Femtosecond laser micro-machining enabled all-fiber mode selective converter

We demonstrate a compact all-fiber mode selective converter enabled by femtosecond laser micro-machining on the few-mode fiber (FMF) facet. By introducing a micro-structure into the FMF core, we can achieve a π spatial phase difference to the fundamental mode of light to be converted. Theoretical optimization reveals that various high-order modes, including LP₁₁, LP₀₂, and LP₂₁ modes, can be selectively converted by various micro-structures on the FMF facet, with a mode extinction ratio of more than 25 dB and mode coupling efficiency of better than 45% over the C-band. Finally, a proof-of-concept experiment is conducted by inscribing a micro-slot on the two-mode fiber facet for the LP₀₁ to LP₁₁ mode conversion. A micro-slot with a width of 6.7 µm and a depth of 5.4 µm is fabricated under the optimal femtosecond laser parameters. A LP₀₁ to LP₁₁ mode conversion with an average insertion loss of 2.7 dB is realized over the C-band, together with a mode intensity profile correlation efficient of more than 65%. Efficient higher-order mode conversion is feasible with a precise femtosecond laser micro-machining.