| 摘要 |
The invention relates to an improvement in a coherent transmission system and a method of transmitting an FSK-modulated and polarisation-switched optical signal with polarisation-insensitive detection and frequency stabilisation at the receiving end. A periodic polarisation variation having a period longer than the bit period of the FSK-modulated signal and shorter than the control time of the frequency stabilisation is applied to the local oscillator signal, but preferably to the received signal. This achieves the result that, for any polarisation state of the local oscillator signal or of the received signal whatsoever, the signal peaks of the FSK-modulated binary elements 'zero' and 'one', are always present in the power distribution of the detected electrical signal within said control time, a frequency-stabilisation problem at start-up thereby being avoided. <IMAGE>
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