Frequency comb generation in a quadratic nonlinear waveguide resonator

We demonstrate a frequency comb generation via a quadratic nonlinear optical waveguide in a resonator as a monolithic device. In the experiment, we constructed the device by a periodically-poled lithium niobate waveguide with high reflective coatings for 1.5- mu m light at both ends of the waveguide. When we set a pump frequency at 1540 nm to on-resonance, we observed a wide spectral broadening up to 60 nm around the pump frequency due to cascaded processes of second harmonic generation and optical parametric oscillation. A narrow radio frequency beat spectrum of the frequency comb appeared at 3.5 GHz which corresponds to the free spectral range of the resonator. Furthermore, when we use two pump lights, we succeeded in a control of the mode spacing of the frequency comb by adjusting the frequency difference of the two pump light.