Near threshold operation of semiconductor lasers and resonant-type laser amplifiers

The characteristics of a semiconductor laser operating in the threshold region are investigated systematically. In this transition region from below to above threshold, the linewidth versus injection current characteristic is found to be nonmonotonic: a local minimum of linewidth just below threshold and a local maximum just above threshold are confirmed experimentally. If a semiconductor laser works below threshold as a resonant optical amplifier or optical filter, the small-signal frequency bandwidth is found to be equivalent to the spontaneous emission linewidth. When the laser amplifier is used simultaneously as a photodetector, the maximum value of photodetection sensitivity is achieved with the laser amplifier biased between 98% and 99% of the threshold current. The Fokker-Planck equation method is employed in the linewidth calculation. A numerical computer simulation is also performed using the rate-equation model. A reasonable agreement between theory and experiment is obtained