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Physical Review Letters

Nonclassical light from a semiconductor laser operating at 4 K

Wayne H. Richardson, Robert M. Shelby

Published Jan 22, 1990• 75 citations
Publisher

Abstract

Amplitude fluctuations of the light from a semiconductor lase, operating 4 K, have been measured to be 11% below the shot-noise limit in the detected photocurrent. Weak optical reflection into the laser reduces the squeezing from the expected detector-current--to--laser-current transfer ratio of 25%. Results compare favorably with a theory which includes effects of optical feedback.

DOI
10.1103/physrevlett.64.400

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Physical Review Letters•Apr 21, 1997•58 citations
Amplitude fluctuations of the light from a semiconductor lase, operating 4 K, have been measured to be 11% below the shot-noise limit in the detected photocurrent. Weak optical reflection into the laser reduces the squeezing from the expected detector-current--to--laser-current transfer ratio of 25%. Results compare favorably with a theory which includes effects of optical feedback.
A new scheme for generating an amplitude-squeezed state is proposed. The photon-flux fluctuation of a semiconductor-laser output wave is measured with a quantum nondemolition (QND) detector, and is negatively fed back to the laser pumping current. The operator Langevin equations are derived by combining the quantum-mechanical analyses on the laser internal-external field fluctuations, the quantum nondemolition detector based on an optical Kerr effect, and a negative-feedback circuit. The output wave features a reduced photon number noise below the standard quantum limit, 〈(\ensuremath{\Delta}n^)2{)}^{2})2〉^〉, and an enhanced phase noise above that, 〈(\ensuremath{\Delta}\ensuremath{\psi}^)2{)}^{2})2〉>(1/4)〈n^${〉}^{\mathrm{\ensuremath{-}}1}$, while the minimum uncertainty product is still preserved. The observed photoelectron statistics in a negative-feedback GaAs laser diode using a conventional p-i-n photodiode (not a QND detector) are shown to exhibit sub-Poissonian statistics with the variance 〈(\ensuremath{\Delta}n^)2{)}^{2})2〉apeq20.26〈n^〉. The measured photocurrent fluctuation spectral density is also indicated to be below the standard quantum limit by a factor of 0.2 (=-7 dB). The experimental results also confirm that such amplitude squeezing is only observed inside the feedback loop and cannot be extracted from the loop unless a quantum nondemolition detector is used.
We detected a portion of the output from a semi-conductor diode laser and used the detected signal as a source of negative feedback to the laser drive current. At frequencies below the characteristic frequency of the feedback loop, the power spectral density of the detected photocurrent was as much as 15 dB below the shot noise limit. This reduction can be described by a semiclassical theory of the effects of feedback on an injected shot noise current. Physically, this corresponds to a process whereby the laser output is changed in response to changes in the noise current.
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.
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 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.