Characterisation of the waveguide dependence of optical mode loss in semiconductor lasers

The improvement of optical mode loss in semiconductor lasers is of significant commercial value, contributing to lower threshold current densities and higher above-threshold slope efficiencies. While it has improved significantly with successive developments in semiconductor laser design resulting from improvements to the fabrication process, characterisation methods for the measurement of optical mode loss have not kept pace. Today values of optical mode losses as low as 1cm-1 are frequently reported, and yet the precision and accuracy of its measurement are rarely better than 1cm-1 itself. To address this, I demonstrate that a modulated approach to the segmented contact method for the measurement of modal absorption and loss can measure optical mode losses with precisions as low as 0.1cm-1. I also demonstrate the removal of non-linearity from detection system using a novel approach to amplitude calibration by flux superposition. I apply this improved methodology in the comparison of InAs QD samples with differing waveguide core dimensions (2440Ǻ and 3740Ǻ), finding a reduction in optical mode loss between these samples from (4.8±0.4)cm-1 to (2.25±0.10)cm-1. In addition to measuring the optical mode loss at the lasing wavelength I investigated the origins of the observed wavelength dependence of optical mode loss below the material band-edge, comparing experimental values of modal absorption and loss spectra with a waveguide model. I show the wavelength dependent optical mode loss of wide waveguide core samples agrees well with losses predicted from model incorporating free carrier absorption, intervalence band absorption and waveguide dispersion. I also show that in both samples optical mode losses and their respective wavelength dependences are insensitive to changes in temperature from 298-370K. I also investigate the causes of oscillating modal absorption and loss seen in narrow waveguide core samples