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Journal of the Optical Society of America B

Quantum-mechanical noise characteristics in a doubly resonant optical parametric oscillator

Sun-Hyun Youn, Jai-Hyung Lee, Joon-Sung Chang

Published Nov 1, 1994• 1 citation
Publisher

Abstract

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)].

DOI
10.1364/josab.11.002282

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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)].
We investigated the intensity noise spectra of a single beam of a pump-enhanced continuous-wave optical parametric oscillator (OPO), which was used to generate quantum-correlated twin beams, as a function of the pump power. With a triply (pump-, signal-, and idler-) resonant cavity, the oscillation threshold of our OPO was 8.5±1.3 mW and the measured slope conversion efficiency was 0.72±0.02. Twin beams with a power of 240 mW were generated at a pump power of 350 mW. The relaxation oscillation frequencies, which depend on the pump power, were observed when the pump power of the OPO was 12.5–28 mW. The experimental results confirm the predicted increase in OPO relaxation frequency with pump power. We experimentally inferred squeezing of the single-beam intensity, for the first time to our knowledge, by exploiting the nature of quantum noise that is dependent on loss.
We investigated the intensity noise spectra of the single beam of a pump-enhanced continuous-wave (cw) optical parametric oscillator (OPO), which was used to generate quantum correlated twin beams, as a function of the pump power. With a triply (pump-, signal-, and idler-) resonant cavity, the oscillation threshold of our OPO was about 8.5+/-1.3 mW and the measured slope conversion efficiency was 0.72+/-0.02. A twin beams with power of 240 mW were generated at pump power of 350 mW. The relaxation oscillation frequencies, which depend on the pump power, were observed when the pump power of OPO was from 12.5 mW to 28 mW. The experimental results confirm the predicted increase in OPO relaxation frequency with pump power. Squeezing of the single beam intensity was for the first time inferred experimentally by exploiting nature of quantum noise dependent on loss.
The dependence of quantum correlations of twin beams on the pump finesse of an optical parametric oscillator is studied via a semi-classical analysis. It is found that the phase-sum correlation of the output signal and idler beams from an optical parametric oscillator operating above threshold depends on the finesse of the pump field when the spurious pump phase noise generated inside the optical cavity and the excess noise of the input pump field are involved in the Langevin equations. The theoretical calculations can explain the previously experimental results quantitatively.
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.
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.