cleaving of fibers (original) (raw)

Definition: preparing fiber ends with clean optical surfaces by controlled breaking

Category: fiber optics and waveguides

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Related: Tutorial on Passive Fiber Optics Part 13: Fiber Accessories and Toolsfiber cleaversfiber splicesfiber endface inspectionpolishing of fibersfiber connectorsfiber opticsfiber joints

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Contents

What is Fiber Cleaving?

Fiber cleaving is an important technique in the area of fiber optics. When optical fibers are connectorized, when they should be spliced or when light should be launched into fibers, the fiber endfaces need to be prepared such that they have clean surfaces. Usually, such surfaces should be as flat as possible, at least over the area of the fiber core (sometimes over the full cross-section), and often it is important that the surface is either perpendicular to the fiber axis or has a well-defined angle to the fiber axis.

Cleaving is the standard method to obtain such surfaces, or sometimes the first step towards that goal. It is a process of controlled breaking of the glass of a bare fiber. It begins with making a tiny fracture (scratch) on the side of the fiber, e.g. with a sharp diamond, carbide or ceramic blade, before or while some defined tension or bending is applied to the fiber. This causes the fiber to break, starting at the mentioned fracture point: the fracture rapidly propagates over the full fiber cross-section. Often, the cleaving leads to a very clean surface of the obtained two fiber parts.

Note that cleaving is not cutting, as the bulk of the process is just breaking. Only the initial tiny break is prepared with a blade.

Before cleaving, a fiber coating needs to be stripped off with a coating stripper tool (fiber stripper), or dissolved with a suitable solvent. The latter technique — chemical stripping — may be required in problematic cases, but takes more time. Thermal stripping may be another option.

Sometimes, a new cleave is required when a fiber end has become dirty, as it is hard to reliably clean fiber ends.

Tools for fiber cleaving are called fiber cleavers. Different kinds of such instruments are explained in the article on fiber cleavers. Simple and inexpensive cleavers, based e.g. on some pen-shaped scribes, are sufficient for simple purposes, when used with proper training. For a higher and more consistent cleave quality, which is less dependent on the operator, mechanical precision cleavers are used, which are substantially more expensive.

Problematic Cases

The optimum settings of a mechanical fiber cleaver — in particular, the applied tension — substantially depend on details like the glass materials and the fiber diameter. Often, fiber cleavers are pre-adjusted for silica fibers with the standard diameter of 125 μm. For fluoride fibers or other mid-infrared fibers, for example, one may have difficulties finding suitable parameters for repeatable cleaving results.

Fibers with particularly large diameters, e.g. beyond 200 μm, can also be difficult to cleave. They need a higher tension force.

Photonic crystal fibers are also more problematic, particularly if they have large air holes (for example, hollow-core fibers). A somewhat reduced fiber tension during the cleaving process may help. Double-clad fibers with an air cladding are particularly challenging.

Cleaving of non-standard fibers may take many experiments and substantial practicing time until it works well. In some cases, it will not work at all, and more cumbersome techniques such as polishing (see below) are then required. Of course, a reasonable cleave is desirable as a starting point for quicker polishing.

Judging the Cleaving Results

The quality of the obtained cleaves has different aspects, the relevance of which depends on the application:

• If fibers should be spliced, the cleaved surfaces should be quite precisely perpendicular to the axis and smooth over the whole fiber cross-section. For example, one could not properly butt together the fiber surfaces if a fiber had a small protrusion (a part standing out), even if that is only near the edge, far away from the fiber core. Also, only for smooth regular surfaces, the surface tension of the softened fibers will optimally self-align the fibers during the fusion process. In addition, a kink shape due to non-perpendicular cleaves can cause substantial coupling losses, particularly for large mode area fibers.
• If one only wants to launch light into a fiber end, or extract light from a fiber end, without having to bring the fiber end into contact with some other solid part, it may be fully sufficient to have a smooth surface only over the area of the fiber core. Some deviation from a normal (perpendicular) cleave may be irrelevant if the orientation of the fiber (or possibly that of a focusing or collimation lens) can be controlled for correcting the tilt angle.
• If the application is very sensitive to back-reflections, a large enough cleave angle is required. As the return loss depends exponentially on the cleave angle, it may not be acceptable e.g. to obtain only 6° instead of 8°. (Note that the larger is the effective mode area of the fiber, the smaller is the required cleave angle.)
• Conversely, if the Fresnel reflection of a fiber end needs to be exploited (e.g. for building a fiber laser), it is important to keep the cleave angle small — well below the beam divergence angle corresponding to the fiber mode. Large mode area fibers are more critical in this respect.

A microscope or other type of fiber endface inspection instrument may be required to properly inspect the obtained fiber surfaces. There are simple hand-held microscopes for such purposes, and fusion splicing equipment also often contains a microscope. Sophisticated instruments for particularly careful analysis are based on interferometers.

Note that it is often very worthwhile to carefully inspect fiber cleaves before using them, as later on it may be much more tedious to locate a fault. Even with a mechanical precision cleaver, the results may not be fully reliable because they require correct settings and can be spoiled by a defective blade, which is not easy to recognize.

Additional Treatment: Polishing

For very high-quality fiber surfaces, it is often necessary to apply some polishing procedure after cleaving. One may, for example, insert the fiber end into a hollow glass tube and fix it there with a glue. The tube gives the fiber a higher strength and is inserted into a polishing apparatus. The fiber is polished down together with the glass tube. This procedure allows one to produce a high-quality surface with an arbitrary well-defined orientation of the fiber surface. However, it takes substantially more time than simple cleaving. See the article on polishing of fibers for details.

Laser Cleaving of Fibers

There are laser-based devices (containing a CO2 laser) which allow the preparation of fiber ends as an alternative to cleaving. Strictly speaking, the used process is not cleaving (which is a mechanical breaking process) but rather something like laser cutting. Nevertheless, it has become common to name it laser cleaving because it produces similar results.

The reliability and resulting quality is substantially better than with real mechanical cleaving, and this of course leads to substantial advantages in terms of yield and working time. That is particularly the case when polishing steps have to follow; the time spent on those (and the amount of used consumables) can be reduced due to the improved starting quality. On the other hand, a laser cleaver is of course substantially more expensive than a mechanical cleaver.

Particularly substantial advantages of laser cleaving are obtained when working with fiber arrays, since multiple fibers can be very quickly and consistently processed together.

Safety Risks from Fiber Scraps

When fibers are cleaved, one obtains small fiber scraps, which have extremely sharp ends. They may stick to a finger and can then be transported into an eye. They can also easily penetrate the skin and are hard to pull out. Fiber scraps should also not be ingested.

For such reasons, it is important to carefully dispose fiber scraps into a properly marked container (collection bin) before they get lost. Also, one should take precautions to make them well visible in the working area, for example by using a black pad below the working area. In addition, one should avoid any eating or drinking near the work area.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is fiber cleaving?

Fiber cleaving is the process of producing a high-quality end face on an optical fiber by making a small scratch and then applying tension to create a controlled break.

Why is proper fiber cleaving important?

A clean, flat fiber end face is essential for low-loss fiber splicing, for efficient connectorization, and for properly launching light into a fiber.

How does cleaving differ from cutting an optical fiber?

Cleaving is a controlled break that starts from a small scratch, resulting in a smooth, flat surface. Cutting implies shearing through the material, which would damage the end face and make it unsuitable for optical applications.

What is an angled fiber cleave used for?

An angled cleave, where the fiber end is not perpendicular to its axis, is used to minimize back-reflections, which can be disruptive in sensitive fiber optic systems.

Which types of optical fibers are difficult to cleave?

Fibers with large diameters (e.g., >200 μm), those made from non-silica glasses like fluoride fibers, and photonic crystal fibers with large air holes can be challenging to cleave reliably.

What is laser cleaving?

Laser cleaving is an alternative to mechanical cleaving that uses a CO2 laser to prepare the fiber end. It offers higher quality and reliability, though it is technically a form of laser cutting rather than a mechanical break.

Are the small fiber scraps from cleaving hazardous?

Yes, fiber scraps are extremely sharp pieces of glass. They can easily penetrate skin or eyes and are difficult to remove, so they must be disposed of carefully.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains 29 suppliers for fiber cleavers. Among them:

⚙ hardware

The AUTOCLEAVER series is a comprehensive product platform with various models for cleaving standard and large diameter optical fibers, all based on our proven and patented tension and scribe cleaving process. Different models are carefully optimized for precision cleaving of standard, specialty and large diameter fibers with cladding diameters between 80 and 1000 μm. A common feature is the automated cleaving process intended to provide consistent results and high production yield in volume manufacturing environments. Low maintenance requirements ensure trouble-free operation during long periods of operation.

We have an open interface for integration into automated production systems. Fiber holder solutions are available for use with fusion splicers from Fujikura, Furukawa/FITEL and Vytran. Dual fiber holder positions for comfortable use with FSM-100 fusion splicers.

See our cleaver comparison sheet.

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