Basic Principles of Fiber Optics Series: Micro and Macro Bending

Written by Ben Hamlitsch, trueCABLE Technical and Product Innovation Manager RCDD, FOI

In fiber optics, "bending" refers to the way in which light travels through a fiber optic cable. When light travels through a fiber optic cable, it is constantly refracted, or bent, as it passes through the cable. There are two types of bending that can occur in fiber optics: microbending and macrobending.

Microbending:

Microbending occurs when the fiber optic cable is bent on a small scale, typically at a radius of less than 1 cm. This type of bending can occur when the cable is subjected to small changes in temperature, pressure, or mechanical stress. Microbending can cause the light traveling through the fiber to be scattered, resulting in signal loss and reduced transmission quality.

Microbends are small distortions of the boundary layer between the core and cladding caused by crushing or pressure. Microbends are very small and may not be visible when looking at the fiber optic cable.

Microbends change the angle of incidence within the fiber. Changing the angle of incidence forces high-order light rays to reflect at the angles that prevent further reflection, causing them to be lost in the cladding and absorbed.

Microbending can also be caused by the coating, cabling, packaging, and installation of the cable. Additionally, one of the most common types of coating on a fiber optic core is Acrylate.

What is Acrylate?

Acrylate coatings are thin layers of polymer that are applied to a variety of surfaces to provide a protective barrier, enhance appearance, and improve the performance of the coated item. They are made from acrylate polymer, which is a type of plastic known for its excellent flexibility, toughness, and UV resistance. Acrylate coatings are used in a wide range of applications, including in the automotive, aerospace, and construction industries. They can be applied through various methods, such as spraying, brushing, or dipping, and can be cured using heat, UV light, or a chemical reaction.

Fiber optic cables use acrylate to coat the fiber core. Because of this, exceeding the operating temperature of the acrylate coating can also cause microbending in fiber cabling, which can also result in significant attenuation.

Macrobending:

Macrobending occurs when the fiber optic cable is bent on a larger scale, typically at a radius of more than 1 cm. This type of bending can occur when the cable is subjected to more significant changes in temperature, pressure, or mechanical stress, such as those caused by cable bending or tension. Macrobending can also cause the light traveling through the fiber to be scattered, resulting in signal loss and reduced transmission quality.

Both microbending and macrobending can have negative effects on the performance of fiber optic systems, and efforts are often made to minimize both types of bending in fiber optic cables. This may involve using special cable designs or installation techniques that reduce the likelihood of bending, as well as using protective measures such as cable clamps or trays to protect the cables from external stresses.

Compared to a microbend, a macrobend has a much larger radius. Macrobends occur when the fiber is bent around a radius that can be measured in millimeters. These tight radii change the angle of incidence within the fiber, causing some of the light rays to reflect outside of the fiber and, as with microbending, be lost in the cladding and absorbed.

Single-mode Macrobending Performance

Single-mode fiber can be broken into two groups: bend-sensitive and bend-insensitive. There is a considerable difference in the bending attenuation of the two groups. The ITU standards G.652 and G.655 define macrobending loss for bend-sensitive optical fiber, and G.657 defines macrobending loss for bend-insensitive fiber.

The macrobending loss in each standard is for an optical fiber that has not been placed in a cable (jacket with aramid yarn etc.). The macrobending loss will vary when the optical fiber is placed in a cable. Any information on these loss values must be obtained from the cable manufacturer. However, to demonstrate how macrobending loss is calculated, it is assumed that optical fiber values listed in the standards are the same as the overall cable loss.

ITU-T G.652 allows for a maximum attenuation of 0.1dB at 1550nm with 100 turns around a 30mm radius mandrel. The 30mm radius of the mandrel is slightly less than the radius of a tennis ball. Reducing the radius of the bend will increase the attenuation. Optical fiber with a macrobending specification like this is not a good choice for installations that may require several sharp bends, especially if they are much less than 30mm.

ITU-T G.657 defines the macrobending loss for bend-insensitive single-mode optical fiber. This type of optical fiber is typically employed in passive optical networks (PONS) that run to your home or residence. Installing a fiber-optic cable in the wall of a home or apartment may require one or more sharp bends. A single sharp 90-degree bend with an optical fiber that is not bend insensitive could cause enough attenuation to prevent the link from functioning.

Multimode Macrobending Loss

Bend-insensitive multimode fiber (or BIMMF) has obvious advantages. In patch panels, it should not suffer from bending losses where the cables are tightly bent around the racks. In buildings, it allows fiber to be run inside molding around the ceiling or floor and around doors or windows without inducing high losses. It's also insurance against problems caused by careless installation.

BI fibers are available in 50/125 MM (OM3, OM4 and OM5) versions. Considering the advantages of BI fiber and the small incremental cost to manufacture it, some manufacturers have decided to make all of their 50/125 MM fiber bend-insensitive fiber.

Bend-insensitive multimode fiber is offered by multiple manufacturers with extremely low bending loss at 850nm and 1300nm. The table below describes the bending loss performance of a generic 50/12Spm BIMMF. The macrobending loss is defined for a number of turns of optical fiber around a mandrel at a specified radius and wavelength.

See table below for more information:

Accounting for micro and macro bends in a fiber cabling system is very important when it comes to ensuring optimum attenuation loss when testing your fiber cabling. Always consider where the fiber will be installed and how it will be installed. Bend-insensitive fiber cable makes this job much easier, but it is always good to ensure good installation practices so your fiber installation will get the best performance possible.

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