Ultimate efficiency of multi-channel spectral beam combiners by means of volume Bragg gratings

Abstract

With recent advances in high-power laser technology, Volume Bragg Gratings (VBG) have been recognized as important elements in different types of beam-combining applications, such as, design of optical correlators, coherent and incoherent power beam-combiners and in particular, spectral beam combiners (SBC) in which the output beams from several distinct laser sources are combined into a single-aperture, diffraction-limited beam. The obvious advantage of VBG's in these applications results from their narrow spectral and angular selectivity compared, for example, to any type of surface gratings. Almost a two order magnitude difference in spectral efficiency (number of channels per usable bandwidth) can potentially allow one to combine a much larger number of lasers into a single spot. The VBG recorded in a photo-thermo-refractive (PTR) glass exhibit long-term stability of all its parameters in high-power laser beams. With power density more than 1 MW/cm2 in the CW beam of total power on a kilowatt level the characteristics of these elements appear to be stable. In order to increase the spectral efficiency of such a "beam-combiner" the overall loss resulting from absorption and cross-talk between channels should be minimized. In this paper we consider architecturespecific beam-combining scheme and address cross-talk minimization problem based on optimal channel positioning. A mathematical model reveals the critical parameters for high efficiency spectral beam combining in which explicit equations are derived to relate the spectral density to the total system efficiency. Issue of system scalability for up to 200 channels is addressed. Coupled wave theory of thick hologram gratings is used in this analysis to characterize.