Reception nodes often receive an optical signal by digital coherent detection when a high-capacity communication or a long-distance communication is performed. Digital coherent receivers generate an electric field information signal that represents a received optical signal, and perform digital signal processing on the electric field information signal so as to recover data. The digital signal processing can compensate for a signal distortion due to a faulty device and/or the features of an optical fiber transmission link. For example, the digital signal processing can compensate for, for example, a dispersion of an optical fiber, a polarization variation in an optical fiber, the frequency response characteristics of an electronic device, phase noise of a laser, and a difference between a carrier frequency of an optical signal and a frequency of a local oscillator (a frequency offset).
On the other hand, polarization multiplexing has been put to practical use in order to realize a transmission of large volumes of data in an optical communication system. The polarization multiplexing transmits a signal using a set of polarizations that are orthogonal to each other. A modulation format of each signal channel is selected according to, for example, a transmission rate, a transmission distance, a desired quality (such as a signal-to-noise ratio: SNR). For example, BPSK, QPSK, 8 PSK, 8 QAM, 16 QAM, and 64 QAM have been put to practical use.
Further, in order to improve the frequency utilization efficiency in an optical communication system, a hybrid modulation that combines signals of different modulation formats in one signal channel and transmits them has been attracting attention. It is possible to use different modulation formats in a polarization domain, a time domain, and a frequency domain. For example, in a polarization multiplexing transmission, modulation formats of two signals transmitted using a set of polarizations may be different from each other. Further, a modulation format may be switched at specified time intervals in each polarization. The hybrid modulation may realize a desired transmission capacity or a desired transmittable distance by adjusting a combination ratio of a plurality of modulation formats.
As a related technology, an optical communication method for setting a signal bandwidth of a multilevel modulated signal to an optimal value according to a frequency spacing is proposed (for example, Japanese Laid-open Patent Publication No. 2013-16978). Further, a method is proposed that realizes a variable bit rate optical transmission using a programmable signal modulation (for example, Japanese National Publication of International Patent Application No. 2011-514736).
As described above, a digital coherent receiver can compensate for a signal distortion due to a faulty device and/or the features of an optical fiber transmission link. However, an accuracy of a compensation value (such as a filter coefficient of a digital filter) that is generated to compensate for a signal distortion depends on the number of bits per symbol of a modulation format. The number of bits per symbol represents the number of bits transmitted for each symbol. For example, the number of bits per symbol of QPSK and 16 QAM are two and four, respectively.
Specifically, an accuracy of a compensation value is lower if the number of bits per symbol of a modulation format is larger. Thus, when a polarization multiplexed optical signal is transmitted by hybrid modulation in which a plurality of different modulation formats are used in time domain and/or polarization domain, the accuracy of a compensation value to compensate for a signal distortion may be decreased. When the accuracy of a compensation value is low, a signal distortion is not sufficiently compensated for, which may result in degrading a bit error rate of recovered data.