A self-phase-locked degenerate femtosecond optical parametric oscillator (OPO) based on the birefringent nonlinear material, bismuth triborate, BiB3O6, synchronously-pumped by a Kerr-lens-mode-locked Ti:sapphire laser at 800 nm is described. By exploiting versatile phase-matching properties of BiB3O6, including large spectral and angular acceptance for parametric generation and low group velocity dispersion in the optical xz plane, stable self-phase-locked degenerate OPO operation centered at 1600 nm is demonstrated using collinear type I (e → oo) interaction in a 1.5-mm crystal at room temperature.
The degenerate OPO output spectrum extends over 46 nm (∼5.4 THz) with 190 fs pulse duration for input pump pulses of 155 fs with a bandwidth of 7 nm. Phase coherence between the pump and degenerate output is verified using f-2f interferometry, and discrete frequency beats caused by different carrier-envelope-offset frequencies are measured using radio frequency measurements.
Photo shows a 1.5-mm BiB3O6 crystal used as a nonlinear gain medium in a degenerate self-phase-locked femtosecond OPO operating at room temperature. The green beam is the result of non-phase-matched sum-frequency mixing between the pump light and the sub-harmonic OPO field at degeneracy.