Single-polarization double-clad fiber for high-power applications

Abstract

A polarizing, ytterbium-doped double clad fiber is demonstrated. Over 100 W of linearly polarized output power with a polarization extinction ratio of 17 dB is achieved from an all-fiber geometry. Single transverse mode fiber lasers have been recently demonstrated to emit output powers of hundreds of watts. 1,2 High power fiber lasers with linearly polarized output have been reported. 3-5 Linearly polarized output is required for phase-matched nonlinear conversion configurations often used to achieve a wider range of operating wavelengths. Moreover, most methods of coherent beam combination, a method of scaling output to even higher powers, also require linearly polarized laser output. In order to achieve linearly polarized output from a laser oscillator, dichroism in the form of either polarization dependent loss or gain is required to prevent laser operation on the unwanted polarization eigenmode. One recent demonstration of linearly polarized operation in a high power fiber laser was through the incorporation of a bulk polarizer into the cavity. 5 Two groups have recently demonstrated two different, all-fiber, linearly polarized lasers: one through bend-induced polarization dependent loss in a birefringent, double clad fiber, 4 the other with a single polarization fiber fusion spliced to a high birefringence, elliptical core double clad fiber. 3 In the present work, an ytterbium-doped, single polarization fiber is used in a cladding pumping geometry. The double clad fiber possessed a dichroism that was based on a novel elliptical core hole-assisted single polarization fiber design. Polarization extinction ratios in excess of 30 dB have been measured in short lengths of similar fibers without ytterbium doping. 6 The single polarization fiber possessed a refractive index profile for which the effective index of the unwanted polarization state was below the index of the cladding. The polarizing fiber had an elliptical core with two air holes placed next to the core along the minor axis of the ellipse. The fiber cross section is shown schematically in Figure 1.