Frequency comb generation in a continuously pumped optical parametric oscillator

Abstract

We experimentally demonstrate that optical frequency combs can be generated in a cw-pumped, nearly degenerate, doubly resonant optical parametric oscillator (OPO). Moreover, we present a time-domain theoretical model of the OPO and derive a non-instantaneous mean field dynamic equation for the parametric field. Numerical simulations of the mean field equation are in good agreement with the observed comb patterns. OCIS codes: 190.4410, 190.4223. In order to generate optical frequency combs (OFCs), the use of passive nonlinear cavities has been shown to represent an attractive alternative to traditional techniques based on femtosecond mode-locked lasers. Indeed, comb generation has been demonstrated to occur in continuosly pumped resonators with third-or second-order nonlinearities [1,2]. In cavities dominated by the second-order nonlinerity, OFCs can be generated when the nonlinear crystal is phase-matched for second harmonic generation and the cavity is pumped above the threshold of the so-called internally-pumped optical parametric oscillation [2,3]. Here, we experimentally demonstrate, for the first time to the best of our knowledge, that quadratic frequency combs can be directly generated in a cw-pumped, nearly degenerate, doubly resonant optical parametric oscillator (OPO). Comb emission is observed both around the pump frequency 2ω 0 , as well as around the parametric spectral signals around ω 0 . We also report on a time-domain mean field equation for the parametric field dynamics, which includes the effect of dispersion, and thus permits for the efficient modelling of the full comb dynamics. Our equation not only gives a deep insight into the physics of comb generation, but also provides a tool to explore the comb dynamics through numerical simulation.The OPO cavity used in our experiments is made by a 15-mm-long, periodically poled, lithium niobate crystal placed inside a four-mirrors bow-tie cavity which resonates only for the (nearly) degenerate signal-idler waves around 1064 nm (ω 0 ). Two curved and one plane mirrors are highly reflecting (99.9%), whilst the fourth outcoupling mirror has a reflectivity of 98%. The OPO is pumped by a frequency doubled Nd:YAG laser up to about 1 W of green power at 532 nm (2ω 0 ). The power threshold for parametric oscillations is about 30 mW. The temperature of each crystal is actively stabilized by a Peltier element. Moreover, each cavity has a mirror mounted on a piezoelectric actuator for cavity length control. The OPO cavity is locked to the laser degeneracy frequency by FM locking technique, thanks to a few-milliwatts laser beam that is phase modulated and coupled to the OPO cavity in the opposite direction of the pump beam. The infrared cavity output is sent to an optical spectrum analyzer (OSA). Fast photodetectors are used for detecting intermodal beat notes.We adjust the crystal temperature such that the fundamental and parametric field wave vectors, k 2 and k 1 , respectively, satisfy the phase matching condition ∆k = 2k 1 − k 2 = 0, correspondi...