Passive Fiber Optics

In part 5 on fiber ends, we have already explained that fiber ends normally need to be stripped and cleaved, and have mentioned some typically used tools. Here is a short overview:

• A common type of fiber stripper tool looks like a pincer. When starting with a fiber cable, one first has to cut into the polymer jacket with the stripper tool and pull the end part of the jacket away. The tool can then also be used to strip off the polymer buffer from the fiber over the last few centimeters. (Some strippers are specialized either for jacket removal or for stripping buffer coatings and smaller polymer coatings.) The device is made such that it cuts into the jacket or the buffer coating, but without damaging the glass fiber inside. Careful handling is important in order to avoid damages to the fiber, which could later cause breaking. With an alcohol-soaked pad one can then remove any residues of the coating, which might later on cause problems.
• The cleaving of fibers can often be done with quite simple mechanical fiber cleaver tools. In the simplest case, one only has a sharp diamond, carbide or ceramic blade (scribe) in a pencil-like form for scratching the fiber, and uses a finger kick to break it. Alternatively, one may put the fiber on a simple holder, hold it with two fingers, scratch it in between, and finally break it by applying some more tension. Such simple blades and holders are contained in simple fiber termination kits (preparation kits).
• Somewhat more sophisticated fiber cleavers have a fiber holder with a V-groove and some mechanics which clamp, pull, scribe and finally cleave fiber; the user only has to insert the stripped fiber and push some lever downward.

• For cleaving with more controlled conditions, leading to more consistent results, there are special apparatuses, called mechanical precision fiber cleavers, which work as follows. Here, one inserts the stripped fiber and fixes it, typically with a V groove and two clamps. One then applies a tension, the magnitude of which can often be adjusted (e.g. with a screwdriver). One now lets a diamond blade approach the fiber, causing the required scratch. The blade may be made vibrating with a small electromagnet or piezo. Finally, the tension is increased such that the fiber breaks. Some semi-automatic fiber cleavers also allow for angle cleaving, i.e., for preparing fiber ends with some angle against the fiber axis.

With standard silica fibers, stripping and cleaving is usually no problem. There are problematic cases, however, e.g. for non-standard fiber diameters, photonic crystal fibers with large air filling fraction, or for fragile fluoride fibers.

See our encyclopedia article on cleaving of fibers for more details.

Fiber splicing means that two fiber ends are put together such that light can get from one fiber to the other without excessive coupling losses. One has to distinguish two techniques:

• Mechanical splicing uses mechanical parts to attach the fiber ends.
• Fusion splicing means that the fiber ends are fused together with a heat treatment.

Mechanical splices can be made with relatively simple consumables and do not require expensive equipment. One simply has to insert the stripped and cleaved fiber ends into the mechanical splice. (Some versions are transparent, so that the fiber end can be seen inside.) With locking nuts, one can fix the fibers. The fibers needs to be inserted so far that essentially no air gap will be between the fiber ends. Some mechanical splices are reusable, i.e., one may take out the fibers and use the splice for other fibers. Others allow the use of an index-matching fluid, which can substantially reduce the insertion loss, but requires additional cleaning when a splice is redone. In some cases, one bonds the fibers with a UV-curable epoxy, which of course leads to permanent (not reusable) splices.

For fusion splicing, there are sophisticated fusion splicing apparatuses. One has to insert the stripped and carefully cleaved fiber ends into clamps and position them properly. Triggered by a button press, the apparatus then puts the fiber ends close together and applies heat from an electrical arc (or possibly with a CO₂ laser), such that the fiber ends are fused together. Fusion splicers are quite expensive, but they produce the most reliable and low-loss splices, and this without requiring expensive consumables.

Although fusion splices are comparably robust as the original fiber, the stripped fiber is less well protected. Therefore, one often uses splice protection sleeves to protect the spliced region.

For more details, see our encyclopedia article on fusion splicing of fibers.

Visual fault locators can help to locate faults of fibers, in particular of splices. Such a device contains a visible (often red) laser source in a little box with a fiber connector, where one can connect the fiber under test. Some visible light is then injected into the fiber. Only at locations where there is a fault, a significant amount of light will be scattered out of the fiber and can be seen. Obviously, mechanical splices should be transparent for that inspection method to work well.

There are also fiber microscopes for inspecting fiber ends. These can be less expensive than all-purpose microscopes, since they can work with a fixed (relatively high) magnification. It can be very useful to regularly inspect e.g. the quality of fiber cleaves and polished surfaces, because this is essential for successful splicing and for low-loss connectorization. Microscopic inspection requires only minimum time – much less than a later search for faults and their repair.

Special interferometers are available for inspecting fiber ends. They can be used to check the surface quality and the dome radius. Typically, the fiber under test is inserted together with its connector.

Fiber-optic connectors are very useful for non-permanent connections, e.g. with fiber patch cords. We have treated connectors in part 6 on fiber ends and in our encyclopedia article on fiber connectors. Fiber patch cables, which are connectorized already by their manufacturer, are available in many different versions:

• There are many different connector types and sometimes different quality levels.
• Different single-mode and multimode fibers are available.
• Depending on the environment of their use, fiber patch cables may have to be made with different resistance to moisture, heat and fire. Indeed, fire safety is an important aspect particularly for indoor applications. Moisture resistance and temperature tolerance are essential for outdoor cables, which however are usually not patch cables. The mechanical protection e.g. against people walking over fiber cables may also be important; there are particularly ruggedized fiber cables.

Note that one typically requires some associated equipment for connectorization, i.e., for attaching a fiber connector. Apart from equipment for stripping and cleaving, this often comprises specialized tools which depend on the type of connector. However, some users of fiber optics can entirely work with pre-fabricated fiber-connected devices and patch cords, and do not have to mount connectors themselves.

There are many kinds of fiber-optic adapters. Many of them allow one to join two fiber ends which already have been equipped with fiber connectors – possibly of different kinds. As many different kinds of fiber connectors exist, a great variety of different adapters is needed.

In many cases, one wants to transform the light exiting a fiber end into a collimated beam. For that purpose, one can attach a fiber collimator to the end. Usually, a collimator is used on a connectorized fiber end, i.e., an end having a fiber connector, for example of FC or SMA type. Essentially, such a collimator contains an anti-reflection-coated lens and an adapter for the fiber connector, or alternatively some kind of mount for a bare fiber. The beam radius of the collimated beam is approximately the focal length of the lens times the beam divergence half-angle from the fiber. For single-mode fibers, the beam divergence is approximately the wavelength divided by π times the mode radius. Larger collimated beams require fiber collimators which are both longer and larger in diameter.

A fiber collimator can also be used for launching a collimated beam into a fiber.

One often uses a V groove with some clamp on top to hold a fiber firmly in a certain position. By arranging such V grooves in an array, one can mount a fiber array, where typically some number of fibers (sometimes even thousands of fibers) are arranged in one line with a constant well-defined spacing. There are also special fiber connectors for fiber arrays.

For launching laser light into a fiber, there are complete fiber launch systems, containing a V groove and clamp(s) mounted on a precision translation stage, and some focusing lens. An alternative to that may be a fiber collimator (see above) mounted on a translation stage.

If some bulk optical elements need to be inserted into a fiber-optic setup, it may be convenient to use an assembly with two fiber collimators and some space in between. Particularly for use with single-mode fibers, a high mechanical stability is important.

This is the end of the RP Photonics tutorial on passive fiber optics. We hope you have enjoyed the tutorial, and will find many more interesting things on our website, e.g. in the RP Photonics Encyclopedia, and in various case studies made with our RP Fiber Power software. For many purposes, you can also profit a lot from the free version or the PRO version of the RP Fiber Calculator software.

Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.

Please do not enter personal data here; we would otherwise delete it soon. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him e.g. via e-mail.