Page 713 - Introduction to Information Optics

12.3. Design of Optical Network Transport 697

Optical fiber

Fig. 12.19. System impairment of chromatic dispersion.

The D parameter, defined as

is commonly used to measure chromatic dispersion, where /. is the wavelength
and c is the speed of light in vacuum, respectively. For a conventional single
mode fiber D is approximately 17ps/nm-km in the 1.55/mi window. As ex-
plained early, chromatic dispersion restricts a maximum optical signal trans-
mission distance. The dispersion-limited transmission distance for a system
2
using external modulated transmitter is approximately 6000/B kilometers,
where B is the bit rate of the signal. It is obvious from the above relationship
that the dispersion limit distance drops rapidly as signal speed increases. For
example, a linear transmission system with a bit rate of 2.5 Gbits/s can
propagate nearly 1000 kilometers without worry about dispersion limitation.
However, the limit is only a few kilometer when the bit rate increases to 40
Gbits/s. Hence, in a high-bit rate optical network, dispersion compensation is
definitely required.

12.3.2. OPTICAL FIBER NONLINEARITY LIMIT

Nonlinear optical effects are the responses of the dielectric material at the
atomic level to the electric fields of an intense light beam. Nonlinear optical
effects in fiber generally fall into two categories: inelastic effects such as
stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS);
and elastic effects such as self-phase modulation (SPM), cross-phase modula-
tion (XPM), and four wave mixing (FWM).

12.3.2.1. Stimulated light Scattering
In the case of stimulated light scattering, a photon associated with the
incoming optical field, often called a pump, interacts with the dielectric