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Physical Review A

High-impedance suppression of pump fluctuation and amplitude squeezing in semiconductor lasers

Y. Yamamoto, S. Machida

Published Jun 1, 1987• 169 citations
Publisher

Abstract

The pump amplitude fluctuation of a current-driven semiconductor laser can be suppressed to below the ordinary shot-noise level by ``high-impedance suppression.'' As a result of this pump-fluctuation suppression, the output of a current-driven semiconductor laser exhibits amplitude squeezing in the frequency region below the cavity bandwidth. The noise equivalent circuits and the quantum-mechanical Langevin equations are derived and discussed for clarifying the difference between an ordinary laser oscillator with shot-noise-limited pump fluctuation and a constant-current-driven semiconductor laser.

DOI
10.1103/physreva.35.5114

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The pump amplitude fluctuation of a current-driven semiconductor laser can be suppressed to below the ordinary shot-noise level by ``high-impedance suppression.'' As a result of this pump-fluctuation suppression, the output of a current-driven semiconductor laser exhibits amplitude squeezing in the frequency region below the cavity bandwidth. The noise equivalent circuits and the quantum-mechanical Langevin equations are derived and discussed for clarifying the difference between an ordinary laser oscillator with shot-noise-limited pump fluctuation and a constant-current-driven semiconductor laser.
This paper clarifies the origins of the standard quantum limit for the amplitude noise of a laser-oscillator outgoing field. The amplitude noise within the cavity bandwidth, \ensuremath{\Omega}\ensuremath{\le}\ensuremath{\omega}/Q, is ultimately caused by the pump amplitude fluctuation, while that above the cavity bandwidth, \ensuremath{\Omega}\ensuremath{\ge}\ensuremath{\omega}/Q, is due to the field zero-point fluctuation. The uncertainty product of the amplitude- and phase-noise spectra at an extremely high pumping level is still larger than the Heisenberg minimum-uncertainty product because of the presence of nonstationary phase-diffusion noise. In this sense, an ordinary laser oscillator is not a quantum-limited device. This paper suggests that a laser oscillator can produce an amplitude-squeezed state in itself if the pump amplitude fluctuation is suppressed below the ordinary shot-noise level. The paper discusses possible schemes for suppressing pump fluctuation, commutator bracket preservation without pump fluctuation, and resulting amplitude and phase spectra. The similarity of and difference between a pump-noise-suppressed laser and a cavity degenerate parametric amplifier are delineated.
Current laser systems have phase and amplitude fluctuations which are partially quantum in origin. The amplitude fluctuations of a highly stabilized laser are shot noise limited, that is they have quantum fluctuations due to the Poissonian fluctuations of photons in a coherent state. These photon number fluctuations have been shown to have an origin in the incoherent pumping mechanism used for standard lasers. It has recently been demonstrated experimentally that by suppressing the amplitude fluctuations of the pump a laser output with reduced amplitude fluctuations is possible. The phase fluctuations in a laser are due to phase diffusion and are substantially above the quantum limit. Techniques to reduce the laser phase fluctuations have been proposed making use of correlated spontaneous emission.
The quantum noise properties of atomic and semiconductor lasers are compared. The intensity fluctuation spectrum of the output field is calculated for the case of regularly, i.e., sub-Poissonian, pumped lasers. The effects of spontaneous emission on the output amplitude fluctuations may be suppressed in regularly pumped atomic lasers by requiring the lifetime of the lower lasing level to be short in comparison with that of the upper laser level. The output amplitude fluctuations then tend to zero at the laser frequency. In regularly pumped semiconductor lasers, net stimulated emission of photons dominates over spontaneous emission because of the combination of pumping and rapid intraband thermalization due to Coulomb scattering.