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Annual Meeting Optical Society of America

Pump noise in an optical parametric oscillator operated above threshold

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

Published Jan 1, 1988• 0 citations
Publisher

Abstract

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.

DOI
10.1364/oam.1988.tul3

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Acta Physica Sinica•Jan 1, 2003•0 citations
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.
Summary form only given. Quasi-phase matched crystals are very promising materials for nonlinear optics because of their high nonlinear coefficient. We have built a CW triply resonant OPO using a periodically poled lithium niobate crystal. It is pumped at 1.06 /spl mu/m and produces signal and idler beams around degeneracy in the range 2-2.3 /spl mu/m. Tunability is achieved by mode hopping when the temperature is varied. This device has an ultra-low threshold of 300 /spl mu/W. It can be operated in a stable way over long period of time. Due to the so-called cascaded nonlinearities, OPO can efficiently reduce the fluctuations of the pump beam reflected by the cavity without reducing too much its intensity. We have been able to measure on the reflected pump beam a noise reduction of 30% (not corrected for detection efficiency), corresponding to 38% reduction of the quantum fluctuations below shot noise at 4 times above threshold. Our set-up constitutes an efficient and simple quantum noise eater for laser beam around 1 /spl mu/m in the mW range.
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 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.
We have used an ultralow threshold continuous-wave optical parametric oscillator (OPO) to reduce the quantum fluctuations of the reflected pump beam below the shot noise limit. The OPO consisted of a triply resonant cavity containing a periodically poled lithium niobate crystal pumped by a Nd:YAG (yttrium aluminum garnets) laser and giving signal and idler wavelengths close to $2.12\ensuremath{\mu}\mathrm{m}$ and a threshold as low as $300\ensuremath{\mu}\mathrm{W}.$ We detected the quantum fluctuations of the pump beam reflected by the OPO using a slightly modified homodyne detection technique. The measured noise reduction was 3030%30 (inferred noise reduction at the output of the OPO 3838%).38
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.
A 106μm all-solid-state single frequency Nd∶YVO4 laser was explored to pump a triply resonant optical parametrie oscillator(OPO) based on the periodically poled lithium niobate. When the OPO operated near degeneracy, the amplitude fluctuation correlation of the signal and idler modes was observed as their wavelengths were separated about 200nm. The measured squeezing is 04dB. The amplitude fluctuation correlation existed in a range of 17nm as we tuned the wavelengths of the signal and idler by change the crystal temperature.