Narrow linewidth, diode laser pumped, solid state lasers

Abstract

The design, construction, evaluation and development of an all solid state, narrow linewidth laser source is presented. The narrow linewidth laser system was based on a miniature standing wave Nd:YAG laser cavity, end-pumped with 100mW of 809nm light from a fibre coupled GaAlAs diode laser array. This basic CW laser generated up to 30mW at 1064nm in a single, diffraction limited transverse mode (TEM00) but multi-longitudinal mode output beam. The laser had a pump power threshold of 24mW and an optical to optical slope efficiency of 39%. A simple rate equation based numerical model of this laser was developed to allow various design parameters such as length of Nd:YAG gain medium and amount of output coupling to be optimised. Excellent agreement between the numerical model predictions of the output power as a function of input pump power and experimental data from the optimised multi-longitudinal mode laser was obtained. To restrict this laser to operate on a single longitudinal mode, twisted cavity mode and intracavity etalon, mode selecting techniques were investigated. Both methods were found to produce reliable single mode laser operation and resulted in output powers at the 10mW level. The relative free running frequency stability between a pair of single longitudinal mode diode laser pumped Nd:YAG lasers was investigated. By isolating these lasers from environmental noise using a small, custom built anechoic chamber the linewidth of the optical heterodyne signal between the two free running lasers was reduced from tens of megahertz to around 10kHz measured on a millisecond time scale. Further improvement in linewidth was achieved by actively locking the laser frequency to a novel ultra high finesse (F~12,500, free spectral range ~500MHz) spherical mirror Fabry-Perot reference interferometer using the technique of Pound-Drever locking. The locked laser displayed a maximum frequency deviation of only 1kHz from the centre of the reference cavity transmission and a frequency noise spectral density of ~20Hz/ √Hz at 1kHz. In one of the first reported demonstrations of an all solid state injection seeded laser system, this single frequency laser was used to injection seed a diode laser array, transversely pumped, Q-switched Nd:YAG laser to produce 0.25mJ, 35ns pulses in a single longitudinal, single transverse mode beam. Preliminary results on injection locking between two single frequency diode laser pumped Nd:YAG laser are also reported. A novel frequency stabilisation scheme based on resonant optical feedback locking iproposed and some preliminary experimental work on this technique is presented.