3.1 Fiber Attenuation
97
Table 3.1 Examples of
absorption loss in silica glass
at different wavelengths due
to 1 ppm of water-ions and
various transition-metal
impurities
Impurity
Loss due to 1 ppm of
impurity (dB/km)
Absorption peak
(nm)
Iron: Fe 2+
0.68
1100
Iron: Fe 3+
0.15
400
Copper: Cu 2+
1.1
850
Chromium: Cr 2+
1.6
625
Vanadium: V 4+
2.7
725
Water: OH –
1.0
950
Water: OH –
2.0
1240
Water: OH –
4.0
1380
arising from these defects are negligible compared with intrinsic and impurity
absorption effects.
The dominant absorption factor in silica fibers is the presence of minute quantities
of impurities in the fiber material. These impurities include OH (water) ions that are
dissolved in the glass and transition metal ions such as iron, copper, chromium, and
vanadium. Transition metal impurity levels were around 1 part per million (ppm) in
glass fibers made in the 1970s, which resulted in losses ranging from 1 to 4 dB/km,
as Table 3.1 shows. Impurity absorption losses occur either because of electron transitions between the energy levels within these ions or because of charge transitions
between ions. The absorption peaks of the various transition metal impurities tend to
be broad, and several peaks may overlap, which further broadens the absorption in a
specific region. Modern techniques for producing a fiber preform have reduced the
transition-metal impurity levels by several orders of magnitude. Such low impurity
levels allow the fabrication of low-loss fibers.
The presence of OH ion impurities in a glass fiber preform results mainly from
the elements in the oxyhydrogen flame used in the hydrolysis reaction of the basic
SiCl 4 , GeCl 4 , and POCl 3 starting materials. Water impurity concentrations of less
than a few parts per billion (ppb) are required if the attenuation is to be less than
20 dB/km. The high levels of OH ions in early fibers resulted in large absorption
peaks at 725, 950, 1240, and 1380 nm. Regions of low attention lie between these
absorption peaks.
The peaks and valleys in the attenuation curves resulted in the designation of
the various transmission windows shown in Fig. 3.2 (see Sect. 1.2.2). By reducing
the residual OH content of fibers to below 1 ppb, standard commercially available
single-mode fibers have nominal attenuations of below 0.4 dB/km at 1310 nm (in the
O-band) and less than 0.25 dB/km at 1550 nm (in the C-band). Further elimination
of water ions diminishes the absorption peak around 1440 nm and thus opens up
the E-band for data transmission, as indicated by the dashed line in Fig. 3.2. Optical
fibers that can be used in the E-band are known by names such as low-water-peak
or full-spectrum fibers.
97
Table 3.1 Examples of
absorption loss in silica glass
at different wavelengths due
to 1 ppm of water-ions and
various transition-metal
impurities
Impurity
Loss due to 1 ppm of
impurity (dB/km)
Absorption peak
(nm)
Iron: Fe 2+
0.68
1100
Iron: Fe 3+
0.15
400
Copper: Cu 2+
1.1
850
Chromium: Cr 2+
1.6
625
Vanadium: V 4+
2.7
725
Water: OH –
1.0
950
Water: OH –
2.0
1240
Water: OH –
4.0
1380
arising from these defects are negligible compared with intrinsic and impurity
absorption effects.
The dominant absorption factor in silica fibers is the presence of minute quantities
of impurities in the fiber material. These impurities include OH (water) ions that are
dissolved in the glass and transition metal ions such as iron, copper, chromium, and
vanadium. Transition metal impurity levels were around 1 part per million (ppm) in
glass fibers made in the 1970s, which resulted in losses ranging from 1 to 4 dB/km,
as Table 3.1 shows. Impurity absorption losses occur either because of electron transitions between the energy levels within these ions or because of charge transitions
between ions. The absorption peaks of the various transition metal impurities tend to
be broad, and several peaks may overlap, which further broadens the absorption in a
specific region. Modern techniques for producing a fiber preform have reduced the
transition-metal impurity levels by several orders of magnitude. Such low impurity
levels allow the fabrication of low-loss fibers.
The presence of OH ion impurities in a glass fiber preform results mainly from
the elements in the oxyhydrogen flame used in the hydrolysis reaction of the basic
SiCl 4 , GeCl 4 , and POCl 3 starting materials. Water impurity concentrations of less
than a few parts per billion (ppb) are required if the attenuation is to be less than
20 dB/km. The high levels of OH ions in early fibers resulted in large absorption
peaks at 725, 950, 1240, and 1380 nm. Regions of low attention lie between these
absorption peaks.
The peaks and valleys in the attenuation curves resulted in the designation of
the various transmission windows shown in Fig. 3.2 (see Sect. 1.2.2). By reducing
the residual OH content of fibers to below 1 ppb, standard commercially available
single-mode fibers have nominal attenuations of below 0.4 dB/km at 1310 nm (in the
O-band) and less than 0.25 dB/km at 1550 nm (in the C-band). Further elimination
of water ions diminishes the absorption peak around 1440 nm and thus opens up
the E-band for data transmission, as indicated by the dashed line in Fig. 3.2. Optical
fibers that can be used in the E-band are known by names such as low-water-peak
or full-spectrum fibers.