Core-less End Caps

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

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

Suppliers

The RP Photonics Buyer's Guide contains eight suppliers for fiber end caps. Among them:

Schäfter + Kirchhoff

Schäfter+Kirchhoff offers single-mode and polarization-maintaining fiber cables with end caps. PCF fiber cables and special broadband RGB fibers with end caps are also available.

Sylex

Fiber end caps, spliced onto optical fibers terminated in FO connector, ensuring reliable data transmission by minimizing reflections and maintaining a consistent refractive index. Their coreless design allows light to expand evenly within the material. They provide high-power protection by reducing intensity at the fiber face, preventing damage from high-power sources. In photonic crystal fibers, end caps prevent water ingress. End caps are essential for fiber-optic sensors, high-gain fiber amplifiers, and protecting laser delivery fibers.

Shalom EO

Hangzhou Shalom EO offers custom fiber end caps made from quartz or fused silica, and various custom glass materials with custom AR coatings. Our fiber end caps are colorless, which enables even beam expansion in inhomogeneous media. The end caps introduce little beam distortion and have a high damage threshold. Our fiber end caps can have either a stem or a tapered lead-in section with a special inclined angle at one terminal of the cap to help splice into the fiber.

Shalom EO's fiber end caps also excel in minimized power absorptions and high precision. With these merits, our fiber end caps become attractive alternatives for clients seeking products with competitive prices in the market and reliable performance in the long run.

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