Carrier acquisition and tracking for coherent systems

Coherent receivers must have the capability of acquiring the carrier of the transmitter and then to switch to carrier tracking before data are received. The authors describe a technique of carrier frequency acquisition which uses the laser temperature to scan the local oscillator frequency over the range of possible transmitter frequencies and then locks to that frequency when the beat IF frequency signal appears in the receiver bandwidth. After acquisition has been achieved, carrier tracking commences. The control of both acquisition and tracking is affected through a built-in microprocessor which gives the system flexibility and the ability to monitor the performance of both subsystems. Acquisition is completely automatic and long term tests showed after 60 hours the system to be operating at the sensitivity limit without the need for reacquisition. These results confirm that coherent systems do possess the stability required for operation in a network environment