Multiplexed tunable holographic phase masks for generation of vortex beams (Conference Presentation)

Vortex beams with different helical modes are used for optical computing, free space optical communications, laser machining, and micro manipulating. We demonstrate holographic vortex phase masks produced in photo-thermo-refractive (PTR) glass. PTR glass is a photosensitive silicate glass that enables permanent refractive index change after UV exposure and thermal development. It is extensively used for recording of volume Bragg gratings (VBGs) and phase masks. A master phase mask is recorded by exposure of a PTR glass plate to UV radiation with a spatial intensity profile produced by a digital micromirror device. It provides a proper phase profile in a transmitted UV beam. This master phase mask is placed in one of the legs of an interferometer used for recording of a transmitting volume Bragg grating (VBG). Therefore, an additional phase profile is holographically encoded into the VBG resulting in the same phase profile in a diffracted beam. Such a device is a holographic phase mask (HPM) that enabled two exceptional features. First, it is tunable and could be used for different wavelengths. Second, holograms in PTR glass could be multiplexed and several HPMs could be fabricated in the single volume of glass. Owing to exceptionally low absorption of PTR glass and high thermal stability of holograms, holographic phase masks recorded in PTR glass can be used for mode conversion of high power laser beams. Such multiplexed HPM can split an incident Gaussian beam into several diffracted beams with different modes encoded.