Fiber cleaving

Fiber cleaving is the controlled fracture of a glass fiber to leave an end face that is flat, smooth to optical quality and nearly perpendicular to the fiber axis. The fiber is stripped of its coating, held under tension or bent, and nicked on its surface with a diamond or carbide blade; the crack crosses the fiber in well under a microsecond and produces a mirror surface without grinding. For standard 125 µm silica fiber a good cleave is within about 0.5° of perpendicular, and 1° is a common rejection threshold before fusion splicing.

How a cleave works

Glass fails by crack propagation from a surface flaw. The cleaver creates a single small flaw with the blade, then applies a stress that is as uniform as possible across the fiber's cross section so the crack front runs straight through it. Near the starting point the fracture surface is smooth and reflective (the mirror zone). If the stress is too high or uneven, the crack accelerates and branches, first producing a finely textured mist zone and then a coarse hackle zone with lips and steps. A good cleave keeps the mirror zone over the entire face, including the core.

Two tool families are common:

• Precision cleavers clamp the stripped fiber on both sides, apply tension or a controlled bend, and bring a rotating circular blade into brief contact. They produce angles typically below 0.5° and are standard alongside fusion splicers.
• Scribe tools (a hand-held diamond or ruby edge) are used to nick a fiber that is then pulled or bent by hand. They are adequate for power monitoring or quick checks with cleave angles of a few degrees.

Specialty cleavers apply calibrated tension for large-diameter fibers, which fracture unevenly under a simple bend. Angle cleavers twist the fiber before scoring to produce a deliberate 8° face.

Cleave angle and what it changes

An end face tilted by $\theta$ refracts the emerging beam. For light leaving glass of index $n$ into air, Snell's law gives a deviation from the fiber axis of

$$\delta = \arcsin(n\sin\theta) - \theta \approx (n-1)\,\theta .$$

With $n$ = 1.444, a 0.5° cleave deviates the beam by 0.22°, a 1° cleave by 0.44°, and an 8° cleave by 3.6°. In a fiber collimator or a butt-coupled test this beam steering reduces coupling to the next element. In fusion splicing, tilted faces touch first at one edge, melt unevenly and leave a small core offset or deformation in the joint, which is why splicers measure and reject cleave angles above a set limit.

A perpendicular cleave in air reflects a fraction

$$R = \left(\frac{n-1}{n+1}\right)^2 = 0.033$$

of the light back into the fiber, from the Fresnel equations, a return loss of 14.8 dB. That reflection can destabilize a laser source, so an open cleaved end in a sensitive setup is angle cleaved, immersed in index-matching gel, or wound into a tight loop. An 8° angle sends the reflected light back at 16° to the axis inside the glass, well outside the roughly 4.8° acceptance half-angle inside the glass of a fiber with NA 0.12, and the Gaussian-mode estimate of the recaptured fraction is about −78 dB. In practice the achieved return loss is limited by surface imperfections and backscatter well before that level.

Inspecting a cleave

Fusion splicers image both ends before the arc and report each cleave angle. For bare fibers without a splicer, a fiber inspection microscope shows chips, lips and hackle at 200–400× magnification. The quick field check is to launch visible light and look at the far-field pattern on a card: a clean, round spot indicates a usable face, while a streaked or split spot indicates a broken or chipped end.

Pitfalls

• Contamination before cleaving. Residual coating or dust under the clamps causes uneven tension. The stripped fiber is wiped with isopropyl alcohol before it enters the cleaver.
• Blade wear. Blades are rotated to fresh positions after a set number of cleaves; a worn position produces higher angles and hackle.
• Wrong settings for the fiber. Fibers with larger cladding, polarization-maintaining stress rods or photonic crystal microstructure need adjusted tension, and crushing the holes during cleaving changes their guidance.
• Handling afterwards. A fresh cleave is contaminated by touching it or by resting it on a surface, and it cannot be cleaned without risk of chipping.
• Safety. Bare fiber shards penetrate skin easily and are nearly invisible, so they are collected in a dedicated container.

Common questions

What is a good cleave angle?

For fusion splicing standard single-mode fiber, under 0.5° is typical of a good precision cleaver, and splicers are commonly set to reject angles above about 1°. Large-diameter fibers and specialty fibers usually achieve and tolerate somewhat larger angles.

What is the difference between cleaving and polishing?

Cleaving produces an optical face in one fracture and is used for bare fiber ends, splicing and mechanical splices. Fiber polishing grinds a fiber already bonded in a connector ferrule down to a controlled domed or angled geometry, and it is needed whenever the end must be mated repeatedly in a connector.

References: Saleh & Teich, Fundamentals of Photonics 3rd ed. 2019, Ch. 10; Hecht, Optics 5th ed. 2017, Ch. 4 (Fresnel reflection); G. P. Agrawal, Fiber-Optic Communication Systems 4th ed. 2010.