High power mode locked lasers monolithically integrated with semiconductor optical amplifiers

This thesis is concerned with the design, fabrication and characterisation of high power semiconductor mode locked lasers (SMLLs), operating at ~ 1.5 μm. The devices are based on a novel epitaxial structure with three quantum wells (QW) in the active region. The novel epitaxial structure was based on a commercially available five-QW AlGaInAs/InP epitaxial structure, with the number of QWs reduced to increase the gain saturation energy and a farfield reduction layer (FRL), and a spacer layer were inserted in the n-cladding layer. SMLLs based on both the five-QW and three-QW material, were fabricated and comparatively investigated. The devices based on the three-QW material exhibited an increased average output power, as well as reduced RF linewidth and pulse widths. The average output power obtained in the mode locked operation from a 40 GHz MLL, based on this three-QW material was limited to 28 mW. Single mode ridge waveguide semiconductor optical amplifiers (SOAs) were monolithically integrated with the SMLLs, which increased the average output power to 130 mW. The devices performance was investigated at both the saturable absorber (SA) and SOA facets. Transform limited pulses with a minimum duration of 3.3 ps and a peak power of > 1 W were obtained at the SOA facet. To take advantage of the higher saturation output power of the tapered SOAs, SMLLs were monolithically integrated with 2° and 6° tapered SOAs, respectively. The devices integrated with 2° tapered SOAs were mounted on Aluminium Nitride (AlN) sub-mounts using Gold-Tin (AuSn) soldering for better heat sinking. These devices resulted in an average output power of 200 mW, with corresponding peak power > 1.2 W. The SMLLs integrated with 6° tapered SOAs, mounted on conventional brass sub-mounts resulted in a lower output power (105 mW), compared to the devices integrated with ridge waveguide and 2° tapered SOAs, respectively.