In recent years, along with the spread of the Internet, the capacity of data traveling over networks (transmission capacity) has been increased. As such, in so-called large artery communication channels linking large cities, optical transmission channels in which the capacity per one channel is 10 gigabit per second (Gb/s) or 40 Gc/s have been introduced.
In the optical transmission of 10 Gb/s, OOK (On-Off-Keying) is used as a modulation system. On the other hand, in the optical transmission of 40 Gb/s, as the optical pulse width is as short as 25 picosecond (ps), an influence of wavelength dispersion is large. As such, if OOK is used, optical transmission of 40 Gb/s is not suitable for long distance transmission. Under such a circumstance, a multilevel modulation system, which is phase modulation, is used, and in the optical transmission of 40 Gb/s, QPSK (Quadrature Phase Shift Keying) is mainly used as a modulation system.
Further, in the ultrahigh-speed optical transmission at a level of 100 Gb/s, it is necessary to widen the optical pulse width by increasing the number of multiplex to reduce the so-called baud rate (modulation rate). This means that it is necessary to further suppress the influence of wavelength dispersion.
In the ultrahigh-speed optical transmission, polarization multiplexing has been known as one method of suppressing an influence of wavelength dispersion. In the polarization multiplexing, surfaces in which the field intensities of a dual optical signal EX and EY oscillate are orthogonally crossed and entered into an optical fiber. The optical signals EX and EY (namely, field intensities) propagate while repeating random rotation in a state where the quadrature relation is maintained in the optical fiber. At the output terminal of the optical fiber, a quadrature multiplexed signal (hereinafter also referred to as quadrature signal) SXY=EX+EY, where the rotation angle θ is unknown, is obtained.
As polarization multiplexing, an optical system and a signal processing system have been known. In the optical system, polarization separation is performed using a polarization control element and a polarization separation element. This means that the quadrature signal SXY=EX+EY is separated by being projected to polarization surfaces X′ and Y′ defined by the polarization separation element. Thereby, optical signals (output signals) represented as EX′=aEX+bEY and EY′=cEX+dEY are obtained (polarization separation: a to d represent coefficients).
Then, with monitoring of the outputs signals after the separation, the output signals are returned to the polarization control element in such a manner that the output signals become maximum, that is, EX′=aEX (b=0) and EY′=dEY (c=0), to thereby estimate the rotation angle θ.
However, as the polarization control element generally has a control frequency (clock frequency) of about 100 MHz, it is difficult to follow high-speed fluctuation in polarization.
On the other hand, in the signal processing system, polarization separation is performed after obtaining an electrical signal by coherently detecting the above-described quadrature signal. As such, in the signal processing system, the quadrature signal EX+EY is projected on the polarization planes X′ and Y′ defined by the local light to be detected, and electric field information in each of the polarization planes X′ and Y′ is obtained as an electric signal.
Here, as an example, a polarization separation system by means of signal processing will be described with use of a typical coherent receiver shown in FIG. 14.
The coherent receiver shown in FIG. 14 includes a local oscillator (LO) 91, a 90° hybrid 92, photodetectors (PD) 93 and 94, an A/D (analog/digital) converter 95, and a DSP (Digital Signal Processing) chip 96.
A quadrature signal SXY=EX+EY is supplied to the 90° hybrid 92. The 90° hybrid 92 also receives a local light SX′Y′ from the LO 91. The quadrature signal SXY interferes with the local light SX′Y′ in the 90° hybrid 92 and is output as interference signals EX′ and EY′. The interference signals EX′ and EY′ are respectively detected by the PDs 93 and 94. These detected signals include electric field information, and are quantized (A/D converted) by the A/D converter 95 and supplied to the DSP chip 96 as quantized signals ex′ and ey′.
For example, the DSP chip 96 has a butterfly filter 96a which operates with a CMA (Constant Modulus Algorithm). The filter coefficient of the butterfly filter 96a is determined according to the CMA operation by the CMA operation section 96b (for example, see Non-Patent Document 1). The butterfly filter 96a filters the quantized signals ex′ and ey′, and cancels the polarization rotation angle θ thereof. As a result, the DSP chip 96 outputs the demodulated signals (electric field information) ex and ey from the ports 97 and 98.
As described above, the interference signal EX′ (or EY′) includes the field intensity (field intensity may be indicated as EX or EY). As such, although the amplitude of the interference signal EX′ (and EY′) fluctuates according to the polarization rotation, the field intensity of the interference signal EX′ (or EY′) is controlled to be constant by the CMA. As a result, the interference signal EX′ (or EY′) converges at the field intensity EX (or EY).
On the other hand, as a coherent receiver, one which receives a high-speed signal light has been known. Such a receiver combines a local oscillation light having polarization-multiplexed quadrature polarization components in which the optical frequencies are different to each other, and a received signal light, in a hybrid circuit, and then photoelectrically converts it in two differential photodetectors. Then, the photoelectrically converted signal is converted to a digital signal in an AD conversion circuit, and signal processing is executed in a digital computing circuit to estimate received data (for example, see Patent Document 1).    [Patent Document 1] JP 2008-153863 A    [Non-Patent Document 1] D. N. Godard, “Self-Recovering Equalization and Carrier Tracking in Two-Dimensional Data Communication System”, IEEE Trans. on Comm., Vol. COM-28, No. 11, pp. 1967-1875, November 1980