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Electronics Letters

Proposal for generation of amplitude-squeezed light (ASL) from laser diodes with electronic feedback

Jacques Arnaud

Published Jan 5, 1989• 10 citations
Publisher

Abstract

It is generally believed that the amplitude fluctuations of a light beam initially in the coherent state cannot be squeezed below those of shot noise by the simple arrangement of a beamsplitter and a detector, the current from the detector being fed back to the light source or to a modulator. A simple semiclassical theory shows that arbitrary amounts of squeezing can in fact be obtained if a negative opticalconductance device such as a constant-voltage-driven (nonself-oscillating) laser diode is used in place of a conventional detector.

DOI
10.1049/el:19890001

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The control of the quantum state of light has recently been studied extensively, aiming at the achievement of ultra-low noise below the shot noise limit. Several experiments showed that amplitude squeezing could be achieved in constant-current-driven semiconductor lasers. However, the generation of amplitude-squeezed light from distributed feedback semiconductor lasers (DFB) has been up to now considerably less successful than that from Fabry-Perot devices. Simulations indicate that DFB structures with high coupling coefficient length product kL are unlikely to be good sources of squeezed light, because of the evanescence of the optical fields along the grating [1]. They also show, that for low kL values the lowest achievable intensity noise depends on the structure of the laser diode and can be higher than expected from the laser quantum efficiency alone.

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Squeezing more out of a laser

Carlton M. Caves

Machida and Yamamoto used negative feedback from a conventional photodetector to stabilize a laser's intensity and thus to obtain sub-shot-noise photon statistics within the feedback loop. Out-of-Ioop light extracted at a beam splitter, however, displayed super-shot-noise statistics. This behavior can be understood both quantum mechanically and semiclassically. Quantum mechanically the culprit is the vacuum field that enters the beam splitter’s second input port. I describe a technique for extracting sub-shot-noise light by replacing the culprit vacuum field with squeezed light generated by a nonlinear device—a squeezer—that is pumped by a portion of the laser light. Moreover, even if the unstabilized laser is shot-noise-limited, the extracted light can have a better intensity SNR; this signal-to-noise improvement requires both feedback and squeezing.

Annual Meeting Optical Society of America•Jan 1, 1987•0 citations
It is generally believed that the amplitude fluctuations of a light beam initially in the coherent state cannot be squeezed below those of shot noise by the simple arrangement of a beamsplitter and a detector, the current from the detector being fed back to the light source or to a modulator. A simple semiclassical theory shows that arbitrary amounts of squeezing can in fact be obtained if a negative opticalconductance device such as a constant-voltage-driven (nonself-oscillating) laser diode is used in place of a conventional detector.
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.
The control of the quantum state of light has recently been studied extensively, aiming at the achievement of ultra-low noise below the shot noise limit. Several experiments showed that amplitude squeezing could be achieved in constant-current-driven semiconductor lasers. However, the generation of amplitude-squeezed light from distributed feedback semiconductor lasers (DFB) has been up to now considerably less successful than that from Fabry-Perot devices. Simulations indicate that DFB structures with high coupling coefficient length product kL are unlikely to be good sources of squeezed light, because of the evanescence of the optical fields along the grating [1]. They also show, that for low kL values the lowest achievable intensity noise depends on the structure of the laser diode and can be higher than expected from the laser quantum efficiency alone.
Machida and Yamamoto used negative feedback from a conventional photodetector to stabilize a laser's intensity and thus to obtain sub-shot-noise photon statistics within the feedback loop. Out-of-Ioop light extracted at a beam splitter, however, displayed super-shot-noise statistics. This behavior can be understood both quantum mechanically and semiclassically. Quantum mechanically the culprit is the vacuum field that enters the beam splitter’s second input port. I describe a technique for extracting sub-shot-noise light by replacing the culprit vacuum field with squeezed light generated by a nonlinear device—a squeezer—that is pumped by a portion of the laser light. Moreover, even if the unstabilized laser is shot-noise-limited, the extracted light can have a better intensity SNR; this signal-to-noise improvement requires both feedback and squeezing.