DESIGN OF A DEVICE FOR PUMPING A DOUBLE-CLAD FIBER LASER WITH A LASER-DIODE BAR

Pumping rare-earth-doped double-clad fiber lasers with GaAlAs laser-di ode bars typically requires one to transform a 1-cm-long one-dimension al linear array of tens of multimode laser-diode sources into a two-di mensional oblong cross section that approximately matches the fiber's first cladding shape. I describe the design of a device, henceforth ca lled a high-brightness geometric transformer, that uses a cylindrical microlens to image the laser-diode bar near field onto a linear array of soft-glass, thin-clad, rectangular fibers. In turn, the fibers outp ut ends are arranged to form a stack that matches the required first c ladding shape. For a typical laser-diode bar with a brightness of 25 m W.mu m(-2).sr(-1), the geometric transformer output brightness is 0.6 mW.mu m(-2).sr(-1), i.e., there is a 40x intermediate loss of brightne ss. If the output of the geometric transformer is used to pump a Nd-do ped double-clad fiber laser, an overall brightness gain of 340x can be achieved.