Measuring Principle OTDR and OFDR Technology: There are 2 principles of measurement for distributed sensing technology. OTDR (Optical Time Domain Reflectometry) and OFDR (Optical Frequency Domain Reflectometry). OTDR was developed more than 20 years ago and has become the industry standard for telecom loss measurements which detects the - compared to Raman signal very dominant - Rayleigh backscattering signals. The principle for OTDR is quite simple and is very similar to the time of flight measurement used for radar. Essentially a narrow laser pulse generated either by semiconductor or solid state lasers is sent into the fibre and the backscattered light is analysed. From the time it takes the backscattered light to return to the detection unit it is possible to locate the location of the temperature event. Alternative DTS evaluation units deploy the method of Optical Frequency Domain Reflectometry (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 principles of OFDR technology are the quasi continuous wave mode employed by the laser and the narrow-band detection of the optical back scatter signal. This is offset by the technically difficult measurement of the Raman scatter light and rather complex signal processing, due to the FFT calculation with higher linearity requirements for the electronic components. Using the Raman OTDR/OFDR DTS technique it is possible to analyse distances of greater than 30 km from one system and to measure temperature resolutions of less than 0. 01^oC