Fully Distributed CO₂ Sensor Based on Optical Frequency Domain Reflectometry With Polyimide-Coated Fiber

In this study, a fully distributed carbon dioxide (CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>) sensor based on optical frequency domain reflectometry (OFDR) system is proposed and experimentally demonstrated. Commercially available polyimide (PI)-coated single mode fibers (fibercore, SM1500(9/125)P) are used as sensing fibers without the additional step of a complex and expensive coating process. Changes of CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> concentration can induce a volume expansion effect in the PI coating layer to transfer strain to the fiber core, and the distributed measurement of CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> concentration can be indirectly realized by demodulating the spectrum shift induced by the strain at each position of the sensing fiber through OFDR. In experiments, the individual sensitivity of PI-coated fiber to CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> was verified with methane (CH<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub>) and nitrogen (N<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>) as references. Distributed CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> concentration measurement was successfully verified with 5cm spatial resolution over 12m sensing fiber. Moreover, the system was proven to have good stability and reliability. This simple, novel, robust, and inexpensive distributed CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> measurement technique has potential applications in downhole fire monitoring, air quality monitoring, and greenhouse gas monitoring.