Fundamental mode amplification in 140 μm core diameter fiber

Many applications require high power good beam quality lasers with fiber delivered output. Very often, the necessary powers are above the damage threshold and nonlinear effects threshold of single mode (SM) fibers. This may be overcome by two approaches. A first approach is to use either large mode area fibers or photonic crystal fibers (PCFs), which have a very large and nearly SM cores. Nevertheless, this approach is limited by the achievable nearly SM core size, which is 40 μm for a flexible PCF. A second possible approach is to impose a Gaussian mode in a highly multimode (MM) step-index fiber with very large core diameter. In this approach, it was found that in passive highly MM fibers the fundamental mode can be maintained over short distances (∼20 cm) in the case of straight fibers [1], or over longer distances in the case of coiled fibers [2]. Additionally, off-centered coupling of the fundamental mode into highly MM (100-400 μm core diameter), 2 m long, passive fibers was studied [4], along with its effect on the output beam quality [3]. Finally, SM excitation in active MM fibers, as means for achieving high quality output in an amplifier configuration, was also reported [2].