Broadband mid-infrared generation with two-dimensional quasi-phase-matched structures

We report the use of highly elliptical pump beams to generate broadband, spatially-chirped mid-infrared light in periodically poled lithium niobate (PPLN). We fabricated PPLN crystals with a fan-out grating period varying continuously from 25.5 to 31.2 μm across a 15-mm width and pumped them in both optical parametric generator and monolithic optical parametric oscillator configurations with a Q-switched Nd:YAG laser. Although the fan-out grating pattern is typically thought of as a continuously varying 1-D quasiphase-matched (QPM) structure, the elliptical pump beam illuminates the full 2-D structure of the fan. The phase-matching and gain characteristics of the crystals prefer noncollinear optical parametric generator operation for elliptical pump beams; however, collinear operation was achieved with polished plane- parallel crystal endfaces such that the Fresnel reflections set up a low- finesse monolithic cavity in the crystals themselves. The generated signal and idler beams were spatially chirped in the near field and angularly chirped in the far field while covering spectral bands as large as 1250 cm^-1. With a simple modification, this system also offers an easy way to generate broadband optical frequency combs across the signal and idler spectral bands. We also present theoretical and modeling considerations for large-aperture pumped or flood illuminated 2-D QPM structures. The Fourier transform of the spatial variation in the nonlinear coefficient in a crystal generates a 2-D map of available grating vectors in wave vector mismatch space. This method can be used to glean phase-matching information from complicated 2-D structures that would be difficult to analyze using other methods