| 摘要 |
PURPOSE:To suppress a maximum value of an optical input level to a light receiving element and to attain stable operation for a reception electric circuit by inserting an optical attenuation element whose attenuation is variable between an optical signal input output terminal and a light receiving element and light emitting element. CONSTITUTION:An optical signal inputted from an optical signal input terminal A is reproduced into an electric signal and outputted from an electric signal output terminal B. On the other hand, a peak value of an electric signal amplified by an equalizing amplifier 14 is detected in an AGC circuit 16 and a control signal proportional to the quantity of the input level is sent to an optical attenuation control circuit 17. The circuit 17 generates a control voltage in a direction to increase the optical attenuation of an optical attenuation element 11 when the input level is larger than the specified value to control the element 11. On the other hand, in the case of the signal transmission direction (from a terminal C to a terminal D), the optical attenuation of an optical attenuation element 23 is varied by a control voltage outputted from the circuit 17. When the distance up to an opposite station is short, the light receiving level of the opposite station is suppressed below the specified value by reducing the optical output level at an optical signal output terminal D automatically. |
