative value of the signal input to the sampling phase deviation detection circuit162and outputs the result to the multiplication unit314. For example, the addition unit313subtracts the signal input to the sampling phase deviation detection circuit162from the signal output from the delay element312and outputs the result to the multiplication unit314. The signal output from the addition unit313may be a difference between signals offset by one symbol. The multiplication unit314multiplies the signal that has been output from the delay element311and that is offset by a ½ symbol by a difference between signals that are offset by one symbol, which has been output from the addition unit313, and outputs the result to the addition unit330.

A Q channel component (H_q or V_q) of signal light input to the sampling phase deviation detection circuit162is input to the delay element321and the addition unit323. The delay element321delays the input signal by an amount equivalent to a ½ symbol and outputs the delayed signal to the delay element322and the multiplication unit324. The delay element322delays the signal outputted from the delay element321by an amount equivalent to a ½ symbol and outputs the result to the addition unit323.

The addition unit323adds the signal outputted from the delay element322and a signal of a negative value of the signal inputted to the sampling phase deviation detection circuit162and outputs the result to the multiplication unit324. For example, the addition unit323subtracts the signal inputted to the sampling phase deviation detection circuit162from the signal outputted from the delay element322and outputs the result to the multiplication unit324. The signal outputted from the addition unit323may be a difference between signals offset by one symbol. The multiplication unit324multiplies the signal that has been outputted from the delay element321and that is offset by a ½ symbol by a difference between signals that are offset buy one symbol, which has been outputted from the addition unit323, and outputs the result to the addition unit330.

The addition unit330adds the signal output from the multiplication unit314and the signal output from the multiplication unit324and outputs the result to a subsequent stage. The processing of the addition unit330may be carried out at a symbol rate (=½ downsampling). Thus, the signal output from the addition unit330may be a phase signal in which the signal with a phase offset by a ½ symbol is the zero cross point.

FIG. 4illustrates an example of a sensitivity monitor circuit.FIG. 4illustrates a configuration of a one side monitor. As illustrated inFIG. 4, the sensitivity monitor circuit166includes an x phase shift unit411and a phase detector412. The x phase shift unit411shifts the phase of a signal input from the wavelength dispersion compensation circuit161by a shift amount x. For example, the x phase shift unit411generates a signal of which the phase is shifted by the shift amount x through an inter-sample interpolation or the like. The x phase shift unit411outputs the signal of which the phase has been shifted to the phase detector412.

The phase detector412detects the phase of the signal output from the x phase shift unit411. The phase detector412may be a phase detector having sensitivity degradation characteristics. The phase detector412outputs a phase signal indicating the detected phase to the wavelength dispersion compensation amount setting unit167and the sensitivity time variation memory168as a sensitivity monitor value (α).

In a case in which a parallel signal is input to the sensitivity monitor circuit166, for example, a downsampling unit413, which is provided at a stage preceding the x phase shift unit411, may carry out downsampling in accordance with the sensitivity variation speed. The sensitivity monitor circuit166may operate at a speed that allows the sensitivity monitor circuit166to follow, among variations in the state of the optical transmission path, a variation that affects the phase detection sensitivity, or for example, a variation in the state of polarization characteristics. Thus, the configuration in which downsampling is carried out may be employed.

In a case in which a parallel signal is input to the sensitivity monitor circuit166, an averaging unit414(Σ), which is provided at a stage following the phase detector412, may average the phase signals for the signals output from the phase detector412. In a case in which each of the signals of the H axis and the V axis is input to the sensitivity monitor circuit166, a polarization diversity addition may be carried out in the averaging unit414. A low pass filter415, which is provided at an output stage of the sensitivity monitor circuit166, may suppress broadband noise of the sensitivity monitor value.

FIG. 5illustrates an example of a sensitivity monitor circuit.FIG. 5illustrates a configuration of a two side monitor. InFIG. 5, configurations that are substantially the same as or similar to the configurations illustrated inFIG. 4are given identical reference numerals, and descriptions thereof may be omitted or reduced. As illustrated inFIG. 5, the sensitivity monitor circuit166may include, in addition to the configurations illustrated inFIG. 4, a −x phase shift unit511, a phase detector512, and an addition unit513.

The phase detector412outputs a phase signal to the addition unit513. The −x phase shift unit511shifts the phase of an input signal by a shift amount −x (opposite direction of the shift amount x). For example, the −x phase shift unit511generates a signal of which the phase is shifted by the shift amount −x through an inter-sample interpolation or the like. The −x phase shift unit511outputs the signal of which the phase has been shifted to the phase detector512.

The phase detector512detects the phase of the signal output from the −x phase shift unit511. The phase detector512may be a phase detector having sensitivity degradation characteristics, as with the phase detector412. The phase detector512outputs a phase signal indicating the detected phase to the addition unit513. The addition unit513adds the phase signal output from the phase detector412and a signal of a negative value of the phase signal output from the phase detector512. For example, the addition unit513subtracts the phase signal output from the phase detector512from the phase signal output from the phase detector412. The addition unit513outputs a phase signal indicating the result of the subtraction to the wavelength dispersion compensation amount setting unit167and the sensitivity time variation memory168as a sensitivity monitor value (α-β).

FIG. 6illustrates an example of a relationship between a deviation of wavelength dispersion compensation and a sensitivity for sampling phase deviation detection. InFIG. 6, the deviation [a.u.] of the wavelength dispersion compensation represented along the horizontal axis indicates a deviation of the compensation amount of the wavelength dispersion compensation circuit161from the wavelength dispersion amount of the reception optical signal. A state in which the wavelength dispersion compensation deviation is 0 indicates that the wavelength dispersion compensation circuit161has been capable of substantially compensating the wavelength dispersion. A state in which the wavelength dispersion compensation deviation is away from 0 indicates that residual wavelength dispersion that could not have been compensated by the wavelength dispersion compensation circuit161is large.

The sensitivity [a.u.] for the sampling phase deviation detection represented along the vertical axis indicates the sensitivity of the sampling phase deviation detection circuit162. A relationship600indicates a relationship between a deviation of the compensation amount of the wavelength dispersion compensation circuit161from the wavelength dispersion amount of the reception optical signal and the sensitivity of the sampling phase deviation detection circuit162.

As indicated by the relationship600, when the wavelength dispersion compensation deviation is 0, the sensitivity for the sampling phase deviation detection reaches the maximum. As the wavelength dispersion compensation deviation shifts away from 0, the sensitivity for the sampling phase deviation detection decreases. When a distortion in the waveform caused by the wavelength dispersion reaches the minimum, or for example, when the dispersion compensation amount becomes optimal, the effect on the sampling waveform due to the wavelength dispersion reaches the minimum, or for example, the wavelength dispersion compensation deviation becomes 0, and thus the sensitivity for the phase deviation detection reaches the maximum.

FIG. 7illustrates an example of a relationship between a signal light input intensity and a sensitivity for sampling phase deviation detection. InFIG. 7, the signal light input intensity [a.u.] represented along the horizontal axis indicates the intensity of the signal light to be input to the sampling phase deviation detection circuit162. The sensitivity [a.u.] for the sampling phase deviation detection represented along the vertical axis indicates the sensitivity of the sampling phase deviation detection circuit162. A relationship700indicates a relationship between the intensity of the signal light to be input to the sampling phase deviation detection circuit162and the sensitivity of the sampling phase deviation detection circuit162.

As indicated by the relationship700, the sensitivity for the sampling phase deviation detection decreases along with an increase in the signal light input intensity. In this manner, the sensitivity for the sampling phase deviation detection depends not only on the deviation of the wavelength dispersion compensation as indicated inFIG. 6but also on the signal light input intensity. In the relationship700, the sensitivity for the sampling phase deviation detection decreases along with an increase in the signal light input intensity. For example, depending on the configuration of the optical reception device100or of the optical transmission system101, the sensitivity for the sampling phase deviation detection may increase along with an increase in the signal light input intensity.

For example, in a case in which the optical amplifier105illustrated inFIG. 1Bis controlled to have a gentle rising slope so that an optical surge does not occur at a rise of an optical signal, it may take time until the optical signal intensity stabilizes. If dispersion compensation is carried out in an environment in which the optical signal intensity varies, the dispersion compensation amount may not be estimated accurately. Consequently, an erroneous determination may be made, and thus appropriate dispersion compensation may not be carried out.

The time it takes for the signal light input intensity to stabilize may differ depending, for example, on the configuration of the optical reception device100or of the optical transmission system101. If a given time it takes for the signal light input intensity to stabilize reliably is preset, it may not be possible to carry out the dispersion compensation until that given time elapses even in a case in which the signal light input intensity has stabilized. For example, the dispersion compensation may not be carried out promptly.

A method for monitoring the presence of a variation in the optical signal intensity includes a method in which the optical intensity is monitored directly by using a device such as a TAP-PD. In this method, a mean intensity of the incident light is monitored. Thus, in terms of the operation of the transmission device, in a case in which control is carried out so as not to stop the output of the optical amplifier provided in the transmission path even when the output of the signal light from the transmission side is being stopped, spontaneous emission light noise emitted from the optical amplifier continues to be inputted to the reception side. Therefore, it may be difficult to determine from the mean intensity of the incident light as to whether or not a signal component is included in the input light.

For example, in order to improve the accuracy of the dispersion compensation, the dispersion compensation is carried out after the intensity of the reception optical signal stabilizes. By monitoring a temporal variation in the sensitivity for sampling phase deviation detection, the stabilization of the intensity of the reception optical signal may be determined with high accuracy.

FIG. 8Aillustrates an example of a wavelength dispersion compensation operation.FIG. 8Billustrates an example of a wavelength dispersion compensation operation. InFIGS. 8A and 8B, the wavelength dispersion compensation operation is carried out by the digital signal processing circuit. InFIGS. 8A and 8B, for example, in a case in which a difference between a maximum value and a minimum value of a plurality of sensitivities detected by the sensitivity monitor circuit166within a certain period has reached or fallen below a threshold value, it is determined that a variation amount in the sensitivity has reached or fallen below a certain variation amount.

In the digital signal processing circuit160, the sensitivity time variation monitor circuit169sets a count value m to 0, sets an acquisition repetition count n, and sets a threshold value Thmon for determining the presence of a temporal variation (operation S801).

The sampling phase deviation detection circuit162sets a PLL loop of sampling synchronization to an open loop so as not to influence the wavelength dispersion compensation operation (operation S802). The sensitivity time variation monitor circuit169sets the wavelength dispersion compensation amount to an initial value (operation S803).

The sensitivity monitor circuit166monitors the sensitivity for sampling phase deviation detection (operation S804). The digital signal processing circuit160stores a monitor value of the phase deviation detection sensitivity in the sensitivity time variation memory168at a memory address that corresponds to the count value m (operation S805). The sensitivity time variation monitor circuit169increments the count value m (operation S806) and also determines whether or not the count value m is equal to or less than the repetition count n (operation S807).

If the count value m is equal to or less than the repetition count n (operation S807: Yes), the processing returns to operation S804. If the count value m exceeds the repetition count n (operation S807: No), the sensitivity time variation monitor circuit169determines whether or not a min-max difference of the monitor values stored in the sensitivity time variation memory168at each memory address falls within the threshold value Thmon (operation S808). A monitor value at a memory address may be a monitor value at each of the memory addresses 0 to m. The min-max difference of the monitor values may be a difference between a minimum value and a maximum value.

If the min-max difference of the monitor values at each memory address does no fall within the threshold value Thmon (operation S808: No), the sensitivity time variation monitor circuit169resets the count value m to 0 (operation S809), and the processing then returns to operation S804. In a case in which the min-max difference of the monitor values at each memory address does not fall within the threshold value Thmon, the environment may be such that the signal intensity has not stabilized after the optical signal has been input.

If the min-max difference of the monitor values at each memory address falls within the threshold value Thmon (operation S808: Yes), the sensitivity monitor circuit166monitors the sensitivity for sampling phase deviation detection (operation S810). The wavelength dispersion compensation amount setting unit167determines whether or not the sensitivity for the sampling phase deviation detection is at a maximum, for example, in the relationship indicated inFIG. 6(operation S811).

If the sensitivity for the sampling phase deviation detection is not at a maximum (operation S811: No), the wavelength dispersion compensation amount setting unit167sets the compensation amount of the wavelength dispersion compensation circuit161(operation S812), and the processing then returns to operation S810. Operation S812may be repeated until the sensitivity for the sampling phase deviation detection reaches the maximum. Upon the sensitivity for the sampling phase deviation detection reaching the maximum (operation S811: Yes), the sampling phase deviation detection circuit162sets the PLL loop of the sampling synchronization to a closed loop (operation S813), and the wavelength dispersion compensation operation is then terminated. After the processing in operation S813, for example, waveform distortion compensation of the adaptive equalization type or the like may be carried out by the adaptive equalization type waveform distortion compensation circuit164.

In the flowchart described above, in a case in which the monitor value (min-max difference) of the phase deviation detection sensitivity after reaching the repetition count n is equal to or less than the threshold value Thmon, the compensation amount of the wavelength dispersion compensation circuit161may be set. For example, the compensation amount of the wavelength dispersion compensation circuit161may be set upon the monitor value of the phase deviation detection sensitivity stabilizing after a certain time has elapsed since the rise, for example, after the repetition count has reached n. Thus, it may be determined with high accuracy that the environment is such that the optical signal intensity has stabilized. As the dispersion compensation is carried in an environment in which the optical signal intensity has stabilized, the accuracy of the dispersion compensation may improve.

InFIGS. 8A and 8B, it is determined whether or not a difference between a maximum value and a minimum value of a plurality of sensitivities detected by the sensitivity monitor circuit166within a certain period has reached or fallen below a threshold value. Although a difference is used as a basis for the determination, a ratio may be used instead.

Each of the processes described above may be carried out when the optical signal rises and may also be carried out when the optical signal rises again after a disconnection. For example, the compensation amount set at the first instance of the rise may be stored in a memory, and the compensation amount stored in the memory may be used at a subsequent instance of the rise. In this case, in place of operations S810to S812, processing of setting a storage value of the memory may be carried out. Thus, processing to be carried out at the subsequent instance of the rise may be simplified.

FIG. 9illustrates an example of a wavelength dispersion compensation operation. InFIG. 9, the wavelength dispersion compensation operation is carried out by the digital signal processing circuit. A method for determining whether or not the optical signal intensity has stabilized in the wavelength dispersion compensation operation indicated inFIG. 9may differ from the determination method in the wavelength dispersion compensation operation indicated inFIGS. 8A and 8B. InFIG. 9, it may be determined that the variation amount of the sensitivity has reached the certain variation amount in a case in which a state in which a difference among the sensitivities detected successively by the sensitivity monitor circuit166is equal to or less than the threshold value continues for a certain period or longer. InFIG. 9, the processing to be carried out after the determination of No in operation S910may be substantially the same as or similar to the processing carried out in and after operation S810indicated inFIG. 8B, and thus descriptions thereof may be omitted or reduced.

InFIG. 9, the sensitivity time variation monitor circuit169of the digital signal processing circuit160sets a count value m to 1 and a count value n to 0, sets the acquisition repetition count nmax, and sets a threshold value Thmon (operation S901).

The sampling phase deviation detection circuit162sets a PLL loop of sampling synchronization to an open loop so as not to influence the wavelength dispersion compensation operation (operation S902). The sensitivity time variation monitor circuit169sets the wavelength dispersion compensation amount to an initial value (operation S903).

The sensitivity monitor circuit166monitors the sensitivity for sampling phase deviation detection (operation S904). The digital signal processing circuit160stores a monitor value of the phase deviation detection sensitivity in the sensitivity time variation memory168at a memory address that corresponds to the count value m (operation S905). The digital signal processing circuit160calculates a difference value of the value of the memory address m relative to the value of a memory address m−1 (operation S906). The sensitivity time variation monitor circuit169determines whether or not the calculated difference value of the value of the memory address m relative to the value of the memory address m−1 falls within the threshold value Thmon (operation S907).

If the calculated difference value of the value of the memory address m relative to the value of the memory address m−1 does not fall within the threshold value Thmon (operation S907: No), the sensitivity time variation monitor circuit169sets the count value n to 0 (operation S908). In a case in which the calculated difference value of the value of the memory address m relative to the value of the memory address m−1 does not fall within the threshold value Thmon, the environment may be such that the optical signal intensity have not stabilized. The sensitivity time variation monitor circuit169updates the value of the memory address m−1 in the sensitivity time variation memory168with the value of the memory address m (operation S909), and the processing then returns to operation S904.

In operation S907, if the difference value falls within the threshold value Thmon (operation S907: Yes), the sensitivity time variation monitor circuit169determines whether or not the value of the count value n is equal to or less than nmax(operation S910). If the value of the count value n is equal to or less than nmax(operation S910: Yes), the sensitivity time variation monitor circuit169increments the count value n (operation S911), and the processing then proceeds to operation S909.

In a case in which the value of the count value n is equal to or less than nmax, a certain period has not elapsed. In operation S910, if the value of the count value n exceeds nmax(operation S910: No), the processing proceeds to operation S810ofFIG. 8B.

In the flowchart described above, in a case in which a difference value between a monitor value of the sensitivity for a previous instance of phase deviation detection and a monitor value of the sensitivity for a current instance of phase deviation detection falls within the threshold value Thmon consecutively nmaxtimes, the compensation amount of the wavelength dispersion compensation circuit161is set. Thus, it may be determined with high accuracy that the environment is such that the optical signal intensity has stabilized. As the dispersion compensation is carried in an environment in which the optical signal intensity has stabilized, the accuracy of the dispersion compensation may improve.

FIG. 10illustrates an example of a wavelength dispersion compensation operation. InFIG. 10, the wavelength dispersion compensation operation is carried out by the digital signal processing circuit. A method for determining whether or not the optical signal intensity has stabilized in the wavelength dispersion compensation operation indicated inFIG. 10may differ from the determination method in the wavelength dispersion compensation operation indicated inFIGS. 8A and 8B. InFIG. 10, it may be determined that a variation amount of the sensitivity has reached a certain variation amount in a case in which a state in which a rate of change (ratio) of sensitivities detected successively by the sensitivity monitor circuit166is equal to or less than the threshold value continues for a certain period or longer. InFIG. 10, the processing to be carried out after the determination of No in operation S1010may be substantially the same as or similar to the processing carried out in and after operation S810indicated inFIG. 8B, and thus descriptions thereof may be omitted or reduced.

InFIG. 10, the sensitivity time variation monitor circuit169of the digital signal processing circuit160sets a count value m to 1 and a count value n to 0 and sets the acquisition repetition count nmaxand a threshold value Th_ratio (operation S1001).

The sampling phase deviation detection circuit162sets a PLL loop of sampling synchronization to an open loop so as not to influence the wavelength dispersion compensation operation (operation S1002). The sensitivity time variation monitor circuit169sets the wavelength dispersion compensation amount to an initial value (operation S1003).

The sensitivity monitor circuit166monitors the sensitivity for sampling phase deviation detection (operation S1004). The digital signal processing circuit160stores a monitor value of the phase deviation detection sensitivity in the sensitivity time variation memory168at a memory address that corresponds to the count value m (operation S1005). The digital signal processing circuit160calculates a rate of change of the value of a memory address m relative to the value of a memory address m−1 (operation S1006). The sensitivity time variation monitor circuit169determines whether or not the calculated rate of change of the value of the memory address m relative to the value of the memory address m−1 falls within the threshold value Th_ratio (operation S1007).

If the rate of change of the value of the memory address m relative to the value of the memory address m−1 does not fall within the threshold value Th_ratio (operation S1007: No), the sensitivity time variation monitor circuit169sets the count value n to 0 (operation S1008). In a case in which the calculated rate of change of the value of the memory address m relative to the value of the memory address m−1 does not fall within the threshold value Th_ratio, the environment may be such that the optical signal intensity have not stabilized. The sensitivity time variation monitor circuit169updates the value of the memory address m−1 in the sensitivity time variation memory168with the value of the memory address m (operation S1009), and the processing then returns to operation S1004.

In operation S1007, if the rate of change falls within the threshold value Th_ratio (operation S1007: Yes), the sensitivity time variation monitor circuit169determines whether or not the count value n is equal to or less than nmax(operation81010). If the value of the count value n is equal to or less than nmax(operation81010: Yes), the sensitivity time variation monitor circuit169increments the count value n (operation81011), and the processing then proceeds to operation S1009.

In a case in which the value of the count value n is equal to or less than nmax, a certain period has not elapsed. In operation81010, if the value of the count value n exceeds nmax(operation81010: No), the processing proceeds to operation S810ofFIG. 8B.

In the flowchart described above, in a case in which the rate of change between the monitor value of the sensitivity for a previous instance of phase deviation detection and the monitor value of the sensitivity for a current instance of phase deviation detection falls within the threshold value Th_ratio consecutively nmaxtimes, the compensation amount of the wavelength dispersion compensation circuit161may be set. Thus, it may be determined with high accuracy that the environment is such that the optical signal intensity has stabilized. As the dispersion compensation is carried in an environment in which the optical signal intensity has stabilized, the accuracy of the dispersion compensation may improve.

As a temporal variation in the sensitivity for the sampling phase deviation detection is monitored, the stabilization of the intensity of the reception optical signal may be determined with high accuracy. The dispersion compensation may be carried out after the intensity of the reception optical signal stabilizes. Thus, the accuracy of the dispersion compensation may improve, and the deterioration of the signal quality may be suppressed.

In a case in which the disparity (min-max difference inFIG. 8A) among the plurality of sensitivities detected within a certain period has reached or fallen below the threshold value (Thmon), it may be determined that the variation amount of the sensitivity has reached or fallen below a certain variation amount. Thus, it may be determined with high accuracy that the environment is such that the optical signal intensity has stabilized. The compensation amount of the wavelength dispersion compensation circuit161may be set after the monitor value of the phase deviation detection sensitivity stabilizes. As the dispersion compensation is carried in an environment in which the optical signal intensity has stabilized, the accuracy of the dispersion compensation may improve, and thus a deterioration in the signal quality may be suppressed.

It may be determined that a variation amount of the sensitivity has reached a certain variation amount in a case in which a state in which the disparity among the sensitivities detected successively, or for example, the difference indicated inFIG. 9or the rate of change indicated inFIG. 10is equal to or less than a threshold value continues for a certain period or longer. The compensation amount of the wavelength dispersion compensation circuit161may be set after it is determined with high accuracy that the environment is such that the optical signal intensity has stabilized and after the monitor value of the phase deviation detection sensitivity has stabilized. Accordingly, the dispersion compensation is carried in an environment in which the optical signal intensity has stabilized, thus, the accuracy of the dispersion compensation may improve, and a deterioration of the signal quality may be suppressed.