High-Power, High-Brightness Holographic Beam Combination of Lasers

The Holographic Beam Combiner, HBC, proposed here can be used to combine incoherently the output from N lasers into a single aperture. It will have an intensity higher than 90% of the sum of the input intensities, and a brightness that is 0.9N timeshigher than that of the individual input beams. The HBC is based on the storage of multiple holographic gratings in the same spatial location. Using a new material called PDA up to 20 holograms each with a diffraction efficiency greater than 90% can storedin a single location. The combination process can be cascaded to increase much further the number of beams to be combined.Under the main task proposed here, we will concentrate on using this technology for combining high-power (4W) multimode diode lasers centered around 978 nm, for pumping dual-clad Yb-doped fiber lasers and amplifiers. Typically, this laser is pumped withlasers at around 915 nm. However, the pumping efficiency is about a factor of 3 higher at 978. The constraint at 978 nm is that the pumping bandwidth is only about 6 nm. If one were to use the blazed grating blazed beam combining, the number of laserscombined would be less than 10, because the wavelength separation between two constituent beams is typically 1 nm. The key advantage of our approach is that because of sharp Bragg selectivity of thick (few mm) holograms, the wavelength separation caneasily be as small as 0.03 nm. As such, up to 200 beams can be combined within the 6 nm bandwidth, using a two stage cascading of HBC's, using a relatively thinner hologram in the second stage. The result would be a nearly 700 W pump at the highlyefficient band around 978 nm, with a brightness of about 180 times that of each input laser. Furthermore, we will investigate the use of this approach for combining incoherently the output of many high-power fiber lasers.