Experimental characterization of an off-axis scheme for pumping high-power photonic crystal fiber lasers

We experimentally investigate a free-space, off-axis method for pumping high-power photonic crystal fiber (PCF) lasers and amplifiers. The method is based on off-axis injection of the pump beam, exploiting the relatively high numerical aperture of the PCF pump cladding. With this method, we obtain pump coupling efficiency of more than 97 % for incidence angles as high as 30 degrees and show that with more than 60-W pump power the laser efficiency and beam quality do not significantly depend on the incidence angle. Our results indicate that this off-axis pumping method, with its significant advantages over the standard on-axis methods, can be exploited in high-power photonic crystal lasers and amplifiers.