Modal sensitivity analysis for single mode operation in large mode area fiber

Most of the current large mode area (LMA) fibers are few-moded designs using a large, low numerical aperture (N.A.) core, which promotes mode coupling between core modes and increases bending losses (coupling with cladding modes), which is undesirable both in terms of mode area and beam quality. Furthermore, short LMA fiber lengths and small cladding diameters are needed to minimize nonlinear effects and maximize pump absorption respectively in high-power pulsed laser systems. Although gain fiber coiling is a widely used technique to filter-out unwanted modes in LMA. fibers, coupling between modes can still occur in component leads and relay fibers. In relay fiber, light coupled into higher-order modes can subsequently be lost in the coiling or continue as higher-order modes, which has the overall effect of reducing the effective transmission of the LP01 mode and degrading the beam quality. However, maximum transmission of the LP01 mode is often required in order to have the best possible beam quality (minimal M-2). Launching in an LMA fiber with a mode field adapter (MFA)(1) provides an excellent way of ensuring maximum LP01 coupling, but preservation of this mode requires high modal stability in the output fiber. Small cladding, low N.A. LMA. fibers have the disadvantage of being extremely sensitive to external forces in real-life applications, which is unwanted for systems where highly sensitive mode coupling can occur. In this paper, we present a detailed experimental and theoretical analysis of mode coupling sensitivity in LMA fibers as a function of fiber parameters such as N.A., core diameter and cladding diameter. Furthermore, we present the impact of higher N.A. as a solution to increase mode stability in terms of its effect on peak power, effective mode area and coupling efficiency.