OBSERVATION OF WAVE-FRONT CURVATURE INSIDE A VERTICAL-CAVITY SURFACE-EMITTING LASER

Gain guiding was clearly shown to be present and can have a significant effect on the beam properties of a vertical-cavity surface-emitting laser (VCSEL). The wave front curvature at the interface of the devices risen in the experiment was measured in an absolute manner. From the wave front quantifications, the beam quality parameter M2 was determined to be characteristically 1.2 or better. These observations suggested that the (complex) refractive index profile inside the VCSEL can be estimated by a quadratic function. For all the measured lasers, a significant decrease of the defocus with current was determined. The trials yielded measured values of the defocus between 8(2) and 0(2) μm, i.e., up to around 25% of the Rayleigh range, for various devices, and for injection currents between 5.6 and 9 mA.