Abstract
We discuss the effect of Fresnel reflection on BOTDR measurement. We
show that the theoretical detection sensitivity with respect to Brillouin
scattering is limited by the pulse modulation effect and phase noise of the
light source that is incorporated into the Brillouin spectrum by way of the
Fresnel reflection, when the Brillouin sensor is located near the Fresnel
reflection point. We describe the detection sensitivity in the presence of
Fresnel reflection in terms of the minimum detectable sensor length. In regard
to the pulse modulation effect, we find that when the pulsewidth is short,
the noise in the Brillouin spectrum is large and the detection sensitivity
deteriorates. We derive quantitative equations and confirm the equations experimentally.
When the pulsewidth is 10 ns with a 1–MHz linewidth laser, the boundary
Fresnel reflection determining whether or not we can measure a 1–m sensor
fiber near the Fresnel reflection is about 30 dB lower than that for a 100
ns pulsewidth. A 1-m sensor fiber near the Fresnel reflection could be measured
with a 10-ns pulse only when the Fresnel reflection was very low at about -50 dB. Moreover, the effect of the laser linewidth is also discussed analytically. The analysis suggests that a laser with a narrower linewidth will provide better sensitivity.
© 2008 IEEE
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