Design, growth, fabrication and characterisation of high power single mode InGaAsP/InP lasers

Abstract

The concept of optical signal amplification in telecommunication networks has resulted in considerable reduction of the bandwidth cost, due to significantly increased distance between the regenerators or in some cases complete elimination from the links. However further cost reduction requires more efficient, reliable and powerful components. For instance the increase of the optical link length or number of channels in the link requires increased pump laser power used for signal amplification. Hence there is a great demand from the telecommunication industry for the development of high power single spatial mode lasers. Single mode InP-based lasers are required for pumping of erbium-doped fiber amplifiers (EDFA), erbium-doped fluoride fiber amplifiers (EDFFA), and Raman amplifiers, while the same structure with antireflection coatings can be used as Semiconductor Optical Amplifiers (SOA). The two main mechanisms limiting the output power in InP-based lasers are gain saturation and two-photon absorption. In this work the avenues to overcome those limitations and increase the output power of single mode InP-based lasers are studied. Broad-waveguide lasers have attracted a lot of attention lately. Increased waveguide thickness results in reduced interaction of the optical field with the highly absorbing heavily doped cladding regions and reduced active region optical confinement factor, thereby allowing longer devices capable of producing high output power to be fabricated. Broad-waveguide structures with optimised p-doping studied in this work have very low internal optical losses of 2.3 (1/cm). However, as shown in this work efficient carrier injection (hole injection in particular) through thick undoped waveguide region is a major issue. The method proposed in this work of improving hole injection in broad-waveguide lasers by placing the active region close to the p-doped cladding show less roll-over and as a result the improvement of laser output power of more than 25% is obtained. The effects of reduced active region optical confinement factor and improved hole injection on laser performance are decoupled in the study, clearly indicating that the improvement is due to improved hole injection. Merged beam laser (MBL) is a more complex method of increasing the output power of a single mode laser proposed in this work. Splitting the optical field into two with Y-coupler reduces gain saturation hence increasing the net gain of the mode, while maintaining single spatial mode. MBL was successfully fabricated and compared with the standard ridge waveguide laser and it shows 30% higher output power for the same injection current. The MBL shows virtually no roll-over and at 3000 mA (limited by current source) produced 229 mW of power. Additionally due to its distinctive design its spectral characteristic is strongly dominated by a single mode. The study of the methods of increasing the laser output powers proposed in this work suggests that they can be successfully implemented in InP-based laser.