Stimulated Brillouin Scattering (SBS) Suppression and Long Delivery Fibers at the Multikilowatt Level with Chirped Seed Lasers

Using chirped seed amplification with a MEMS VCSEL seed to scale the output power of a ytterbium fiber amplifier.

Army Research Laboratory, Adelphi, Maryland

One obstacle in the scaling of high-power fiber lasers arises because of nonlinear effects (e.g., stimulated Brillouin scattering [SBS]) due to the large intensity times length product. Efforts to raise the power threshold include: 1

reducing the Brillouin gain by combining materials with positive and negative elasto-optic coefficients or tailoring the acoustic index to avoid guiding the acoustic wave,

reducing the effective Brillouin gain by using a seed linewidth much wider than the Brillouin bandwidth,

enlarging the Brillouin bandwidth relative to the seed linewidth,

lowering the laser intensity by enlarging the fiber core, and

minimizing the required active fiber length by pumping at the wavelength of maximum absorption and doping as heavily as possible.

Conventional approaches to broadening the seed linewidth reduce the coherence length, making it difficult to coherently combine multiple fiber amplifiers. For example, a seed bandwidth of 40 GHz (coherence length in fiber = 5 mm) will require path-length matching of much less than 1 mm to maintain high coherence.