Coherence and Two-Color Squeezing in Doubly Resonant Optical Parametric Oscillators
Optical parametric oscillators (OPO's) are enjoying a revival of interest. New materials and improved pump sources are enabling OPO's to live up to their decades -old promise of providing efficient, narrowband, widely tunable sources of light. In this thesis I describe advances in efficient, high coherence OPO's achieved through the use of improved pump lasers and monolithic OPO resonator designs. A doubly resonant optical parametric oscillator (DRO) fabricated from MgO:LiNbO_3 was pumped by a frequency-doubled, diode-laser-pumped Nd:YAG laser. Threshold for cw operation was 12 mW, and maximum DRO output power was 8.15 mW at two times above threshold, with a conversion efficiency for the incident pump of 34%. The DRO was temperature tuned from 1007 nm to 1129 nm, operated on a single axial mode pair over most of the range, and could be electric field tuned by as much as 38 nm near degeneracy. An alternate laser pump source was developed for greater power at 532 nm and increased reliability. A lamp -pumped, 13-watt cw Nd:YAG ring laser at 1064 nm was injection locked using a 40-mW single-frequency diode-laser-pumped Nd:YAG laser as the master oscillator. Over 200 mW of light at 532 nm could be obtained by single-pass frequency doubling in a 2-cm crystal of MgO:LiNbO_3 . The monolithic doubly resonant OPO exhibited very high coherence. The linewidth of the DRO signal was less than 13 kHz, the DRO was shown to generate a phase-locked subharmonic of the pump at degeneracy (acting as a coherent div2 optical frequency divider), and the signal and idler were shown to be mutually coherent with the pump and phase anti-correlated with each other away from degeneracy. The signal-idler heterodyne linewidth was 500 Hz, and pump phase modulation was shown to transfer to the DRO phase at degeneracy. 'Two-color squeezing,' or sub-shot-noise correlations of the signal and idler amplitudes were observed in DRO's with various output couplings. The best results were obtained with a DRO with nominal 1.2% output coupling where two -color squeezing 54% below the shot-noise level was observed, in good agreement with theory.
- Pub Date:
- Physics: Optics