Fiber Attenuation

ATTENUATION

Attenuation is the loss of optical power as light travels through fiber. Measured in decibels per kilometer, it ranges from over 300 dB/km for plastic fibers to around 0.21 dB/km for singlemode fiber.

Attenuation varies with the wavelength of light. In fiber there are two main causes: Scattering and Absorption

Scattering

Scattering (Figure 2-7), the more common source of attenuation in optical fibers, is the loss of optical energy due to molecular imperfections or lack of optical purity in the fiber and from the basic structure of the fiber.

Scattering, does just what its name implies. It scatters the light in all directions including back to the optical source. This light reflected back is what allows optical time domain reflectometers (OTDRs) to measure attenuation levels and optical breaks

Absorption

Absorption (Figure 2-8) is the process by which impurities in the fiber absorb optical energy and dissipate it as a small amount of heat, causing the light to become “dimmer.” The amount converted to heat, however, is very minor.

Microbend Loss

Microbend loss (Figure 2-9) results from small variations or “bumps” in the core-to-cladding interface. Transmission losses increase due to the fiber radius decreasing to the point where light rays begin to pass through the cladding boundary. This causes the fiber rays to reflect at a different angle, therefore creating a circumstance where higher order modes are refracted into the cladding to escape. As the radius decreases, the attenuation increases.

Fibers with a graded index profile are less sensitive to microbending than step-index types. Fibers with larger cores and different wavelengths can exhibit different attenuation values.

Macrobend Loss

Macrobend losses (Figure 2-10) are caused by deviations of the core as measured from the axis of the fiber. These irregularities are caused during the manufacturing procedures and should not be confused with microbends.