Thermally Controlled Onset of Spatially Incoherent Emission in a Broad-Area Vertical-Cavity Surface-Emitting Laser

We present an experimental study of the physical process that leads to spatially incoherent, nonmodal emission in broad-area vertical-cavity surface-emitting lasers. We show that this special emission regime that occurs in pulsed operation of these lasers is due to a combination of a spatially distributed thermal or refractive index gradient (thermal lens) and thermal expansion of the cavity during the pulse (thermal chirp). Our measurements are based on preinstalling a thermal lens through a current bias, and subsequently, modulating a pulse onto the bias. This approach allows us to independently investigate the role of both thermal effects in the onset of nonmodal emission.