| 摘要 |
Between clockwise and counterclockwise light beams having spent a time ? in propagating through a loop-shaped optical transmission line are provided phase differences accordance with a biasing signal. Light of interference between the clockwise and counterclockwise light beams is converted to an electric signal corresponding to the intensity of the interference light. A step value signal for phase negative feedback is derived from a synchronously detected output and the step value is accumulated by an accumulator. When the accumulated output exceeds a threshold value corresponding to 2m.pi. rad, the accumulator outputs the overflow value as the accumulated result and, at the same time, outputs an overflow signal and an overflow sign signal representing its polarity. The accumulated output is converted by a D/A converter to an analog signal, by which an optical phase modulator is driven. A control signal generator outputs a 0 when the overflow signal is zero, outputs -1 when supplied with a positive overflow signal, and outputs +1 when supplied with a negative overflow signal. The output of the control signal generator is applied to a multiplier. The level difference between the outputs of a synchronous detector before and after the biasing signal is changed over at the k? time intervals is detected by a conversion error detector. The detected output is applied to the multiplier, wherein it is multiplied by the control signal from the control signal generator. The multiplied output is integrated by an integrator and the integrated output is used to control the conversion gain of the D/A converter by a negative feedback, thereby compensating for a change in the conversion gain of the optical phase modulator.
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