Pump noise in an optical parametric oscillator operated above threshold

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.