VCSEL beam waists from optical spectra

We relate the optical beam waist of a vertical-cavity surface-emitting laser (VCSEL) to a measurement of the transverse mode splitting found in the device's optical output spectra and to a measurement of beam divergence. We show experimentally that the results are in good agreement. Accurate knowledge of beam waists is useful for tailoring devices for single-mode or for high-speed operation and to better understand and interpret the mode spectrum.