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

Pumping of a rare-earth-doped double-clad fiber laser with a GaAlAs la ser diode bar requires imaging the near field output of a 1 cm long on e-dimensional linear array of tens of multi-mode laser diode sources o nto a two-dimensional oblong cross section approximately matching the fiber's first cladding shape. This work describes the design and preli minary implementation of a device (here called a high brightness geome tric transformer) that achieves this transformation with minimum brigh tness loss. This is done by imaging the laser diode bar near field usi ng a fast cylindrical micro-lens onto a linear array of N (approximate to 10) soft-glass, thin-clad rectangular fibers; in turn, the fibers' output ends are arranged to form a stack that matches the required fi rst cladding shape. For a typical 20 W CW laser diode bar with brightn ess of 25 mW .mu m(-2). sr(-1), the geometric transformer output brigh tness is 0.6 mW .mu m(-2). sr(-1), i.e., there is a x40 intermediate l oss of brightness. If the output of the geometric transformer is used to pump a Nd-doped double-clad fiber laser, an overall brightness gain of x340 can be achieved.