| 摘要 |
OPTICAL SIGNAL CONTROL METHOD AND APPARATUS Apparatus for processing one or more optical signals to produce a desired polarisation transformation on a Poincaré sphere has particular application in a receiver for a coherent communication system. One or more birefringent elements 11 receive a polarised optical signal from a local oscillator 12, and vary the state of polarisation of the local optical signal in order to match it to a received optical signal from a remote source on a line 13. The transformed local signal is fed on a line 14 to a directional coupler 15 which compares the local and remote optical signals. A receiver 16 extracts data by interference between the two signals in the directional coupler 15, and feeds data out at a line 17. A level detector 18 detects the level of the output signal 17 and supplies a feed-back signal to a polarisation controller 19, which controls the birefringent element or elements 11. The birefringent device 11 is adapted to provide a variable rotation on a Poincaré sphere about an axis of rotation which itself may be varied in direction. The control means 19 varies the amount of rotation on the Poincaré sphere produced by the birefringent device, and varies the direction of the axis about which the rotation takes place, so as to achieve the desired polarisation transformation. A preferred form of the birefringent device 11 comprises a waveguide of electro-optic material, and means for producing in the electro-optic material an electric field of variable strength and variable orientation whereby required polarisation transformation can take place. Preferably the electro-optic material is lithium niobate. |
