Design of a tunable single photon interferometer based on modal engineered tapered optical fibers

We propose the use of modal engineered tapered optical fibers for single photon interferometry experiments. Tapered optical fibers (TOF) can be designed to function as tunable Mach-Zehnder (MZ) type modal single photon interferometer. The MZ design criteria of tunability, visibility, and wavelength range are inherently interconnected in TOF's, which makes the design complex. Moreover, novel TOF related design criteria, like loss minimization, have to be taken into account. Design guidelines can be based on a semi-analytical approach assuming multiple two mode beat equations for different mode power amplitudes. Linearization permits to obtain a set of simple and robust analytical relations that correlate the TOF design parameter with the TOF and MZI optical properties. By focusing on the tunability, we have demonstrated a device that permits to be tuned between local minima and maxima.