| The recent development of periodically poled lithium niobate (PPLN) has spawned a whole new technology for achieving efficient nonlinear frequency conversion of coherent radiation. The high nonlinearity and designable phase-matching characteristics have made PPLN a truly novel material, capable of performing nonlinear conversion in regimes never before accessed. Although many experiments have demonstrated the use of PPLN in low-threshold, continuous-wave (cw) optical parametric oscillators, the operation often requires sophisticated dielectric mirror coatings which limit the wavelength tuning range. Furthermore, in many applications cw operation is not necessary. In such cases, the high nonlinearity of PPLN (27 pm/V) coupled with the high peak power of nanosecond pulses (1 GW/cm2) can produce a nonlinear gain sufficient to reach optical parametric oscillation (OPO) in short crystals with a low finesse cavity, or even cavityless single-pass optical parametric generation (OPG) in slightly longer crystals.; In this work, we discuss the fabrication of plane-parallel monolithic PPLN OPO's and single-pass OPGs, and investigate the operating characteristics when pumped with a 3.5-ns Nd:YAG laser in a variety of configurations. With no external mirrors, these compact monolithic devices require trivial alignment and readily produce simple, stable, rugged, broadly-tunable, low-threshold operation. When pumped with small beams, the devices provide low-energy, near-diffraction-limited output beams with excellent stability. Pumping with large elliptical beams exploits the full crystal aperture and increases the energy handling capability beyond 100 mJ. However, aperture-scaling also permits oscillation of off-axis signal angles, resulting in severe beam quality degradation and spectral broadening for large diameter beams. These effects are measured and explained in terms of a general noncollinear phase-matching geometry, and are suppressed in two systems presented here. First, beam divergence and spectral compression are obtained by tuning the devices through degeneracy. Pumping in a 10:1 elliptical beam allows throughput energies of 175 mJ, and produces over 100 mJ of degenerate broadband output. Second, the separated gratings of a multi-grating crystal provide individual channels of nonlinear gain that confine the signal beams and limit the number of transverse modes in each grating.; This work also addresses several other notable PPLN devices including: diffusion- bonded PPLN stacks; high-reflectivity monolithic PPLN OPO's; and 20°-angle fan-grating OPG's. Advances in the fabrication process, including the real-time visualization of domain formation in PPLN, are also presented. |
