Mid-infrared fibre lasers for use in wavefront coe=rrection in advanced gravitational wave detectors

Abstract

This thesis outlines the development and characterisation of mid-InfraRed (IR) fibre lasers for the correction of wavefront distortion in the next generation gravitational wave detectors. These distortions are caused by the absorption of circulating laser light and manufacturing tolerances. This project was motivated by the fact that the fused silica mirror used in advanced Laser Interferometer Gravitational wave Observatory (aLIGO) absorb heavily for wavelength beyond 3.5μm and also around 2.7μm if the glass contains high concentration of hydroxyl ions. Fibre lasers systems that emit at these wavelengths were identified as promising light sources for thermal compensation and actuation systems. This was primarily due to their compactness, high output power in the mid-IR and excellent beam quality. Two erbium-based fibre laser systems were considered, one which operated around a wavelength of 2.8μm and the other around 3.5μm. We developed a watt-level mid-IR fibre laser operating around 2.8μm. The output beam quality and lasing wavelength were ideal for use in a new thermal actuation system being developed for aLIGO. The laser had a slope efficiency of 24.9%, a value which is consistent the values quoted in the literature for other similar Er:ZBLAN fibre lasers. A preliminary experiment with this laser in a thermal actuation system yielded some promising results. Understanding the power scaling capabilities of mid-IR fibre lasers is critical in evaluating their potential usefulness as light sources for thermal actuation. With this in mind, a new steady state algorithm was developed to predict the performance of the 2.8μm and 3.5μm systems and explore possible optimisation and power scaling techniques. The developed algorithm was verified against multiple simulation results which were achieved using different numerical methods. The output of the 2.8μm laser couldn’t be scaled significantly as the parameters of the system were close to their optimal values. A wavelength of 3.6μm was chosen as a suitable wavelength for maximising the absorption of laser power in the compensation plates at the next generation detectors. Ways to optimise the output laser power of the 3.6μm system were explored using the newly developed algorithm. Our investigation revealed that the output of the current system will be limited to 15W. However, there is still potential for power scaling the output of this system by either using a different resonator configuration or by increasing the radius of the fibre core. We also characterised the performance of various fibres from a fibre manufacturer. Various techniques were used to understand the low numerical aperture of highly-doped ZBLAN fibres. These included numerical aperture, scanning electron microscopy and ellipsometry measurements.