Ultra-Low-Loss Broadband All-Fiber Mode Selective Couplers for MIMO-Less MDM Transmission

Ultra-low-loss all-fiber mode selective couplers (MSCs) have been designed and fabricated using fused biconical taper technique with precise control on phase matching condition. An LP
₀₁
MSC with minimum insertion loss about 0.1 dB is fabricated. It is promising for single-mode excitation in large-mode-area few-mode fibers. The insertion loss of the demonstrated LP
₁₁
MSC is 0.3 dB at 1540 nm with mode extinction ratio higher than 28 dB. It has a broad operation wavelength from 1480 nm to 1640 nm with wavelength dependent loss about 0.3 dB. To the best of our knowledge, it shows the lowest loss in the reported broadband LP
₁₁
mode converters with high mode purity. The fabrication process is reliable and repeatable with smooth power curves, which is of great importance to the practical fabrication of these components. MIMO-less 2 × 25-Gb/s OOK signal and 2 × 20-Gb/s PAM-4 signal transmission over 15-km few-mode fiber have been demonstrated using the fabricated components. The low loss, large bandwidth and the high fabrication repeatability of the mode selective couplers make them promising for low-cost MIMO-less WDM-MDM transmission.