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Optics Letters

Pulsed squeezed-light measurement: a new technique

Orhan Aytür, Prem Kumar

Published Apr 1, 1990• 22 citations
Publisher

Abstract

A transient noise measurement scheme is described to observe pulsed squeezing generated by means of parametric downconversion of a Q-switched laser. In the usual implementation of balanced-homodyne detection, the vacuumstate noise level is determined by the average local oscillator power. However, if the local oscillator is derived from a pulsed laser, as is the case in our experiments, the associated peak power can easily saturate the detectors along with the subsequent electronics. To overcome this limitation we have implemented a hybrid frequency-time-domain measurement scheme in which the vacuum-state noise level is determined by the peak, instead of the average, local oscillator power, thus avoiding the saturation problem.

DOI
10.1364/ol.15.000390

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Physical Review Letters•Nov 30, 1987•283 citations
A transient noise measurement scheme is described to observe pulsed squeezing generated by means of parametric downconversion of a Q-switched laser. In the usual implementation of balanced-homodyne detection, the vacuumstate noise level is determined by the average local oscillator power. However, if the local oscillator is derived from a pulsed laser, as is the case in our experiments, the associated peak power can easily saturate the detectors along with the subsequent electronics. To overcome this limitation we have implemented a hybrid frequency-time-domain measurement scheme in which the vacuum-state noise level is determined by the peak, instead of the average, local oscillator power, thus avoiding the saturation problem.
Squeezed light with partial coherence is generated via optical parametric amplification (OPA) in a KTiOPO4 crystal that is pumped by the second harmonic of a Q-switched multimode Nd:YAG laser.1 More than 1 dB of squeezing is observed when a proper mode of the downconverted light is homodyned with the 380-ns multilongitudinal-mode local oscillator (LO) pulse. In such a system, large OPA gain is easily obtained in a single-crystal signal-pass setup. The detection of squeezed light is, however, not straightforward. In the usual implementation of a balanced homodyne detection scheme,2 the vacuum-noise level, as measured on a radio frequency spectrum analyzer, is determined by the average LO power. If the LO is derived from a Q-switched laser, as is the case in our experiment, the associated peak power can easily saturate the detectors and the subsequent electronics. To overcome this limitation, we have implemented a hybrid frequency/time domain measurement scheme in which the vacuum noise level is determined by the peak (instead of the average) LO power. The LO average power requirement is thus relaxed and the detector saturation problem avoided.
Pulsed squeezed light and intensity-correlated twin beams are generated by means of a traveling-wave optical parametric amplifier (OPA) consisting of a type-II phase-matched KTiOPO4 crystal that is pumped by a Q-switched and frequency-doubled Nd:YAG laser. Q switching of the laser results in a high peak pump intensity and a large peak parametric gain. Squeezed light is generated by amplifying the vacuum input field to the OPA, and it is detected by a balanced homodyne configuration that uses the fundamental beam of the laser as a local oscillator.1 Twin beams are generated by supplying a small input signal to the OPA. The amplified signal and idler beams are direct-detected individually and yield quantum-noise reduction when the two photocurrents are subtracted.
The degree of squeezing generated by parametric downconversion or four-wave mixing can be limited by the ratio of the nonlinear coupling to the linear losses in many practical systems. This ratio can be enhanced by increasing the pump power used for the nonlinear coupling in many solid nonlinear systems where the linear losses are not directly related to the nonlinear squeezing process. Pulsed pump sources are attractive for optimizing the nonlinear coupling-to-loss ratio. We have shown1 that homodyne detection can be used to detect the pulsed squeezing generated by a mode-locked pulse train from the pump laser. A portion of the pump beam is used as the local oscillator which stroboscopically samples portions of the squeezed light pulse train. The amount of squeezing which can be measured as a function of the relative durations of the pump and local oscillator pulses is shown. Progress of recent experiments using both four-wave mixing and parametric downconversion is described.
We report pulsed squeezed-light generation by means of an optical parametric downconverter that is pumped by the second harmonic of a mode-locked Q-switched laser. Using the fundamental beam of the laser as a local oscillator, we observe 2-dB squeezing for a parametric gain of 2.0. This local oscillator, however, is nonoptimal because of its spatiotemporal mode mismatch with the squeezed mode generated by the downconverter. We describe an experiment in which a matched local oscillator is generated with the use of an optical parametric amplifier that is pumped by the same laser as is the downconverter. In this case, 2-dB squeezing is observed for a parametric gain of 1.5. The present experimental setup is limited by the uncontrollable phase fluctuations that prohibit us from making squeezing measurements at higher parametric gains.
A photodetector's electronic bandwidth is the frequency range over which beats between different frequencies in the input can be observed in the output. The output of a balanced homodyne detector, the prototypical device for detecting squeezed light, consists of beats between a signal to be measured and a local oscillator. Beats between the signal and quantum local-oscillator fluctuations lead to noise that degrades the observed squeezing. Through wide-band calculations, we show that this noise contains contributions from the entire signal bandwidth, even when this bandwidth is orders of magnitude larger than the detector's electronic bandwidth. This allows us to place an upper limit on the total power that can enter the detector's signal port (both signal and extraneous light) within the detector's optical bandwidth in order for a given level of squeezing to be observable.
Squeezed light in the form of a train of pulses, each approximately 100 ps in width, has been generated by parametric down-conversion in KTiOPO4{\mathrm{O}}_{4}O4​ crystals. The measured noise reduction in the quiet quadrature of the field is 0.6 dB below the shot-noise limit. The quadrature noise components are measured with a balanced homodyne detector using a pulse train for the local oscillator. The noise reduction extends over a broad spectrum and has important implications for precision measurement at very short time scales.