Our study describes the development of coherent beam combining of an array of nine fiber lasers using an all-optical ring cavity feedback loop based on a diffractive optical element to achieve a single-aperture output. Nine 300-mW Yb-doped fiber amplifier beams arranged in a 1 × 9 end-cap array were combined to achieve a single-aperture beam with a power of 739 mW and a beam quality (M2) of 1.18 with 21.5% combining efficiency. The optical spectra, far-field distributions, and time-domain characteristics of the combined beams were investigated under open- and closed-loop conditions. Under open-loop conditions, the far-field coherent visibility changed constantly from 72.1% to 90.9% and the fluctuation intensity was strong. Under closed-loop conditions, the system achieved a steady state with a visibility of 98.6% and an average feedback intensity of 0.4 V, indicating the occurrence of phase locking. Furthermore, mode hopping was observed when there were more than four channels in a combination. However, the system interference pattern remained stable. Comprehensive research on the relevant literature indicated that novel filled-aperture CBC was achieved using an all-optical ring cavity feedback loop based on a DOE.
Laser-induced fluorescence (LIF) of high-purity fused silica irradiated by ArF excimer laser is
studied experimentally. LIF bands of the fused silica centered at 281nm, 478nm and 650nm are
observed simultaneously. Furthermore, the angular distribution of the three fluorescence peaks is
examined. Microscopic image of the laser modified fused silica indicates that scattering of the
generated fluorescence by laser-induced damage sites is the main reason for the angular distribution
of LIF signals. Finally, the dependence of LIF signals intensities of the fused silica on laser power
densities is presented. LIF signals show a squared power density dependence, which indicates that
laser-induced defects are formed mainly via two-photon absorption processes.
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