work on the design and fabrication of DCF with negative dispersion and
negative dispersion slope began in the early 1990s [26–29]. Successful large-
scale manufacturing was in place by 1998 [30].
Transmission fibers used in terrestrial applications have positive dispersion
and positive dispersion slopes, as shown in Fig. 5.3. DCFs have high negative
dispersions in the range of 100 to 250 ps=nm-km, as well as negative slopes.
To achieve the negative dispersion and dispersion slope, a large amount of
waveguide dispersion must be added by strongly confining the mode in a narrow
and high D Ge-doped core, surrounded by a low D deeply F-doped trench, with
an additional ring of positive D Ge-doped silica surrounding that. DCF index
profiles may include central cores with D as high as 2%, compared to 0.3–0.6%
for transmission fibers. As the wavelength grows longer, the optical mode
progressively spreads out of the core, having more power in the trench and
ring. The effective index is, thus, forced to change rapidly with wavelength,
and both the dispersion and the slope can be designed to be negative [31]. One
consequence of building-in the large waveguide dispersion is a necessarily small
in the range of 15–21 mm
. Because of the high D, which results in a softer
glass at processing temperatures, the core ovality of a DCF is generally more
difficult to control than that of a Tx fiber, and the stress asymmetry resulting
from core ovality is also higher, resulting in somewhat higher PMD values.
However, good process control and the use of spinning [17] allows modern
DCFs to have PMD values of 0.1 ps/rt(km) or lower [25]. The large Ge doping
in the core increases Rayleigh scattering and the large change in material prop-
erties at core–trench interface elevates the loss of DCF to a typical range of
approximately 0.5 dB/km. Typical slope-compensating module insertion losses
and PMD are noted in Section 5.6.3.
Because of the higher loss and PMD, as well as the desire to minimize the size
of the fiber bobbin, it is desirable to maximize the magnitude of the negative
dispersion as practical of the fiber to minimize the length of DCM. As a conse-
quence, the dispersion divided by the fiber loss is often used as a figure of merit
for DCM. Well-designed DCF should have a figure of merit larger than 200.
5.5.3 Full-Band Dispersion Compensation
To guarantee that a DCM precisely compensates the dispersion across the
entire C- or L-band, it is necessary for the relative dispersion slope (RDS) of the
Tx fiber and the DCF to be equal. Let us approximate the dispersion of an
optical fiber as a linear function over a wavelength band,
D(l) ¼ D(l
) þ (l l
), (5:8)
where the center of the band is l
and D
¼ dD=dl is the dispersion slope. Then
Mendez / Specialty Optical Fibers Handbook ch05 Final Proof page 143 26.10.2006 6:44pm
Optical Fiber Design Principles for Wideband and High Bit Rate Transmission 143

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