Optical Backscatter Reflectometer 4600 187
User Guide
uniform, the frequency window function will not significantly alter the measured
Rayleigh scatter amplitude.
Distributed Sensing
Distributed strain and temperature measurements are made by sensing the spectral
shift in the fiber Rayleigh scatter. Since the higher and lower parts of the scanned
spectrum are attenuated by the frequency domain filter, the effective measurable
temperature and strain range is cut roughly in half. For this reason it is advisable
but not necessary to leave the Frequency Domain Window off when making
distributed strain or temperature measurements. It is possible but not advisable to
use a
reference
with the window applied and obtain
subsequent
data with the window
off, or vice-versa.
Optional Distributed Sensing Parameters
Five distributed sensing parameters can also be measured by the OBR, with a
purchased software option. The OBR uses swept-wavelength interferometry (SWI)
to measure the Rayleigh backscatter as a function of length in optical fiber with high
spatial resolution. Rayleigh backscatter in optical fiber is caused by random
fluctuations in the index profile along the fiber length. Scatter amplitude is a random
but static property of a given fiber and can be modelled as a continuous weak fiber
Bragg grating (FBG) with a random period.
The physical length and index of refraction of the fiber are intrinsically sensitive to
environmental parameters: temperature and strain, and to a lesser extent, pressure,
humidity (if the fiber coating is hydroscopic), electromagnetic fields, etc. In most
practical cases the effects of temperature and strain will dominate the spectral
response of the Rayleigh backscatter, so we concentrate our examination on these
parameters.
Changes in the local period of the Rayleigh scatter cause temporal and spectral shifts
in the
locally-reflected spectrum.
These shifts can then be scaled to form a distributed
sensor. This SWI-based technique enables robust and practical, distributed
temperature and strain measurements, with millimeter-range spatial resolution over
tens to hundreds of meters of standard fiber, with strain and temperature resolution
as fine as 1
µε and 0.1 °C.
To Next Page
