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Journal of Modern Optics

Temporally Incoherent Twin Beams of Light

Orhan Aytür, Prem Kumar

Published May 1, 1991• 1 citation
Publisher

Abstract

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.

DOI
10.1080/09500349114550801

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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.
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.
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.
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.
Twin beams generated from a nondegenerate optical parametric amplifier (NOPA) of gaing possess1 an intensity correlation that falls below the quantum limit by (2g – 1). This occurs whenever the input idler to the NOPA is in the vacuum state. Such input is practically advantageous because it makes the NOPA output intensities phase insensitive. When a non-vacuum idler input state is chosen, the NOPA output intensities become phase sensitive. For a proper input state and a suitable pump phase the output intensity correlation falls below the quantum limit by ( g + g − 1 ) 2 , which is 3 dB superior to the vacuum-idler-input result for g >> 1 and is equivalent to the quantum-noise reduction obtainable from a squeezed state generated with the same gain. Thus the sensitivity improvement achievable with NOPA twin beams by using direct detection is equivalent to that obtainable with squeezed states by using homodyne detection. The above equivalence also provides a new scheme for the generation of pulsed squeezed light that, in addition to being simple, automatically generates a matched local oscillator2 for homodyne detection of the squeezed pulses.
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.