Optical receiving device and communication system

A communication system comprising:

• a plurality of optical transmitting devices classified into a plurality of groups;
• a plurality of optical couplers that are provided corresponding to the groups, respectively, each of the optical couplers outputting light transmitted from an optical transmitting device included in a corresponding group among the groups;
• and
• an optical receiving device that receives light output from the optical couplers, wherein
• the optical receiving device includes:
• a plurality of input ports to which light output from the optical couplers is input;
• a plurality of amplifiers that are arrayed and provided corresponding to the input ports, respectively, each of the amplifiers amplifying and outputting light input from a corresponding input port among the input ports;
• a photo diode that converts light into an electrical signal
• a lens that inputs to the photo diode, light output from the amplifiers;
• and
• a control circuit that obtains input information indicating whether light is input into the amplifiers, based on the obtained input information, controls gain of an amplifier among the amplifiers and to which no light is input, to be lower than gain of an amplifier among the amplifiers and to which light is input.