A detailed model of diffraction of plane and Gaussian beams on plane uniform phase Bragg gratings based on a Kogelnik's theory of coupled waves is presented. The model describes reflecting gratings (Bragg mirrors) with arbitrary orientation in a plane-parallel plate having no material losses. It takes into account spectral width and angular divergence of laser beams. The results of modeling are compared with experimental data for Bragg mirrors in a photo-thermo-refractive glass. © 2012 Society of Photo-Optical Instrumentation Engineers (SPIE).

Selective Laser Melting (SLM) of metal powder bed layers, whereby 3D metal objects can be printed from a digital file with unprecedented design flexibility, is spurring manufacturing innovations in medical, automotive, aerospace and textile industries. Because SLM is based on raster-scanning a laser beam over each layer, the process is relatively slow compared to most traditional manufacturing methods (hours to days), thus limiting wider spread use. Here we demonstrate the use of a large area, photolithographic method for 3D metal printing, using an optically-addressable light valve (OALV) as the photomask, to print entire layers of metal powder at once. An optical sheet of multiplexed ~5 kW, 20 ms laser diode and ~1 MW, 7 ns Q-switched laser pulses are used to selectively melt each layer. The patterning of near infrared light is accomplished by imaging 470 nm light onto the transmissive OALV, which consists of polarization-selective nematic liquid crystal sandwiched between a photoconductor and transparent conductor for switching.

Spectral beam combining (SBC) by volume Bragg gratings (VBGs) recorded in photo-thermo-refractive (PTR) glass is a powerful tool for laser applications that require higher radiance than a single laser unit can achieve. The beam-combining factor (BCF) is introduced as a tool to compare various beam-combining methods and experiments. It describes the change of radiance provided by a beam-combining system but is not affected by the initial beam quality of the combined lasers. A method of optimization of VBGs providing the maximum efficiency of SBC has been described for an arbitrary number of beams. An experiment confirming the proposed modeling for a two-beam SBC system by a single VBG has demonstrated a total combined power of 301 W with a channel separation of 0.25 nm, combining efficiency of 97%, close to diffraction limited divergence with M(2)=1.18, BCF of 0.77, and spectral radiance of 770 TW/(sr·m(2)·nm), the highest to date for SBC.

Lasers that produce 100 kW level diffraction limited power will require beam combining due to fundamental thermal and nonlinear limitations on the power of single aperture lasers. Towards this goal, we present high power, high spectral density beam combining by volume Bragg gratings of five 150 W beams with a spectral separation of 0.25 nm between beams, the narrowest to date for high power. Within 1 nm, 750 W of total power is combined with greater than 90 % efficiency. Combined beam quality is discussed including the effect of unequal individual beam divergences on the combined beam quality. The individual input beams may have unique divergences as they enter the system, and the heated volume Bragg gratings (VBGs) may introduce very slight changes in divergence to each beam. These small differences in beam divergence between the beams will not degrade the M 2 of the individual beams, but the composite M 2 after combination can be adversely affected if the beams do not have equivalent divergence at the output of the system. Tolerances on beam divergence variation are analyzed and discussed. High power beams transmitting through or diffracting from a VBG can experience different distortions resulting from thermal effects induced in the VBGs. Each beam also experiences a different aberration, as no two beams pass through the same number of identical VBGs. These effects are studied with experiment compared to modeling. Possible methods of beam quality improvement are discussed.