Distributed Temperature Alarm System Based on Incoherent Optical Time-Domain Reflectometry and Side Air-Hole Optical Fibers

• Published in: Journal of lightwave technology Vol. 42; no. 18; pp. 6599 - 6607
• Main Authors: Yang, Yuting, Soto, Marcelo A., Thevenaz, Luc
• Format: Journal Article
• Language: English
• Published: New York IEEE 15.09.2024; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Alarm systems; Attenuation; Carbon dioxide; Distributed sensing; fiber optics; Interrogation; Light sources; Liquid phases; Liquids; Optical attenuators; Optical fiber sensors; Optical fibers; Optical reflection; Optical surface waves; optical time-domain reflectometry; Position indicators; Questioning; Short fibers; Spatial resolution; Temperature; Temperature sensors; Time domain analysis
• ISSN: 0733-8724; 1558-2213
• DOI: 10.1109/JLT.2024.3403189

This article proposes and experimentally validates a distributed temperature alarm system based on carbon dioxide (CO 2 )-filled 18 m side air-holes fiber (SAHF), interrogated through a conventional (incoherent) optical time-domain reflectometer (OTDR). Customizable alarm threshold temperatures can be designed and set by adjusting the pressure of the CO 2 filling the air-hole region, which in turn determines a threshold temperature under which CO 2 liquefies. The observed slopes of the backscattered Rayleigh intensity trace as a function of the position along the fiber serve as indicators of the CO 2 states, allowing the identification of both gaseous and liquid phases through variations in the optical attenuation. Utilizing a comparative analysis of curve slopes, the proposed method enables the distributed identification and localization for both cold and hot spots along the fiber. The spatial resolution and the required loss accuracy are subject to the classical tradeoff inherent to OTDR interrogation. In this study, two interrogation wavelengths (namely, 1310 nm and 1550 nm) are exploited. Results point out that the selection of the operating wavelength gives rise to optical attenuation factors with distinct contrasts along the OTDR traces, allowing the optimization of the system to meet specific measurement objectives. In particular, the use of an optical source at 1550 nm demonstrates superior measurement accuracy within short fiber lengths, revealing a higher loss contrast between gas and liquid phases. Conversely, a 1310 nm light source is deemed more suitable for long-distance monitoring due to the lower optical loss in cold spots (CO 2 in liquid phase), providing relevant measurements over extended cooled fiber sections. Based on our experimental results using a spatial resolution of 30 cm, the maximum detection length is limited to 36.0 m or 16.0 m at 1310 nm or 1550 nm, respectively, which take place when all CO 2 liquefies inside the fiber.