Optical frequency conversion of high energy pulses with high beam quality and application in standoff detection of biological aerosols

Abstract

Three main topics are addressed in this dissertation. The first is concerned with the techniques that can be used to improve beam quality from optical parametric oscillators (OPOs) generating high energy pulses in the nanosecond time regime. Such OPOs require the use of wide beams to avoid optical damage, in combination with a short resonator to enable a sufficient number of roundtrips to ensure high conversion efficiency. The resulting high Fresnel number of the resonator tends to favor multi-mode operation, and thereby a poor beam quality. A solution to this problem, studied theoretically and experimentally in this work, is the orthogonal critical planes optical parametric oscillator (OCP-OPO), which takes advantage of the beam quality improvement induced by the spatial walk-off between the signal and idler beams in the nonlinear crystals used in the OPO. OPOs using birefringent nonlinear materials and type 2 collinear critical phase matching have walk-off between signal and idler. When only a single nonlinear crystal is used, walk-off takes place in only one of the transversal directions, namely the critical plane, where there is a nonzero angle between the propagation vector (~k) and the Poynting vector (~S ). However, using two different types of nonlinear crystals with orthogonal critical planes, walk-off takes place in orthogonal transversal directions in the two crystals when they are phase matched for the same process. The same is achieved when two identical crystals in an OPO are separated by a half-wave plate that rotates the linearly polarized pump and signal beams by 90 , and the crystals are oriented accordingly. Both types of OPO were demonstrated experimentally. The latter type has previously been proposed in the literature. The OCP-OPO permits the use of a simple and short linear resonator, allowing two-pass pumping, easy alignment, and wavelength tuning, and it has proven to be a reliable frequency conversion source for laboratory and field use. Secondly, the thesis presents experimental results of sum frequency mixing of the output of an OCP-OPO with its pump pulse (or higher harmonics of the pump) to generate high-energy pulses in the ultraviolet. The results were obtained in a compact experimental setup. The achieved conversion efficiencies are comparable to the previously best published work on OPOs of similar complexity. Simulations indicate that even higher conversion should be possible with an optimized pump laser, but modifying this laser or building our own pump laser were out of scope of this work. Finally, the ultraviolet sum-frequency device described above was employed in an experimental lidar setup for standoff detection of biological aerosols based on laser induced fluorescence. The instrument was used to acquire fluorescence spectra in an experiment where different biological agents were released in a semiclosed chamber at two hundred meters standoff distance. The fluorescence spectra of the biological aerosols were compared for different excitation wavelengths. The results showed that excitation at 294 nm was more effective than the more common excitation at the third harmonic of a Nd:YAG laser (355 nm). The ratio between induced fluorescence and excitation pulse energy was as expected higher at the shorter wavelength. More importantly, however, was the diversity of the fluorescence spectra at the shorter wavelength excitation. This holds promise of improving automatic classification of potentially harmful biological agents from background aerosols such as pollen and fungi.