What causes macrobending loss, and how can it be identified using an OTDR?

Macrobending loss is caused by excessive bending of an optical fiber, where the bend radius is significantly larger than the fiber's diameter but still too tight for the fiber's specifications. When a fiber is bent, the light traveling within the core attempts to follow the curve. If the bend is too sharp, some of the light rays exceed the critical angle for total internal reflection and escape from the core into the cladding. This escaping light is lost, resulting in increased attenuation, known as macrobending loss. The amount of macrobending loss depends on several factors, including the bend radius, the wavelength of light, and the fiber's design. Tighter bends, longer wavelengths, and fibers with poor bend performance will experience higher macrobending loss. An OTDR (Optical Time Domain Reflectometer) can be used to identify macrobending loss by analyzing the OTDR trace. Macrobending loss typically appears as a gradual increase in attenuation over a section of the fiber. Unlike a sharp spike indicating a connector or a break, macrobending creates a more gradual downward slope on the trace. The OTDR measures the loss in dB/km, and a section of fiber with macrobending will show a higher loss than expected for that type of fiber. Furthermore, macrobends often exhibit wavelength dependence. This means that the loss is more pronounced at longer wavelengths. By performing OTDR measurements at multiple wavelengths (e.g., 1310 nm and 1550 nm), macrobending loss can be more easily identified, as the loss will be noticeably higher at the longer wavelength. For example, if an OTDR trace shows a section of fiber with a loss of 0.5 dB/km at 1550 nm, while the same section shows a loss of only 0.3 dB/km at 1310 nm, this is a strong indication of macrobending. The location of the macrobend can be determined by the distance on the OTDR trace where the increased attenuation begins.