Tapered-fiber interferometric wavelength response: the achromatic fringe

In considering the wavelength-dependent transmission of tapered fibers we provide an observation of the fringe that shows the least response to variations in wavelength and that is, by definition, the achromatic fringe. This is a manifestation of the equalization of the HE11–HE12 group velocity occurring at V = 4.4. The fact that it was observed in a tapered single-mode fiber confers some particular features that are discussed and compared with those of other two-mode guides with regard to sensor applications.