OFDR Technology

The LIOS product line utilizes the method of Optical Frequency Domain Reflection (OFDR). OFDR scans the optical sensor cable using light. With OFDR technology you can measure the precise location of reflective events over the length of the entire optical sensor cable without drop-outs or dead-zones. Therefore, OFDR enables pin-pointing of locations that requires immediate attention or of needs additional surveillance.   

The essential benefits of OFDR technology are the quasi continuous wave mode employed by the laser diode and the narrow-band detection of the optical backscatter signal, whereby a significantly higher signal to noise ratio is achieved compared to conventional pulse technology (OTDR). This technical benefit allows the use of affordable semiconductor laser diodes and electronic assemblies for signal averaging. LIOS’ OFDR Raman DTS is classified as a Laser Class 1M Product.

The OFDR principle ensures a temperature survey even over long distances at an appealing spatial resolution of 1 m or even down to 25 cm. The technology provides an almost invariant spatial resolution along the entire sensor length, which ensures identification and clear measurements of 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.

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.

High Reliability and Industrial Strength

The OFDR technology – with its approved optical components from the telecom industry – enables to provide sophisticated fiber 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.