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Physical Review A

Use of quantum-noise correlation for noise reduction in semiconductor lasers

Anders Karlsson, Gunnar G. Bjork

Published Dec 1, 1991• 24 citations
Publisher

Abstract

Both the junction voltage fluctuations and the frequency fluctuations of a semiconductor laser are highly correlated with its intensity noise. We analyze three schemes where the correlation information is used to reduce the intensity noise. In conventional semiconductor lasers, starting with intensity noise at the standard quantum limit, ideally 3-dB amplitude squeezing is possible. The influence of nonideal factors such as parasitics for junction voltage feedforward and the linewidth floor for frequency noise feedforward are discussed. It is shown that under certain conditions the number-phase uncertainty can reach the ultimate limit set by Heisenberg's uncertainty principle. These conditions are clarified, and examples are given for both conventional semiconductor lasers and microcavity lasers.

DOI
10.1103/physreva.44.7669

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Both the junction voltage fluctuations and the frequency fluctuations of a semiconductor laser are highly correlated with its intensity noise. We analyze three schemes where the correlation information is used to reduce the intensity noise. In conventional semiconductor lasers, starting with intensity noise at the standard quantum limit, ideally 3-dB amplitude squeezing is possible. The influence of nonideal factors such as parasitics for junction voltage feedforward and the linewidth floor for frequency noise feedforward are discussed. It is shown that under certain conditions the number-phase uncertainty can reach the ultimate limit set by Heisenberg's uncertainty principle. These conditions are clarified, and examples are given for both conventional semiconductor lasers and microcavity lasers.
The quantum correlation between junction voltage fluctuations1/frequency fluctuations2 and the intensity fluctuations of a semiconductor laser may be used to reduce the intensity noise. Here we address squeezed-state generation discussing both fundamental and practical limitations on noise reduction in conventional semiconductor lasers and microcavity lasers. In conventional semiconductor lasers, starting with intensity noise at the standard quantum limit, ideally a 3-dB amplitude squeezing is possible. This fundamental limit is set by spontaneous emission noise. Another fundamental limit is that noise reduction beyond the level set by optical losses and pump noise is not possible. It is also shown that the number-phase uncertainty in some conditions can reach the ultimate limit set by Heisenberg’s uncertainty principle. These conditions are clarified and a comparison made between junction voltage and frequency noise feedforward. Feedforward schemes realizing intensity noise reduction are discussed.
Summary form only given. Optical phase is conjugate to the photon-number in quantum optics and therefore is important for nonclassical states in which the photon-number operator is reduced below the semiclassical shot noise limit, i.e., for photon-number squeezed states. Nonclassical emission from semiconductor lasers has the added facet that excess phase fluctuations are correlated with fluctuations in the photon-number through the nonlinear gain, and this correlation extends below shot noise. In this experiment, quantum-limited number-phase correlations are examined using an external-cavity semiconductor laser with heavily suppressed (>40 dB) side modes and near ideal single-mode laser characteristics. A feedforward gain matching condition is found to limit the observation of a strong nonclassical effect.
A second-order correlation function approach is adopted to focus on the mechanism for generating amplitude-squeezed light in a semiconductor diode laser pumped with suppressed amplitude fluctuations. Second-order correlation functions between the electromagnetic field intensities and between the electromagnetic field intensities and the charge carrier numbers are calculated. Sub-Poissonian statistics and antibunching effects inside the laser cavity are analyzed; the analysis focuses on various quantities responsible for generating this state of the field. The influence of these quantities on the noise spectra of the detected field and conditions for quantum-noise reduction are considered.
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.
The quantum noise properties of injection-locked semiconductor laser oscillators are investigated. Included in the analysis are pump-noise suppression and squeezing of the injection signal. It is shown that the main effect of injection locking is the reduction of the phase noise of the outgoing laser light field. The influence that squeezing of the injection signal state has on the output state is shown to be strongly dependent on the value of the linewidth enhancement factor. In general, little or no noise reduction stands to be gained by squeezing the injection signal. It is also shown that under idealized conditions, the spectral uncertainty product of the outgoing field will approach, but never go below, the minimum uncertainty value given by the Heisenberg uncertainty relation.