It is required to further enlarge the capacity in a backbone transmission system as the amount of information (traffic) in the Internet increases. A coherent optical transmission technology has drawn attention as one of technologies for high capacity. In the coherent optical transmission technology, an AC (Alternating Current) signal component is received which is amplified by mixing signal light and local oscillation (LO) light in a coherent optical receiver device. At that time, the larger the optical output of the local oscillation (LO) light becomes, the larger amplifying operation acts on the signal light. Therefore, the receiving characteristics with the high S/N (Signal/Noise) ratio can be obtained by inputting the high-power local oscillation (LO) light compared with the signal light.
An example of such coherent optical receiver device is described in patent literature 1. The coherent optical receiver device described in patent literature 1 includes an optical hybrid, an optical receiver, an analog-to-digital converter (ADC), and a signal processing unit, and further includes a signal quality monitor, a controlling amount providing unit, an input light level controller, and a local light source.
The input light level controller controls the level of the light signal to be introduced into the optical hybrid, that is, the level of the light signal propagated through the optical transmission path, under the control of the controlling amount providing unit. The level of the local light to be output to the optical hybrid is controlled under the control of the controlling amount providing unit. The signal quality monitor monitors the quality of received signals with reference to information obtained through digital signal processing performed in the signal processing unit.
The controlling amount providing unit provides the input light level controller and the local light source with controlling amounts based on the monitoring results by the signal quality monitor, by which the controlling amount providing unit controls the power ratio between input light and local light that are to be introduced into the optical hybrid and be mixed with each other. In addition, the controlling amount providing unit controls the amplitude of signal light and local light keeping the power ratio constant.
By adopting the configuration, it becomes possible to keep the power ratio between the signal light power and the local light power to a constant level at least. On the other hand, when the local oscillation light power is kept constant, the signal quality of received signals becomes optimum with the signal light power in a prescribed range. As a result, according to the related coherent optical receiver device disclosed in patent literature 1, it is said that the signal quality of received signals can be kept high by controlling the power ratio between the signal light power and the local light power. In addition, it is said that the amplitudes of the signal light and the local light can also be optimized to contribute to further improvement in the quality of received signals.    Patent literature 1: Japanese Patent Application Laid-Open Publication No. 2010-245772 (paragraphs [0031] to [0071])