Page 608 - Introduction to Information Optics

592 10. Sensing with Optics

Example 10.2. An OTDR instrument has a pulse width of 10 ns; assume that
the refractive of the fiber is n — 1.5. Calculate the spatial resolution of this
OTDR.
8
8
Solve: Substituting c = 3 x 10 m/s, T = 1(T s, and n = 1.5 into Eq, (10.12),
we get AZ min = 1 m.
From the above example, it can be seen that the spatial resolution OTDR is
in the order of meter. This may not be high enough for certain distributed
sensing requirements, such as smart structure monitoring. To increase the
spatial resolution, one has to reduce the pulse width. However, the reduction
in the pulse width may cause a decrease in the launched pulse energy so that
the detected light signal also decreases. Thus, the detected signal-to-noise ratio
may become poor. In particular, when long sensing range is required, a certain
power level is needed to guarantee that the backscattering signal is detectable
in the whole sensing range. Thus, there is a basic trade-off between spatial
resolution and sensing range. The pulse width and pulse energy must be
optimized based on their application requirements.
To enhance the capability of detecting a weak backscattering signal, several
techniques were developed (e.g., coherent OTDR [25] and pseudorandom-
coded OTDR [26]). In coherent OTDR, the weak returned backscatter signal is
mixed with a strong coherent local oscillator optical signal to provide coherent
amplification. In pseudorandom-coded OTDR, correlation techniques are used
in conjunction with pseudorandom input sequences. Due to the use of multiple
pulses, the pulse energy is increased without sacrificing the spatial resolution.

10.3.1.2, Optical Time-Domain Reflectometry Based on Raman Scattering
Rayleigh scattering is caused by density and composition fluctuations frozen
into the material during the drawing process. This type of scattering is largely
independent of ambient temperature provided that the thermo-optic coeffi-
cients of the fiber constituents are similar. To sense the ambient temperature
distribution, optical time-domain reflectometry based on Raman scattering
was developed.
Raman scattering involves the inelastic scattering of photons. The molecular
vibrations of glass fiber (induced by incident light pulse) cause incident light
to be scattered, and as a result, produce components in a broadband about the
exciting (pump) wavelength comprising Stokes (A s, lower photon energy) and
anti-Stokes (x a, higher photon energy) emissions [27]. Its usefulness for
temperature sensing is that the intensity ratio between Stokes and anti-Stokes
is temperature dependent, as given by

4 hi
- ™\ (10.13)