| 摘要 |
PURPOSE:To operate normally an alarm function even if an APD bias voltage is set higher in the presence of a signal by detecting a current flowing through an avalanche photodiode(APD) and the presence or absence of a transmission signal and controlling the APD bias voltage low in the absence of signal and high in the presence of signal. CONSTITUTION:A current detection circuit 20 generates an output when a current flowing to the APD 1 exceeds a prescribed value in a region where a dark current increases rapidly. A transmission signal detection circuit 30 generates when an optical transmission signal of a prescribed level or over is inputted to the APD 1. A control circuit 40 acts like raising an output voltage of a high voltage generating circuit 50 and increasing a bias voltage of the APD 1 through the combination of both outputs only when the optical transmission signal is sent at a prescribed level or over. In the case of no optical input to the APD 1, the APD 1 receives a low bias voltage. When an output is given from the APD 1 by the input of the optical transmission signal, the bias voltage of the APD 1 is increased, and in the absence of the optical transmission signal, the dark current is increased rapidly, and an output appears from the current detection circuit 20, an output voltage of the high voltage generating circuit 50 is dropped to lower the bias voltage. |
