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Measuring Principle - OFDR Technology

Measuring Principle - OFDR Technology

For the locally distributed measurement of the Raman scattered light (DTS) one requires a high performance optical backscatter process. Compared to the Rayleigh scattered light, the Raman scattered light measurement displays a backscatter signal which is smaller by a factor of 1000.

The LIOS DTS evaluation unit deploys the method of Optical Frequency Domain Reflection  (OFDR).  The OFDR system provides information on the local characteristic only when the backscatter signal detected during the entire measurement time is measured as a function of frequency in a complex fashion, and then subjected to Fourier transformation. The essential benefits of OFDR technology are the quasi continuous wave mode employed by the laser and the narrow-band detection of the optical back scatter signal, whereby a significantly higher signal to noise ratio is achieved than with conventional pulse technology (OTDR). This technical benefit allows the use of affordable semiconductor laser diodes and electronic assemblies for signal averaging. This is offset by the technically difficult measurement of the Raman scatter light and rather complex signal processing, due to the FFT calculation (FFT, Fast Fourier Trans-formation) with higher linearity requirements for the electronic components.

The optical frequency domain reflectometry has been developed as a high-resolution measurement process for the characterisation of optical wave guides with length dimensions of just a few millimetres. In contrast, its application for the Raman backscatter measurement was newly introduced by LIOS Technology.

The temperature measuring system consists of a controller (frequency generator, laser source, optical module, HF mixer, receiver and micro-processor unit) and a quartz glass fibre (fibre optic) as line-shaped temperature sensor.

The design is three-channel, since an additional reference channel is required besides the two measurement channels (Anti-Stokes and Stokes). Corresponding to the OFDR system, 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 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 fibre 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. The fibre temperature along the sensor cable results from the amplitude ratio of the two measurement channels.

High Reliability and Industrial Strength
The OFDR technology - with its approved optical components from the telecom industry - enables to provide sophisticated fibre optic temperature surveillance at commodity prices.

The applied light source is a modern and durable semiconductor laser diode, instead of a rather complex solid state laser which powers typically Raman OTDR systems.

The semiconductor laser diode has been critically type tested according the Telcordia GR-468 standard and is fulfilling telecom standards with a medium lifetime of > 25 years. Not only the laser diode has been tested, the entire system was comprehensively evaluated by various independent international bodies (e.g. the Germany VdS, the association of German asset insurers), which includes EMI tests as well as endurance tests at accelerated aging environments.

Invariant spatial resolution along the entire sensor length
The applied Optical Frequency Domain Reflectometry (OFDR) principle ensures a temperature survey even over long distances at an appealing spatial resolution of 1 m or even 25 cm.

The OFDR technology provides an almost invariant spatial resolution along the entire sensor length, which ensures to identify and clearly measure atypical hotspots at early stages, even at most remote distances.

This is in contrast to other measurement principles (e.g. laser pulse principle, OTDR), which are sensitive to dispersion effects and therefore affected by a broadened spatial resolution at longer measurement distances; in other words, the hot spot sensitivity of pulse type measurements degrade with a function of distance.