Development and characterisation of a diode laser based tunable high-power MOPA system

The aim of this work is to develop and characterize widely tunable high-power diode lasers emitting at 976 nm, targeting non-linear frequency conversion applications. This was realized by a master oscillator power amplifier (MOPA) configuration, where the MO provides wavelength stabilization and tuning, while the PA ensures power amplification to the watt level. Two approaches of developing tunable laser sources were investigated. In the first approach, three types of monolithic multi-arm lasers were developed and characterized: two-arm (Y-branch), four- and six-arm distributed-Bragg reflector ridge waveguide (DBR-RW) lasers. Among the main findings of this work is that the performance of these lasers is strongly influenced by the intersection point between the individual arms and less affected by the bend structure. Thermal wavelength tuning of the developed laser sources is obtained by resistor-based micro-heaters embedded on top of the grating sections, where tuning of up to 7.5 nm can be achieved from each arm. The second approach uses sampled-grating (SG) structures to obtain wide wavelength tuning from a single laser cavity. While InP based SG lasers are well established, a milestone of this work is the first-time demonstration of fully functional GaAs-based SG lasers. Up to 21 nm of discrete wavelength tuning is achieved when operating a single SG heater with a SG mode spacing of about 2.3 nm. By operating both heaters, 23.5 nm of quasi-continuous tuning is obtained with a mode spacing of about 115 pm. Among the constructed MOPA systems is a Y-branch based MOPA system with 9.7 nm wavelength tuning and 5.5 W output power. Another system utilizes an SG laser, providing 23 nm of quasi-continuous wavelength tuning with an output power in the watt range.