Germanium-Free Graded-Index-Core Few-Mode Fibers

Germanium-free graded-index-core fibers supporting 6 spatial modes are designed, fabricated, and characterized. Their graded-index-core profile is precisely controlled using the plasma chemical vapor deposition process, which allows for high Fluorine incorporations with high accuracy. This approach results in low differential mode group delays and low Rayleigh scattering losses. A 1st realization, however, exhibits high total losses from 0.23 to 0.26 dB/km due to unwanted couplings between the guided modes of the core and the highly lossy modes of the silica outer cladding (deposition tube). In a 2nd realization, the tube is removed after deposition, eliminating these coupling effects and reducing losses to 0.202–0.211 dB/km at 1550 nm for all guided modes, despite some contaminations during manufacturing in our R&D lab. In a controlled and optimized environment, these losses can be further reduced to <0.18 dB/km. The plasma chemical vapor deposition process enables these Ge-free structures to be optimized for low differential-mode-group-delay few-mode fibers supporting up to 55 spatial modes, with losses ≤0.20 dB/km at 1550 nm. Such fibers, combined with cyclic mode permutation techniques, are promising candidates for mode division multiplexed long-distance applications.