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High power, stable single frequency laser sources are required for gravitational wave interferometry. The next generation of interferometers may require laser sources in the 1.3-1.65 µm band for use with Si test masses and InGaAs photodetectors. We propose a high power cryogenic Er:YAG laser operating at 1.618 µm for this purpose, adapting existing knowledge about cryogenic Yb:YAG lasers developed at the University of Adelaide. To produce such a laser, further information is required about the viability of Er:YAG in high power, single frequency operation. In this thesis, I report this investigation of the spectroscopy of Er:YAG at room temperature and at cryogenic temperatures (≈ 77 K) and investigate a variety of wavelengths for diode pumping of an Er:YAG slab laser. Spectroscopy indicates that diode pumping for the 77 K laser slab will be most effective in the 1450-1480 nm absorption band, most specifically at the 1453 nm absorption peak. I describe methods for cooling a 1470 nm diode below 0 °C to pump this 1453 nm Er:YAG absorption. The cooled diode exhibits up to 9 % increase in slope effciency and improved beam divergence compared to room temperature operation. I then describe the construction and characterisation of CW Er:YAG lasers at both 300 K and 77 K, tuning the pump wavelength in the 1450-1480 nm band. At 300 K, I demonstrate an Er:YAG laser with 4.5 W output power when pumped with 30 W of diode power at 1468 nm, and just under 4 W of output power when pumped with 34Wof diode power at 1456 nm. Both lasers have a threshold of approximately 12 W incident pump power. The laser pumped at 1468 nm also demonstrates a greater slope effciency relative to incident pump power: 28 % compared to 20 % when pumped at 1456 nm. The development of a preliminary cryogenic Er:YAG laser is also reported. Despite sub-optimal mounting materials and geometries, we demonstrate a cryogenic laser with 5.5 W output power and approximately 6 W threshold under comparable pumping conditions to the 4.5 W 300 K laser pumped at 1468 nm. Unfortunately subsequent studies of the cryogenic slab laser are not comparable to the 300 K Er:YAG laser due to electrical damage to the diode that significantly reduced diode power and changed pumping conditions. Nevertheless, these results provide valuable information on the sensitivity of end-pumped cryogenic lasers to mounting conditions and pump focusing that are useful for a future high power design.