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Physical Review A

Spectrally asymmetric mode correlation and intensity noise in pump-noise-suppressed laser diodes

Christoph Becher, Edeltraud Gehrig, Klaus J. Boller

Published May 1, 1998• 28 citations
Publisher

Abstract

We report on the experimental observation of a spectrally asymmetric quantum correlation between the intensities of the main mode and a large number of weakly excited side modes in a pump-noise-suppressed laser diode. The total intensity noise of the laser diode results from a generally incomplete cancellation process between these mode intensities, which influences the generation of amplitude squeezed light. Nonlinear gain due to cross mode gain saturation in the semiconductor lasing medium is found to be the origin of the asymmetric intensity noise distribution between short- and long-wavelength side modes. The experimental results are in good agreement with the predictions of a multimode Langevin rate equation model including asymmetric nonlinear gain saturation.

DOI
10.1103/physreva.57.3952

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We report on the experimental observation of a spectrally asymmetric quantum correlation between the intensities of the main mode and a large number of weakly excited side modes in a pump-noise-suppressed laser diode. The total intensity noise of the laser diode results from a generally incomplete cancellation process between these mode intensities, which influences the generation of amplitude squeezed light. Nonlinear gain due to cross mode gain saturation in the semiconductor lasing medium is found to be the origin of the asymmetric intensity noise distribution between short- and long-wavelength side modes. The experimental results are in good agreement with the predictions of a multimode Langevin rate equation model including asymmetric nonlinear gain saturation.
Summary form only given.Laser diodes with pump noise suppression are reliable sources for the generation of amplitude-squeezed light. Recently, it was shown that this amplitude squeezing is not always a single mode effect but can result from a cancellation among anticorrelated fluctuations between the main mode and competing side modes. To obtain more details on this correlation effect, we performed spectrally resolved intensity noise measurements with a pump noise suppressed SDL 5410-C (810 nm) diode laser in different configurations: free-running, injection-locked, and with an external Littrow grating feedback.
We demonstrate experimentally that the intensity noise of so-called "free-running single-mode" laser diodes results from a cancellation between very large anticorrelated fluctuations of the main mode, on one hand, and of many weak longitudinal side modes, on the other hand. When line narrowing techniques are used, intensity squeezing can be observed at room temperature, but this noise reduction is not always single-mode squeezing. These experimental results are in agreement with a simple phenomenological model using Langevin-type equations.
We derive a quantum macroscopic model for a multimode semiconductor laser, starting from a complete microscopic description. Nonlinear gain terms describing saturation and mode–mode interaction are obtained by a suitable treatment of the spectral hole burning in the distribution of carriers. In the case in which there are two neighbouring active longitudinal modes above threshold and for quiet pumping, we calculate the spectrum of the total intensity fluctuations at zero frequency. For moderate pumping level the nonlinear gain terms do not reduce the degree of anticorrelation between modes, leaving the squeezing in total intensity unchanged with respect to the single-mode case. Only for very high pumping does the nonlinear gain suppression become relevant, and the total intensity noise is increased.
We study the impact of weak dispersive optical feedback on the intensity noise of semiconductor lasers by using two-mode quantum Langevin equations with nonlinear gain and loss. The nonlinear gain arises due to spectral hole-burning and population pulsation and the nonlinear loss arises from saturable absorption by deep trap levels. We find that both the total noise and the main mode noise can be reduced with weak dispersive optical feedback; for moderate side mode suppression ratio, the main mode noise can be reduced by as much as 10 dB. Furthermore, the AM noise spectrum is found to be sensitive to the feedback phase.
Amplitude-squeezed states are generated from a room-temperature semiconductor laser using a combination of pump suppression and dispersive optical feedback. The laser amplitude noise is found to be sensitive to extremely weak feedback levels, of the order of 10-8 of the output power. a reduction of the noise from 2% below the standard quantum limit (SQL) under free-running conditions to 19% below the SQL under optimal feedback conditions is obtained. A single mode theory is presented but is found to be inadequate in explaining the measured dependence of the noise reduction on the feedback power. A multimode theory including asymmetrical cross-mode nonlinear gain is proposed to explain this discrepancy.