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

Photon-noise reduction experiments with a Q-switched Nd:YAG laser

Prem Kumar, Jianming Huang, Orhan Aytür

Published Mar 1, 1991• 2 citations

Abstract

ABSTRACT We demonstrate the use of a commercial mode-locked and Q-switched Nd:YAG laser in photon-noisereduction experiments. There are many frequency bands wherein the photodetection of such a laser isshot-noise limited. By appropriately filtering the photocurrent produced in direct detection of light froman optical parametric amplifier that is pumped with this laser, we have successfully generated twin-beam states of light. We demonstrate that such twin beams can be used to make measurements below the shot-noise limit. 2. INTRODUCTION The fundamental noise in optical measurements using lasers results from the fact that photons of light are detected at random. In a lossless system, one can ascribe the random detection of photons to the random emission of photons by the laser itself. It can be shown from the detailed laser theory' that anideal laser emits photons in a Poissonian fashion2 as shown in Fig. 1. This fact implies that there willalways be fluctuation in the number of photons that are counted in a given time interval. If = ii photonsare counted by the detector in an observation time T, then the variance in the photon count will be givenby the Poisson formula Var(N) = ii. This fluctuation in the detected photon number leads to the followingmaximum signal-to-noise ratio (SNR) that can be obtained using laser light:

DOI: 10.1117/12.48065

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ABSTRACT We demonstrate the use of a commercial mode-locked and Q-switched Nd:YAG laser in photon-noisereduction experiments. There are many frequency bands wherein the photodetection of such a laser isshot-noise limited. By appropriately filtering the photocurrent produced in direct detection of light froman optical parametric amplifier that is pumped with this laser, we have successfully generated twin-beam states of light. We demonstrate that such twin beams can be used to make measurements below the shot-noise limit. 2. INTRODUCTION The fundamental noise in optical measurements using lasers results from the fact that photons of light are detected at random. In a lossless system, one can ascribe the random detection of photons to the random emission of photons by the laser itself. It can be shown from the detailed laser theory' that anideal laser emits photons in a Poissonian fashion2 as shown in Fig. 1. This fact implies that there willalways be fluctuation in the number of photons that are counted in a given time interval. If = ii photonsare counted by the detector in an observation time T, then the variance in the photon count will be givenby the Poisson formula Var(N) = ii. This fluctuation in the detected photon number leads to the followingmaximum signal-to-noise ratio (SNR) that can be obtained using laser light:

Recently, we have observed as much as 7 dB of quantum-noise reduction in the direct difference detection of twin-beam signal and idler outputs from a traveling-wave optical parametric amplifier (OPA).1 The OPA, which consists of a type-II phase-matched KTiOPO4 crystal, is pumped by the second harmonic of a mode-locked and Q-switched Nd:YAG laser. Here we describe an experiment in which we used these twin-beam pulses to improve on the optimal (shot-noise-limited) direct-detection signal-to-noise ratio (SNR) that can be achieved with coherent-state pulses of light of the same peak power.

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.

Pulsed twin beams of light have been generated using an optical-parametric amplifier that is pumped by the second harmonic of a mode-locked and Q-switched neodymium:yttrium aluminum garnet laser. The intensity noise levels on the direct-detected signal and idler beams are found to be correlated by more than 15 dB, and the subtracted noise falls below the quantum limit by more than 6 dB (75%). To our knowledge, this is the first observation of pulsed twin beams of light, and yields the highest quantum-noise reduction observed in any experiment to date. Our measurements are in excellent agreement with the quantum theory of a nondegenerate optical-parametric amplifier.

Temporally incoherent twin beams of light are generated by means of an optical parametric amplifier that is pumped by the second harmonic of a Q-switched multi-longitudinal-mode Nd:YAG laser. For a parametric gain of 1·12, the intensity noise levels on the direct-detected signal and idler twin beams are found to be correlated by 1·4 dB, whereas the subtracted noise falls below the quantum limit by 0·7 ± 0·1 dB (15%). Although no direct coherence experiment is performed on the twin beams themselves, their temporal incoherence is manifested by the stringent path-length matching requirement between the pump and input signal beams.

Measurement slight amounts of absorption of light with accuracy beyond the standard quantum limit has been experimentally demonstrated. The quantum-correlated twin beams used in the measurement were generated from a nondegenerate optical parametric oscillator including an alpha-cut KTiOPO(4) crystal pumped by an intracavity frequency-doubled Nd:YAG laser. The noise in the intensity difference between the twin beams was reduced by 88% below the standard quantum limit (SQL). The signal-to-noise ratio was improved by 7 dB with respect to the SQL of the total light employed in the experiment and by 4 dB with respect to that of the signal light.