Page 611 - Introduction to Information Optics

10.3. Distributed Fiber-Optic Sensors
Longitudinal
strain

-13.0 -12.6

v
Frequency difference v pr ~ pu (GHZ)
Fig. 10.17. An illustration of Brillouin scattering-frequency shift under different longitudinal strain
conditions.

Since the Brillouin frequency shift is not much, to detect this frequency shift,
a very stable, single-frequency, tunable laser is needed. Currently, 50-m spatial
resolution with 0.01% strain resolution over > 1 km fiber lengths is achieved
by using a Nd:YAG ring laser, which is capable of delivering 1 mW power into
the single mode fiber with remarkably narrow (5 kHz) spectra [29].

10.3.1.4. Optical Frequency-Domain Reflectometry

As discussed earlier, to obtain high spatial resolution, a very narrow light
pulse is required, which results in a proportionally lower level of the backseat-
tering signal and an increased receiver bandwidth requirement for detecting
these pulses. Thus, a large increase in the noise level is expected so that only
strong reflections can be detected in noise. To increase the spatial resolution
without sacrificing backscattering signal intensity, optical frequency-domain
reflectometry (OFDR) was developed [30, 31,32].
Figure 10.18 shows the configuration of OFDR. A highly monochromatic
light is coupled into a single mode fiber, and the optical frequency, ox is
modulated in a linear sweep. Assume that the power loss constant is a (due to