A major problem encountered in using AlGaAs lasers for optical heterodyne phase-shift-keying communication is that of broad linewidth caused by phase noise in the laser. Typical linewidth/bit rate ratio requirements are ≤0.1 % for PSK and ≤0.3% for DPSK. When these are met, the performance degradation due to the linewidth of the source becomes negligible. For AlGaAs lasers with 10-20-MHz linewidths this means a transmission rate in excess of several gigahertz, which exceeds the needs of many applications and which is difficult to achieve. For more common data rates (50-500 MHz) the linewidth becomes a serious problem.
© 1988 Optical Society of AmericaPDF Article