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Nonclassical intensity correlation from a type I phase matched optical parametric oscillator

N. C. Wong, K. W. Leong, Jeffrey H. Shapiro

Published May 21, 1990• 0 citations
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Abstract

We report here the observation of nonclassical correlation in the intensities of the nondegenerate signal and idler outputs from an optical parametric oscillator (OPO).1 Our cw OPO consists of a 25-mm long type I phase matched LiNbO3:MgO crystal inside a single ended cavity, which is resonant for both the signal and idler. The output coupling for the IR signal and idler is ~4.5%, while the internal round trip power loss is estimated to be 1.9%.

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European Quantum Electronics Conference (EQEC)•Aug 24, 2005•0 citations
We report here the observation of nonclassical correlation in the intensities of the nondegenerate signal and idler outputs from an optical parametric oscillator (OPO).1 Our cw OPO consists of a 25-mm long type I phase matched LiNbO3:MgO crystal inside a single ended cavity, which is resonant for both the signal and idler. The output coupling for the IR signal and idler is ~4.5%, while the internal round trip power loss is estimated to be 1.9%.
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.
A cw optical parametric oscillator (OPO) operated above threshold is of particular interest in current squeezing experiments. Quadrature squeezing in the signal and idler modes and in the pump mode is predicted. Intensity squeezing in the difference intensity of the signal and idler is predicted and has recently been observed.1 We have built a doubly resonant cw OPO with a magnesium oxide- doped lithium niobate crystal that is single-pass-pumped with the 530.9-nm line of a kryptonion laser. Mode hopping instability near degeneracy puts stringent requirements on the frequency stability of the pump laser and the mechanical stability of the OPO cavity. We have oberved such instability near degeneracy and determined the stability requirements for this OPO. Stable operation can be obtained as the OPO is operated away from degeneracy but at a higher threshold. Classically the single-pass pump output is predicted to be clamped at the threshold value so that the OPO in this configuration acts as a pump power limiter. Quantum mechanically, we have found that the pump intensity noise is enhanced. Measurements on the pump intensity noise spectrum in this particular configuration have been made and compared to our theoretical predictions.
We have carried out a linearized quantum analysis of the steady-state operation of an optical parametric oscillator (OPO) without cavity detunings; our analysis emphasizes the effects of pump quantum noise and pump excess noise. We show that pump excess noise reduces the observable signal/idler intensity correlation at low frequencies only in the presence of a cavity-loss mismatch between these subharmonic waves. This effect is so strong that the combination of pump excess noise and cavity-loss mismatch explains the rapid low-frequency noise increase well above the shot-noise level that has been seen in recent signal/idler correlation measurements. Moreover, the critical dependence of intensity correlation on cavity-loss mismatch opens a new possibility for ultrasensitive intracavity absorption spectroscopy. Specifically, sinusoidal amplitude modulation of the OPO pump beam with lock-in amplification of the signal/idler photocurrent affords a sensitive probe of subharmonic cavity-loss mismatch. Although this robust measurement capability exploits the strong intracavity correlation between the signal and idler waves, it does not require that nonclassical correlation be externally observable, nor is it limited to near-threshold OPO operation.
Quantum images, that is inhomogeneous field distributions purely generated by quantum fluctuations, persist when passing from the degenerate to the non-degenerate case of optical parametric oscillators (OPO). Below the threshold for parametric oscillation where the near-field distributions are homogeneous both in intensity and phase, appropriate spatial correlation functions anticipate the transverse spatial pattern that appears above threshold. In particular, the angular dependence of the far field spatial correlation function is able to reveal the travelling-wave nature of the phase pattern above threshold typical of nondegenerate OPOs. Cross-correlation functions between signal and idler intensities show clear evidence of the non-classical nature of the output light.