It is also possible to perform stronger tapering, as shown in Fig 1, where the diameter of the center tapered fiber region can be only a few microns over a length of a few millimeters (or even longer than 15-20mm). Under these conditions, the original fiber core becomes so small that it has no significant influence any more, and the light is guided only by the air–glass interface. Provided that the transition regions from the full fiber diameter to the small waist and back again are sufficiently smooth, essentially all the launched light can propagate in the taper region and (more surprisingly) find its way back into the core of the subsequent full-size fiber region. These are work more in fiber optics sensors application.
It is even possible to merge several fibers to taper as one fiber over a laser fusing techniques, forming a common taper region and we can use those techniques for fiber optics couplers. Optical fiber tapering will be useful for telecom and biomedical and tapered fibers with few-micron taper regions are interesting for a number of applications, such as super-continuum generation, fiber optics sensors or acousto-optic, or fiber modulators.
Recently, it has been demonstrated that with somewhat refined tapering techniques (involving indirect heating of the glass via a sapphire taper or a sapphire capillary) it is possible to carry out even very extreme tapering, leading to nano-fibers with diameters of a few hundred nanometers or sometimes even well below 100nm.