3.3 Design and Characteristics of SMFs
123
3.3.1 Tailoring of Refractive Index Profiles
When creating single-mode fibers, manufacturers pay special attention to how the
fiber design affects both chromatic and polarization-mode dispersions. Such considerations are important because these dispersions set the limits on long-distance and
high-speed data transmission. As Fig. 3.11 illustrates, the chromatic dispersion of
a classic step-index silica fiber is lowest at 1310 nm. However, if the goal is to
transmit a signal as far as possible, it is better to operate the link at 1550 nm (in
the C-band) where the fiber attenuation is lower. For high-speed links the C-band
originally presented a problem for standard single-mode fibers because chromatic
dispersion is much larger at 1550 nm than at 1310 nm. Consequently, fiber designers
devised methods for adjusting the fiber parameters to shift the zero-dispersion point
to longer wavelengths.
The basic material dispersion is hard to alter significantly. However, it is
possible to modify the waveguide dispersion by changing from a simple stepindex design to more complex index profiles for the cladding, thereby creating
different chromatic-dispersion characteristics in single-mode fibers. Figure 3.13
shows representative refractive-index profiles of four fiber-design categories. These
are 1310-nm-optimized fibers, dispersion-shifted fibers, dispersion-flattened fibers,
and large-effective-core-area fibers.
Popular single-mode fibers that are used widely in telecommunication networks
are near-step-index fibers, which are optimized for use in the O-band around 1310 nm.
These 1310-nm-optimized single-mode fibers are of either the matched-cladding or
the depressed-cladding design, as shown in Fig. 3.13a. Matched-cladding fibers have
a uniform refractive index throughout the cladding. Typical mode-field diameters are
9.5 μm and the core-to-cladding index differences are around 0.35%. In depressedcladding fibers the cladding material next to the core has a lower index than the outer
cladding region. Mode-field diameters are around 9.0 μm, and typical positive and
negative index differences are 0.25 and 0.12%, respectively.
As Eqs. (3.35) and (3.42) show, whereas material dispersion depends only on the
composition of the material, waveguide dispersion is a function of the core radius,
the refractive index difference, and the shape of the refractive index profile. Thus
the waveguide dispersion can vary dramatically with the fiber design parameters.
By creating a fiber with a larger negative waveguide dispersion and assuming the
same values for material dispersion as in a standard single-mode fiber, the addition
of waveguide and material dispersion can then shift the zero dispersion point to
longer wavelengths. The resulting optical fiber is known as a dispersion-shifted fiber
(DSF). Examples of refractive-index profiles for dispersion-shifted fibers are shown
in Fig. 3.13b.
Because the zero-dispersion value of a DSF falls at 1550 nm, the chromatic dispersion is negative for wavelengths less than 1550 nm and positive for longer wavelengths. These positive and negative dispersions will seriously affect closely spaced
WDM signals within the C-band because of nonlinear effects in the fiber, as Chap. 12
describes. To reduce the effects of fiber nonlinearities, fiber designers developed
123
3.3.1 Tailoring of Refractive Index Profiles
When creating single-mode fibers, manufacturers pay special attention to how the
fiber design affects both chromatic and polarization-mode dispersions. Such considerations are important because these dispersions set the limits on long-distance and
high-speed data transmission. As Fig. 3.11 illustrates, the chromatic dispersion of
a classic step-index silica fiber is lowest at 1310 nm. However, if the goal is to
transmit a signal as far as possible, it is better to operate the link at 1550 nm (in
the C-band) where the fiber attenuation is lower. For high-speed links the C-band
originally presented a problem for standard single-mode fibers because chromatic
dispersion is much larger at 1550 nm than at 1310 nm. Consequently, fiber designers
devised methods for adjusting the fiber parameters to shift the zero-dispersion point
to longer wavelengths.
The basic material dispersion is hard to alter significantly. However, it is
possible to modify the waveguide dispersion by changing from a simple stepindex design to more complex index profiles for the cladding, thereby creating
different chromatic-dispersion characteristics in single-mode fibers. Figure 3.13
shows representative refractive-index profiles of four fiber-design categories. These
are 1310-nm-optimized fibers, dispersion-shifted fibers, dispersion-flattened fibers,
and large-effective-core-area fibers.
Popular single-mode fibers that are used widely in telecommunication networks
are near-step-index fibers, which are optimized for use in the O-band around 1310 nm.
These 1310-nm-optimized single-mode fibers are of either the matched-cladding or
the depressed-cladding design, as shown in Fig. 3.13a. Matched-cladding fibers have
a uniform refractive index throughout the cladding. Typical mode-field diameters are
9.5 μm and the core-to-cladding index differences are around 0.35%. In depressedcladding fibers the cladding material next to the core has a lower index than the outer
cladding region. Mode-field diameters are around 9.0 μm, and typical positive and
negative index differences are 0.25 and 0.12%, respectively.
As Eqs. (3.35) and (3.42) show, whereas material dispersion depends only on the
composition of the material, waveguide dispersion is a function of the core radius,
the refractive index difference, and the shape of the refractive index profile. Thus
the waveguide dispersion can vary dramatically with the fiber design parameters.
By creating a fiber with a larger negative waveguide dispersion and assuming the
same values for material dispersion as in a standard single-mode fiber, the addition
of waveguide and material dispersion can then shift the zero dispersion point to
longer wavelengths. The resulting optical fiber is known as a dispersion-shifted fiber
(DSF). Examples of refractive-index profiles for dispersion-shifted fibers are shown
in Fig. 3.13b.
Because the zero-dispersion value of a DSF falls at 1550 nm, the chromatic dispersion is negative for wavelengths less than 1550 nm and positive for longer wavelengths. These positive and negative dispersions will seriously affect closely spaced
WDM signals within the C-band because of nonlinear effects in the fiber, as Chap. 12
describes. To reduce the effects of fiber nonlinearities, fiber designers developed