Tutorial Passive Fiber Optics, Part 13: Fiber accessories and tools (original) (raw)

Key questions:

This is part 13 of a tutorial on passive fiber optics from Dr. Paschotta. The tutorial has the following parts:

After having treated a lot of the physics background for optical fibers, we finally turn to a couple of practical aspects: some tools and accessories which are often needed for using fiber optics.

Tools for Stripping and Cleaving of Fibers

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:

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.

Equipment for Splicing of Fibers

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 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 endfaces. 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.

Inspection of Fiber Ends and Joints

Visual fault locators can help to localize 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 Connectors and Patch Cables

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:

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.

Fiber Adapters

There are many kinds of fiber-optic adapters. They allow one to join two fiber ends which already have been connectorized (i.e., equipped with fiber connectors). Some adapters can be used to join connectors of different kinds. As many different kinds of fiber connectors exist, a great variety of different adapters is needed.

Fiber Collimators

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 ($\pi$) 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.

Opto-mechanical Parts for Fiber Optics

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.

Fiber Management

When working with many fibers, one may use special solutions for managing fibers:

There are also software solutions for managing fibers. Essentially, such software can provide a virtual model of an existing system and keep track of all its relevant properties — for example, which signal channels (often with different wavelengths for wavelength division multiplexing systems) go through which fibers. It can be used for evaluating performance bottlenecks and the efficiency of resource use. This can be vital for planning further extensions, for example in response to a growth of performance demand.

The End

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.