Frequency comb generation by a continuous-wave-pumped optical parametric oscillator based on cascading quadratic nonlinearities

摘要

We report optical frequency comb generation by a continuous-wave pumped optical parametric oscillator (OPO) without any active modulation. The OPO is configured as singly resonant with an additional nonlinear crystal (periodically poled MgO:LiNbO_3) placed inside the OPO for phase mismatched second harmonic generation (SHG) of the resonating signal beam. The phase mismatched SHG causes cascading χ~(2) nonlinearities, which can substantially increase the effective χ~(3) nonlinearity in MgO:LiNbO_3, leading to spectral broadening of the OPO signal beam via self-phase modulation. The OPO generates a stable 4 THz wide (-30 dB) frequency comb centered at 1.56 μm.