Mode suppression employing periodical large-small-core (PLSC) filter in strongly pumped large mode area fiber amplifier

Mode instability (MI) has become one of the main restrictions for further power scaling of large mode area (LMA) fiber systems. A periodical large-small-core (PLSC) filter in a strongly pumped LMA fiber amplifier based on multimode interference and self-imaging effect is proposed to suppress high order modes and improve the beam quality. An investigation on the modeling of light propagation within this filter structure is presented. Transverse mode competition and individual transverse mode power distribution in a strongly pumped fiber amplifier are theoretically analyzed. By employing this PLSC mode filter in the 30/400 strongly pumped LMA fiber amplifier, the percentage of the fundamental mode rises from 27.8% (without filter) to 95.6%, and the percentage of the high order modes correspondingly reduces from 72.2% (without filter) to 4.4%. By the modal power decomposition, the optical field pattern in free space is established and the M-2 parameter of beam quality decreases dramatically from 2.24 to 1.12 (0 relative phase) and from 3.01 to 1.28 (pi/2 relative phase). This study provides a new method to achieve high beam quality in LMA fiber amplifiers and this filter would be extended to larger mode area fiber amplifiers.