disclosed herein is a terminal device including a photodetector , amplitude detecting circuit , light source , drive circuit , and control circuit . the photodetector receives a first optical signal . the amplitude detecting circuit detects the amplitude of an output from the photodetector . the drive circuit supplies a drive current to the light source so that the light source outputs a second optical signal . the control circuit decreases and increases the drive current according to an increase and decrease in the amplitude detected by the amplitude detecting circuit , respectively . in the case that this terminal device is used with another terminal device to construct a network , the dynamic range of the other terminal device can be reduced . accordingly , the use of this terminal device can improve the extensibility of the network .