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Brazilian Journal of Physics

Classical and quantum properties of optical parametric oscillators

M. Martinelli, Carlos L. Garrido Alzar, P. H. Souto Ribeiro, P. Nussenzveig

Published Dec 1, 2001• 17 citations
Publisher

Abstract

We present a review of the Optical Parametric Oscillator (OPO), describing its operation and the quantum correlation between the light beams generated by this oscillator. We show the construction of an OPO using a Potassium Titanyl Phosphate crystal (KTP), pumped by a frequency doubled Nd:YAG laser, and discuss the stability of the system and related thermal effects. We have measured the quantum correlation of signal and idler beams in a transient regime, obtaining a noise correlation level 39 % below the shot noise level.

DOI
10.1590/s0103-97332001000400011

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Journal of the Optical Society of America B•Nov 1, 1994•1 citation
We present a review of the Optical Parametric Oscillator (OPO), describing its operation and the quantum correlation between the light beams generated by this oscillator. We show the construction of an OPO using a Potassium Titanyl Phosphate crystal (KTP), pumped by a frequency doubled Nd:YAG laser, and discuss the stability of the system and related thermal effects. We have measured the quantum correlation of signal and idler beams in a transient regime, obtaining a noise correlation level 39 % below the shot noise level.
We have constructed a mode-locked optical parametric oscillator (OPO), synchronously pumped by the second harmonic of a continuously mode-locked Nd:YAG laser and based on type II phase matching in a KTP crystal. The two subharmonics (signal and idler waves) are nearly degenerate in frequency but are distinct orthogonal polarization states, and the oscillator is thus a nondegenerate OPO. Above threshold, the intensities of the signal and idler output beams should be highly correlated, as recently demonstrated for a similar single-frequency OPO.1 Ideally, the quantum noise on the corresponding photocurrent difference signal is completely suppressed for frequencies that are small compared to the OPO cavity bandwidth. Since this configuration produces intense light beams with potentially very good noise suppression, it is an important prototype for possible applications, e.g., to spectroscopy. Initial results with our OPO show an excess uncorrelated polarization noise, possibly due to depolarized light scattering in the KTP crystal. When the OPO cavity is stabilized with the signal and idler degenerate in frequency, they can be mixed to form a superposition by manipulating their polarization. When the oscillator is pumped below threshold, a pulsed squeezed vacuum state is produced in two such mixtures.2 When the polarizations are mixed inside the OPO cavity by a Faraday rotator, the backaction evasion necessary for a quantum nondemolition (QND) measurement of one of the field quadratures of an externally injected beam is obtained.3 The pulsed local oscillator used in observing the squeezing or QND effects is derived from the fundamental output of the pump laser. Progress in implementing these experiments with our mode-locked OPO is described as well as possible applications to time-domain measurements with noise below the shot-noise limit.
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.
The nondegenerate optical parametric amplifier (OPA) has many uses for investigating quantum fluctuations of light. Among them are EPR-type quadrature-phase correlations,1 QND measurement,2 and phase-sensitive quantum amplification.3 To study these problems, a reliable high-gain low-loss optical parametric amplifier (OPA) is crucial. Although OPAs operated at 1064 nm with KTP crystals pumped by a frequency doubled YAG laser exist,1 their performance is compromised because of problems with beam walk-off and polarization mixing for pairs of critically phase-matched KTP crystals. To avoid such problems, we have constructed a different nondegenerate OPA, which is operated at 1080 nm and consists of a single KTP crystal with type II noncritical phase matching. The pump beam at 540 nm for the amplifier is obtained from intracavity frequency doubling of a single-mode Nd: YAP laser with a similar KTP crystal as in the OPA.
Quantum-correlated twin beams were successfully generated from an optical parametric oscillator (OPO) pumped by a frequency-doubled diode laser. The 50-mW diode laser output at 1080 nm was first converted to 22.5-mW green light, and then was used to pump an OPO to yield signal and idler beams with a total power of 5 mW. Noise reduction of 4.3 dB below the shot-noise level was observed in the intensity-difference spectrum. The setup opens a simple way to a compact and low-cost source of diverse nonclassical states of light.
We have generated bright squeezed light and quantum-correlated twin photon beams by using a cw semimonolithic KTP Optical Parametric Oscillator (OPO), which exhibits a low oscillation threshold and is capable of oscillating even in the free-running (no external-feedback) condition. The pump beam reflected by the OPO, after a cascading nonlinearity process in the crystal, appears as bright squeezed light. The squeezing observed on the reflected pump beam from the semimonolithic OPO was 30% (inferred squeezing: 48%). The signal and idler beams generated by the optical parametric oscillator are highly quantum-correlated twin photon beams. Since the squeezing property of the intensity difference between the twin photonbeams is robust in the condition where the two fields are balanced, the squeezing can be observed even in the freerunning condition of the OPO. This intensity-difference squeezing observed between the twin photon beams from the free-running semimonolithic OPO was 70% (inferred squeezing: 87%).
We obtain information about the amplitude fluctuations of both signal and idler beams from a doubly resonant optical parametric oscillator (DRO) without an optical isolator or a frequency stabilizer. To get a total noise power spectrum, we digitized the photocurrents of the cross-polarized twin beams from the DRO and calculated the noise spectra of the two beams from the digitized data; we also calculated the sum and the difference of the two beams. We fabricated a DRO with a KTiOPO4 (KTP) crystal pumped by a single-mode Ar+ laser (514.5 nm). The experimental results of the quantum-mechanical correlations of the twin beams are well fitted by a theoretical prediction [ J. Opt. Soc. Am. B4, 1520 ( 1987)].