Basically, DTS consists of an optical fiber, a laser light source, an optical splitter, a signal processing unit and a display unit. Due to the properties of the optical fiber cable, when the laser launches a pulse of light through the fiber, most of the light will bounce back unchanged. However a small amount of light will change – called the Raman scatter. The Raman Effect is thermally influenced by temperature, pressure, and applied forces that can affect glass fibers and thereby locally change the characteristics of light transmission in the fiber.
A portion of this scattered light is reflected back toward the laser source and a directional coupler separates the anti-stokes from stokes. Where the anti-stokes band is temperature dependent, the stokes band is practically independent of temperature. DTS registers the potential change in the returning light pulse and can hereby measure the difference in intensity between the signals. Temperature is hereby determined by measuring the different light intensity. The position of a temperature change reading is determined by measuring the time it takes for the signal to return to the source and can therefore pin-point a very exact position of the change.
Rather than a single temperature snap-shot DTS provides a very precise full temperature profile giving the operator a here-and-now look at temperature changes as they happen.
DTS Raman measurement principle
DTS system set up
The LIOS product line consists of a controller (frequency generator, laser source, optical module, HF mixer, receiver, and micro-processor unit) and a quartz glass fiber (fiber optic) as line-shaped temperature sensor.
The power output of the laser runs through the sinus-shaped frequency starting from a starting frequency in the kilohertz range through the ending frequency in the high megahertz range within a measurement time interval with the help of the High Frequency (HF) modulator. The resulting frequency shift is a direct measurement of the local resolution of the reflectometer. The frequency-modulated laser light is connected to the fiber optic-sensor via the optical module.
The continuously back-scattered Raman light is spectrally filtered in the optical module and converted into electrical signals by means of photo detectors. Then the measurement signals are amplified and mixed in the Low Frequency spectral range (LF range). The Fourier transformation of the averaged LF signals results in the two Raman backscatter curves. The amplitudes of these backscatter curves are proportional to the intensity of the Raman scattering of the viewed location.