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

Abstract

Abstract-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 98-99% of the threshold current. The Fokker-Planck equation method is employed in the linewidth calculation. A numerical computer simulation is also performed using the rateequation model. A reasonable agreement between theory and experiment is obtained.

Ab initiomultimode linewidth theory for arbitrary inhomogeneous laser cavities

“…We check the approximation of |δ| a 0 a posteriori and we find that it generally holds (as was also shown in [40]), except near threshold (a 0 → 0), which is a case we discuss in Sec. VI C.…”

“…We believe that this phenomenon could be explored using a future generalization of our formalism, with some modifications (extending earlier work [28,40] on linewidth enhancement from ASE).…”

“…Nonlasing modes contribute to ASE, which has a significant effect on the spectrum near and below the lasing thresholds [40,59] and will be treated in future work. In Appendix A, we derive the N-SALT TCMT equations of motion for a μ (t) starting with the full vectorial MaxwellBloch equations.…”

“…The theory is valid well above threshold, and whenever a new mode turns on, this inverse-power relation produces a divergence due to the failure of the linearization approximation near threshold. However, we show that this divergence is spurious and can be avoided by solving the nonlinear N-SALT TCMT equations numerically [40]. (Our formalism can be extended to treat the near-threshold regime analytically by including noise from subthreshold modes, as discussed in Sec.…”

“…Moreover, while the average (SALT) behavior is unaffected by nonlasing poles, they do affect the noise properties, and N-SALT in its current form only accounts for a finite number of poles in the Green's function (Appendix A 2). [We only include lasing poles (i.e., poles on the real axis), but extension to include nonlasing poles, which determine the amplified spontaneous emission (ASE) [40,59], is straightforward (Sec. VIII).]…”