Abstract

Single spatial mode operation of a fiber laser leads to perfect output beam quality, which is important for industry. Increasing of the effective mode area allows for achieving higher laser power at the wave field intensity lower than the limit imposed by non-linear effects. However, the large mode area (LMA) fiber lasers are open to higher order mode (HOM) appearance. Single mode operation was demonstrated for core diameter up to 40 μm in conventional double-clad fibers [1], and up to 100 μm in photonic crystal fiber amplifiers [2]. Recently, the leaky channel fiber amplifier with a core diameter up to 53 μm was demonstrated [3]. The single mode operation in all these constructions is due to the highest modal gain and smallest loss of the fundamental mode (FM). Generally, the higher small signal modal gain is insufficient condition for stable single mode operation at high powers. The spatially non-uniform gain saturation leads to HOM excitation. This effect can be eliminated if the FM has flat top profile in the gain region [4]. Flattened FM constructions of cylinder and ribbon fibers were suggested in [5] as possessing higher efficiency for inversion consumption.

© 2009 IEEE