This technical report is intended to characterize the microbend sensitivity of optical fibers, thereby guiding fiber and cable manufacturers regarding the design of various coatings and basic fibers as they apply to the design and performance of cable.

For the purposes of this technical report, the term microbending should be properly distinguished from macrobending. The spirit of these two terms is to identify whether the bending is small or large. Through many years, experiments with small and large bends provided a link between bend size and specific qualities of the resulting loss spectra of the optical fiber under test, thereby producing new, functional definitions for micro- and macrobending. The following definitions will attempt to capture some of the art and intuition behind these historic terms, while maintaining the integrity of their original intent. Moreover, since bend loss is a function of wavelength, these terms should be defined in the context of a given wavelength range. This report will use the typical wavelength region of interest for telecommunication fibers: 1200 to 1700 nm.

Macrobending is usually characterized by a constant, moderately large bend radius leading to an exponential increase in the loss as a function of wavelength. This loss in known to be physically induced by an optical tunneling phenomena, where the light from a propagating mode leaks out toward the outside of the bend. In the event that the bend radius is not constant, the type of bending could still be considered macrobending provided the different bend radii are all generally large, and the loss is still dominated by the optical tunneling phenomena with exponential wavelength dependence. Microbending, on the other hand, is random microscopic fiber axis perturbations along the length of the fiber. Such random microbending is typically indicated by a uniform loss across the te leco m mu n ica t io ns wave length band . Of cou rse , ce rtai n m icro be nd scenarios can be created (small, periodic bending) where the loss response is not uniform, but these cases are typically not accidental and may in fact be engineered into the fiber for a specific purpose. Regardless of the statistics of the microbends however, they can generally be described by some form of coupled-mode theory, treating the bends as small perturbations to the otherwise straight fiber. Thus, micro- and macrobends can be distinguished both by the underlying physics and the appearance of the resulting loss spectra, provided the spirit of these terms is maintained. The transition from microbending to macrobending as bend radius is increased is a continuous process, meaning that the boundary typically contains features of both types of loss. Moreover, such a boundary would be more appropriately described in terms of the bend statistics (random versus constant, for example) than the absolute bend radius. Therefore this report will not attempt to limit its scope with absolute specifications of bend radii, but will rather leave this discrimination to those reviewing the above qualities of the fiber under test.

For the purposes of this technical report, microbend and macrobend will be defined in terms of bend loss in optical fiber. The spirit of these two terms is to identify whether the bending is small or large. Through many years, several different regions of bend quality emerged. Since bend loss is a specified function of wavelength, the typical region of interest for telecommunication fibers is 1200 to 1700 nm.

Macrobending is usually characterized by a constant moderately large bend radius leading to an exponential increase in the loss as a function of wavelength physically induced by an optical tunneling phenomena, where the light from a propagating mode leaks out toward the outside of the bend. Spectrally uniform microbending is microscopic in nature, usually characterized as a collection of random small bends, rapidly varying in both radius and orientation induced by factors that are usually described by coupled-mode theory. Microbend is dependent on the type of bend - usually random, periodic. The boundary between microbend / macrobend (combination region) is continuous and dependent on the wavelength properties usually in the mm range.

Although they can be very useful tools for evaluating fiber and cable designs, the current state-of-the-art of these test procedures are such that test results may prove to be misleading. These tests are not recommended for use in comparing different types of optical fibers, nor should test results be compared from one facility to another nor from one technique to another.

The ability of a coating to prevent microbend loss can be dependent upon temperature. The temperature(s) at which this test is to be performed for expandable drum, fixed drum and wiremesh shall be specified in the Detail Specification. The basketweave technique is a temperature dependent, microbend sensitivity test with the recommended temperatures defined within the text.