| 摘要 |
PURPOSE:To hold the efficiency of conversion from a signal light component to an electric signal constant by using an optical fiber coupler which employs a polarization maintaining fiber from its local oscillation light incidence terminal to its multiplexed light projection terminal for an optical multiplexing part. CONSTITUTION:Local oscillation light 2 which contains a linear polarized component is coupled with the polarization maintaining fiber 20 of the optical fiber coupler 5 and an adjustment is so made that the characteristic axis of the fiber 20 coincides with the polarization direction of the linear polarized component. Signal light 4 is coupled with the incidence terminal of a single-mode optical fiber 21 and multiplexed output light 6 is demultiplexed by a polarization separating element 7 into light beams 8 and 9 having mutually orthogonal linear polarized waves. Further, an adjustment is so made that the characteristic axis of the polarization maintaining fiber 20 and the polarization axis of the polarization separating element 7 contains 45 deg. at a multiplexed light projection terminal, and local oscillation light components of light beams 8 and 9 are equal in level and stable. The light beams 8 and 9 are supplied to photodetection parts 10 and 11 for heterodyne detection, and electric signals 12 and 13 are outputted as a base-band demodulated signal 19 through a multiplexer 16 and a detecting circuit 18. |
