Power Scaling Architectures for Solid-State and Fiber Lasers

Abstract

This thesis focuses on developing power scaling architectures for solid-state and fiber lasers. The thermally-guided fiber-rod (TGFR) laser is suggested as a novel power scalable concept. This device lies in a domain between bulk rod lasers and traditional fiber lasers. The motivation is to benefit from the excellent thermal management properties of fibers, whilst negating deleterious nonlinear effects owing to the tight beam confinement and long interaction lengths that plague high power fiber lasers. An elegant thermal guiding technique is proposed to provide mode control with the TGFR. We derive the refractive index profile that ensues as a result of end-pumping the TGFR with a fiber-coupled diode laser. Furthermore, we construct a model that predicts the resulting impact on Gaussian beam propagation through the TGFR for various pump configurations. A model describing the gain within the device is derived from the laser rate equations. These two models allow us to predict amplifier and laser performance of the TGFR device.
We initially suggest soft glass as a host material for the TGFR, owing to the ability to dope this material with rare-earth ions in significantly higher concentrations than silica which is the traditional material of choice for fiber lasers, thus allowing the realisation of shorter devices. The requirements of a soft glass host are discussed in terms of both device fabrication and laser operation. Three potential sources are identified, including an in-house manufactured neodymium-doped and undoped phosphate glass, a commercial neodymium-doped and undoped silicate glass, and a neodymium-doped and undoped phosphate glass obtained through collaboration. The fabrication of potential TGFR devices with these three sources is described. This is followed by a laser investigation of these devices, where the issues of glass homogeneity and transmission loss become apparent, which are largely attributed to poor glass quality and unsuitable compatibility between the doped and undoped glasses. The neodymium-doped phosphate obtained through collaboration performed best, with a maximum output power around 1054nm of 2.5W, with a slope efficiency with respect to launched pump power of 28.5%. However, the poor glass quality prevented the thermal guiding investigation, and thus the beam quality was dictated by the highly multimode guide, resulting in a beam propagation factor of M2 = 60. Additionally, although this device had the lowest loss of the three sources, a significant loss of 5.7dB/m was measured using the Findlay-Clay analysis.
In light of these glass quality issues, the TGFR concept was fully tested using an extra-large mode area silica fiber. A mode guiding investigation revealed that an in-built non-uniform refractive index profile was responsible for providing a degree of guiding, even in the absence of pumping. This guiding was well predicted by assuming a parabolic refractive index profile and utilising the mode guiding model. Furthermore, the thermal guiding model gave excellent agreement with measured data across a range of launched pump powers up to 30W. The device was operated as an amplifier for seed beams at 976nm and 1030nm, and good agreement with the gain model was observed. At 976nm a maximum gain of 4.1dB was achieved for a 60mW seed resulting in an output power of 155mW, and 2.2dB for a 450mW seed resulting in an output power of 750mW. For 1030nm a maximum gain of 5.0dB was achieved for a 50mW seed resulting in an output power of 160mW, and 3.9dB for a 1.1W seed resulting in an output power of 2.7W. Excellent beam quality was maintained throughout amplification with M2 < 1.1 at the maximum gain levels for both 976nm and 1030nm. The concept was extended to a laser configuration at for both the 975nm and 1030nm transition. A device operating at 1032.5nm achieving a maximum output power of 13.1W with a slope efficiency of 44% with respect to launched power and 53% with respect to absorbed power. Excellent beam quality was achieved at maximum output power with M2 < 1.1. Additionally, a device operating at 978.5nm achieved a maximum output power of 1W with a slope efficiency of 8% with respect to launched power. Again, excellent beam quality was achieved at maximum output power with M2 < 1.1. The slope efficiencies of both of these devices, particularly the latter, are expected to increase with higher pump powers.
An Yb:YAG thin-slab architecture is suggested as a power scalable architecture for cylindrical vector (CV) beams, which have promising applications within materials processing. A seed source is constructed for operation at 1030nm, which exploits thermally-induced bi-focusing to produce a radially polarised output beam with a maximum output power of 6.9W, with a conversion efficiency of 41% with respect to absorbed pump power. The beam quality was measured as M2 = 2.3, whilst the radial polarisation extinction ratio (RPER) was > 15dB. It was demonstrated that the seed source could be amplified in a highly asymmetric thin-slab gain medium whilst maintaining radial polarisation purity. The implications of the Gouy phase shift owing to astigmatic focusing within the slab are discussed. Amplifier experiments yielded a gain of 7.5dB for a 25mW seed input power, and 4.4dB for a 1.45W seed input power, resulting in a maximum output power of 4W.The beam propagation factor at the maximum gain level was maintained at the lowest seed input power at M2 = 2.3, and was only slightly degraded to M2 = 2.4 at the highest seed input powers. Crucially, the RPER was maintained at >15dB for both cases.

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