Silicone Rubber Based Highly Sensitive Fiber-Optic Fabry–Perot Interferometric Gas Pressure Sensor

Gao H., Jiang Y., Zhang L., Cui Y., Jiang Y., Jia J., Jiang L.

A self-temperature-calibrated gas pressure sensor with a sandwich structure made of single-mode fiber (SMF)-hollow core fiber (HCF)-SMF is proposed and experimentally demonstrated. A Fabry–Perot interferometer (FPI) is formed by the SMF-HCF-SMF structure along the axial direction, and an antiresonant reflecting optical waveguide (ARROW) is formed by the ring-cladding of the HCF along the radial direction. A micro-channel is drilled on the ring-cladding of the HCF using a femtosecond laser to facilitate air entering/exiting the HCF. The FPI functions as the pressure sensor, and the ARROW functions as the temperature sensor. The initial wavelength and pressure sensitivity of the FPI can be calibrated from the temperature obtained by measuring the optical thickness of the ARROW. The experimental results show that the ARROW exhibits a temperature sensitivity of ~0.584 nm/°C, and the pressure sensitivity of the FPI ranges from 3.884 to 0.919 nm/MPa, within the temperature range of 37–1007 °C. The simplicity and durability of the sensor make it suitable for reliable gas pressure measurement in high-temperature environments.

Luo C., Liu X., Liu J., Shen J., Li H., Zhang S., Hu J., Zhang Q., Wang G., Huang M.

To effectively control the critical thickness of a polydimethylsiloxane (PDMS) film and enhance the sensitivity characteristics of the fiber pressure sensor, we propose a new method to optimize the thickness of the PDMS film in a fiber tube. It is characterized by analyzing the relationship between the diffusion rate of the PDMS and its viscosity, and using an oven to solidify the PDMS to a certain extent to accurately control the diffusion rate and diffusion length of the PDMS in the fiber tube. We also used multiple transfer methods to control the volume of the PDMS in the fiber tube to minimize the thickness of the formed PDMS film. Fabry-Perot interference occurs when the surface of the PDMS film layer filled into the fiber tube and the adjacent single mode fiber/fiber tube form a joint surface. This method forms a new fiber-optic Fabry-Perot pressure sensor that is very sensitive to external pressure parameters. The experimental results show that the optimized film thickness will be reduced to an order of 20 μm. Correspondingly, the fiber-optic pressure sensor has a sensitivity of up to 100 pm/kPa, which is about 100 times that reported in the literature. The structure also has better resistance to temperature interference. To our knowledge, this is the first in-depth study of the effects of the PDMS viscosity coefficient, diffusion rate, and fiber pressure sensitivity in fiber. The film thickness optimization method has some advantages, including a low cost, good controllability, and good application value in high sensitivity pressure and sound wave detection.

Guo Z., Lv W., Wang W., Chen Q., Zhang X., Chen H., Ma Z.

A white light non-scanning correlation interrogation system was proposed and built to interrogate absolute length of the air cavity of fiber-optic compound Fabry–Perot pressure sensors for the extraction of pressure value. By carefully choosing thickness range and tilt angle of the optical wedge used for cavity length matching, correlation interferometric signal of the basal cavity can be naturally filtered out. Based on peak positioning by Fourier transform, bandpass filtering in frequency domain, inverse Fourier transform back to time domain, envelope fitting and zero fringe finding through a gravity center method, cavity length can be determined with an accuracy of 0.04%. The system was used for the interrogation of a fiber-optic compound Fabry–Perot pressure sensor under different pressures. For a pressure range of 0.1~2.9 Mpa, the linear relationship between the air cavity length and the gas pressure imposed was successfully extracted.

Yao M., Ouyang X., Wu J., Zhang A., Tam H., Wai P.

Domingues M.F., Rodriguez C.A., Martins J., Tavares C., Marques C., Alberto N., André P., Antunes P.

In this work, a cost-effective procedure to manufacture optical fiber pressure sensors is presented. This has a high relevance for integration in robotic exoskeletons or for gait plantar pressure monitoring within the physical rehabilitation scenarios, among other applications. The sensing elements are based on Fabry-Perot interferometric (FPI) micro-cavities, created from the recycling of optical fibers previously destroyed by the catastrophic fuse effect. To produce the pressure sensors, the fiber containing the FPI micro-cavities was embedded in an epoxy resin cylinder used as pressure transducer and responsible to transfer the pressure applied on its surface to the optical fiber containing the FPI micro-cavity. Before the embedding process, some FPI sensors were also characterized to strain variations. After that, the effect of the encapsulation of the FPI structure into the resin was assessed, from which a slight decrease on the FPI interferogram fringes visibility was verified, indicating a small increase in the micro-cavity length. Up on the sensors characterization, a linear dependence of the wavelength shift with the induced pressure was obtained, which leads to a maximum sensitivity of 59.39 ± 1.7 pm/kPa. Moreover, direct dependence of the pressure sensitivity with the micro-cavity volume and length was found.

Liang H., Jia P., Liu J., Fang G., Li Z., Hong Y., Liang T., Xiong J.

Zhang Z., Liao C., Tang J., Bai Z., Guo K., Hou M., He J., Wang Y., Liu S., Zhang F., Wang Y.

We demonstrate a novel polyvinyl chloride (PVC) diaphragm-based fiber-tip Fabry-Perot interferometer for gas-pressure measurements with ultrahigh sensitivity. The PVC diaphragm has been coated to the end facet of a well-cut standard single-mode fiber by use of a plastic welder. An ultrahigh-pressure sensitivity of ~65.5 nm/MPa at 1565 nm and a low-temperature cross sensitivity of ~-5.5 kPa/°C have been experimentally demonstrated. The proposed sensor has advantages of high pressure sensitivity, miniature size, low cost, and easy fabrication.

Chen W.P., Wang D.N., Xu B., Zhao C.L., Chen H.F.

We demonstrate an optical Fabry-Perot interferometer fiber tip sensor based on an etched end of multimode fiber filled with ultraviolet adhesive. The fiber device is miniature (with diameter of less than 60 μm), robust and low cost, in a convenient reflection mode of operation, and has a very high gas pressure sensitivity of −40.94 nm/MPa, a large temperature sensitivity of 213 pm/°C within the range from 55 to 85 °C, and a relatively low temperature cross-sensitivity of 5.2 kPa/°C. This device has a high potential in monitoring environment of high pressure.

Wang R., Huang P., He J., Qiao X.

In this paper, we present a gas refractometer based on an open cavity optical fiber Fabry–Perot interferometer. The Fabry–Perot cavity is fabricated by sandwiching a short section of preprocessed side hole fiber between two single-mode fibers. A chemical etching process creates a side-open cavity through which gas can enter and leave freely. By tracking the wavelength shift of the fringe pattern, the measurement of the refractive index change of nitrogen can be realized. The experiment results show the gas RI sensitivity is up to 1290  nm/RIU.

Xu B., Wang C., Wang D.N., Liu Y., Li Y.

A micro-cavity fiber Fabry-Perot interferometer based on dual capillaries is proposed and demonstrated for gas pressure measurement. Such a device is fabricated by fusion splicing of a tiny segment of a main-capillary with a feeding-capillary on one end, and a single mode fiber on the other, to allow gas enters the main-capillary via the feeding-capillary. The reflection spectrum of the interferometer device shifts with the variation of gas pressure due to the dependence of gas refractive index on the pressure applied. During the device fabrication process, a core-offset fusion splicing method is adopted, which turns out to be highly effective for reducing the detection limit of the sensor. The experimental results obtained show that the proposed device exhibits a high gas pressure sensitivity of 4147 pm/MPa, a low temperature cross-sensitivity of less than 0.3 KPa/°C at atmospheric pressure, and an excellently low detection limit down to ~4.81 KPa. The robust tip structure, ultra-compact device size and ease of fabrication make the device an attractive candidate for reliable and highly sensitive gas pressure measurement in a precise location.

Poeggel S., Tosi D., Duraibabu D., Leen G., McGrath D., Lewis E.

Sun B., Wang Y., Qu J., Liao C., Yin G., He J., Zhou J., Tang J., Liu S., Li Z., Liu Y.

We investigated a novel and ultracompact polymer-capped Fabry-Perot interferometer, which is based on a polymer capped on the endface of a single mode fiber (SMF). The proposed Fabry-Perot interferometer has advantages of easy fabrication, low cost, and high sensitivity. The variation of the Fabry-Perot cavity length can be easily controlled by using the motors of a normal arc fusion splicer. Moreover, the enhanced mechanical strength of the Fabry-Perot interferometer makes it suitable for high sensitivity pressure and temperature sensing in harsh environments. The proposed interferometer exhibits a wavelength shift of the interference fringes that corresponds to a temperature sensitivity of 249 pm/°C and a pressure sensitivity of 1130 pm/MPa, respectively, around the wavelength of 1560 nm.

Wang R., Qiao X.

miniature fiber-optic Fabry-Pérot cavity interferometerA miniature fiber-optic Fabry-Pérot cavity interferometer is presented. The interferometer is fabricated at the fiber tip by wet chemical etching using hydrofluoric acid. A concave well is created on the fiber end face. Interference happens between the reflections from the fiber core and the end of the cladding. We demonstrate applications of the interferometric sensor for temperature and gas refractive index sensing.

Xu L., Wei N., Zheng Y.

Defects are generally believed to deteriorate the superlative performance of graphene-based devices but may also be useful when carefully engineered to tailor the local properties and achieve new functionalities. Central to most defect-associated applications is the defect coverage and arrangement. In this work, we investigate, by molecular dynamics simulations, the mechanical properties and fracture dynamics of graphene sheets with randomly distributed vacancies or Stone-Wales defects under tensile deformations over a wide defect coverage range. With defects presented, an sp-sp(2) bonding network and an sp-sp(2)-sp(3) bonding network are observed in vacancy-defected and Stone-Wales-defected graphene, respectively. The ultimate strength degrades gradually with increasing defect coverage and saturates in the high-ratio regime, whereas the fracture strain presents an unusual descending-saturating-improving trend. In the dense vacancy defect situation, the fracture becomes more plastic and super-ductility is observed. Further fracture dynamics analysis reveals that the crack trapping by sp-sp(2) and sp-sp(2)-sp(3) rings and the crack-tip blunting account for the ductile fracture, whereas geometric rearrangement on the entire sheet for vacancy defects and geometric rearrangement on the specific defect sites for Stone-Wales defects account for their distinctive rules of the evolution of the fracture strain.

Ma J., Jin W., Ho H.L., Dai J.Y.

A miniature fiber-tip pressure sensor was built by using an extremely thin graphene film as the diaphragm. The graphene also acts as a light reflector, which, in conjunction with the reflection at the fiber end–air interface, forms a low finesse Fabry–Perot interferometer. The graphene based sensor demonstrated pressure sensitivity over 39.4  nm/kPa with a diaphragm diameter of 25 μm. The use of graphene as diaphragm material would allow highly sensitive and compact fiber-tip sensors.

Wang T., Zuo C., Wang Y., Liu X., Xia Q., Zhu J., Wu X., Luo J., Yu B.

Jia C., Chen Z., Lin X., Yang H., Zhang X.

Ojaghloo A., Parvin P., Moradi H., Shahi F., Karimi R., Sanderson J.

Li R., Zhao X., Ma B., Lin Z., Tan Y.

Shu Y., Jiang C., Hu C., Deng L., Li L., Gao J., Huang H.

We proposed and experimentally demonstrated a gelatin diaphragm-based Fabry-Perot interferometer (FPI) structure ultra-sensitive fiber optic air pressure sensor. The Fabry-Perot (F–P) cavity of the sensing probe FPI1 is made of gelatin diaphragm, with a thickness of approximately 0.8 μm and the probe length of approximately 136 μm. The air pressure sensitivity of the probe obtained in the experiment is as high as 334 nm/MPa. In order to further improve the sensitivity of the probe, we fabricated an FPI2 with an air F–P cavity that is approximately the length of the FPI1 cavity, and then paralleled FPI2 with FPI1 to generate a Vernier effect. The average sensitivity of the Vernier probe obtained in the experiment reached −2650 nm/MPa, which magnified the sensitivity of a single FPI1 by about 7.9 times. The temperature cross-sensitivity is only 3.6 kPa/°C. This proposed air pressure sensor has the advantages of ultra-high sensitivity, simple manufacturing, low cost, good repeatability and stability, and has broad application prospects in high sensitivity pressure and acoustic detection.

Qu J., Zhang Y., Zheng Z., Liang J., Miao C.

In this paper, an ultra-sensitive gas pressure and temperature sensor based on vernier effect is presented. It consists of a single mode fiber (SMF) and hollow cylindrical waveguides (HCW). Through the self-forming film technique, a gelatin film is manufactured at the air core of HCW, and the thickness of the gelatin film manufactured by this technique is only 3.16 μm. The vernier spectrum is formed by the superposition of the spectra of the air cavity (AC) and the mixing cavity. The mixing cavity is composed of AC and gelatin cavity (GC). The thin gelatin film makes the optical path difference between the AC and the mixing cavity smaller, which is beneficial to increase the vernier magnification factor. When the gas pressure changes, the gelatin film will deform abnormally. This abnormally deformation squeezes the film, causing the cavity length of the air cavity to become longer. The experimental results show that the sensor exhibits a high gas pressure sensitivity of 788.22 nm/MPa, and the temperature sensitivity is 0.39 nm/℃. The proposed sensor enables simultaneous measurement of gas pressure and temperature. It also processes the advantages of compact size, low cost, and strong mechanical strength, which makes it suitable for aerospace, Biomedical and Natural gas detection.

Jing C., Xing M., Zhou Q., Yao W., Huang H., Wu H., Wen H., Zhou A., Zhao Y.

Morozov O., Agliullin T., Sakhabutdinov A., Kuznetsov A., Valeev B., Qaid M., Ponomarev R., Nurmuhametov D., Shmyrova A., Konstantinov Y.

The paper describes the design and manufacturing process of a fiber optic microphone based on a macro cavity at the end face of an optical fiber. The study explores the step-by-step fabrication of a droplet-shaped macro cavity on the optical fiber’s end surface, derived from the formation of a quasi-periodic array of micro-cavities due to the fuse effect. Immersing the end face of an optical fiber with a macro cavity in liquid leads to the formation of a closed area of gas where interfacial surfaces act as Fabry–Perot mirrors. The study demonstrates that the macro cavity can act as a standard foundational element for diverse fiber optic sensors, using the droplet-shaped end-face cavity as a primary sensor element. An evaluation of the macro cavity interferometer’s sensitivity to length alterations is presented, highlighting its substantial promise for use in precise fiber optic measurements. However, potential limitations and further research directions include investigating the influence of external factors on microphone sensitivity and long-term stability. This approach not only significantly contributes to optical measurement techniques but also underscores the necessity for the continued exploration of the parameters influencing device performance.

Gao Z., Song J., Zhao Y., Xu X.

Wang Y., Li J., Meng F.

Janani R., Majumder D., Scrimshire A., Stone A., Wakelin E., Jones A.H., Wheeler N.V., Brooks W., Bingham P.A.

The full realisation of optical fibres in devices such as sensors is reliant on the stability of their polymer coating under in-service conditions. Depending on the application, resistance to several environmental factors may be required, such as high or low humidity level, temperature, pressure, or exposure to aggressive solids, liquids or gases. Changes in mechanical or chemical properties as a result of harsh environments can lead to stresses in the coating and subsequent deterioration of the physical or optical properties of the optical fibre. A variety of coating materials are available on the global market, offering optical fibre manufacturers a plethora of options. This review provides a comparison among four most utilised, commercially available types of coating material: conventional and specialty acrylates, polyimides and silicones. It details the history of their development, reported physiochemical properties and some of their main limitations in the context of optical fibre coating applications.

Zhu C., Zheng H., Ma L., Yao Z., Liu B., Huang J., Rao Y.

Qiu H., Tian J., Yao Y.

Chen Y., Zheng Y., Xiao H., Liang D., Zhang Y., Yu Y., Du C., Ruan S.

A high-sensitivity optical fiber acoustic probe based on extrinsic Fabry-Perot interferometer (EFPI) is designed, fabricated, and analyzed. The sound sensitive diaphragm fabricated by silicone rubber exhibits excellent properties including good adhesion, high mechanical strength, flexibility, aging resistance, and biocompatibility. The acoustic pressure change introduces the periodic vibration of silicone rubber diaphragm, which causes the fluctuation of the output voltage. The FP cavity is formed by a cleaved end face of fiber and the silicone rubber diaphragm. The silicone rubber is prepared by a simple spin-dip method, which is easy to fabricate and suitable for mass production. A high acoustic pressure sensitivity and a high signal-to-noise ratio (SNR) of 387 mV/Pa and 48 dB at 1.5 kHz were obtained, respectively. The sensor is expected to be suitable for photoacoustic spectroscopy, week acoustic detections and biological application.

Liu C., Tao W., Chen C., Liao Y.

In this paper, a hollow core fiber was spliced with standard single-mode fibers to form a fiber optic gas pressure sensor, and its sensing characteristics with single hole or multi-holes punched on the hollow core fiber with femtosecond laser pulses were investigated. The experiments demonstrate that the air pressure sensitivity of the single hole sensor was −3.548 nm/MPa, with a linearity of 99.45%, while its response times for air pressure’s rise and fall were 4.25 s and 2.52 s, respectively. The air pressure sensitivity of the ten-hole sensor was up to −3.786 nm/MPa, with a linearity of 99.47%, while its response times for air pressure’s rise and fall were 2.17 s and 1.30 s, respectively. Theoretical analysis and experimental results indicate that the pressure sensitivity of the sensor with an anti-resonant reflecting guidance mechanism mainly comes from the refractive index change of the air inside the hollow core fiber. The proposed device with multi-holes drilled by a femtosecond laser has the advantages of fabrication simplicity, low cost, fast response time, good structural robustness, high repeatability, high sensitivity to air pressure, and insensitivity to temperature (only 10.3 pm/°C), which makes it attractive for high pressure sensing applications in harsh environments.