When the signal light input to an optical receiving module applied in an optical communication system has large power, a large amplitude signal is input to a receiving circuit after a light receiving element such as an Avalanche Photo Diode (APD). As a result, the duty variation due to the amplitude saturation may deteriorate the received signal characteristic. In a long distance optical communication, since many optical amplifier may be provided at some points of an optical transmission line, a wide optical power range (dynamic range) is requested which can be received by an optical receiving module. On the other hand, for example, a technology has been known which has a variable optical attenuator before an internal light receiving element of an optical receiving module and attenuates signal light when input signal light has large power.
In recent years, an input-power monitoring function which monitors the input power of a signal light to an optical receiving module may be requested in an optical receiving module. (Refer to Japanese Laid-open Patent Publication No. 2006-203179, for example). The input-power monitoring may be required to indicate the input power of a signal light before attenuated by a variable optical attenuator even while the signal light is being attenuated. On the other hand, for example, a technology has been known in which an optical coupler before the variable optical attenuator partially splits signal light, and the split signal light is monitored for implementing the input power monitoring.
However, the technology in the past may not monitor input power with high precision. For example, a branch ratio of an optical coupler may depend on the wavelength of signal light or temperature. Thus, the technology in which signal light is partially split by an optical coupler before a variable optical attenuator for monitoring may provide input-power monitoring results which vary in accordance with the changes in branch ratio of the optical coupler even for constant input power of signal light.
On the other hand, a look-up table may be created for wavelengths or temperatures in advance, and split signal light and the look-up table may be used to estimate the input power. However, creating a look-up table with high precision for correcting monitoring result variations due to the wavelength or temperatures may require many experiments and/or simulations, for example, which may increase the manufacturing costs for the optical receiving module.
The technology which partially splits and monitors signal light with an optical coupler before a variable optical attenuator may unconditionally split a part of signal light, which may cause signal light loss. Therefore, when signal light has small input power, the minimum receiving sensitivity may deteriorate.