Parametric generation of picosecond radiation with high spectral $Q$ factor and diffraction-limit divergence in a resonator by mode-locked pumping

A study was made of the resonator configuration of a picosecond optical parametric oscillator in which $\alpha-HJO_3$ crystals were excited by phase-matched pumping. The configuration ensured a high energy efficiency of the process $(\eta_E\approx10\%)$ even at low pump intensities. The product of the pulse duration and the band width reached $\tau\triangle\nu=0,7$ and the divergence of the radiation was $10^{-3}$ rad, demonstrating the advantage of such resonator oscillators compared with the traveling-wave systems. The oscillator was pumped by a train of pulses of the second harmonic of a highly stable phosphate glass laser at a repetition frequency up to 2 Hz.