Polishing of Fibers
In fiber optics, it is often necessary to prepare the ends of optical fibers such as to obtain a high mechanical and optical quality. For example, that is necessary when fibers are to be equipped with fiber connectors or when they must be spliced together.
This article focuses mostly on the polishing of fiber ends, but briefly treats side polishing in the last section.
When is Fiber Polishing Required?
In many practical cases, cleaving provides sufficiently high quality, and is relatively easy and quick to apply. (For details, see the article on cleaving of fibers.) In some cases, however, cleaving is not appropriate; some examples:
- It may be very important to obtain a fiber surface which is precisely perpendicular to the fiber axis. In that respect, results of fiber cleaving are sometimes not sufficiently reliable.
- Even more difficult is the preparation of a fiber surface such that the normal direction is precisely at a certain angle against the fiber axis. Although there are methods of angle cleaving (i.e., cleaving such that one obtains such an angled surface), they usually exhibit a substantial variation of the obtained cleave angle.
- While cleaving usually provides a good surface flatness within a fiber core area of limited size, it often produces substantial irregular structures near the outer boundary, which can be disturbing in some cases. In particular, there can be protrusions which do not allow a good contact between fibers in a mechanical splice, for fusion splicing or in a fiber connector.
- Cleaving also usually does not work well for fibers with relatively large cladding diameters of e.g. >200 μm, and for some non-standard fiber glasses, which can e.g. be quite fragile.
In such respects, polishing methods applied to fiber ends can provide better results. Polishing may also be required in cases with special requirements – for example, preparing a fiber bundle such that all fiber ends quite accurately match a particular plane.
Unfortunately, polishing is usually more time-consuming than simple cleaving. However, the time for careful polishing and inspection of results may be well invested, avoiding possibly much more time-consuming fault searches at later stages. For example, if the performance of a optical fiber communications system or depends on many components, it is advisable to minimize the risk that some of the fiber connectors exhibit too high insertion loss or a too low return loss. (Note that such faults may not be easy to locate later on.) The same applies to complex fiber laser or amplifier setups.
Typical Fiber Polishing Process
Removal of Cladding and Coating
If a fiber is equipped with a fiber coating, that coating needs to be removed over some length before polishing. In the case of a fiber cable, the jacket and buffer must be removed beforehand.
Applying a Ferrule
Optical fibers can usually not directly be polished, since they are too small and mechanically not sufficiently stable. Therefore, one usually first inserts a fiber into a fiber ferrule consisting of ceramic, glass or metal material. (The ferrule may be provided as part of a fiber connector or as a separate part.) Beforehand, the ferrule is filled with a suitable adhesive (e.g. a two-component epoxy), which will quickly set (become solid) after inserting the fiber.
The fiber end can be cleaved just at the end of the ferrule. Now it is ready for polishing. Generally, one uses a series of polishing pads (e.g. three of them), starting with a relatively rough grit of polishing film (e.g. mostly for removing protrusions) and then using finer grits.
The applied procedures can differ in various details. In simple cases (e.g. often used for fiber connectors), the polishing is done manually. For example, one may insert the ferrule into a metallic holder (polishing puck) and manually swipe this over a flexible polishing pad for some time. More reproducible (but not necessarily quicker) results can be achieved with a polishing machine, where a polishing pad is rotated while a well defined load force is applied to the fiber puck. One may also apply a steady flow of a liquid polishing agent during the polishing process. Generally, some further cleaning (e.g. with isopropyl alcohol) is required after the polishing.
By using a flexible polishing pad, usually one does not prepare an exactly flat fiber end, but rather a slight dome, i.e., a slightly convex structure. This makes it easier to get a well-defined physical contact, e.g. in a fiber connection, if the radius of curvature is large enough for the intended contact area.
A slight undercut of the fiber may result, i.e., the final fiber end surface will be slightly behind the surface of the surrounding ferrule. It is often important to keep that undercut small enough.
The polishing process can be modified for angle polishing, i.e., for obtaining a well-defined angle between the surface normal and the fiber axis. Another possibility is polishing several fibers (e.g. all of a fiber bundle) together.
Getting Consistent High Quality Results
Consistent high quality polishing results require that any substantial variations of the process are avoided, including variations of the used consumables. Also, the environment must be kept sufficiently clean.
When using a fiber polishing machine or a manual polishing set (hand polisher), it is advisable to carefully study the user instructions beforehand until a clear understanding of all details is achieved. A clearly formulated and helpful instruction manual should be part of any high quality polishing machine or material set.
When applying a polishing process to fibers with different characteristics, for example different glass materials or cladding diameters, one may need to adapt the polishing process and some details. For example, the load force and the polishing duration may need to be modified.
Even for well-established polishing processes, it is advisable to regularly (perhaps even every time) carefully inspect the obtained results, for example using a fiber microscope. The target is a scratch-free, smooth end face with the fiber accurately sitting in the center of the ferrule. If that is reached, one should usually achieve connections with low insertion loss and high return loss. One should appropriately document the findings, including possibly encountered problems and chosen methods to fix them.
In relatively rare cases, fibers need to be polished from the side.
For example, it is possible to produce special fiber couplers by polishing two fibers close to their fiber cores and fixing them together, so that light from one fiber core can reach the other core. A similar procedure may be applied to make certain fiber-optic pump combiners.
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