We report the experimental demonstration of tunable optical frequency division based on an efficient, one-step parametric downconversion process in a potassium titanyl phosphate (KTP) optical parametric oscillator (OPO). Our compact OPO generated two nearly degenerate subharmonic outputs that can be tuned over a 1.5 THz range in the signal-idler beat frequency by crystal angle tuning and cavity length scanning. We achieved continuous tuning of the beat frequency over a range of 0.5 GHz through the use of temperature and electro-optic tuning of the KTP crystal. A tunable optical frequency divider was realized by phase locking the signal- idler beat frequency to an external microwave frequency source, which yielded a beatnote linewidth at the sub-Hz level. The output frequencies of the phase-locked OPO were therefore precisely controlled by the tunable microwave source and the pump frequency. Characteristics of three different OPO cavity designs and their operations are discussed.
Ngai C. Wong,
"High-performance tunable optical parametric oscillator", Proc. SPIE 1837, Frequency-Stabilized Lasers and Their Applications, (30 April 1993); doi: 10.1117/12.143695; https://doi.org/10.1117/12.143695