Conversion of Mode-Locked States within an Empty Optical Resonator

Optical resonators are routinely used as optical spectrum analyzers or mode-cleaners [1], [2]. However, as shown in this work, their spectral and spatial filtering properties can also be utilized to convert longitudinal to transverse mode-locked states and vice versa. For longitudinal to transverse conversion the spectral components of an incident longitudinal mode-locked beam are matched to transverse mode resonances of an empty optical cavity. As a result, all spectral components are transmitted simultaneously and converted into transverse modes. Thus, an amplitude-modulated, i.e. longitudinal mode-locked, beam is converted into a transverse mode-locked beam that rapidly oscillates across the transverse plane. This conversion scheme allows to control the set of locked transverse modes and their respective amplitudes separately, which improves the generation of transverse mode-locked states compared to their excitation in laser-active cavities [3]–[5].