Development of an Er:YAG laser for Range-finding

Abstract

Long distance, high precision range-finding requires high energy, short duration laser pulses that exceed 1MW peak power, as specified by defence companies. However, eye safety is a particularly important consideration for a laser system that will be used outdoors. Currently, this is often achieved by frequency shifting the 1064nm output from a Q-switched Nd:YAG laser to the ‘eye-safe’ band (1.5 um to 1.8 um). A 1617 nm or 1645 nm Er:YAG laser operating on the 4I13/2 to 4I15/2 transition can satisfy this need without additional frequency shifting systems. Although this transition has long been used in a variety of erbium-doped gain media, advancements in diode pumping have made these lasers more compact and efficient. The lower level of the 1.6 um transition in erbium is within the ground state manifold, making it a quasi-three-level system, leading to re-absorption from the lower laser level. It also suffers from low gain due to Stark splitting of the upper laser level. The gain is further reduced by up-conversion that depletes ions from the upper laser level. Er:Glass and Er:YAG lasers previously constructed by our research group were useful for coherent laser radars (CLR's) and Doppler wind-field mapping. This project aimed to build an Er:YAG laser with higher pulse energy and shorter (< 10 ns) pulse duration. This was expected to be a serious challenge based on the outcomes of all previous work. Our approach was to use a Co-Planar Folded zig-zag Slab (CPFS) architecture to provide a sufficiently long path length in the gain medium to compensate for the low inherent gain. The CPFS geometry was first demonstrated using Nd:YAG in the mid 1990’s, but was never previously attempted with Er:YAG. In this thesis, I describe the design and construction of Q-switched Er:YAG lasers for this purpose, followed by a more powerful cavity-dumped Er:YAG laser. Both lasers were resonantly pumped using 1.47 um laser diode bars. To the best of our knowledge these are the first CPFS Er:YAG lasers demonstrated. Our first Q-switched laser produced 15.5 mJ pulses with FWHM duration of less than 25 ns, corresponding to a peak power of 620kW, from 60W peak pump power. The final version of the Q-switched laser used a shorter crystal and resonator to reduce the pulse duration, and produced 6 mJ pulses with pulse durations of less than 15 ns, but with a peak power of only 400kW. These are the equal shortest duration pulses from an actively Q-switched Er:YAG laser at the present time. The results from both lasers are compared with the theoretical predictions adapted from a model of the dynamics of a quasi-threelevel laser. These investigations demonstrate the difficulties in building a Q-switched Er:YAG laser capable of safely meeting the project requirements. This thesis also describes the development and characterisation of a cavity-dumped CPFS Er:YAG laser. Cavity-dumped lasers are generally capable of shorter pulse durations and can therefore produce higher peak powers. This laser produced 10 mJ pulses with 4.5 ns duration, corresponding to a peak power of over 2MW. This represents the highest peak power at 1645 nm reported at the present time.