High-power phase shift keyed optical transmitter using a phased array optical amplifier
Description
The communications performance of a novel transmitter for potential use in space-based laser communications systems was experimentally measured. The transmitter uses an integrated phased array of strongly gain-saturated AlGaAs semiconductor optical amplifiers as a power amplifier to amplify a phase-shift-keyed master oscillator signal. The bit-error-rate performance of the transmitter was measured and compared to the bit-error-rate performance of the master oscillator alone A self-homodyne technique was used to measure the bit-error-rate performance of the transmitter. A portion of the unmodulated master oscillator signal was split off and used as the local oscillator in the receiver to allow the data to be recovered. A 3 dB fiber-optic coupler was used to combine the modulated signal and the local oscillator so that the mixing efficiency of the homodyne detection process could be made nearly one. The phased array of semiconductor optical amplifiers was a graded-index, separate-confinement heterostructure, single-quantum well device that was operated under strongly gain-saturated conditions in order to maximize the energy extracted from it. A 2$\sp7$-1 length pseudorandom data sequence at a data rate of 200 Mbps was used to measure the bit-error rate. The impact of the unequal path lengths to each amplifier of the integrated phased array was analyzed. In addition, the impact of amplified spontaneous emission noise was analyzed for an intersatellite laser communications system No degradation in bit-error-rate performance was observed from using the integrated phased array of strongly gain-saturated semiconductor optical amplifiers to amplify the phase-shift-keyed master oscillator signal. In addition, the phased array of strongly gain-saturated semiconductor optical amplifiers did not measurably increase the linewidth of the amplified, unmodulated master oscillator signal. The unequal path lengths of the integrated phased array cause the on-axis intensity to drop off and recover at each phase transition, which causes a small penalty in communications performance and limits the data rate. The amplified spontaneous emission noise causes no degradation because only a small fraction of it is received by the receiver of an intersatellite laser communications system due to the beam forming properties of the phased array