An investigation of laser oscillators and amplifiers using high intensity diode-pumping

Abstract

The work presented in this thesis is split into two related areas. The first area of research was the construction of high gain, high power, all-solid-state laser amplifiers for use in master oscillator, power amplifier (MOPA) systems. The second area was the operation of solid-state lasers on low gain transitions. The two areas are related by the fact that the primary aim in each was to maximise the available gain on a given laser transition.

Two diode-pumped travelling wave amplifiers are described, both using Nd:YLF as the active medium, and both employed to amplify the output from a modelocked 1047nm Nd:YLF laser. The first amplifier was pumped by a 4W diode, and produced a small signal gain of 34. To suppress gain saturation in the amplifier, the input signal to the amplifier was formed into pulse trains of duration 10µs. The average gain achieved during these pulses was 20, giving rise to an average output power of 5W during the pulse. The amplifier output was subsequently frequency-doubled by a single pass through an LBO crystal. An average conversion efficiency of 57% was obtained, giving an average green power of 2.9W. The green output was subsequently used to pump both an OPO based on LBO, and a Ti:Sapphire laser.

The second amplifier had a pump power of 28W. This produced a small-signal gain of 40 at 1047µm, and yielded 6W of amplified modelocked output on a cw basis rather than pulsed as in the first amplifier. The output from this amplifier was used to pump an OPO based on PPLN, and this was able to oscillate at a maximum wavelength of 6.2µm.

The work on low gain lasers was addressed at the 1123nm transition in Nd:YAG. This has a cross-section ~15 times lower than at 1064nm. The pump source was a 7W diode-bar, and using this 1.7W of TEM00 output at 1123nm was obtained in a beam with an M2 of 1.1. This output was subsequently used as the pump for a Tm:ZBLAN fibre laser, which produced a maximum of 230mW of 480nm blue light. A second application envisaged for the 1123nm output was to sense atmospheric water vapour by using a differential absorption LIDAR. For this, narrow linewidth operation of the laser was required. To this end, a single-frequency ring laser was constructed, and this produced a maximum power of 180mW, again in a TEM00 mode with M2 = 1.05. The output power was restricted by limitations on available components.

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