A PLC based mode multi/demultiplexer based on asymmetric directional coupler has advantages in terms of compactness, mass productivity, low insertion loss, and matured reliability. However, it has relatively large wavelength dependence due to the difference of coupling length. To expand the bandwidth, we have designed two-mode (LP01/LP11a) multi/demultiplexer by wavefront matching method and demonstrated the broadband and low-loss characteristics. This paper reviews the device design by wavefront matching method and investigates the mechanism of its broadband characteristics.