NEAR-THRESHOLD OPERATION OF SEMICONDUCTOR-LASERS AND RESONANT-TYPE LASER-AMPLIFIERS

The characteristics of a semiconductor laser operating in the threshol d region are investigated systematically. In this transition region fr om below to above threshold, the linewidth versus injection current ch aracteristic is found to be non-monotonic: a local minimum of linewidt h just below threshold and a local maximum just above threshold are co nfirmed experimentally. If a semiconductor laser works below threshold as a resonant optical amplifier or optical filter, the small-signal f requency bandwidth is found to be equivalent to the spontaneous emissi on linewidth. When the laser amplifier is used simultaneously as a pho todetector, the maximum value of photodetection sensitivity is achieve d with the laser amplifier biased between 98-99% of the threshold curr ent. The Fokker-Planck equation method is employed in the linewidth ca lculation. A numerical computer simulation is also performed using the rate-equation model. A reasonable agreement between theory and experi ment is obtained.