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A unified picture of threshold behaviour in strained quantum well lasers

Arvind Surendranath Baliga, University of Massachusetts Amherst


The focus of this investigation is to provide a unified understanding of the relative impact of compressive and tensile strain on thresholds in III-V separate-confinement-heterostructure single-quantum-well (SCH SQW) lasers. First, a strained-layer laser model for SCH SQW lasers that calculates gain spectra, differential gain, light-current characteristics, and threshold current densities is developed. This model is based on a six valence-band Luttinger-Kohn finite-element dispersion calculation. Second, an extensive theoretical and experimental study on tensile-strained GaAsP-AlGaAs SCH SQW broad-area stripe lasers is performed to understand a complex interplay of TE and TM gains and modal losses unique to tensile-strained lasers. Threshold current density measurements for sample sets encompassing 10 phosphorus compositions ranging from 0 to 30% and 5 cavity lengths ranging from 300 to 1500 $\mu$m are reported. The theoretical model is used to replicate detailed features of the experimental data including absolute magnitudes and polarization-switching behaviour. A constant gain contour approach is introduced to explain the dependence of the measured thresholds on strain and cavity length as a result of competition between a TM gain advantage and a TM electromagnetic disadvantage. Tensile strain is shown to have a minimal impact on threshold current densities for GaAsP-AlGaAs lasers. Third, a comparative analysis of strain effects on laser performance in the InGaAs-GaAs-AlGaAs, GaInAs-GaInAsP and GaInAsP-GaInAsP material systems is presented. Different approaches to analyzing strained-laser performance such as constant-well-width, constant-wavelength and Seki pure strain studies are employed. The constant gain contour approach combined with the pure strain strategy is shown to provide a powerful tool for the understanding of strain effects in 1.3 $\mu$m GaInAsP-GaInAsP lasers. It is explained that tensile-strain lowers thresholds relative to unstrained and compressively strained lasers only in certain high optical gain regimes and that tensile-strain is expected to provide high differential gain in all regimes of operation for the material systems investigated in this study.

Subject Area

Electrical engineering

Recommended Citation

Baliga, Arvind Surendranath, "A unified picture of threshold behaviour in strained quantum well lasers" (1995). Doctoral Dissertations Available from Proquest. AAI9541079.