An all-fiber interferometric technique for identification of the coupled- and throughput ports of a fused fiber directional coupler

Optical fiber directional couplers are widely used in various devices for applications in optical communication networks and fiber optic sensors. For a given input port, the two output ports of the coupler are designated as 'throughput port' and 'coupled port'. Any ambiguity in identifying the coupled and the throughput ports in configuring devices that involve multiple couplers could degrade their performance. For example, in multi-stage all-fiber Mach-Zehnder interferometer (MZI) based wavelength interleavers, the peak-to-peak extinction ratio in the passband would drop down substantially; also, ripples appear in the flattop passband of the spectral response. In this paper, a simple all-fiber interferometric method is presented to resolve such ambiguity. The applicability of the method is experimentally demonstrated by fabricating an all-fiber MZI. Further, the variation in the extinction ratio at the output of the MZI due to asymmetry in the splitting ratios and excess losses of the couplers has been also studied.