Optical sensing systems based on specialty optical fibers

Abstract

Novel optical fibers are typically designed to introduce special properties that are difficult to achieve in standard single-mode fiber (SMF). Developing novel fibers is a promising research field for advancing fiber sensors. Meanwhile, proposing novel sensing signal demodulation algorithms to achieve functions, such as multi-point multiplexing measurement, can expand the applications of the present sensing system. Moreover, reducing the cost and operational difficulty of the sensing system, as well as improving its portability and robustness, can facilitate its application in everyday life. Therefore, this thesis proposes several novel sensing systems based on two kinds of specialty optical fibers for the above requirements. Firstly, a novel optical fiber sensor is proposed for the simultaneous measurement of high temperature and gas pressure. The Hollow-core Bragg fiber (HCBF) functions as an anti-resonant reflecting waveguide and acts as pressure and temperature sensing unit. The sample was fabricated in our lab, exhibiting a high linear pressure and temperature sensitivity of -3.747 nm/MPa and 25.925 pm/°C, respectively. As a temperature monitor, the regenerated fiber Bragg grating (RFBG) is integrated to compensate for the temperature crosstalk of the HCBF. By utilizing a 2 × 2 matrix obtained from the experimental results, simultaneous temperature and gas pressure measurements can be achieved. Secondly, a highly sensitive relative humidity (RH) sensor based on Fabry-Perot interferometers (FPIs) is presented. The sensor is fabricated by splicing a segment of HCBF with SMF and functionalized with hygroscopic polymer film at the end of the HCBF. The reflection beams from the fibers’ splicing points and the two surfaces of the polymer film generate the Vernier effect in the reflection spectrum, which significantly improves the humidity sensitivity of the sensor. To demodulate the envelope based on the Vernier effect and realize multi-point sensing, a digital signal processing (DSP) algorithm is proposed to process the reflection spectrum. The proposed sensor demonstrates a high sensitivity of 1.45 nm/% RH for RH ranging from 45% RH to 90% RH. Thirdly, a sensing system based on a homemade seven-core fiber (SCF) is proposed for the simultaneous measurement of two critical vital signs, including the breath rate (BR) and the heartbeat rate (HR). A section of SCF is spliced between two SMF to form an inline Mach-Zehnder interferometer (MZI), which is sensitive to curvature. The sensing structure is packaged in a mattress to detect respiration and ballistocardiogram (BCG) signals. The results demonstrate a good correlation with those obtained from commercial devices, thereby indicating our sensor’s effectiveness and practical applicability in vital signs monitoring. At last, a novel inline all-silica sensor based on the HCBF is proposed for portable human breath monitoring. When water vapor is exhaled from the subject, the smoothness of the fiber cladding can be disrupted, causing a reduction in surface reflectivity and increasing light loss at the antiresonant wavelength. The sensor’s performance has been evaluated, demonstrating a fast response and recovery time. The curvature and temperature response of the sensor has also been investigated, and results show that our sensor can realize reliable measurements in actual applications.