Core-less End Caps
Author: the photonics expert Dr. Rüdiger Paschotta
Definition: end pieces attached to optical fibers, containing no fiber core
Categories: 
DOI: 10.61835/ooh Cite the article: BibTex plain textHTML Link to this page LinkedIn
In many cases, optical fibers are used with bare ends. For example, fiber ends may be inserted into fiber connectors or mechanical fiber splices, or just mounted for release of the guided light into air. In some cases, however, one prefers to attach a transparent fiber end cap to a fiber end. That can be done for different reasons, which are explained in the following sections.
Fiber end caps are usually cylindrical homogeneous pieces of optical glass with a length of a few millimeters, for example, and a diameter matching a fiber diameter (typically 125 μm). As the end caps contain no waveguide (fiber core), light propagates in these regions as beams which expand toward the ends.
Most fiber end caps are flat, but possibly with slightly non-normal light incidence on the air–glass interface. They may be equipped with dielectric coatings, for example with anti-reflection coatings. It is also possible to have curved surfaces, for example for use as a beam collimator.
Making and Attaching Core-less Fiber End Caps
Fiber end caps can be made from larger blocks of glass with different techniques (e.g., drawing like in fiber fabrication). They can be attached to cleaved fiber ends with fusion splicing or with an optical adhesive. In the case of photonic crystal fibers, it may be sufficient to heat the fiber end, e.g. with a fusion splicer, so that the microscopic holes collapse.
Functions of Fiber End Caps
The possible functions of core-less end caps are explained in the following sections:
Protection of Fiber Ends
After being equipped with an end cap, a fiber end can hardly be damaged mechanically. On the other hand, the other surface of the end cap may be damaged. Enhanced mechanical protection is achieved mainly for photonic crystal fibers, which are somewhat more delicate due to their air holes.
Another aspect, which is often more important, is that the optical intensity at the glass–air interface is strongly reduced by beam expansion in the end cap. The risk of laser-induced damage thus gets strongly reduced, for example for Q-switched lasers or high-power fiber lasers and amplifiers, also for launching intense light into a fiber through an end cap.
Ends of photonic crystal fibers are also more delicate due to the risk that liquids may enter those holes. As the refractive index of any liquid is very different from that of air, that can profoundly spoil the guiding characteristics. (Note also that such a liquid could hardly be removed from the fiber due to strong capillary forces.) When such a fiber end is equipped with a solid end cap, forming a hermetically sealed end facet, that problem is eliminated.
Reducing Reflections
The optical interface between a glass fiber and air has a large refractive index difference, which implies a substantial reflectance due to Fresnel reflection. By attaching a core-less end cap, ideally having the same refractive index as the fiber, the reflection at that glass–glass interface is essentially eliminated, but a similar reflection remains at the interface from the end cap to air. That reflected light, however, will mostly not get back into the fiber core, but rather into the fiber cladding, since the beam diameter of the reflected light is usually far larger than the core diameter. This can be quantified with a large return loss.
High return loss is essential in various cases. For example, at the output of a fiber amplifier, one needs a high return loss since any light reflected back into the fiber core will again be strongly amplified on the way back through the amplifier. It could then either damage some other components or at least degrade the amplifier performance by extracting additional power, causing gain saturation. Suppressing parasitic lasing in high-gain fiber amplifiers can also be important. Some fiber lasers are also sensitive to back-reflections, and also superluminescent sources (ASE sources).
More to Learn
Encyclopedia articles:
Suppliers
The RP Photonics Buyer's Guide contains seven suppliers for fiber end caps. Among them:
Bibliography
| [1] | Y. O Aydin et al., “Endcapping of high-power 3 μm fiber lasers”, Opt. Express 27 (15), 20659 (2019); https://doi.org/10.1364/OE.27.020659 |
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2023-07-19
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.