Abstract: We used elliptical beams to demonstrate aperture scaling effects in nanosecond single- and multi-grating periodically poled lithium niobate (PPLN) monolithic optical parametric oscillators (OPOs) and generators (OPGs). Increasing the cavity Fresnel number in single-grating crystals broadened both the beam divergence and the spectral bandwidth. Both effects are explained in terms of the phase matching geometry. These effects are suppressed when using a multi- grating PPLN crystal since the individual gratings provide small effective subapertures. A flood-pumped multi-grating OPG displayed a low output beam divergence, and contained nineteen pairs of signal and idler frequencies. We also demonstrated beam quality improvement in a pulsed periodically poled lithium niobate monolithic optical parametric oscillator (OPO) by operating at degeneracy, which eliminated the off-axis non-collinear phase-matched interactions. Using a 10:1 elliptical beam, we obtained 110 mJ of degenerate output when pumping with 175 mJ. We observed narrowing of the degenerate OPO beam divergence and output spectrum when temperature tuning beyond the collinear degenerate phase-matching condition. !15
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