Page 71 - Optical Communications Essentials

Optical Fibers

Optical Fibers 61

TABLE 4.6. Generic Parameter Values of an Erbium-Doped Fiber for
Use in the C-Band
Parameter Specification
Peak absorption at 1530nm 5 to 10dB/m
Effective numerical aperture 0.14 to 0.31
Cutoff wavelength 900 50nm; or 1300nm
Mode field diameter at 1550nm 5.0 to 7.3
Cladding diameter 125µm standard; 80µm for tight coils
Coating material UV-cured acrylic

mode field diameter, numerical aperture, and cladding diameter for these fibers.
Higher erbium concentrations allow the use of shorter fiber lengths, smaller
claddings are useful for compact packages, and a higher numerical aperture
allows for the fiber to be coiled tighter in small packages. Table 4.6 lists some
generic parameter values of an erbium-doped fiber for use in the C-band.

Photosensitive fiber. A photosensitive fiber is designed so that its refractive
index changes when it is exposed to ultraviolet light. This sensitivity may be
provided by doping the fiber material with germanium and boron ions. The
main application is to create a fiber Bragg grating, which is a periodic variation
of the refractive index along the fiber axis (Chap. 9 has details on this).
Applications of fiber Bragg gratings include light-coupling mechanisms for
pump lasers used in optical amplifiers, wavelength add/drop modules, optical
filters, and chromatic dispersion compensation modules.
Bend-insensitive fiber. A bend-insensitive fiber has a moderately higher numer-
ical aperture (NA) than that in a standard single-mode telecommunication
fiber. The numerical aperture can be varied to adjust the mode field diameter.
Increasing the NA reduces the sensitivity of the fiber to bending loss by confin-
ing optical power more tightly within the core than in conventional single-mode
fibers. Bend-insensitive fibers are available commercially in a range of core
diameters to provide optimum performance at specific operating wavelengths,
such as 820, 1310, or 1550nm. These fibers are offered with either an 80-µm or
a 125-µm cladding diameter as standard products. The 80-µm reduced-cladding
fiber results in a much smaller volume compared with a 125-µm cladding diam-
eter when a fiber length is coiled up within a device package. Whereas Table 4.2
shows there is a high bending loss for tightly wound conventional single-mode
fibers, the induced attenuation at the specified operating wavelength due to 100
turns of bend-insensitive fiber on a 10-mm-radius mandrel is less than 0.5dB.

Attenuating fiber. These fibers have a uniform attenuation in the 1250- to 1620-
nm band. This makes an attenuating fiber useful for WDM applications to lower
the power level at the input of receivers or at the output of an EDFA. The fibers
are offered commercially with attenuation levels available from 0.4dB/cm to

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.