| 摘要 |
PURPOSE:To detect stably the output of an optical signal by mixing and detecting an optical signal from a transmitter and an optical signal from a semiconductor laser by a photodetector, and providing a coil and a polarization plane correcting device at just before the semiconductor laser. CONSTITUTION:A magnetic garnet crystal 7' is placed on the optical path of a semiconductor laser 2 and a beam splitter 4, and a coil 6' is wound to the periphery of the magnetic garnet crystal 7'. Whether the polarization direction of signal light which is transmitted through an optical fiber 3 coincides with the polarizartion plane of a local oscillation output light or not can be judged by whether the product of amplitude components corresponding to interference components of both rays of light is large or small. In case the planes of polarization coincide completely with each other, the quantity of light shows the maximum value. Accordingly, the coincidence of the plane of polarization can be attained by rotating the plane of polarization of the local oscillator output light by the Faraday effect of the magnetic garnet crystal 7'. In such a manner, a heterodyne photodetecting device which is excellent in the stability, good in the accuracy and inexpensive is obtained. |
