a stable optical reference oscillator consisting of conventional low power cw source of coherent optical radiation having a relatively narrow bandwidth is mixed , i . e ., beat against a sample of an incoherent laser transmit signal and the phase of the resultant signal is recorded . this is then compared to the phase of an ideal pulse of a perfect laser transmitter which was previously generated and recorded . the result is a phase correction term which is used in the subsequent signal processing of the received signals to realign the received laser pulses so that they are phase coherent .