All-fiber three-path Mach-Zehnder interferometer.

We report the realization of a three-path Mach-Zehnder interferometer using single-mode fibers and two integrated 3 x 3 fiber couplers. We observed enhanced phase sensitivity, as compared with two-path interferometers, with a visibility of the interference pattern of more than 97%. This interferometer has an analog in two-photon interferometry, and we believe it to be the first nontrivial example of N x N multiport interferometers.