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Journal of Optics B Quantum and Semiclassical Optics

Quantum amplitude modulation noise of semiconductor lasers with weak optical feedback

Hua Deng, Yaakov Shevy

Published Mar 26, 2004• 1 citation
Publisher

Abstract

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.

DOI
10.1088/1464-4266/6/4/010

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•May 24, 1999•0 citations
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.
This thesis describes the phase and amplitude noise properties of semiconductor lasers subjected to weak, dispersive optical feedback. In the first half, experiments demonstrating reductions in the laser frequency noise power spectrum and spectral linewidth by several orders of magnitude are presented. Weak optical feedback is applied to the laser by an external cavity containing an atomic vapor. The presence of the vapor adds to the dispersion of the cavity and simultaneously locks the laser to a fixed frequency reference. The role of 1/f frequency noise in limiting the effectiveness of this linewidth reduction technique is investigated and 1/f noise is found to be the dominant contribution to the linewidth under strong optical feedback conditions. An electronic feedback scheme utilizing FM sideband locking is then implemented alongside the optical feedback, and an additional reduction in the low frequency 1/f frequency noise power spectrum by over two orders of magnitude is obtained. With both systems operating simultaneously, the spectral linewidth is narrowed from its free-running value of about 20 MHz to 1.4 kHz. Excellent absolute frequency stability is also achieved. In the second half, the effects of optical feedback on the quantum mechanical amplitude noise properties of the laser are examined. A fully quantum mechanical theory of amplitude and phase noise for a semiconductor laser with weak optical feedback is developed, and the nature and limits of the noise reduction using this technique are established. Particular attention is given to the feedback-induced enhancement of the amplitude squeezing which can be obtained in a pump-suppressed semiconductor laser: an improvement in the squeezing by 3 dB is predicted under moderate pumping. Somewhat larger noise reductions are expected when the laser is operating closer to threshold. Measurements performed on a laser biased close to threshold are then described and a reduction in the low frequency amplitude noise power spectrum by 7 dB is obtained. An experimental investigation of the effects of optical feedback on the amplitude squeezing in a semiconductor laser is then discussed. The low frequency squeezing in a room temperature device is increased from 3% below the standard quantum limit (SQL) under free-running conditions to 19% below the SQL with optimal feedback. The experimental results are found to agree poorly with the single-mode model and a multi-mode model including the effects of asymmetrical cross-mode non-linear gain is developed to explain the discrepancy. Finally, further experimental investigation into the generation of amplitude squeezed light is presented using a commercial laser with no intentional external modifications. Squeezing as large as 29% below the SQL is measured using a balanced homodyne detector with the laser operating near room temperature, corresponding to 41% below the SQL at the output facet of the laser.
We present the theory and measurements of the amplitude noise spectrum from a semiconductor laser with weak optical feedback (P(fb)/P(out) approximately 10(-6)) from an external cavity containing an element of dispersive loss. The laser noise is found to be reduced over most of the low-frequency spectrum, although an increase in the noise is observed at frequencies corresponding to multiples of the external-cavity free spectral range. The low-frequency noise reduction closely follows theoretical predictions, and a reduction of as much as 7 dB is measured at an injection current of 1.5 times the threshold current. The potential of this method for contributing to the production of amplitude-squeezed light is discussed.
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.
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.
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.