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

This article proposes and experimentally validates a distributed temperature alarm system based on carbon dioxide (CO2)-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 CO2 filling the air-hole region, which in turn determines a threshold temperature under which CO2 liquefies. The observed slopes of the backscattered Rayleigh intensity trace as a function of the position along the fiber serve as indicators of the CO2 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 1550nm demonstrates superior measurement accuracy within short fiber lengths, revealing a higher loss contrast between gas and liquid phases. Conversely, a 1310nm light source is deemed more suitable for long-distance monitoring due to the lower optical loss in cold spots (CO2 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 36.0m or 16.0m at 1310 nm or 1550 nm, respectively, which take place when all CO2 liquefies inside the fiber.