With regard to for example optical transmission systems that are used for long-range communications, such as 40 Gbps (bits per second) or 100 Gbps, digital-coherent receiving devices that use a phase modulation method, such as Quadrature Phase Shift Keying (QPSK), have been known in recent years. The receiving device causes the received signal to interfere with local oscillator light so as to acquire a primary optical signal and performs digital signal processing on the acquired primary optical signal.
For example, a digital signal processor (DSP) in the receiving device compensates for distortion that occurs in an optical transmission path, distortion due to incompleteness of a communication device, or the like, through digital signal processing. FIG. 24 is a block diagram that illustrates an example of the functional configuration of the DSP 100.
A DSP 100, illustrated in FIG. 24, includes a chromatic dispersion compensator (CDC) 101, an adaptive equalizer (AEQ) 102, a frequency offset compensator (FOC) 103, and a carrier phase recovery (CPR) 104. The CDC 101 is a wavelength-dispersion compensation circuit that compensates for wavelength dispersion that occurs in an optical transmission path. The AEQ 102 is an equalizing circuit for, for example, polarized-wave separation, band compensation, or linear distortion compensation. For example, the AEQ 102 performs a polarized-wave separation process to adaptively follow time fluctuations, such as polarized-wave fluctuations or polarized-wave mode dispersion, a compensation process to compensate for residual dispersion that has not been compensated during the previous wavelength-dispersion compensation, or a compensation process to compensate for signal band narrowing that occurs in an electric device, an optical device, or the like.
The FOC 103 is a frequency-offset compensation circuit that estimates the difference between the frequency of the light source at the side of the transmitting device and the frequency of the local-oscillator light source at the side of the receiving device and that compensates for the difference. The CPR 104 is a carrier-wave phase synchronizing circuit that compensates for phase noise of the local-oscillator light source or fluctuation components of the high-speed residual frequency offset that has not been compensated by the FOC 103.
The received signal, processed by the DSP 100, contains for example phase errors of the light source at the side of the transmitting device or the local-oscillator light source at the side of the receiving device, or phase errors due to residual frequency offset, or the like, which has not been compensated by the FOC 103. With regard to the CPR 104, as a method for compensating for a phase error of a received signal, Viterbi-Viterbi algorithm, such as exponentiation by n, to remove modulated components by raising the nPSK signal to the power of n, is known.
According to exponentiation by n, the nPSK signal is raised to the power of n to remove phase modulated components, n constellation points on the IQ plane are consolidated into the neighborhood of the single constellation point, and the consolidated constellation point at one area is averaged by multiple symbol numbers so that noise other than phase errors, e.g., gaussian noise, may be reduced. FIG. 25 is an explanatory diagram that illustrates an example of the constellation (there is phase noise) after exponentiation by n on the QPSK signal. The constellation before exponentiation by 4 on the QPSK signal has constellation points at 4 areas on the IQ plane. Each constellation point has a phase error ϕe due to the presence of phase noise. Furthermore, the constellation of the QPSK signal after exponentiation by 4 is in a state where the constellation points at 4 areas on the IQ plane have been consolidated into the neighborhood of the constellation point at one area.    [Patent Literature 1] Japanese National Publication of International Patent Application No. 2013-530619    [Patent Literature 2] International Publication Pamphlet No. 2012/132103
However, in the CPR 104 that uses exponentiation by n, the phase-estimation possible range is limited to 2π/n; therefore, if a phase error of equal to or more than 2π/2n occurs between successive symbols, a phase error of equal to or more than ±2π/n, i.e., phase slip, occurs. As a result, a burst error occurs in a received signal after the symbol in which the phase slip occurs.