We demonstrated optical frequency comb generation in a doubly resonant cavity SHG system. The experiment is based on a lithium niobate nonlinear crystal placed in a traveling-wave optical cavity, pumped by a cw Nd:YAG laser emitting 0.5 W at 1064 nm [Fig. 1(a)]. The cavity is resonant for frequencies around both the fundamental pump and its second harmonic at 532 nm, while the nonlinear crystal is a 15-mm-long, 5%-MgO-doped periodically poled lithium niobate sample, quasi phase-matched for SHG. One of the mirrors is mounted on a piezo-electric actuator, in order to finely control the cavity path length. An intracavity adjustable silica window allows to separately set the detunings of the pump and its second harmonic from the corresponding nearest cavity resonances, respectively.

The cavity is locked to the pump laser frequency by acting on the piezoelectric actuator via the Pound-Dever-Hall offset locking technique, which exploits the natural birefringence of the nonlinear crystal. To this end, while the cavity is pumped by a vertically polarized high-power beam (primary beam), an orthogonally polarized low-power auxiliary beam is phase modulated through an electrooptic modulator and sent to the cavity in the opposite direction. In general, the two beams resonate for two different cavity lengths, however, by properly choosing the modulation frequency of the auxiliary beam, it is possible to make a sideband resonate in the vicinity of a resonance of the primary beam [Fig. 1(b)]. The cavity is locked in correspondence of that sideband, and, by varying the modulation frequency, we are able to change the relative position of the primary beam frequency and the corresponding resonance.

This scheme permits to achieve stable cavity locking with detunings up to several cavity linewidths, and to observe a large variety of comb regimes, with different teeth spacing and spectral span, as shown in Fig. 2(a) and (b). Figure 2(c) shows the cavity power profile at the fundamental frequency observed by slowly scanning the laser-cavity detuning. Here, zero frequency marks the center of the bell-shaped cavity resonance of the “cold” cavity. The high-power profile is strongly displaced to lower detunings because of photothermal effects, while the power jumps in the profile mark the separation between different comb regimes. The lowest experimental threshold for comb formation is as low as 7 mW, showing the possibility to significantly reduce the input pump power with respect to singly resonant configurations with threshold around 100 mW.