Symmetric Few-Mode Fiber Couplers as the Key Component for Broadband Mode Multiplexing

All-fiber broadband mode multiplexers (MMUXs) for mode and wavelength division multiplexing transmission systems are designed and analyzed. The MMUXs are based on cascaded 2-D or 3-D symmetric few-mode fiber (FMF) couplers. The MMUXs are optimized for operation over the C band and multiplex modes LP₀₁, LP_11a, LP_11b, LP_21a, LP_21b, and LP₀₂ with a nearly flat response and an average insertion loss around 1.6 dB, depending on the design approach. The operation of the FMF couplers and the MMUXs is analyzed numerically by means of a full-vectorial beam propagation method. If the two polarization states of each LP mode are further considered, such all-fiber MMUXs can be used to combine 12 spatial channels, supporting an order of magnitude capacity increase-compared to a single spatial channel system-in optical fiber transmission systems through space (mode and polarization) division multiplexing.