Stability and frequency tuning of thermally loaded continuous-wave AgGaS_2 optical parametric oscillators

We analyze and investigate experimentally the output stability and the frequency tuning characteristics of weakly triply resonant silver gallium sulfide $({\mathrm{AgGaS}}_2)$ 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 $C_p/K_c.$ A thermal feedback servo gain as high as 180 is obtained, owing to the large thermal figure of merit $\eta =(\mathrm{d}n/\mathrm{d}T)/K_c$ of ${\mathrm{AgGaS}}_2,$ 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 $(\mathrm{\Delta }{\nu }_s/\mathrm{\Delta }{\nu }_p\approx 0.66),$ and over 100 MHz, by means of the crystal temperature $(\mathrm{\Delta }{\nu }_s/\mathrm{\Delta }T\approx 250 \mathrm{MHz}/°\mathrm{C}),$ is obtained. These tuning ranges are in good agreement with calculations based on a cold doubly resonant oscillator.