CHUNG E. LEE
Reference
Fiber fiber Fiber-optic splitter connector
Sensing
Photodetectors fiber cable
Temperature/ distance
Optional reference
Laser
Optical filter
S, AS
DTS Unit
Signal
processor
ptical fibers have been used as a major tool in telecommunications for decades. But recently, the adoption rate of fiber-optic sensors and sensing technologies has grown rapidly because of unique advantages over other measurement devices and methods. 1 Fiber-optic sensors offer the capability of handling much higher bandwidth and reduced operational risk (no electrical shocks or sparks), and optical fiber is inherently immune to electromagnetic interference (EMI) and does not emit EMI.
The most prominent feature of Raman distributed temperature sensing (DTS) is its ability to provide true distributed parameter monitoring. Utilizing DTS technology, temperature profiles, including distance information, can be monitored along the entire length of the sensing fiber—over 15 km. Distributed temperature monitoring provides information equivalent to tens of thousands of conventional point temperature sensors with simplified deployment and lower operational costs. Thus, Raman DTS provides an economical means of long-distance temperature monitoring.
p W 30 Rayleigh 25 Stokes
20
15
10 Anti-stokes
5
0 1000
1075
nm
15 nm / D
1150
FIGURE 1. In a DTS unit, the laser output injected into the reference-fiber section is transmitted to a sensing fiber through a fiber splitter that can be easily connected and disconnected with a fiber-optic connector (top). The backscattered portions from the reference fiber and the sensing fiber are guided back to an optical filter. Then the Stokes and anti-Stokes components, separated by a filter, are fed to photodetectors. Finally, the signal processor calculates the temperature profile along the sensing fiber. The backscattered Stokes and anti-Stokes spectra appear in plot of measured Raman bands excited by a 1064 nm laser source in an average mode (bottom).
Spontaneous Raman scattering When an optical fiber is pumped by a light source, most of the light is transmitted, but small portions of incident light are scattered along the fiber. They are categorized
CHUNG E. LEE is the corporate scientist for Sensor Tran, 4401 Fre-idrich Lane, Suite 307, Austin TX 78744; e-mail: lee@sensortran.com; www.sensortran.com.
into Rayleigh, Raman, and Brillouin scatterings. In Raman scattering, two opposite scattered bands called Stokes and anti-Stokes are generated with respect to the pump spectrum. The intensity of anti-Stokes is highly dependent on temperature, as compared to the Stokes component, because of the Boltzmann energy distribution. The Raman temperature-sensing method is based
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