External and common-cavity high spectral density beam combining of high power fiber lasers

Abstract

Spectral beam combining (SBC) has been extensively used for power scaling of laser systems. SBC is an incoherent technique of combining laser radiation from multiple sources with offset wavelengths into a single near-diffractionlimited beam with increased energy brightness. SBC by means of volume Bragg gratings (VBGs) recorded in photo-thermo- refractive (PTR) glass has been shown to be a simple and robust technique for combining high-power laser radiation. High-efficiency large-aperture VBGs were fabricated in PTR glass wafers. While being photosensitive in the UV, PTR glass offers high transmittance in the near-IR and visible parts of spectrum. Excellent mechanical properties and refractive index independent of temperature enable VBGs in PTR glass to withstand high-power laser radiation, making them ideal elements for high-power SBC. We report spectral combination of five randomly polarized fiber lasers with 0.5 nm spectral separation between channels around 1064 nm using reflecting VBGs in PTR glass. Maximum output power of the system is 773 W, corresponding to 91.7% combining efficiency. It is shown that VBGs introduce no significant beam distortions under high-power operation. Additionally, a common-cavity configuration for SBC with automatic wavelength control of sources by intra-cavity VBGs is suggested. Two fiber lasers are combined using this technique and automatic wavelength control is demonstrated. We show how simple power scaling allows obtaining multi-kW near-diffraction-limited laser radiation via SBC with volume Bragg gratings in PTR glass.