| 摘要 |
The frequency shift keying (FSK) format used in the prior art, radio frequency carrier, PCM communication systems generally comprises a train of pulsed carriers having sufficiently different frequencies so as to permit frequency separation and identification by means of either filters or frequency discriminators. At optical frequencies, however, the relatively small amount of frequency shift readily obtainable by the use of available optical devices operating on the output of a laser may not allow complete frequency separation by an amount sufficient to employ either of these conventional detection systems. In the system described herein, the received pulse train is divided into two pulse trains, one of which is delayed the equivalent of one pulse repetition period relative to the other. The two pulse trains are then coupled to a frequency mixer whose output is indicative to the frequency shift between pulses in adjacent time slots. Means are provided at either the transmitter or the receiver for converting between the standard binary-encoded signal and a differential binary encoded signal.
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