| 摘要 |
In an optical superheterodyne receiver, received light is heterodyned with the light generated by a local laser. In addition to the use of coherent light, the prerequisite for the heterodyning is that both light waves have the same polarisation. Since the plane of polarisation of the light transmitted via optical waveguides can change, the receiving end requires either an arrangement for controlling polarisation, or the diversity principle is used in which four optical combination signals are generated from the received light and the light of a local laser, of which in each case two signals have the same polarisation with respect to one another and the polarisations of both pairs are orthogonal to one another. After being supplied to two push-pull photo receivers, two electrical signals are obtained which, after demodulation, are added weighted so that, when the signal component is low in one of the two receiver branches, only a low noise component is added to the output signal.
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