To realize an ultrafast light transmission system of 40 Gbits/s or higher, attention is paid to adoption of the polarization multiplexing technique. The polarization multiplexing technique is a method of, paying attention to the fact that a single wavelength has two polarization states which are orthogonal to each other, transmitting two independent pieces of signal information by using the two polarization states.
It is known that, in a polarization multiplexing transmission system, degree of deterioration in transmission quality due to the fiber nonlinear effect and PMD (Polarization Mode Dispersion) varies according to a relative delay time difference between polarization channels orthogonal to each other. For example, from the viewpoint of fiber nonlinearity tolerance, an interleave polarization multiplexing method in which the relative delay time difference between polarization channels orthogonal to each other is set to the half of symbol time is advantageous. On the other hand, from the viewpoint of PMD tolerance, a time align polarization multiplexing method in which the phases become the same is advantageous.
In the polarization multiplexing transmission system, to control deterioration in transmission quality due to the nonlinear tolerance and PMD, the relative delay time difference between polarization channels orthogonal to each other has to be controlled surely. However, even if the relative delay time difference between polarization channels is set at the initial setting, the relative delay time difference between the polarization channels which are orthogonal to each other may vary due to temperature change or deterioration with age. To address the problem, it is considered to compensate for a phase difference among drive signals of a plurality of modulators by using temperature monitor information. However, in this case, information such as temperature dependency, a characteristic of fluctuation with time, individual variations, and the like is preliminarily necessary for feed forward control using the temperature monitor information. For a simple and high-precision phase difference control, a further technical development is demanded.    Patent Document 1: Japanese Patent Application Laid-Open No. 2002-344426    Patent Document 2: Japanese Patent Application Laid-Open No. 2003-338805    Patent Document 3: Japanese Patent Application Laid-Open No. 2005-65027    Non-patent Document 1: D. van den Borne, et al., Journal of Lightwave Technology, Vol. 25, No. 1, pp. 222-232, January 2007.