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2.4 Fundamental of OTDR
OTDR——Optical Time Domain Reflector is a high precision optical testing
meter that use the theory of Rayleigh scattering and Fresnel reflection. It is
widely used in the maintenance, construction and monitoring of an optical line.
All the important parameters like fiber length, optical loss, connection loss,
broken or twisted point and etc. of a fiber can be shown on the OTDR. When
the a light transmits along a fiber, it would be scattered to various directions
caused by the difference of come properties of the transmission medium, this
phenomenon called Rayleigh scattering. During the scattering process, some
of the light will be scattered along the absolutely converse direction, this
phenomenon is called Rayleigh back-scattering. It provides some details about
the fiber length. The parameters about fiber length can be got by calculation
with the parameter of time(This is the derivation of TD in OTDR——Time
Domain).
These back-scattering signals shows the loss level of a fiber and through
these information, OTDR can generates a backward oblique trace which
reflects several important attributes of a optical fiber. When the light,
transmitting downward along the fiber, meet a different density medium, a part
of the light will be reflected, this phenomenon is called Fresnel reflection.
There are many reasons can cause the changing of the medium density like a
little slot at the splicing point, a broken of fiber or etc. This phenomenon is
usually used to locate the discontinuous point. Compare to the Rayleigh
scattering, the consuming amount of the light in Fresnel reflection is much
more then it is in Rayleigh scattering. The power of Fresnel reflection is tens of
thousands times to the back-scattering's. The reflection level depends on the
changing grade of refraction ratio.
Formula of the distance: distance = (c/n)
×
(t/2).
Here:
c
is the light speed traveled in vacuum(2.998
×
10
8
m/s).
t
is the delay between launching pulse and receiving pulse.
