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

SPR fiber optic sensor based on MMF-NCF doped with MgF2 for dual-parameter measurement of temperature and humidity

Dual-parameter temperature and humidity sensors based on optical fiber sensing have wide applications. Among various optical fiber sensors, surface plasmon resonance (SPR) sensors exhibit excellent sensing sensitivity. To address the bandwidth issue and expand the sensitivity, this paper proposes a multimode fiber-no core fiber (MMF-NCF) SPR sensor. The humidity channel covers an Ag/PVA composite film, while the temperature channel covers an Ag/MgF2/PDMS composite film. The MgF2 modulates the shape of the resonance curve, shifts the resonance curve towards the infrared direction and enhancing the sensitivity by 0.245 nm/oC. Compared with previous temperature and humidity detection ranges up to 20°C ∼ 100°C and 40% ∼ 100%, this has obvious improvement. The maximum sensitivities are -0.5683 nm/%RH and -3.259 nm/oC. The mean temperature sensitivity is -1.88877 nm/oC and the mean humidity sensitivity is -0.51249 nm/%RH. The polynomial fitting degrees reach 0.99914 and 0.99875, and FOM values are 2.715 × 10-2/oC and 4.735 × 10-3/%RH, which is expected to be in agriculture, food industry, such as biological implementation is widely used in production activities.

Construction of multiple anti-resonant light guidance mechanisms in a hollow-core fiber structure for simultaneous measurement of multiple parameters

The construction of multiple light guidance mechanisms in a hollow-core fiber (HCF) structure is a popular way to realize the simultaneous measurement of multiple parameters. In this work, a partial coating method to excite multiple anti-resonant light guidance mechanisms (ARLGMs) in an HCF structure for the simultaneous measurement of multiple parameters is proposed. As an example, a double ARLGM based on a partially polyimide (PI)-coated HCF structure for the simultaneous measurement of relative humidity (RH) and temperature is demonstrated theoretically and experimentally. The dip (dip II) produced by the PI-coated HCF section shifts linearly with surrounding RH changes with a sensitivity of circa 58.6 ± 0.77 pm/%RH, while the dip (dip I) produced by the bare HCF section (with an air coating layer) is insensitive to RH changes. In addition, both types of dips have linear responses to temperature variations, with similar sensitivities of ∼ 17 pm/°C. Hence, the proposed sensor structure can be used as an RH sensor that is also capable of compensating for local temperature fluctuations. More importantly, the simultaneous measurement of multiple parameters (such as biomarkers) is possible using the proposed method provided the proper sensing materials are partially coated onto the HCF surface.

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.