Estimation of LP11 mode content in LMA fiber laser output based on quadrant photodiode measurements

Large-mode-area (LMA) fibers are enablers for high-power fiber lasers but may lead to multimode operation. The LP11 mode is the most problematic higher-order mode, typically appearing first and causing beam pointing instability due to its antisymmetric shape. Existing methods to quantify LP11 content are complex and lack high temporal resolution, crucial for transient regimes or TMI. Variations in relative LP01-LP11 phase shift the beam centroid and can be captured by a quadrant photodiode. We propose a simple technique to estimate LP11 content and beam size based on quadrant photodiode measurements, and validate it with simulations and experiments.