fiber end
Preparing Fiber Ends for Cleaning: Stripping, Cleaving, Polishing
In most cases, when working with fiber optics, clean ends must be prepared. The first step is usually to strip the last few centimeters of the polymer coating using a mechanical stripper. In problematic cases, solvents may have to be used (chemical stripping). The outer shell of fiberglass will usually be clean, but the fiber end, if it’s just snapped off, will still have an irregular shape. So we need some way to get a good surface – usually a plane, which is perpendicular to the fiber axis, or sometimes with some other angle. The most common method of preparing clean ends is by cutting. Essentially, this means controlled breakage of the bare fiber glass. One way is to make a tiny scratch on the side of the fiber, for example with a sharp diamond, carbide or ceramic blade, before or while applying some tension or bending to the fiber. This causes the fiber to break from the break point mentioned above. Typically, the resulting surface is very smooth. Cutting is usually done with a simple diamond blade. One scratches the fiber slightly and breaks it, for example with the end of a fingertip. This process takes some practice and the results are somewhat variable. For more consistent results, cleaving with a precision fiber cleaver will need to be performed under more controlled conditions. Some of these devices can also be used to prepare angled cuts (see Figure 2), where the angle between the cut surface and the fiber axis is relatively well controlled. Cutting becomes more difficult in non-standard situations, such as large fiber diameters or non-standard glass compositions. For example, when cutting fluoride fibers, at least proper parameters need to be used for precision cutters. For more details, see our encyclopedia article on fiber optic cutting. Section 13 on Fiber Optic Accessories and Tools also provides more details on cleavage tools. Re-cutting the fiber can be an alternative to cleaning, as it is difficult to reliably clean the fiber end. For very high quality fiber surfaces, or when using large diameter fibers, or when connecting fiber optic connectors, some post-cleavage polishing procedures may be required. For example, the fiber end can be inserted into a ferrule (hollow ceramic, glass or metal tube) and held there with glue. The fiber is then polished together with the glass tube using a special polishing machine. This process allows the production of high-quality surfaces with arbitrary, well-defined fiber surface orientations. However, it takes more time than simple cutting, and of course all details of the polishing machine (e.g. load force, speed and time) and polishing agent must be well adapted to the ferrule and fiber material and size. Hand polishing is also Possible, but usually leads to poor results. Polished fiber ends, other than the cleaved end, may have some convex curvature due to the use of flexible polishing pads. This “domed surface” facilitates good contact between, for example, two single-mode fibers in a connector set.
Dependence on cutting angle
In some cases it is important to have a cut fiber surface just perpendicular to the fiber axis. For example, this typically occurs when a fiber is inserted into a fiber optic connector (see Section 6), although some connectors require angled cuts. Mechanical joints are also not suitable for non-vertical ends (see Figure 1).
Figure 1: Fiber splices will not function properly when the fiber cut is not perpendicular: air gaps will form, or kinks will form.
Note that abnormal cleaving can cause the output beam to be oriented off the fiber axis due to refraction at the fiber end (see Figure 2). Additionally, a properly tilted input beam is required for efficient emission. This makes the use of angle cuts somewhat inconvenient.
Figure 2: When light exits an angle-cut fiber, it is somewhat deflected. Also shows the direction of the reflected light; it does not return to the core.
The splitting angle also has a significant effect on backreflected light. If it is small, the light reflected at the output surface (Fresnel reflection due to the difference in refractive index with air) will essentially travel backwards in the core. However, for a sufficiently large cut angle, the light will completely enter the cladding and be lost there. This means that despite significant reflections there is still a very large return loss (say 60 dB), which for a normal split is only 14 dB. Depending on the details of the fiber, what cut angle is needed to achieve high feedback rejection. For example, with typical single-mode fiber, the mode has a beam divergence of a few degrees. For example, a cutting angle as large as 8° may be required. It can be larger for fibers with high numerical aperture. However, for large mode area fibers, a rather small cut angle is sufficient to suppress feedback. In some cases, Fresnel reflections from the end of an optical fiber are used, such as for efficient output couplers in fiber lasers, or in optical time domain reflectometers (OTDRs).
other end shapes
In most cases, the fiber ends are simply flat – either cut vertically or at an angle to the fiber axis as discussed above. However, in some cases different geometries of fiber ends are used:
The lensed fiber end has a strong curvature, which causes collimation or at least reduces the beam divergence of the beam leaving the fiber. Due to the usually rather small core size, relatively small radii of curvature are required to obtain a significant lensing effect. One specific implementation is the fiber optic ball lens, in which a tiny glass sphere is fused to the end of a fiber optic. A special fusion splicer can be used for this. The natural surface tension of glass facilitates the manufacture of high-quality fiber optic ball lenses.
The above-mentioned glass sphere can also be further processed; for example, it can be equipped with a reflective plane that reflects the outgoing beam to the side. This is useful, for example, for some medical applications where optical fibers are embedded in endoscopes.
With fiber optic axicons, the fiber diameter rapidly decreases to essentially zero near the end of the fiber. This can be achieved by polishing (resulting in a pencil shape) or by tapering techniques. Only in the latter case will the core size also gradually decrease; however, this aspect may not be critical for the performance of the device. Light coming from an optical fiber and passing through such an axicon end is focused to a rather small diameter so it can be launched into a very small waveguide such as a photonic integrated circuit. Instead, light from such a waveguide can be efficiently transmitted into a single-mode fiber.
The fiber ends can be tapered (→ Tapered Fibers) and then cut in the area of reduced fiber diameter. Such components can be used in mode field converters if the reduced mode size at the smaller end is suitable for different kinds of fibers.
Coreless End Caps are homogeneous glass components that are spliced to the end of an optical fiber. (In the case of photonic crystal fibers, the holes in the end region can simply be folded over using a fusion splicer.) Light from the fiber core will expand within the coreless end cap, so its beam radius is greatly increased (with a corresponding decrease in intensity) Once the glass/air interface is reached. This device allows light to be transmitted from fiber optics into the air and vice versa at very high power levels.
