Trend in Optical Fibers and Cables for Fiber-To-The-Home

60
Feature Articles: Optical Fiber Communications
cost directly account for a cost per subscriber, and
should be able to be handled easily for quick and
economical installation.
A surplus optical fiber took out from the cables is
wound and stored in a closure or a cabinet. In
particular, the fiber in the drop cable is required to
be able to be bent small to downsize the closure
and the cabinet. Furthermore, a fiber in an indoor
cable may be bent with a small curvature for wiring
along a wall and for storage.
Fig�� Fig�� Fig�� Fig�� Fig�� Configuration Configuration of of cables cables in in in FTTH� FTTH� FTTH� FTTH� FTTH�
3. OPTICAL FIBER USED IN FTTH
The structure of a silica optical fiber, which is the
most popular optical fiber in telecommunications, is
shown in Fig.2. The fiber consists of a cladding
which is made from silica and a core whose refractive
index is raised slightly higher than that of the
Fig�� Fig�� Fig�� Fig�� Fig�� Structure Structure of of silica silica optical optical fiber� fiber� fiber� fiber� fiber� (a) (a) dimensions� dimensions� dimensions� dimensions� dimensions� (b)
(b)
refractive refractive index index profile� profile� profile�
profile� profile�
cladding by doping germanium.
Light can be confined within the core by the coreto
-cladding index difference. The structure is characterized
by the core diameter 2a and the relative
refractive index difference Light confined in the
fiber propagates at a very small attenuation, e.g. 0.
2-0.3 dB/km. The cladding diameter is 125 µm, and
the cladding is coated with plastic resin to a diameter
of 250 µm.
The optical fiber has two main categories; a multimode
fiber and a single-mode fiber. The
core diameter of the multimode fiber is 50
µm and the fiber is easy to connect with each
other and with an inexpensive light source,
so that the fiber is applied to short distance
communications. The single-mode fiber
whose core diameter is about 10 µm is applied
to high-speed and/or long-distance communications
since more sophisticated techniques
are required to connect with each
other and with optical devices.
The fiber with an index profile as shown in
Fig.2 can be operated as a single-mode fiber
when the normalized frequency V defined as the following
equation is less than 2.405 at a wavelength of .
(1)
(2)
Possible combinations of parameters which
result in single-mode operation are infinitely.
However, the dimensions and properties of "the
single-mode fiber" for telecommunications are
defined in ITU-T Recommendation G.652.
In Japan, the single-mode fiber complying G.
652 are used even in FTTH whereas FTTH
covers short distance and requires economical
materials. This is because arc-fusion splice and
connector for the fiber have been highly developed
and the cost of optical devices for the fiber
has become reasonable.
3.1 Solution for small bending radius
An optical fiber for a drop cable or an indoor
China Communications December 2005