Half-disk laser: insight into the internal mode structure of laser resonators.

Usually electromagnetic modes inside a laser resonator are a matter of the theoretical studies. In a sense we manage "to have a look into a whispering gallery mode (WGM) resonator" and observe how the resonator modes arrange in reality. The picture occurs to be quite different from the commonly used Bessel modes in a disk resonator. A chance to explore optical modes inside a resonator appears in a WGM laser with a cleaved cavity. The flat laser facet gives an opportunity to study both far and near field patterns formed by different modes. In this research we use a high resolution technique of detection of laser emission based on an atomic force microscope, which allowed us to visualize even high Q modes normally sealed inside the resonator. This information was completed with spatially resolved emission spectra and far-field patterns measured using an infrared camera. The analysis of the obtained results using both wave and geometrical optics approaches and finite elements simulations showed that emission of the studied devices is governed by a few low order optical modes experiencing a small number of reflections from the resonator walls. These modes can be considered as counter propagating Gaussian beams and their interference at the laser facet was also observed in the experiment. This work showed that, contrary to conventional ridge or surface emitting lasers, in such deformed disk resonators outputs of different optical modes are spatially separated and can be studied individually along the cleaved facet of the laser.