Recent work  has shown that the etching of deep trenches in close proximity to GaAs VCSEL apertures consistently causes the linear TE polarisation of the emission to be pinned in a direction parallel to the line etch. Further enhancement of this polarisation pinning has been achieved by post-annealing after etching. Initial studies have been carried out using photoluminescence and Raman measurements of the VCSEL wafer before and after etching as well as after annealing. Modelling the observed shift in the optical cavity (longitudinal) mode has indicated that etching introduces strain perpendicular to the etch in the active region of the VCSEL of 4 × 108 dyn/cm2. The strain causes the cavity mode to shift to longer wavelength and reduces the spontaneous emission in the direction perpendicular to the etched trenches. The strain introduced by etching is believed to be the origin of this polarisation pinning effect. Measurements of the spontaneous emission profile across the VCSEL facet after etching give valuable information on the mechanisms involved.