Negative Curvature Hollow Core Fiber Based All-Fiber Interferometer and Its Sensing Applications to Temperature and Strain

Light transmission mechanisms in a SMF-capillary fiber-SMF structure and its application to bi-directional liquid level measurement

Capillary fiber (CF) has been extensively investigated in a singlemode fiber (SMF)-CF-SMF (SCS) sensing structure since multiple light guiding mechanisms can be easily excited by simply tuning the air core diameter (cladding diameter) and length of the CF. Understanding the light guiding principles in an SCS structure is essential for improved implementation of a CF based fiber sensor. In this work, light guiding principles in a relatively large air core diameter (≥ 20 µm) and long length of CF (> 1 mm) are investigated theoretically and experimentally. It is found that both multimode interference (MMI) and Anti-Resonant Reflecting Optical Waveguide (ARROW) light guiding mechanisms are excited in the SCS structure in the transmission configuration. However, MMI dips are not observed in the spectrum for the air core diameters of CF smaller than 50 µm in the experiment due to large transmission loss in small air core CFs. Further experimental results demonstrate that a CF with a bigger air core diameter shows a higher sensitivity to curvature, and the highest sensitivity of -16.15 nm/m^-1 is achieved when an CF-100 was used. In addition, a SMF-CF-20-CF-30-SMF (SCCS) structure is proposed for high sensitivity bi-direction liquid level measurement for the first time, to the best of our knowledge. Two types of ARROW dips (Dip-20 and Dip-30) are simultaneously excited in transmission, hence both liquid level and liquid flow direction can be detected by tracing the dip strength changes of Dip-20 and Dip-30, respectively.

Double Antiresonance Fiber Sensor for the Simultaneous Measurement of Curvature and Temperature

Antiresonant hollow core fibers (ARHCFs) have gained some attention due to their notoriously attractive characteristics on managing optical properties. In this work, an inline optical fiber sensor based on a hollow square core fiber (HSCF) is proposed. The sensor presents double antiresonance (AR), namely an internal AR and an external AR. The sensor was designed in a transmission configuration, where the sensing head was spliced between two single mode fibers (SMFs). A simulation was carried out to predict the behaviors of both resonances, and revealed a good agreement with the experimental observations and the theoretical model. The HSCF sensor presented curvature sensitivities of -0.22 nm/m^-1 and -0.90 nm/m^-1, in a curvature range of 0 m^-1 to 1.87 m^-1, and temperature sensitivities of 21.7 pm/°C and 16.6 pm/°C, in a temperature range of 50 °C to 500 °C, regarding the external resonance and internal resonance, respectively. The proposed sensor is promising for the implementation of several applications where simultaneous measurement of curvature and temperature are required.

Strain-, curvature- and twist-independent temperature sensor based on a small air core hollow core fiber structure

Cross-sensitivity (crosstalk) to multiple parameters is a serious but common issue for most sensors and can significantly decrease the usefulness and detection accuracy of sensors. In this work, a high sensitivity temperature sensor based on a small air core (10 µm) hollow core fiber (SACHCF) structure is proposed. Co-excitation of both anti-resonant reflecting optical waveguide (ARROW) and Mach-Zehnder interferometer (MZI) guiding mechanisms in transmission are demonstrated. It is found that the strain sensitivity of the proposed SACHCF structure is decreased over one order of magnitude when a double phase condition (destructive condition of MZI and resonant condition of ARROW) is satisfied. In addition, due to its compact size and a symmetrical configuration, the SACHCF structure shows ultra-low sensitivity to curvature and twist. Experimentally, a high temperature sensitivity of 31.6 pm/°C, an ultra-low strain sensitivity of -0.01pm/µε, a curvature sensitivity of 18.25 pm/m^-1, and a twist sensitivity of -22.55 pm/(rad/m) were demonstrated. The corresponding temperature cross sensitivities to strain, curvature and twist are calculated to be -0.00032 °C/µε, 0.58 °C/m^-1 and 0.71 °C/(rad/m), respectively. The above cross sensitivities are one to two orders of magnitude lower than that of previously reported optical fiber temperature sensors. The proposed sensor shows a great potential to be used as a temperature sensor in practical applications where influence of multiple environmental parameters cannot be eliminated.