Specialty Optical Fibers Handbook

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

A

eff

in the range of 15–21 mm

2

. 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

c

) þ (l  l

c

)D

0

(l

c

), (5:8)

where the center of the band is l

c

and D

0

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