Ultra-broadband fiber mode converter based on apodized phase-shifted long-period gratings

We have demonstrated an ultra-broadband fiber mode converter based on CO₂-laser inscribed length apodized phase-shifted long-period gratings (LPGs) with a three-section linear length apodization profile, where a π-phase shift was introduced between two adjacent grating sections. The grating parameters were optimized theoretically to achieve the broadband mode conversion between the LP₀₁ mode and LP₁₁ mode. The demonstrated device provides mode conversion efficiency higher than 90% over an ultra- broad bandwidth of ∼182nm. The insertion loss of the LPGs is negligible. Orbital angular momentum modes with left- and right-handed circular polarization can be generated from the demonstrated ultra-broadband mode converter successfully.