Free space coupling into deformed whispering gallery mode disk resonators – analysis of the far field distribution

Free space coupling of focused laser beams into optical microresonators has been shown to provide an alternative to conventional tapered optical fiber coupling. This results from the high stability of the resonator-beam alignment and a high coupling efficiency. We investigate a specific type of free space coupling using lithographically etched diffractive optical elements (DOE) integrated on top of the resonator. Choosing the correct DOE parameters, the overlap integral between the incident laser field and the excited whispering gallery mode needs to be maximized in order to enable efficient coupling. For a demonstration experiment, a deformed whispering gallery mode disk resonator exhibiting strong far-field emission into a single direction (also referred to as a shortegg resonator) is fabricated lithographically with the DOE integrated. The free space coupling efficiency is examined with the help of far-field intensity measurements. Finally, a comparison with simulations conducted in COMSOL Multiphysics is presented.