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Distributed Temperature Sensing (DTS) principle

AP Sensing’s distributed temperature measurement instruments are based on the proven Raman-Optical Time-Domain-Reflectometry (OTDR) technique. An optical laser pulse propagating through the fiber gets partially scattered back to the transmitting end, where it is analyzed. The backscattered light consists of different spectral components:

• Rayleigh Backscattering
• Brillouin Backscattering
• Raman Backscattering

The Raman backscattering intensity depends on temperature and can be used as a measure for the temperature along the fiber. The Raman backscattered light has two components above and below the incident light been: the Raman Stokes and Raman Anti-Stokes peak.

The backscattered light is spread across a range of wavelengths. Some of these wavelengths are affected by temperature changes while others are less affected. Using a very accurate detector the difference in the signal strength is measured and the temperature is derived from these measurement results.

Because the light propagation speed in an optical fiber is well known, the distance can be determined from the round trip delay time of the returning backscattered light. The exact position of the temperature reading is determined by measuring the arrival timing of the returning light pulse similar to a radar echo showing the distance of a car or plane.

Sir Chandrasekhara Venkata Raman, an Indian physicist, discovered already in 1928 through its experiments about the scattering of light the effect which is today named after him. At the age of 42 he became laureate of the Nobel Price for his work and the discovery of the Raman Effect.
Even before Raman was able to prove the effect in 1928, Adolf Smerkal predicted this effect already in 1923 – therefore the effect is sometimes also referred to as Rama-Smerkal-Effect.

Even though the physical discovery of the effect is more then 80 year back, it took decades until technology companies, such as Agilent Technologies, had been able to exploit the effect and take it out of the research labs into reliable test and measurement instruments.

The use of passive optical fibers as distributed temperature sensors is today considered a powerful way of monitoring temperature over long distances and in areas covered with

• dirt, dust,
• humidity,
• corrosive conditions,
• strong electromagnetic fields, or
• extreme temperatures

were conventional detection technologies often fail to offer a reliable and cost effective solution.
With a continous - linear - fiber optical temperature sensor there is no need to install many conventional punctual sensors, neither the requirement to have an a priori knowledge about the exact positioning of the sensors, because the fiber optical sensor leaves no are unmonitored, by quasi simultaniously monitoring thousends of temperature points.