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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Generation of amplitude-squeezed light from a pump-suppressed semiconductor laser with dispersive optical feedback

John E. Kitching, Amnon Yariv, Yaakov Shevy

Published Apr 26, 1995• 2 citations
Publisher

Abstract

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.

DOI
10.1117/12.208247

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Optics Letters•Sep 1, 1994•30 citations
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.
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.
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.
Experimental results are reported for amplitude-squeezed states generated from two kinds of 1550-nm distributed-feedback semiconductor laser at room temperature. Both lasers exhibit excess noise under free-running conditions, but the origin of this noise is different for the two lasers. For one of the lasers the excess noise is reduced by enhancement of the side-mode–suppression ratio. For the other the excess noise that degrades the squeezing is reduced by weak dispersive optical feedback produced by a grating detuned from the maximum of its first-order reflection. Thus the grating is used to convert phase noise into intensity noise.
Amplitude-squeezed light has been generated by semiconductor quantum well lasers in numerous optical configurations, at both room and cryogenic temperatures. Various intensity noise sources associated with nonideal laser operation, in particular mode partition and polarization-dependent noise, can introduce intensity fluctuations that limit or remove amplitude squeezing. We discuss amplitude noise reduction in low-temperature free-running and injection-locking laser configurations and room-temperature injection locking and external-cavity laser configurations. Drive-current modulation with a nonclassical SNR improvement is achieved with an injection-locked 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.