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2 Optical Fiber Structures and Light Guiding Principles
Fig. 2.24 Variation in refractive index as a function of doping concentration in silica glass
until at very high temperatures it becomes a viscous liquid. The expression “melting
temperature” is commonly used in glass manufacture. This term refers only to an
extended temperature range in which the glass becomes fluid enough to free itself
fairly quickly of gas bubbles.
The largest category of optically transparent glasses from which optical fibers
are made consists of the oxide glasses. Of these, the most common is silica (SiO 2 ),
which has a refractive index ranging from 1.458 at 850 to 1.444 at 1550 nm. To
produce two similar materials that have slightly different indices of refraction for the
core and cladding, either fluorine or various oxides (referred to as dopants), such as
B 2 O 3 , GeO 2 , or P 2 O 5 , are added to the silica. As shown in Fig. 2.24, the addition of
GeO 2 or P 2 O 5 increases the refractive index, whereas doping the silica with fluorine
or B 2 O 3 decreases it. Because the cladding must have a lower index than the core,
examples of fiber compositions are
• GeO 2 –SiO 2 core; SiO 2 cladding
• P 2 O 5 –SiO 2 core; SiO 2 cladding
• SiO 2 core; B 2 O 3 –SiO 2 cladding
• GeO 2 –B 2 O 3 –SiO 2 core; B 2 O 3 –SiO 2 cladding.
Here, the notation GeO 2 –SiO 2 , for example, denotes a GeO 2 -doped silica glass.
The principal raw material for silica is high-purity sand. Glass composed of pure
silica is referred to as silica glass, fused silica, or vitreous silica. Some of its desirable
properties are a resistance to deformation at temperatures as high as 1000 °C, a high
resistance to breakage from thermal shock because of its low thermal expansion, good
chemical durability, and high transparency in both the visible and infrared regions
of interest to fiber optic communication systems.
2 Optical Fiber Structures and Light Guiding Principles
Fig. 2.24 Variation in refractive index as a function of doping concentration in silica glass
until at very high temperatures it becomes a viscous liquid. The expression “melting
temperature” is commonly used in glass manufacture. This term refers only to an
extended temperature range in which the glass becomes fluid enough to free itself
fairly quickly of gas bubbles.
The largest category of optically transparent glasses from which optical fibers
are made consists of the oxide glasses. Of these, the most common is silica (SiO 2 ),
which has a refractive index ranging from 1.458 at 850 to 1.444 at 1550 nm. To
produce two similar materials that have slightly different indices of refraction for the
core and cladding, either fluorine or various oxides (referred to as dopants), such as
B 2 O 3 , GeO 2 , or P 2 O 5 , are added to the silica. As shown in Fig. 2.24, the addition of
GeO 2 or P 2 O 5 increases the refractive index, whereas doping the silica with fluorine
or B 2 O 3 decreases it. Because the cladding must have a lower index than the core,
examples of fiber compositions are
• GeO 2 –SiO 2 core; SiO 2 cladding
• P 2 O 5 –SiO 2 core; SiO 2 cladding
• SiO 2 core; B 2 O 3 –SiO 2 cladding
• GeO 2 –B 2 O 3 –SiO 2 core; B 2 O 3 –SiO 2 cladding.
Here, the notation GeO 2 –SiO 2 , for example, denotes a GeO 2 -doped silica glass.
The principal raw material for silica is high-purity sand. Glass composed of pure
silica is referred to as silica glass, fused silica, or vitreous silica. Some of its desirable
properties are a resistance to deformation at temperatures as high as 1000 °C, a high
resistance to breakage from thermal shock because of its low thermal expansion, good
chemical durability, and high transparency in both the visible and infrared regions
of interest to fiber optic communication systems.