Ultrafast laser inscribed mode-group-selective 6-mode photonic lanterns for mode-Division multiplexing

Global Internet data demand is rapidly approaching the fundamental transmission capacity limit of single-mode optical fibres. One of the most promising technologies to avoid the looming capacity crunch is mode-division multiplexing. Mode-division multiplexing exploits higher-order modes supported by few-moded or multi-moded optical fibres as individual, parallel data channels. As a result, the fibre transmission capacity can be scaled proportional to the number of utilized modes. Key components for future mode-division multiplexed communication systems are mode-multiplexers that enable the selective excitation and detection of the individual modes of the fibres. In this paper, the first 6-mode integrated mode-group-selective photonic lantern is demonstrated. The 70 mm long device consists of 6 waveguides. The 6 single-mode waveguides are arranged at the input in a linear array to match a 127 μm pitch fibre array. After remapping into a pentagon with a central waveguide, the waveguides gradually transition over a 50 mm length to an 8 μm radius pentagon with one waveguide in the centre, forming a composite few-moded waveguide. Unlike in previous reports for 3-mode lanterns, the careful choice of the inscription parameters and glass substrate has resulted in a uniform multimode waveguide at the output with no regions of negative index contrast which would limit the packing density of the individual waveguides at the output. The waveguides were inscribed in a single-pass using the slit method (1 kHz repetition rate, 800 nm, 120 fs) into Schott AF-45 glass with pulse energies of 1300 nJ (LP02), 1350 nJ (LP21), 1400 nJ (LP11) and 1450 nJ (LP01), respectively. The output near-field profiles, imaged by injecting 1550 nm light into the individual single-mode waveguides. These devices are a promising solution for future ultrahigh bandwidth communication networks.