Power-scaling of diode-end-pumped solid-state lasers

Abstract

This thesis presents a strategy for power-scaling diode-end-pumped solid-state lasers to multiwatt output power whilst retaining the high-efficiency and diffraction-limited beam quality, that have been characteristic of operation at low powers. This strategy reduces the detrimental effect of energy transfer upconversion (ETU), which can decrease the lifetime of the upper-laser-level and also increase the heat generated inside the laser material. An analytical description of the lifetime quenching and increased thermal lensing due to ETU is presented. Using this analytical model it is shown that ETU can be reduced by decreasing the absorption coefficient, by increasing the spot size or by decreasing the upconversion parameter, W. This strategy is applied to a Nd:YLF laser. Before designing the laser the published value of the upconversion parameter for Nd:YLF is confirmed as W = (1.7 ± 1) x 10-16cm3s-1, by observing the fluorescence saturation with increasing pump power. Also, without applying the power-scaling strategy, the thermal lensing in Nd:YLF in a typical pump set-up is determined, under lasing and non-lasing conditions. A significant difference between the thermal lensing under non-lasing and lasing conditions is observed which is attributed to the increase in heat input caused by ETU. In a standing-wave cavity, under cw lasing conditions and at the maximum pump power (29.5 W incident, 27.4 W absorbed) an output power of 11.1 W is obtained. This laser has excellent beam quality (M2x,y, < 1.1) throughout the pump power range. An intracavity-frequency doubled ring laser is also demonstrated. This laser produces >10W of single-frequency output with a 10% output coupler (~9.7 W at 1.053μm and ~0.6 W at 526.5 nm) at the maximum pump power. With a high reflector, for 1.053 μm, 6.2 W of polarised output (8.3 W generated inside the LBO), at 526.5nm is obtained. The output has excellent beam quality (M2x,y < 1.2) and amplitude stability (<± 0.5%). The mode-hopping suppression obtained in this laser is also investigated. A large tuning range (~42 GHz, ~80 axial mode-spacings) with the laser optimised for 1053nm output (i.e. with the 10% output coupler) is achieved with both diode-bars at full pump power. Also a large tuning range is achieved with the laser optimised for green output with one diode-bar (~72 GHz, ~150 axial mode-spacings). The upconversion problems are also sufficiently reduced to successfully demonstrate a highly efficient Q-switched ring laser. This laser overcomes the slow switching speed of the A-O modulator by taking the output through the diffracted beam. ~3.5mJ of single-frequency TEM00 energy for ~25W of incident pump power is demonstrated. This laser is then used to pump an OPO and preliminary results are presented.

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