An approach for recording complex holograms using two beam interference setup incorporating a light modulating device in one of the arms of the holographic setup is proposed. This allows the intensity profile of the beam reflected by the DMD to be adjusted to a pre-determined profile. Using an UV laser, the created interference pattern is recorded into PTR glass plate. The variable light amplitude exposure of the glass is converted to a refractive index change after thermal treatment step. The DMD device makes possible the incorporation of zones in the hologram where there is no interference pattern and therefore there is no hologram. Such custom designed holograms can find applications in laser resonators or as spatially sensitive wavelength filters. In addition, the holograms recorded in the volume of the PTR glass are practically insusceptible to surface damage, changes in the environment and other factors from which holograms recorded in polymers suffer.
We present a high-power capable, simple, tunable, and robust vortex beam generator. Initial phase profiles are generated using a Spatial Light Modulator and then holographically encoded into a Transmitting Volume Bragg Grating (TBG) resulting in fabrication of a Holographic Phase Mask (HPM). Such HPMs could be used for mode conversion (e.g. Gaussian to vortex) at high power due to low absorption and low nonlinear refractive index of photo-thermo-refractive glass. Unlike monochromatic conventional phase mask, adjustment of an incidence angle on the HPM results in conservation of the phase information incurred into the beam making HPM tunable.
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.
Phase masks for mode conversion and other laser beam transformation are usually produced by surface profiling using spatially selective etching or deposition. Such fabrication techniques for making complex phase masks take significant time, effort, and expense. Surface damages and contaminations restrain the wide application of such phase elements as well. We propose an alternative, where volume phase masks were produced by spatially selective refractive index change in the bulk of plane-parallel plate of photo-thermo-refractive (PTR) glass. Those phase masks show high tolerance to harsh environments and high-power laser radiation. The approach uses a light amplitude modulating element called digital micromirror device (DMD) which has millions of micro-mirrors that tilt to ‘on’ or ‘off’ position based on the voltage applied to them. Selecting different time intervals for the mirrors to be ‘on’ or ‘off’, allows for grey level images to be generated and projected by the DMD device. Using a broadband UV light source, the desired amplitude image was projected onto a PTR glass plate for a designated period of time. The variable light amplitude exposure of the glass is converted to a refractive index change after thermal treatment step. The phase masks are recorded in the volume and are practically unsusceptible to surface damage, changes in the environment and other factors from which surface created phase masks suffer. The simplicity in fabricating grey level phase masks, the flexibility to computer design, and the robustness are the main advantages of the approach when compared to the standard phase mask fabricating techniques.
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