Optical fibers attenuation losses

An important characteristic for optical fibers is loss or attenuation. Attenuation in fibers is normally measured in decibels per kilometer (dB/km). The optical loss, in dB, measured over 1km is written as 10 log/in//out where / and /out are the input and output optical power. 

Attenuation, or transmission loss, is one of the most important performance indicators of optical fibers. Attenuation is the main determinant of the maximum transmission distance of optical communication systems without amphfiers or repeaters, as well as of the maximum output power from the light source and... 

The development of optical fiber is a fascinating story, especially with regard to the introduction of CVD as the primary method of fabrication, but the details are beyond the scope of this text. Suffice to say that traditional glass-forming technologies, which were based on melt processing, did not provide fibers free from defects such as bubbles or with sufficient purity to prevent enormous attenuation losses. The lowest losses of optical quality glasses were on the order of 1000 dB/km, whereas optical fibers today... 

This CVD procedure is somewhat different from that used to deposit semiconductor layers. In the latter process, the primary reaction occurs on the substrate surface, following gas-phase decomposition (if necessary), transport, and adsorption. In the fiber optic process, the reaction takes place in the gas phase. As a result, the process is termed modified chemical vapor deposition (MCVD). The need for gas-phase particle synthesis is necessitated by the slow deposition rates of surface reactions. Early attempts to increase deposition rates of surface-controlled reactions resulted in gas-phase silica particles that acted as scattering centers in the deposited layers, leading to attenuation loss. With the MCVD process, the precursor gas flow rates are increased to nearly 10 times those used in traditional CVD processes, in order to produce Ge02-Si02 particles that collect on the tube wall and are vitrified (densified) by the torch flame. 

Explain the connection between the attenuation loss of the optical fiber used for transmission of information and the sensing principles utilizing an evanescent field. 

At a fairly late stage in 1966 optical fibers were first considered seriously to be a possible transmission medium by Kao and Hockham1. In 1969 Jones and Kao2 demonstrated that minimum attenuation losses in bulk silica can be as low as 5 db/km. The actual breakthrough was achieved in 1970 with the preparation of fibers with optical losses below 20 db/km3. Since then rapid progress has been made in this field all over the world. 

The optical fibers resulting from the plasma deposition process show no particularly different properties as regards attenuation between 600 nm and 1100 nm compared with those of fibers prepared by thermal means. Figure 18 shows a typical attenuation curve with the losses near the intrinsic limit. The water peak at about 950 nm corresponds to an impurity level of 1 ppm OH. 

There are two types of optical fibers SM and MM fibers. The important parameters characterizing fiber properties are the core and cladding indices, core diameter, attenuation ot, and numerical aperture (NA). The attenuation represents optical losses caused by absorption or scattering of light propagating through the fiber per unit length, as ... 

Attenuation Loss of hght power as the signal travels through fiber-optic cable. 

Attenuation is the combined loss of optical pulse power resulting from internal and external factors as the light propagates within the optical fiber. Mathematically, attenuation is expressed as... 

Attenuation The combination of optical energy losses from a light pulse traversing a length of optical fiber resulting from absorption and scattering, typically referred to as intrinsic attenuation, and bending losses, typically referred to as extrinsic attenuation, expressed in dB. However, attenuation is often used as a synonym for attenuation coefficient, expressed in dB/km. 

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