An optical communication transmission system which uses an optical fiber as a transmission path medium carries out an important role in a backbone communication network based on its long-distance transmission capability and large-volume transmission capability.
As one of the indicators for the economical efficiency of the optical communication transmission system, there is a way of determining it based on how many transmission signals can be shared with an optical fiber as a transmission path and a relay optical amplifier. The wavelength division multiplex optical transmission technology is a technique which can easily increase the signal total capacitance with a single optical fiber, so that it is employed to most of the backbone transmission system that is required to perform great amount of transmissions. The mainstream of the signal speed per wavelength used currently on commercial systems is 2.5 Gb/s or 10 Gb/s. However, it is expected to improve the signal speed up to 40 Gb/s or 100 Gb/s, for example.
If the signal speed is to be increased without improving the capacity of the transfer path, it becomes difficult to design the structure of the transmitter-receiver in accordance with the increased operation speed. Further, influences of the various signal quality degradations generated within the optical fiber as the transmission path become more prominent. That is, the technique regarding the transmitter-receiver and the optical fiber becomes complicated.
In order to cope with the degradations of the receiving sensitivity caused due to the increased transfer speed, it is desired to employ a modulation-demodulation method which is excellent in the receiving sensitivity. For the optical communication transmission system whose signal speed is 10 Gb/s or below, a simple IM/DD (Intensity Modulation/Direct Detection) method which superimposes a signal on the intensity of the signal light has been the mainstream. However, for the optical communication transmission system whose signal speed is 40 Gb/s or more, it has been studied to employ a method which superimposes information not only to the intensity of the signal light but also to the phase in a dividing manner.
Further, influences of the accumulation of the wavelength dispersions become heavier in accordance with the increased signal speed. It becomes necessary to have a slight compensation for each wavelength, for example, while conventionally the multiple-wave lights can be compensated collectively. In a case where the compensation is performed for each wavelength, a compensator is loaded inside each optical transmitter-receiver. Further, same processing as that of the wavelength dispersions is to be conducted for polarization dispersions.
As a related literature where the invention related to the present invention is depicted, there is Patent Document 1.    Patent Document 1: Japanese Unexamined Patent Publication 2002-33703