| 摘要 |
PURPOSE:To set a modulation index at a comparatively small value by providing a light receiving part with a differential detection circuit, the delay time of which is n-bits, and inputting a first intermediate frequency signal to it, and demodulating a signal by a second intermediate frequency signal, appearing in the output of it. CONSTITUTION:The first intermediate frequency signal is obtained by transmitting an FSK modulated signal light from a light transmitting part, and receiving the FSK modulated signal light by the light receiving part, and the first intermediate frequency signal is inputted to the differential detection circuits 5, 6, the delay time of which is n-bits (n is one or larger integer), and a demodulated signal is obtained by demodulating the second intermediate frequency signal, appearing in the output of the differential detection circuits 5, 6. Accordingly, because the considerable part of the influence of the FM noise of a light source is absorbed by the differential detection circuits 5, 6, this influence comes difficult to appear in the demodulation of the signal. Thus, the dependency of a required modulation index upon the spectrum width of the light source and a bit rate can be extremely reduced, and in addition, the modulation index can be set at a comparatively small value. |
