A tapered optical fiber can be produced by gently stretching an optical fiber while it is heated e.g. over a flame, such that the glass becomes soft. This procedure makes the fiber thinner over some length of e.g. a few millimeters or centimeters. The fiber core also gets thinner by the same factor as the total fiber.
Tapers for Mode Matching
Moderate tapers are sometimes used for the purpose of mode matching: it is possible, e.g., to reduce the mode area for one end of a standard single-mode fiber in order to achieve an improved coupling to some small-area waveguide (→ mode field converters).
Tapers for Mode Filtering
Another application is mode filtering: the higher-order guided modes become quite weakly guided or disappear completely in a moderately stretched fiber region, so that largely only light in the fundamental fiber mode remains. Figure 1 shows how light in the LP11 mode is completely lost in the tapered region (here with a 50% reduction in core diameter), whereas light in the fundamental mode (see Figure 2) hardly experiences any losses and even does not undergo a substantial change of mode size (for the chosen parameters).
In the non-processed fiber after the taper region, the light may then remain in the fundamental mode, if mode mixing e.g. by bending the fiber is avoided.
That technique can be utilized for high-power fiber amplifiers, for example, which are based on few-mode fibers, since such fibers can reach higher performance than single-mode fibers. Different taper formats have been used. For example, the core and cladding diameter may gradually increase along the whole fiber. Alternatively, there may be a tapered region (with reduced dimensions) at the input end, and possibly another one at the output end (double-tapered design), in order to support operation dominantly on the fundamental mode, even though there are also some higher-order modes in the section with larger dimensions.
Strongly Tapered Fibers
It is also possible to perform stronger tapering, as shown in Figure 3, where the diameter of the tapered fiber region can be only a few microns over a length of a few centimeters (or even longer than 10 cm). 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.
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 nanofibers with diameters of a few hundred nanometers or sometimes even well below 100 nm.
If two or more fibers are heated over a flame, they may form a common taper region. That configuration is often used in fiber couplers. Here, some of the light launched into one fiber can couple over to the other fiber, but only into a mode with the same propagation direction – it is a directional coupler.
If the parameters of the original fibers are somewhat different, a null coupler may result, where light launched into one fiber will emerge only from the corresponding end, and coupling occurs only e.g. under the influence of a sound wave propagating in the taper region. That way, one obtain an unusual kind of acousto-optic modulator.
Multi-core Tapered Fibers
Tapering can also be applied to multi-core fibers. Each fiber core will then be subject to the same taper ratio. However, as at most one core can be on the fiber axis, the others will also experience a lateral shift, which implies bending in the taper region. That may lead to additional bend losses. Another problem may be unwanted coupling of light between the cores in the region with small fiber diameter. One may need to optimize the taper design in order to minimize such effects – for example, based on numerical simulations of flight propagation in such devices.
Instead of single fibers, one can also taper fiber-optic plates containing many, sometimes even millions of fibers. Such fiber-optic tapers are mainly used for imaging applications (e.g. with fiber-optic endoscopes) where some amount of (de)magnification is required.
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See also: fibers, nanofibers, fiber couplers, photonic crystal fibers, supercontinuum generation, mode field converters, fiber-optic tapers
and other articles in the category fiber optics and waveguides