定义：
长度超过一定直径的光纤通过加热光纤，例如在火焰上，使光纤变软，就可以产生锥形光纤.。这个过程使纤维在一些长度如几毫米或厘米。纤维芯也得到了相同的因素较低的总纤维。

模式匹配的锥度
适度的锥度有时用于模式匹配的目的：它是可能的，例如，减少模式为一端为标准单模光纤实现一种改进的耦合到一些小面积的波导（→模式尺寸转换器）。

滤模锥度
另一种应用是模式滤波：高阶引导模式变得相当弱引导或完全消失在适度拉伸纤维区域，所以基本上只有光的基本纤维模式仍然存在。图1显示了在LP11模式如何完全失去了光在锥形区域内，而光的基本模式（见图2）几乎没有经历任何损失甚至不接受模式规模大幅变化（所选择的参数）。

图1:发射到LP11模式沿锥形光纤光振幅分布。在变薄的区域，LP11模不存在，以及相应的光进入包层。该数值束传播模拟已经完成与软件RP光纤功率。

图2:在基本模式发射振幅分布（LP01）
强烈的锥形纤维
它也可以执行更强的锥形，如图3所示，其中的锥形纤维区域的直径可以只有几微米的长度为几厘米（甚至长于超过10厘米）。在这种情况下，原来的光纤芯变得如此之小，它没有任何显着的影响，和光被引导只有由空气-玻璃界面。但从全纤维直径地区小腰，再次是足够光滑，基本上所有的发射光可以传播在锥形区域（更令人惊讶的是）发现到后续的全光纤核心区域的方式。

图3：玻璃纤维在一定长度上呈锥形。
它已被证明是有点精尖技术（包括玻璃间接加热通过蓝宝石锥度或蓝宝石毛细管）可以执行甚至非常极端的紧缩，导致几百纳米的纳米纤维的直径或甚至低于100 nm。

光纤耦合器
如果两个或更多的纤维在火焰上加热，它们可以形成一个共同的锥形区域。这种配置通常用于光纤耦合器。在这里，一些光发射到一个光纤可以耦合到其他光纤，但只有到一个模式具有相同的传播方向-这是一个定向耦合器。

如果原纤维的参数有所不同，一个空耦合器可能会导致，其中光发射到一个纤维将只出现从相应的端部，和耦合只发生例如在锥形区域中传播的声波的影响下。

其他应用
几微米的锥形区域的锥形纤维多的应用是有趣的，如超连续谱的产生，光纤传感器，或光纤声光调制器。

Definition: optical fibers which are over some length stretched out to a very small diameter

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).

Figure 1: Amplitude distribution of light along a tapered fiber for launching into the LP₁₁ mode. In the thinned region, the LP₁₁ mode does not exist, and the corresponding light gets into the cladding. This numerical beam propagation simulation has been done with the software RP Fiber Power.

Figure 2: Amplitude distribution for launching in the fundamental mode (LP₀₁).

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.

Figure 3: A glass fiber which is strongly tapered over some length.

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.

Fiber Couplers

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.

Other Applications

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.

Multi-fiber Tapers

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 where some amount of (de)magnification is required.

Bibliography