Veitch, PeterNg, SebastianHolmes, Zachary James2025-06-252025-06-252025https://hdl.handle.net/2440/145417Next-generation gravitational wave detectors may use cryogenic-silicon test mass mirrors to reduce the impact of thermal noise and absorption-induced wavefront distortion. Unfortunately, silicon is opaque at 1064 nm, which is the wavelength used by the current generation of detectors. A single-frequency, diffraction-limited, linearly-polarised, high power laser with a wavelength in the 1.5-2.05 μm band must therefore be developed. The preferred architecture is a master oscillator followed by a chain of power amplifiers, sometimes referred to as a master-oscillator-power-amplifier (MOPA). Additionally, the coherent combination of several high power laser beams may be required. This thesis reports the investigation of several technologies that could be used in such a laser source: stabilised laser diodes for use as a master oscillator, a low power polarisation-maintaining fibre pre-amplifier, and the coherent combination of multiple laser beams using a ring resonator. Laser diode linewidths have been progressively narrowing and they exhibit wide frequency modulation bandwidths. Thus, in Chapter 2, I report on the stabilisation limitations of a 1645nm distributed feedback (DFB) laser diode using an optical phaselock. Bandwidth limitations and achievable noise suppression magnitudes were investigated, and the likelihood that DFB diodes could be a potential master or seed oscillator is discussed. The investigation of a wavelength-tunable 2 μm external cavity diode laser (ECDL) is described in Chapter 3. The power, polarisation, wavelength stability, and noise characteristics were investigated in the context of detector requirements. The ECDL is also compared to the DFB diode and the feasibility of such diodes as master oscillators is evaluated. The development of a thulium-doped silica fibre pre-amplifier is described in Chapter 4. This consists of the amplification of 2 μm laser diodes to investigate amplifier gain, efficiency, and noise performance. The maximum output power of narrow-linewidth fibre amplifiers is limited by optical nonlinearities. Larger core diameters and lower numerical apertures are used, although this can result in transverse mode instability and unacceptable beam jitter. Thus, in Chapter 5, I describe the coherent combination of multiple laser beams using a ring resonator. The scheme is investigated and high combination efficiency is demonstrated, along with the simultaneous spatial mode filtering of the input beams.enLaser diodesgravitational wave detectorsfibre amplifierscoherent beam combinationoptical phaselocking2 umThesis