We analyze and investigate experimentally the output stability and the frequency tuning characteristics of weakly triply resonant silver gallium sulfide (AgGaS<sub>2</sub>) optical parametric oscillators. These oscillators are subject to thermally induced bistability and passive self-frequency-locking phenomena. The robust self-frequency-locking on a single-mode pair (thermal lock) originates from the material’s ability to correct external cavity-length perturbations, which would normally cause a mode hop, by increasing or decreasing the optical path length of the cavity through the thermo-optic effect triggered by the intracavity signal–idler power fluctuations. The Fourier frequency bandwidth of this passive servo is limited to ∼1 kHz by the thermal diffusion time constant, which is proportional to the ratio of the specific heat to the thermal conductivity C<sub>p</sub>/K<sub>c</sub>. A thermal feedback servo gain as high as 180 is obtained, owing to the large thermal figure of merit η= (dn/dT)/K<sub>c</sub> of AgGaS<sub>2</sub>, leading to routine passive mode-hop-free operation for more than 30 min, without the need for an external cavity-length servo. Analysis of the stability range of the thermally loaded standing-wave resonator shows that thermal lensing is less critical for shorter doubly resonant optical parametric oscillator cavities employing shorter-curvature mirrors, in agreement with experimental observations. When a doubly resonant oscillator operates near the boundary of the power stability range a self-pulsing behavior is observed on a number of axial mode pairs. This self-pulsing is found to originate from the destabilization of a self-locked cw state, and the transition from self-pulsing to the stable self-frequency-locked state is found to be controlled by the pump frequency detuning. The passive stability allows the single-parameter frequency tuning to be studied. Under pure thermal lock operation the oscillators show a tendency to resist the pump frequency and temperature tuning processes. When an external cavity-length servo is implemented continuous tuning over 850 MHz, by means of the pump frequency tuning (Δν<sub>s</sub>/Δν<sub>p</sub>≈0.66), and over 100 MHz, by means of the crystal temperature (Δν<sub>s</sub>/ΔT≈250 MHz/°C), is obtained. These tuning ranges are in good agreement with calculations based on a cold doubly resonant oscillator.
© 1999 Optical Society of America
(160.4330) Materials : Nonlinear optical materials
(190.1450) Nonlinear optics : Bistability
(190.4870) Nonlinear optics : Photothermal effects
(190.4970) Nonlinear optics : Parametric oscillators and amplifiers
A. Douillet, J.-J. Zondy, A. Yelisseyev, S. Lobanov, and L. Isaenko, "Stability and frequency tuning of thermally loaded continuous-wave AgGaS2 optical parametric oscillators," J. Opt. Soc. Am. B 16, 1481-1498 (1999)