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Mach-Zehnder interferometers (MZIs) are used in many optical applications, such as measurement of the coherence length of a laser, thermal dynamic flow, flatness of plane optical plates, thickness of thin films, etc. In this type of interferometer, light passing through a sample region in one direction recombines with a second leg without traversing the sample twice.In telecommunication, MZIs are used for demodulating differential phase-shift-keyed (DPSK) signals. DSPK has attracted increased attention in fiber optic transmission in recent years because of its 3-dB optical signal to noise ratio improvement over standard intensity modulated transmission, as well as for its high tolerance to nonlinear effects and coherent crosstalk. In a standard fiber MZI, two wideband fiber couplers are spliced together with one arm providing a one-bit delay to convert the phase difference into an intensity modulation. In our alternate type of MZI, the two-mode interferometer, the time delay is obtained through the difference between the propagation constants of two modes instead of through a physical path length difference.We present here a novel single multimode fiber modal interferometer for DPSK demodulation. In this design, a second mode is excited by splicing a standard fiber to a multimode fiber length such that two modes beat together before recombining in a second splice. A numerical analysis and an experimental verification of the multimode fiber parameters to maximize the extinction ratio and minimize the length of the interferometer are presented. We investigate coupling, insertion losses, temperature sensitivity and polarization effects of using modes with and without radial symmetry. The design is extremely low-cost, easily manufactured and is intrinsically less temperature-sensitive than standard MZI. Although balanced detection is lost, DPSK may still be advantageous because of its high tolerance to nonlinear effects and coherent crosstalk.

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