Intensity noise of pump-enhanced continuous-wave optical parametric oscillators

We investigate the intensity noise spectra of a pump-enhanced continuous-wave (cw) optical parametric oscillator (OPO) as a function of the pump power. When the OPO is operating sufficiently high above threshold, the intensity noise spectra of the signal wave output exhibit a pronounced peak, that shifts to higher noise-frequencies in proportion to the fourth root of the above-threshold pump power. The width of the peak remains approximately constant. The observed shapes of the noise spectra and the measured dependence on the pump power are in good agreement with a theoretical model derived from the coupled field equations. The agreement shows that the intensity noise is caused by relaxation oscillations of the OPO which resonantly enhance the spectrally white intensity noise of the pump laser around the OPO's relaxation oscillation frequency. The pump laser is an A1GaAs diode laser master-oscillator power-amplifier (MOPA) system which delivers up to 250 mW of single-frequency, 808-nm radiation in a near diffraction limited beam. The cavity of the OPO consists of two mirrors to enhance the pump laser power and resonate the signal and idler waves. The triply resonant OPO is based on noncritical type-II phasematching in RbTiOAsO4 (RTA), which provides a threshold pump power of 16 mW. The maximum signal wave power is 23 mW at a wavelength of 1243 nm. The spectral bandwidth of the signal wave is less than 750 kHz.