The fiber optic Distributed Temperature Sensing (DTS) method using the Raman-effect was developed at the beginning of the 1980s at Southampton University in England, UK. The DTS method is based on Optical Time-Domain Reflectometer (OTDR) technology and uses a technique derived from telecommunication cable testing.
The fiber optic-based DTS method measures temperature using optical fiber instead of thermocouples or thermistors, as has typically been the case in the past. DTS systems represent a cost-effective method for obtaining thousands of accurate, high-resolution temperature measurements.
Fiber optic-based distributed temperature monitoring is of a particular advantage in several common situations:
When there are a large number of sensors to be placed. If it is necessary to place a lot of temperature sensors for complete monitoring, then the ease of installation of fiber optic DTS becomes apparent. A single optical fiber can replace many point sensors, so all that is necessary is to route the fiber so that it provides the necessary measurement coverage and density.
When there is no a priori knowledge of sensor placement. When the preliminary engineering is performed on a project, it is not always possible to determine the correct location for temperature sensors. The high spatial resolution and long range of a fiber optic sensor allows the operator to select which parts of the fiber to monitor after the project is complete.
When electrical temperature monitoring is impractical. In a situation where there is a large amount of electromagnetic noise, the data being read from thermocouples and thermistors can be corrupted. However, the data being read by the DTS is purely optical and thus immune to contamination in this kind of environment.
When electrical temperature monitoring is unsafe. There is a risk of sparking inherent in all electrical systems. If the atmosphere in the area being monitored is in danger of becoming volatile, then the fact that a fiber optic DTS does not present a spark hazard can be a significant safety advantage.
DTS Technique Described
In the DTS technique, a pulsed laser is coupled to an optical fiber through a directional coupler. Light is backscattered as the pulse propagates through the fiber, owing to changes in density and composition as well as to molecular and bulk vibrations. In a homogeneous fiber, the intensity of the backscattered light decays exponentially with time.
Because the velocity of light propagation in the optical fiber is well known, the distance can be determined from the time-of-flight of the returning backscattered light. The backscattered light consists of different spectral components due to different interaction mechanisms between the propagating light pulse and the optical fiber.
These backscattered spectral components include Rayleigh, Brillouin, and Raman peaks or bands. The Rayleigh backscattering component is the strongest due to density and composition fluctuations and has the same wavelength as the primary laser pulse. The Rayleigh component controls the main slope of the intensity decay curve and may be used to identify the breaks and heterogeneities along the fiber. The Rayleigh component is not sensitive to temperature.
The Brillouin backscattering components are caused by lattice vibrations from the propagating light pulse. However, these peaks are spectrally so close to the primary laser pulse that it is difficult to separate the Brillouin components from the Rayleigh signal.
The Raman backscattering components are caused by thermally influenced molecular vibrations from the propagating light pulse. Thus, their intensity depends on temperature. The Raman backscattered light has two components that lie symmetric to the Rayleigh peak: the Stokes peak and Anti-Stokes peak.
The intensity of the Anti-Stokes peak is lower than that of the Stokes peak, but is strongly related to temperature; whereas the intensity of the Stokes peak is only weakly related to temperature. By calculating a ratio of the Anti-Stokes to Stokes signal intensities, an accurate temperature measurement can be obtained. Combining this temperature measurement technique with distance measurement through time-of-flight of light, the DTS provides temperature measurements incrementally along the entire length of the fiber.