Mode-Selective Couplers for Two-Mode Transmission at 850 nm in Standard SMF

The optimal design of a low-loss fused fiber mode-selective coupler for two-mode fiber transmission in the 850-nm band is presented. The coupler is based on precise phase matching of the propagation constants in each arm of a weakly fused fiber coupler. The designed component permits both mode converter and mode multiplexer/demultiplexer operation, thus enabling modal multiplexing transmission in this band with no additional component. The presented design is evaluated by simulation considering two types of structures, leading to asymmetric and symmetric coupler configurations. Mode converter and mode multiplexer operation is achieved with 93.5% efficiency in the band of 845-855 nm. Mode demultiplexer operation is achieved with an extinction ratio better than 20.4 dB in the same band.