High energy nanosecond laser systems for nonlinear optics and remote sensing applications

During my PhD research activity carried out at the Laser Source Laboratory of the University of Pavia, I had the opportunity to study and develop complex high-energy (from several mJ to several tens of mJ) and high spectral purity, even single longitudinal mode (SLM), nanosecond laser systems operating at 1 μm and 2 μm. High spectral purity, high energy nanosecond pulses are requested in many applications ranging from nonlinear optics frequency up- and down-conversion, high resolution spectroscopy, LIDAR and remote sensing. The first laser system I had the opportunity to developed is a table-top pulsed Master Oscillator Power Amplifier (MOPA) laser originally intended as a part of a more complex laser system aimed to work as a transmitter for High Spectral Resolution LIDAR (HSRL) applications. Given the very demanding specifications in terms of both spectral and spatial quality, and high pulse energy, we opted for a MOPA architecture based on a high quality, compact, low-energy seeder and a suitable number of amplifiers. As a seeder we designed and built a Nd:YVO4 microchip laser operating in the gain switching regime. The gain switching technique has the main drawback of producing low-energy pulses. This required a careful design of a multi-stage, high-gain amplification chain. This MOPA laser was employed as a pump source for a nonlinear optics experiment consisting in the demonstration of Raman conversion in the visible with a SrWO4 crystal. During the second year of my PhD, I took part in a 6-month research activity at KTH Royal Institute of Technology in Stockholm, Sweden. During this research activity I had the opportunity to take part in a project aimed to build a tunable double-wavelength nanosecond excitation source, with pulse energies in the range of tens of mJ. This rather complex MOPA laser system was intended to be a universal laboratory source for testing different THz platforms which were developed in parallel by other colleagues at KTH. In order to achieve higher power conversion efficiency in THz generation the tunable source was designed to operate in the spectral range around 2 μm providing two wavelengths close enough to permit THz generation through difference frequency generation in proper nonlinear materials. The architecture chosen for the 2-μm laser source was an Optical Parametric Oscillator (OPO) followed by an Optical Parametric Amplifier (OPA) both pumped at 1 μm, since this was the most practical solution for achieving the required pulse energies in the spectral range of interest. During the final year of my PhD I focused my research activity on high-energy (multi-mJ), Q-switched nanosecond laser oscillators architecture suitable to be employed in remote sensing and military applications. This kind of applications are usually very demanding for what concerns the robustness of the laser, since it is supposed to be working in harsh conditions. To this purpose, Porro prisms have been widely employed in substitution of usual flat or concave mirrors in Fabry-Perot resonators. Porro prisms are retro-reflecting prisms with 90◦-cut reflecting surfaces that allow the laser beam to be reflected into the same direction where it comes from, up to a small angle of misalignment from the normal of the input plane face of the prism. To this purpose, I investigated both a Passively Q-switched (PQS) and an Electro-Optical Modulator (EOM) Actively Q-Switched (AQS) resonator configurations. The interest in these two different Q-switching techniques is motivated by the trade-off required from the specific application, i.e., generally AQS can provide higher energies as well as a higher pulse-to-pulse jitter stabilization, whereas PQS architectures generally are far simpler and therefore more cost-effective and reliable.