RESUMO
Wavelength tuning of conventional mirror-based optical parametric oscillators (OPOs) exhibits parabolically-shaped tuning curves (type-0 and type-I phase matching) or tuning branches that cross each other with a finite slope (type-II phase matching). We predict and experimentally prove that whispering gallery OPOs based on type-0 phase matching show both tuning behaviors, depending on whether the mode numbers of the generated waves coincide or differ. We investigate the wavelength tuning of optical parametric oscillation in a millimeter-sized radially-poled lithium niobate disk pumped at 1 µm wavelength generating signal and idler waves between 1.7 and 2.6 µm wavelength. Our experimental findings excellently coincide with the theoretical predictions. The investigated whispering gallery optical parametric oscillator combines the employment of the highest nonlinear-optical coefficient of the material with a controlled type-II-like wavelength tuning and with the possibility of self-phase locking.
RESUMO
We demonstrate optical parametric oscillation in a millimeter-sized whispering gallery resonator suitable for broadband infrared spectroscopy. This nonlinear-optical process is quasi-phase-matched using a radial domain pattern with 30 µm period length, inscribed by calligraphic poling. The output wavelengths are selected in a controlled way over hundreds of nanometers. We achieve this by increasing the temperature of the resonator in steps such that the azimuthal mode number of the pump wave rises by one. As a proof-of-principle experiment, we measure a characteristic resonance of polystyrene in the spectral range of 2.25 - 2.45 µm.