Linewidth modelling and measurements of single mode-VCSELs. The vertical-cavity surface-emitting laser (VCSEL) is used for many applications and is today produced in large volumes for optical interconnects in data centres, gas sensors, and smart-phones. Some of these applications require the VCSEL to be single-mode, in other words to achieve a single spectral emission line. The linewidth, which is related to the phase noise and coherence of the laser, represents the width of such emission. It depends on the design of the single-mode VCSEL and on the operating conditions. This thesis work covers the analysis of VCSEL lasers up to the linewidth developed at Chalmers University of Technology.