Distributed High Temperature Monitoring of SMF under Electrical Arc Discharges Based on OFDR

Distributed Temperature Sensing through Network Analysis Frequency-Domain Reflectometry

In this paper, we propose and demonstrate a network analysis optical frequency domain reflectometer (NA-OFDR) for distributed temperature measurements at high spatial (down to ≈3 cm) and temperature resolution. The system makes use of a frequency-stepped, continuous-wave (cw) laser whose output light is modulated using a vector network analyzer. The latter is also used to demodulate the amplitude of the beat signal formed by coherently mixing the Rayleigh backscattered light with a local oscillator. The system is capable of attaining high measurand resolution (≈50 mK at 3-cm spatial resolution) thanks to the high sensitivity of coherent Rayleigh scattering to temperature. Furthermore, unlike the conventional optical-frequency domain reflectometry (OFDR), the proposed system does not rely on the use of a tunable laser and therefore is less prone to limitations related to the laser coherence or sweep nonlinearity. Two configurations are analyzed, both numerically and experimentally, based on either a double-sideband or single-sideband modulated probe light. The results confirm the validity of the proposed approach.

Rayleigh-Based Distributed Optical Fiber Sensing

Distributed optical fiber sensing is a unique technology that offers unprecedented advantages and performance, especially in those experimental fields where requirements such as high spatial resolution, the large spatial extension of the monitored area, and the harshness of the environment limit the applicability of standard sensors. In this paper, we focus on one of the scattering mechanisms, which take place in fibers, upon which distributed sensing may rely, i.e., the Rayleigh scattering. One of the main advantages of Rayleigh scattering is its higher efficiency, which leads to higher SNR in the measurement; this enables measurements on long ranges, higher spatial resolution, and, most importantly, relatively high measurement rates. The first part of the paper describes a comprehensive theoretical model of Rayleigh scattering, accounting for both multimode propagation and double scattering. The second part reviews the main application of this class of sensors.

Optical Fiber Sensors for High-Temperature Monitoring: A Review

High-temperature measurements above 1000 °C are critical in harsh environments such as aerospace, metallurgy, fossil fuel, and power production. Fiber-optic high-temperature sensors are gradually replacing traditional electronic sensors due to their small size, resistance to electromagnetic interference, remote detection, multiplexing, and distributed measurement advantages. This paper reviews the sensing principle, structural design, and temperature measurement performance of fiber-optic high-temperature sensors, as well as recent significant progress in the transition of sensing solutions from glass to crystal fiber. Finally, future prospects and challenges in developing fiber-optic high-temperature sensors are also discussed.

Distributed temperature profile in hydrogen flame measured by telecom fiber and its durability under flame by OFDR

The distributed temperature profile of hydrogen flame based on optical frequency-domain reflectometry (OFDR) was experimentally demonstrated for the first time. Spatial temperature field at different flow rate of H₂ flame was monitored by OFDR via a telecom fiber (Corning SMF-28, CPC6) inside the flame over seconds, and the highest temperature is on the sides of center flame separated by ∼1.4mm with difference of 140∼190°C over the flame dimension of 2.5 mm above 900°C. Uniformity level of temperature is studied by varying the distance between fiber and tube entrance, and the largest uniform region over 1-millimeter length of fiber is obtained. Rayleigh scatters correlation coefficient decreases with temperature to 90% around 400°C, further reduces to 70% about 800°C, and 50% roughly at 1000°C. It indicates that a nonlinear thermal sensitivity of SMF is expected for temperature higher than 400°C with OFDR measurement. The durability of single-mode fiber under H₂ flame is studied via decorrelation time at various temperature. It maintains 20s at 880°C with correlation coefficient around 68% and drops to 50% decorrelation at 1000°C over 20s. This information is important for high temperature measurement using telecom fiber over 800°C based on OFDR. A maximum temperature of 1100°C was measured by OFDR, and it is possible for higher temperature measurement beyond of 1100°C with quicker system response time (<1s).

Graphical Optimization of Spectral Shift Reconstructions for Optical Backscatter Reflectometry

Optical backscatter reflectometry (OBR) is an interferometric technique that can be used to measure local changes in temperature and mechanical strain based on spectral analyses of backscattered light from a singlemode optical fiber. The technique uses Fourier analyses to resolve spectra resulting from reflections occurring over a discrete region along the fiber. These spectra are cross-correlated with reference spectra to calculate the relative spectral shifts between measurements. The maximum of the cross-correlated spectra-termed quality-is a metric that quantifies the degree of correlation between the two measurements. Recently, this quality metric was incorporated into an adaptive algorithm to (1) selectively vary the reference measurement until the quality exceeds a predefined threshold and (2) calculate incremental spectral shifts that can be summed to determine the spectral shift relative to the initial reference. Using a graphical (network) framework, this effort demonstrated the optimal reconstruction of distributed OBR measurements for all sensing locations using a maximum spanning tree (MST). By allowing the reference to vary as a function of both time and sensing location, the MST and other adaptive algorithms could resolve spectral shifts at some locations, even if others can no longer be resolved.