Simplex coded raman OTDR-based distributed temperature sensing

Abstract

Temperature sensing for environmental and structural monitoring is of utmost importance for certain applications, e.g., monitoring of wildfires, gas pipelines, electric lines, temperature of facilities such as nuclear reactors, etc. Conventional devices to measure temperature along many kilometers are not cost-effective, and are impractical to install each measurement device. The solution heralded was to utilize the back-scattered Raman signal inside the optical fiber to measure the temperature, as the Raman scattering is a manifestation of the interaction of light with molecular vibrations (optical phonons). Each scattering point along the fiber serves as an individual sensing element, and for that reason, this tech nology is called ‘Distributed Temperature Sensing’ (DTS). However, the problem with Raman OTDR-based DTS originates due to the Ra man signal being weak as much as 70dB below the launched optical power, re sulting in a low signal-to-noise ratio (SNR). Increasing optical power is not viable because of optical nonlinearities. To overcome this problem, advanced noise filter ing techniques and interrogation methods can be used. An interrogation method named Simplex coding is widely used in the literature, which involves the modu lation of the amplitude of the laser, based on a predetermined code word. In this way, more than one pulse can be present during each interrogation, which then suppresses the uncorrelated additive white Gaussian noise. Optimizing the SNR enables longer sensing distances and higher temperature resolution performance in Raman OTDR-based DTS systems. In this thesis, first, a commonly used Wavelet-transformation-based denoising is experimentally demonstrated in a multi-mode fiber equipped RDTS system. The method has improved the temperature resolution from 0.45◦C to 0.10◦C at 5.5km. Finally, 7-bit Simplex coding is experimentally demonstrated in a single mode fiber equipped RDTS system. The Simplex coding method has improved the temperature resolution from 1.3◦C to 0.8◦C at 5km.