Effect of spatial filtering on the spontaneous emission spectrum of a sub-threshold VCSEL

We have measured the optical spectrum of a vertical cavity surface-emitting laser (VCSEL) operating below threshold. The spectrum consists of a strongly asymmetric central peak with weak structures in the wings that denote the edges of the DBR stopband. We show that the striking asymmetry of the central resonant part is caused by rays emitted at an angle with respect to the surface normal. The asymmetry can be removed by spatial filtering in the far-field. Analysis of the so obtained Lorentzian spectrum yields the current-dependent cavity loss rate and resonance frequency and thus, provides a new tool to analyze VCSEL´s