For materials processing, laser manufacturing and machining, medical applications, etc., it is important to shape intensity and phase profile of the laser beam. To maintain optimum processing of materials, e.g., in drilling, the beam profile should be dynamically changed as the crater depth grows. Existing methods of dynamic shaping, viz., using an array of mechanically moving mirrors, are bulky and expensive. Methods based on using spatial light modulators work only for low-power lasers. For fast, non-mechanical beam shaping, we suggest using an innovative compact method based on dynamic, real-time holography: beam coupling during self-diffraction of laser beams in photosensitive materials. High pass spatial filtering (Figure 1), essential for generating flat-top beams, has been recently theoretically and experimentally demonstrated to yield edge enhancement of images in the transmitted optical field during diffraction from volume Bragg reflection gratings in photosensitive materials at the University of Dayton. Figure 1 shows the magnitude squared of the spatial transfer functions for the (a) transmitted and (b) reflected optical fields during diffraction from volume reflection grating written in photorefractive lithium niobate. The transmitted field shows high pass spatial filtering leading to edge enhancement.

Our device consists of a sandwich combining a volume Bragg diffractive element (VBDE) and a layer of a photosensitive material. A laser beam illuminating the VBDE splits into two beams (transmitted and Bragg-diffracted) that record dynamic gratings in the photosensitive material. Diffraction by the VBDE and subsequent self-diffraction by the dynamic grating in the photosensitive material will cause energy and phase transfer needed for simultaneous shaping of the two beams.

At this stage, we have developed a theoretical model that predicts the ability of shaping Gaussian beams in space (and/or time) by self-diffraction, and experimentally tested the ability of the VBDE to shape two Gaussian-shaped (in space) laser beams. For applications related to high-energy lasers (HELs), we have tested a possibility of beam shaping by holographic optical elements (HOEs) recorded in photo-thermo-refractive (PTR) glasses developed at CREOL, University of Central Florida. The gratings in PTRs are stable up to 400° C with laser-induced breakdown energy threshold of 10 J/cm^2 in 1 ns pulses at 1064 nm. The high diffraction efficiency (> 95%) of gratings recorded in PTR glass enables high energy transfer that is vital for commercial laser applications. Preliminary results show that flattop laser intensity distribution may be realized in the diffraction orders. Figure 2 shows experimental results for spatial beam shaping of the self-diffracted beam from recorded grating in PTR glass illustrating transformation of initial Gaussian beam to flattop shape. Figure 3 shows the results of modeling of simultaneous beam spatial (x) shaping as function of grating thickness (z) for 2 input Gaussian beams by energy exchange through beam coupling during self-diffraction. The left beam changes to flat-top shape while transferring energy to the right beam, which becomes sharper.

The suggested method of laser beam combining and shaping based on dynamic holography and volume gratings allow for combination and shaping of CW and pulsed lasers for laser machining and laser processing.