rt 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.
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