EFFECT OF SPATIAL-FILTERING ON THE SPONTANEOUS EMISSION-SPECTRUM OF ASUBTHRESHOLD VCSEL

We have measured the optical spectrum of a vertical-cavity surface-emi tting laser (VCSEL) operating below threshold. The spectrum consists o f 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 remove d by spatial filtering in the far-field. Analysis of the so obtained L orentzian spectrum yields the current-dependent cavity loss rate and r esonance frequency and thus, provides a new tool to analyze VCSEL's.