Development of continuous-wave Tm:YAlO₃ lasers for high power applications

Abstract

Thulium and holmium lasers operating near 2 μm are required for applications in various industries, such as remote sensing and detection, spectroscopy, surgery and optical countermeasures. High power lasers with high brightness are necessary for many of these applications, with fibre lasers often preferred due to their various advantages. However, a major drawback of fibre lasers is the brightness requirement on the pump source, which needs to have moderate to high brightness in order to couple light into the fibres efficiently. Such pump sources are often prohibitively expensive. A possible solution is the use of brightness converters. Brightness converters are lasers that are designed such that the output brightness is significantly greater than that of its pump source, and is sufficiently bright to pump fibre lasers efficiently. For example, by using a thulium-doped solid-state laser as a brightness converter, holmium-doped fibre lasers can be pumped efficiently by cheap, high power but low brightness diode stacks. Thulium-doped YAlO₃ lasers are ideal for this purpose: their emission wavelength corresponds to the peak absorption of holmium in silica, high power diode stacks are readily available at its absorption wavelength, and it is a crystal with a high damage threshold. However, Tm:YAlO₃ lasers suffer from significant self-pulsing, which can lead to unstable gain-switching of the holmium-doped fibre laser as well as risking damage due to the high peak power. In this thesis, I describe the investigation and development of Tm:YAlO₃ lasers as high power brightness converters and pump sources. A detailed analysis of the self-pulsing is conducted using a 6.5 W Tm:YAlO₃ laser. The self-pulsing is shown initially to be consistent with an unstable relaxation oscillation in the gain medium. A model based on significant excited-state absorption at the lasing wavelength is shown to reproduce the experimental results. The assumed cross-section required for this process is tested in a further experiment, which rules out this theory. The Tm:YAlO₃ laser is then analysed as a chaotic system, with results from time delay embedding and the 0–1 test for chaos indicating strongly that the laser system is chaotic. To the best of my knowledge, this is the first analysis and evidence of Tm:YAlO₃ lasers as a chaotic system. I describe the suppression of the self-pulsing using a method applicable to high power. Using this feedback system, the Tm:YAlO₃ laser is shown to produce a stable, continuous-wave output. To the best of my knowledge, this is the first demonstration of the suppression of such strong self-pulsing. This thesis also describes the design and development of a high power Tm:YAlO₃ laser using a novel geometry, which in principle is capable of several hundred watts of output power. This design aims to combine the superior thermal handling of disk lasers with the ease of pumping and laser design of the slab laser. A comprehensive model of such a laser is described, and the development of the laser up to the construction stage is presented.