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Optics Letters

Amplitude noise reduction in semiconductor lasers with weak, dispersive optical feedback

John E. Kitching, Yaakov Shevy, Robert W. Boyd, Avigail Yariv

Published Sep 1, 1994• 30 citations
Publisher

Abstract

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.

DOI
10.1364/ol.19.001331

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Physical Review Letters•Apr 21, 1997•58 citations
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.
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.
Amplitude-squeezed states are produced from a room-temperature Fabry-P\'erot quantum-well semiconductor laser using a combination of pump suppression and weak, dispersive optical feedback. Up to 0.9 dB of squeezing is measured using a balanced homodyne detector corresponding to 1.5 dB of squeezing at the output facet of the laser. A comparison of the data to theoretical predictions suggests that the excess noise measured under free-running conditions could be a result of incomplete side-mode suppression in combination with asymmetrical cross-mode nonlinear gain saturation.
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.
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 light from a quantum-well semiconductor laser with weak optical feedback from a highly dispersive grating is employed for laser Doppler anemometry. Up to 2 dB noise reduction below the shot-noise level is observed with a feedback factor of $1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. Enhanced sensitivity is demonstrated in the Doppler measurement of a gas flow velocity with an improvement in the signal to noise ratio of 1.0 dB above the shot-noise limit.