Why does light travelling through fiber optic cables suffer low loss?
Answers
Light traveling in an optical fiber loses power over distance. The loss of power depends on the wavelength of the light and on the propagating material. For silica glass, the shorter wavelengths are attenuated the most (see Fig. 1). The lowest loss occurs at the 1550-nm wavelength, which is commonly used for long-distance transmissions.
Transmission of light by fibre optics is not 100% efficient. There are several reasons for this including absorption by the core and cladding (caused by the presence of impurities) and the leaking of light from of the cladding. When light reflects off the cladding /core interface it actually travels for a short distance within the cladding before being reflected back. This leads to attenuation (signal reduction) by up to 2db/Km for a multi-mode fibre. For example, with this level of attenuation, if light travelled over 10kM of cable only 10% of the signal would arrive at the following end.
The amount of attenuation for a given cable is also wavelength dependent. Figure 1 shows the attenuation profile for the two main types of fibre; multi-mode and single-mode cable (described in detail below). The absorption peak at 1000nm is caused by the peculiarities of single mode fibre while the peak at 1400nm is caused by traces of water remaining in the fibre as an impurity. Due to this water absorption peak there are two standard single-mode wavelengths in use, 1310nm and 1550nm. 1310nm has been a standard for many years, only now is there a trend towards using 1550nm brought about by the need to extend the distances between repeaters.
The loss of power in light in an optical fiber is measured in decibels (dB). Fiber optic cable specifications express cable loss as attenuation per 1-km length as dB/km. This value is multiplied by the total length of the optical fiber in kilometers to determine the fiber’s total loss in dB.