Diode-pumped Nd:YLF lasers for high-power operation

Abstract

This thesis presents the results of investigation into a variety of high-power, miniature Nd:YLF lasers end-pumped by diode-lasers. Diode-pumping enables more compact, efficient and reliable operation than alternative laser sources. Three main areas of research have been investigated, namely mode-locking, novel laser-amplifiers and high-brightness end-pumping. The main motivation was to develop simple picosecond-oscillators, suitable for pumping synchronously-pumped optical parametric oscillators. These devices deliver widely tunable radiation and have applications in ultrafast spectroscopy.

Ultrashort-pulse generation via mode-locking offers a particularly attractive way of scaling the peak-power output by many orders of magnitude above the average level. Nd:YLF lasers were chosen for investigation as they are suited to diode-pumping, have excellent thermal properties and are capable of generating pulses of ~1 ps duration.

Different amplification techniques were studied as a means of further scaling the output power from these lasers. A novel technique termed mode-locked injection-seeding was demonstrated. A steady train of ~3.6 ps pulses, 135 MHz repetition rate, from an additive-pulse mode-locked laser was used to injection-seed a slave laser. The slave laser was pumped by a single 3 W diode-laser. An average signal-power of ~25 mW, at 1053 nm gave an amplified pulse duration of 5.2 ps and an average output power of 840 MW. This corresponded to a power gain of 34 (15.3 db) and a peak output power of 1.3 kW.

Optimised resonator designs were investigated for a simple and scalable mode-locking technique known as Kerr-lens mode-locking. New resonator designs show improved Kerr-lens sensitivity but this technique is shown to be problematic in Nd:YLF.

The first application of beam-shaping of broad-stripe diodes (4W) is described using a two-mirror device. This scheme does not significantly reduce the brightness of the diode-laser and allows focusing to intense circular spots. This reconfigured beam is used to investigate high-brightness pumping of a simple double-pass amplifier. Small-signal gains of 17 dB were demonstrated. Also two beam-shaped diode-bars, delivering a combined power of ~30 W were used to investigate pump-power scaling of an actively mode-locked laser. This laser produced 54 ps pulses (240 repetition rate) and 7.6 W average power.

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