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Core-less End Caps

Definition: end pieces attached to optical fibers, containing no fiber core

Categories: fiber optics and waveguides, laser devices and laser physics


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For some fiber optics applications, particularly in the area of fiber amplifiers and lasers, it can be helpful to use optical fibers with core-less end caps. These end caps are essentially homogeneous transparent pieces with a length of a few millimeters, for example. As the end caps contain no waveguide (fiber core), light propagates in these regions as beams which expand toward the ends.

core-less end cap
Figure 1: A fiber with core-less end cap. A beam from air is adjusted such that its focus is where the waveguide structure in the fiber begins.

Such end caps can be made by attaching (e.g. fusion splicing) small pieces of homogeneous glass to the fiber ends. In the case of photonic crystal fibers, it can be sufficient to heat the fiber end e.g. with a fusion splicer, so that the microscopic holes collapse.

The possible advantages of core-less end caps are:


[1]Y. O Aydin et al., “Endcapping of high-power 3 μm fiber lasers”, Opt. Express 27 (15), 20659 (2019), DOI:10.1364/OE.27.020659

See also: fibers, fiber lasers, fiber amplifiers, photonic crystal fibers, fiber optics

Questions and Comments from Users


How can one calculate beam expansion for a single-mode fiber spliced to a beam expanding coreless endcap? Gaussian beam propagation equations I have found only apply for beam expansion in air/vacuum. How do you account for beam expansion in silica with index n = 1.46?

The author's answer:

You can use the same equations, just with the wavelength of light in the medium – reduced by the refractive index.

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