| 摘要 |
PROBLEM TO BE SOLVED: To secure stable communication by stopping automatic tracking when extraneous light other than a light beam is made incident and a tracking error occurs. SOLUTION: When extraneous light other than received light beams is incident on a quadripartite optical receiver 37, a reverse bias current caused to flow to photodetectors 51a to 51d is increased in accordance with the intensity of incident light compared with when the extraneous light is not incident, and the change of the reverse bias current appears as the change of the voltage drop in a resistor R2. A voltage drop voltage is inputted to a differential amplifier constituting a low-pass filter and outputted as an appropriate direct current voltage. Then, the output voltage of the differential amplifier 53 is changed by the intensity of spots image-formed on the photodetectors 51a to 51c, and when the output voltage surpasses a threshold in a control circuit part 54, a command for stopping the driving of a movable mirror control circuit is transmitted. When the extraneous light is incident on an optical space transmitting device to bring about a tracking error in the automatic tracking operation, the automatic tracking operation is stopped at the position of the movable mirror. When the incidence of the extraneous light disappears and the output voltage falls below the threshold, the movable mirror restarts the automatic tracking operation from the position where the automatic tracking operation has stopped. |
