Observation of intrinsic quantum fluctuations in semiconductor lasers

The first experimental observation of the excitation of the natural resonance of a semiconductor laser by the quantum fluctuations intrinsic to the laser is reported. Such excitation and the resulting resonant-like peaks in the microwave noise spectrum of the laser intensity were initially predicted by McCumber and later calculated in detail for semiconductor lasers by Haug. In addition, we present experimental and theoretical results that show that high-level excitation of the resonance by internal or forced modulation of the population inversion lowers the resonant frequency due to the nonlinearity present in the rate equations. In the experiments to be described, intensity noise spectra of continuously operating GaAs injection lasers were observed at microwave frequencies with a high-speed photodiode and a microwave spectrum analyzer. Because of the low level of the photocurrent produced by these fluctuations, it was necessary to reduce the intrinsic noise level of the system by using phase-sensitive detection techniques. In this way, the following experimental observations have been made. First, a sharp peak in the intensity noise spectrum has been observed at currents
I
from 1.5 to 100 percent above threshold (
Ith
). Second, at constant heat-sink temperature, the frequency of the peak varies, with current as
(I/I_{th} - 1)^{1/2}
. Third, while the intensity fluctuations relative to the square of the laser intensity continuously decrease (by three orders of magnitude) with increasing current, the absolute value of the noise peak increases (by more than two orders of magnitude) to a maximum value that is maintained with further increases in current. Finally, the frequency of the noise peak at constant current level above threshold shows no variation with heat-sink temperatures between 80°K and 150°K. The above observations were made on lasers in which the resonance was not strongly excited by combination tones present in the active medium and consequently for which there were no deep intensity pulsations. For lasers in which the intensity spontaneously pulses, the resonance peak has also been observed at currents very near threshold. However, in these diodes, the frequency at the peak increases with current as predicted by the theory only over a small range near threshold. Beyond this range, the resonance is excited by the combination tones, causing the frequency to decrease to a minimum value before increasing farther as the current is increased. Such behavior can be qualitatively understood in terms of the decrease in the average inversion (and consequently a reduction of the resonant frequency), which accompanies the self-induced pulsing of the laser intensity. Computer calculations based on nonlinear rate equations have confirmed this behavior.