Attenuation is loss of power. During transit, light pulses lose some of their energy. Attenuation for a fibre is specified in decibels per kilometer (dB/km). For commercially available fibers, attenuation ranges from approximately 0.5 dB/km for Singlemode fibre to 1000 dB/km for larger core plastic fibres.

Attenuation varies with the wavelength of light. There are three low-loss "windows" of interest: 850 nm, 1300 nm, and 1550 nm. The 850 nm window is perhaps the most widely used because 850 nm devices are inexpensive. The 1300 nm window offers lower loss, but at a modest increase in cost for LED’s. The 1550 nm window today is mainly of interest to long-haul telecommunications applications.