Spectral beam combining of fiber lasers with increased channel density

Incoherent spectral beam combining (SBC) by means of volume Bragg gratings (VBGs) has been shown to be a simple and robust technique for generating high-power laser radiation. Combination of laser radiation from multiple sources into a single near-diffraction-limited beam results in energy brightness increase, while spectral brightness is preserved. High-efficiency VBG recording in photo-thermo-refractive (PTR) glass has been recently developed. While being photosensitive in the UV, PTR glass offers high transmittance in the near-IR and visible parts of spectrum. Moreover, this glass has excellent mechanical properties and refractive index independent of temperature. These features enable VBGs in PTR glass to withstand high-power laser radiation, making them ideal elements for high-power SBC. We present experimental results of successful 5-channel SBC with reflecting VBGs in PTR glass with small channel spacing (similar to 0.43 nm around 1064 nm). Absolute system efficiency of 93.5% is demonstrated. Combined beam is shown to be near-diffraction-limited with M-2 = 1.11. Behavior of narrow-band reflecting VBGs in high-power beams is studied. VBGs are shown to withstand 570 W CW radiation around 1064 run without significant performance degradation. Diffraction efficiency in excess of 94% is demonstrated at the maximum power, only similar to 0.5% lower than low-power efficiency of the grating. Pathway to near-diffraction-limited high-power laser systems via SBC with VBGs is shown. High-efficiency SBC system with 0.2 nm channel spacing is designed.