Microbend loss

Microbend loss is the attenuation caused by small, random deformations of a fiber's axis, with lateral displacements of a micrometer or less repeated over lengths of the order of a millimeter. Each deformation is invisible and harmless alone, but a fiber pressed against a slightly rough surface, squeezed by a cable or a shrinking coating, or wound on a drum under tension has thousands of them per meter, and together they couple light from the guided mode into modes that the core does not guide. Macrobend loss, from bends of millimeters to centimeters in radius that can be seen, is a different effect with its own scaling; the bend loss entry covers it.

Which deformations matter

A perturbation along the fiber couples two modes efficiently only if its spatial frequency matches the difference in their propagation constants, $\mathrm{\Omega }=\mathrm{\Delta }\beta$, just as a grating couples the modes it is phase matched to. For random deformations, coupled-mode theory gives a power loss proportional to the power spectrum of the axis displacement at that spatial frequency (Marcuse). In a single-mode fiber the guided mode couples to cladding and radiation modes, whose propagation constants lie below $k_0{n}_{\text{cl}}$, so only deformation periods shorter than $2\pi \mathrm{/}({\beta }_{01}-k_0{n}_{\text{cl}})$ remove light. Computed for a standard single-mode fiber (core radius 4.1 µm, index step 0.36%):

The displacement spectrum of a real fiber in a coating or cable falls steeply toward short periods, so the loss is dominated by periods near that limit. At longer wavelengths the limit moves to longer periods, where the deformation spectrum is stronger, and the mode reaches further into the cladding, so microbend loss rises steeply with wavelength and with mode field diameter (Petermann). The same fiber in the same cable therefore loses more at 1625 nm than at 1550 nm, and more at 1550 nm than at 1310 nm, the opposite trend to Rayleigh scattering.

In a multimode fiber the guided modes are spaced closely in propagation constant, and microbends couple them to each other as well as to the cladding. Adjacent mode groups of a graded-index fiber with a 25 µm core radius and a 1% index step are coupled by periods of about 1.1 mm. This mode coupling moves light toward an equilibrium distribution of modes, which is why controlled microbends are used as mode scramblers, and it removes light from the highest-order modes near cutoff (Olshansky).

Causes

• Coatings. A fiber's coating cushions it against lateral forces. The usual dual-layer coating, a soft primary layer under a hard secondary layer, spreads a local force over a length long enough that the axis deforms mainly at long periods, which do not couple light out (Gloge).
• Cabling. Pressure from buffer tubes, tight buffers, strength members and neighboring fibers, and the stress of cable installation.
• Temperature. Cable materials contract more than glass when cold, and the compression buckles the fiber inside its tube or buffer, so microbend loss typically appears in the cold end of temperature cycling.
• Winding. Fiber wound under tension on a rough spool or drum. Measurement methods use this deliberately.

Reducing it

A fiber with a smaller mode field diameter, from a larger index step or a trench in the cladding, confines the mode more strongly and is less sensitive to microbending, which is why the trench-assisted designs of bend-insensitive G.657 fiber reduce microbend loss along with macrobend loss. Coatings with a soft primary layer and cable designs that decouple the fiber from the cable structure, such as loose tubes in which the fibers lie with some excess length, keep the deformations small and their periods long. In polarization-maintaining fiber, deformations with periods near the beat length couple the two polarization axes and lower the extinction ratio as well as adding loss.

Testing and detection

IEC TR 62221 describes methods for comparing the microbend sensitivity of fibers: the fiber is wound under controlled tension onto a drum covered with a rough surface such as sandpaper or a wire mesh, or pressed between meshes under a load, and its attenuation is compared with a loose, unstressed winding of the same fiber. Cables are checked by temperature cycling while their attenuation is monitored. In installed cable, microbend loss shows as a raised slope of an OTDR trace over a section rather than a single event, larger at 1625 nm than at 1550 nm, whereas a macrobend at a single point appears as a localized step. Comparing traces at the two wavelengths is the usual field check for cable stress.

Microbend sensors

Microbending is also a sensing mechanism. In a microbend sensor, a fiber passes between two corrugated plates whose period matches the spacing of the fiber's mode groups; pressure or displacement pushes the plates together, the microbends couple light out of the core, and the transmitted power falls. Such sensors were demonstrated for acoustic and pressure measurement around 1980 (Fields and Cole). They are intensity sensors, unrelated to the Brillouin and Raman distributed sensors that read strain and temperature from backscattered light.

Common questions

What is microbend loss?

The loss caused by small, random deformations of a fiber's axis, micrometers or less in size and around a millimeter in period, which couple light from the core into cladding and radiation modes. It comes from pressure by coatings, cables, rough surfaces and low temperatures.

What is the difference between microbending and macrobending?

A macrobend is a visible bend of millimeters to centimeters in radius, and its loss rises exponentially as the radius shrinks. Microbends are microscopic, random distortions distributed along the fiber, and their loss depends on the spectrum of the deformations at the spatial frequencies that couple light out of the core.

Why does microbend loss increase with wavelength?

At longer wavelengths the guided mode reaches further into the cladding and its propagation constant lies closer to those of the cladding modes, so longer-period deformations, which are stronger in a real fiber, couple light out. Loss at 1625 nm is therefore a sensitive indicator of microbending in a cable.

How is microbend loss reduced?

With fibers that confine the mode more tightly, such as bend-insensitive G.657 designs, with soft primary coatings that cushion the glass, and with cable designs that keep lateral pressure off the fibers across the operating temperature range.

References: D. Marcuse, Theory of Dielectric Optical Waveguides, 2nd ed. (Academic Press, 1991). D. Gloge, "Optical-fiber packaging and its influence on fiber straightness and loss," Bell System Technical Journal 54, 245 (1975). K. Petermann, "Theory of microbending loss in monomode fibres with arbitrary refractive index profile," Archiv für Elektronik und Übertragungstechnik 30, 337 (1976). R. Olshansky, "Mode coupling effects in graded-index optical fibers," Applied Optics 14, 935 (1975). J. N. Fields and J. H. Cole, "Fiber microbend acoustic sensor," Applied Optics 19, 3265 (1980). IEC TR 62221, Optical fibres: Measurement methods: Microbending sensitivity. ITU-T Recommendation G.657, Characteristics of a bending-loss insensitive single-mode optical fibre and cable.