Effect of gain-dependent phase shift for tunable abrupt-tapered Mach-Zehnder interferometers.

A differential gain-dependent phase shift between the core mode and the cladding mode can be all-optically tuned by a cw 975 nm pump light for Mach-Zehnder interferometers using two abrupt fiber tapers in a 1.2-cm-long highly Er/Yb codoped fiber. The highly doped fiber has large absorption and emission cross sections to efficiently introduce the phase shift when the inversion rate is changed by varying the pump power. The first abrupt taper converts part of core mode into the cladding mode, while the second abrupt taper recombines both modes to interfere after propagating for a length of a few millimeters.