Electro-Optics - Applied Optics Fundamentals and Device Applications - page 50

core to the cladding generally takes two forms—either a sharp step function
change (step-index) or a sharp change followed by a graded change (graded-
index) in the core.
The two key differences between single- and multimode fibers are the dis-
persion characteristics and the dimensions. Dispersion causes signal deg-
radation and therefore limits the distance over which transmission can be
effectively accomplished. The dimensions of the guiding cores differ greatly
between the two types of fibers causing ease of coupling and effectiveness of
alignment to become issues.
Thin-film planar waveguides confine light using the same principle of dif-
fering refractive indices. In principle, the practical determining factors as
to which materials can be used for waveguide fabrication are the refractive
index, loss coefficient, and available dimensions. It is the last factor that pro-
motes the use of thin films for the guides. For many acceptable materials, the
refractive indices are such that the required waveguide dimensions are on
the order of microns. Thin-film processing has advanced to the stage where
films of this thickness are easily obtainable. Of special interest are wave-
guides in semiconductor materials. GaAs in particular has become increas-
ingly attractive because of the compatibility with monolithic electronic and
electro-optic device fabrication.
2.6.2 Single-Mode versus Multimode Fibers
Single-mode step index fibers possess the ultimate bandwidth capability
due primarily to favorable dispersion characteristics. This means that single-
mode fibers are the optimal choice for long haul applications in data trans-
mission. In some applications, however, the lengths of transmission are such
that multimode fiber can be utilized. A prime advantage of multimode fibers
is the greater ease of coupling. Multimode fibers have acceptance areas of
many orders of magnitude greater than single-mode fibers and thus have
greater tolerance to misalignment and provide higher system gain.
Fiber dispersion can be classified as either intermodal or intramodal disper-
sion. Intermodal dispersion is found primarily in multimode fibers and is due
to the differential delay between modes at a single frequency. This is elimi-
nated in single-mode fiber since only one mode can propagate. Intramodal or
chromatic dispersion is due to the variation of group velocity (the speed at
which energy in a particular mode travels along the fiber) with wavelength.
For multimode graded-index fibers, the dominant cause of pulse spread-
ing is intermodal dispersion. Experimentally determined values of this pulse
dispersion have been found to range from 20 to 500 ps/km [76]. Dispersion
values can be calculated exactly for quadratically graded index fibers. The
results for calculations using an equation derived by Yariv [77] and particu-
lar fiber characteristics are shown in Figure 2.31. A reasonable bit rate for LSI
applications would be 1.6 Gb/s. This assumes time division with sixty-four