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Definition: optical fibers which are over some length stretched out to a very small diameter
A tapered fiber can be produced by gently stretching an optical fiber while it is heated e.g. over a flame. 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 size converters).
Figure 1: A glass fiber which is tapered over some length.
It is also possible to perform stronger tapering, as shown in Figure 1, 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 is even possible to merge two or more fibers over a flame, forming a common taper region. 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.
Tapered fibers with few-micron taper regions are interesting for a number of applications, such as supercontinuum generation, fiber-optic sensors, or acousto-optic 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 nanofibers with diameters of a few hundred nanometers or sometimes even well below 100 nm.
Bibliography
| [1] | T. A. Birks et al., “The acousto-optic effect in single-mode fiber tapers and couplers”, J. Lightwave Technol. 14 (11), 2519 (1986) |
| [2] | T. A. Birks and Y. W. Li, “The shape of fiber tapers”, J. Lightwave Technol. 10, 432 (1992) |
| [3] | C. E. Chryssou, “Theoretical analysis of tapering fused silica optical fibers using a carbon dioxide laser”, Proc. SPIE 38 (10), 1645 (1999) |
| [4] | T. A. Birks et al., “Supercontinuum generation in tapered fibers”, Opt. Lett. 25 (19), 1415 (2000) |
| [5] | G. Brambilla et al., “Ultra-low-loss optical fiber nanotapers”, Opt. Express 12 (10), 2258 (2004) |
| [6] | F. Warken et al., “Ultra-sensitive surface absorption spectroscopy using sub-wavelength diameter optical fibers”, Opt. Express 15 (19), 11952 (2007) |
| [7] | N. Vukovic et al., “Novel method for the fabrication of long optical fiber tapers”, IEEE Photon. Technol. Lett. 20 (14), 1264 (2008) |
See also: fibers, nanofibers, photonic crystal fibers, supercontinuum generation, mode size converters
