Spatial mode engineering in mid-infrared optical parametric oscillators

Abstract: The purpose of this thesis was to investigate the possibilities of enhancing the temporal and spatial coherence of parametric devices in the mid-infrared spectrum by exploiting inherent coherence selection of Bessel beams. Several optical parametric oscillators (OPO) were constructed and characterized for various types of resonators and pump lasers. In the first part of this work an OPO was constructed out of a periodically poled KTP (PPKTP) crystal within a stable resonator. It was pumped by a Q-switched Nd:YAG laser in transversal and longitudinal single mode operation. The OPO was pumped at 1064 nm and operated at signal wavelength of 1595.9 nm with a bandwidth at FWHM of 0.2 nm and idler wavelength 3192.9 nm and bandwidth of 0.5nm at FWHM. The stable resonator was constructed out of a curved input coupler and a planar output coupler to generate a spatially coherent beam. The beam quality factor of the OPO was measured to M2=1.3 and M2=1.7 in the horizontal and the vertical direction respectively. In the second part of this work conical beams, and hence noncollinear phase matching, was used. This was done to improve and study the temporal and spatial coherence of the generated beam if the OPO is pumped by a laser operating in multiple transversal and longitudinal modes. Using noncollinear phase matching provides a possibility of generating a highly coherent signal (or idler) wave when using a pump laser which operates in multiple transversal and longitudinal modes. Usually, such pump laser makes it difficult to generate highly coherent beams since the properties, such as bandwidth and beam quality factor, of the pump laser are transferred to the generated waves. So called multimode lasers, which operate in multiple modes, can be considered inexpensive and reliable while providing high pulse energies, and they are therefore attractive as pump sources for nonlinear processes. Throughout this second part a Q-switched Nd:YAG laser operating in multiple transversal and longitudinal modes at a wavelength 1064 nm was used. Optical parametric generation (OPG) was used to compare the effects between pumping with a Gaussian wave, and collinear phase matching, and pumping with a conical, a Bessel beam and hence noncollinear phase matching. Thereafter, optical parametric oscillation was studied with collinear and noncollinear phase matching using a Fabry-Perot cavity. The OPO pumped by a Q-switched Gaussian beam, and hence using collinear phase matching had a signal wave at 1596.3 nm and a bandwidth at FWHM of 4 nm. The OPO pumped by a conical beam, hence a Bessel beam and noncollinear phase matching, had a signal wave at 1596.5 nm and a bandwidth at FWHM of 0.2 nm. Both OPOs had a beam quality factor M2~2 in horizontal and vertical direction.