Comparative study of light propagation and single-mode operation in large-mode area fibers designed for 2-μm laser applications

Output performances of fiber-based optical systems, in particular fiber lasers and amplifiers, can be controlled using tailored fiber designs, gain profiles, and pump light overlap with the gain medium. Here, the performances of 2-μm light, propagating in three large-mode area fibers, a step-index fiber, a photonic crystal fiber (PCF), and a leakage channel fiber (LCF), designed to deliver a single-mode (SM) beam at this wavelength, were compared. Using the S2 imaging technique, the transverse mode content has been decomposed, and propagation losses, SM purity, and mode-field area (MFA) were measured for various input mode overlap and coiling diameters. It was experimentally demonstrated that coiling the PCF and LCF to 40 and 20 cm in diameter, respectively, resulted in efficient higher-order mode suppression, pure SM beam delivery, moderate (∼1 dB) coil-induced losses in the fundamental mode, and nondistorted, large MFA (∼1600 μm2) beam delivery. © 2015 Society of Photo-Optical Instrumentation Engineers (SPIE) [DOI: 10.1117/1.OE.54.1.011006]