Fiber-Optic Hydraulic Sensor Based on an End-Face Fabry–Perot Interferometer with an Open Cavity

Turov A.T., Barkov F.L., Konstantinov Y.A., Korobko D.A., Lopez-Mercado C.A., Fotiadi A.A.

This work studies the application of low-cost noise reduction algorithms for the data processing of distributed acoustic sensors (DAS). It presents an improvement of the previously described methodology using the activation function of neurons, which enhances the speed of data processing and the quality of event identification, as well as reducing spatial distortions. The possibility of using a cheaper radiation source in DAS setups is demonstrated. Optimal algorithms’ combinations are proposed for different types of the events recorded. The criterion for evaluating the effectiveness of algorithm performance was an increase in the signal-to-noise ratio (SNR). The finest effect achieved with a combination of algorithms provided an increase in SNR of 10.8 dB. The obtained results can significantly expand the application scope of DAS.

Agliullin T., Anfinogentov V., Morozov O., Sakhabutdinov A., Valeev B., Niyazgulyeva A., Garovov Y.

The work is dedicated to a comparative analysis of the following methods for fiber Bragg grating (FBG) spectral response modeling. The Layer Sweep (LS) method, which is similar to the common layer peeling algorithm, is based on the reflectance and transmittance determination for the plane waves propagating through layered structures, which results in the solution of a system of linear equations for the transmittance and reflectance of each layer using the sweep method. Another considered method is based on the determination of transfer matrices (TM) for the FBG as a whole. Firstly, a homogeneous FBG was modeled using both methods, and the resulting reflectance spectra were compared to the one obtained via a specialized commercial software package. Secondly, modeling results of a π-phase-shifted FBG were presented and discussed. For both FBG models, the influence of the partition interval of the LS method on the simulated spectrum was studied. Based on the analysis of the simulation data, additional required modeling conditions for phase-shifted FBGs were established, which enhanced the modeling performance of the LS method.

Tian Y., Xiao G., Luo Y., Zhang J., Yuan L.

• This paper review the research status of microhole fiber based nanoliter liquid sensors. • This paper illustrates the microhole fiber based nanoliter liquid sensors for several variables measurement. • The microhole fiber gives a new possibility for nanoliter liquid measurement. • The future development directions of microhole fiber-optic sensors for nanoliter liquid are discussed. This article reviews the development of nanoliter liquid sensors based on microhole fiber. To measure liquid characteristics in microscale is crucial in the field of environmental pollution monitoring, food and drug safety testing, life science research, clinical application, chemical analysis and biomedical research. Microhole optical fiber with micro air holes can provide fitting space and circulation channel for the measurement of miniature volume liquid. Microhole optical fiber sensors have the characteristics of simple structure, enclosed environment, strong light and liquid interaction, which can meet the requirements of multi-parameter, high sensitivity and pollution-free measurement of minimal specimen. Here, we outline the cumulative introduction and the previous experimental research work on microhole fiber-optic sensors for nanoliter liquid, and ends with some conclusions and prospects. We hope this review can provide rigorous and organized literature to researchers/students for the basic understanding, trends, and practical utility of microhole fiber sensors.

Li J., Jia P., Fang G., Wang J., Qian J., Ren Q., Xiong J.

• A fiber-optic Fabry-Perot pressure sensor for high-temperature applications up to 800 °C is proposed. • The sensor heads are batch-produced using a silica precise micromachining method, which can reduce cost and variability. • The manufacturing process is simple and the pressure sensor has desired pressure measurement range and sensitivity. • Owing to the all-silica adhesive-free design, the pressure sensor exhibits good thermal stability and a low thermal drift. Fiber-optic pressure sensors have attracted significant attention for high-temperature applications. However, conventional sensors suffer from large thermal drifts owing to the large coefficient of thermal expansion of the sensing materials. In this study, an all-silica fiber-optic Fabry–Perot pressure sensor featuring excellent high-temperature survivability was developed and experimentally demonstrated. To reduce cost and variability, the sensor heads were batch-produced using silica wafer high-temperature direct bonding technology and a precise micromachining method. The sensor head, hollow silica tube, and gold-coated optical fiber were fused using a CO 2 laser to form the all-silica pressure sensor. Experimental results indicated that the all-silica pressure sensor can function under 1 MPa from room temperature up to 800 °C, with a pressure sensitivity of 3.25 µm/MPa at 800 °C. This all-silica design affords high reliability and a low thermal drift of 0.435 nm/°C. Given the all-silica adhesive-free design and the batch-producible sensing heads, the proposed all-silica pressure sensor shows promise for application in gas pressure detection under high-temperature environments.

Zhang Y., Zhang S., Gao H., Xu D., Gao Z., Hou Z., Shen J., Li C.

This paper proposes a Fabry–Perot pressure sensor based on AB epoxy adhesive with ultra-high sensitivity under low pressure. Fabry–Perot interference, located between single-mode fiber (SMF) and hollow-core fiber (HCF), is an ultra-thin AB epoxy film formed by capillary action. Then the thick HCF was used to fix the HCF and SMF at both ends with AB epoxy adhesive. Experimental results show that when the thickness of AB epoxy film is 8.74 μm, and the cavity length is 30 μm, the sensor has the highest sensitivity. The sensitivity is 257.79 nm/MPa within the pressure range of 0–70 kPa. It also investigated the influence of the curing time of AB epoxy on the interference spectrum. Experiments showed that the interference spectrum peak is blue-shifted with the increase of curing time. Our study also demonstrated the humidity stability of this pressure sensor. These characteristics mean that our sensor has potential applications in the biomedical field and ocean exploration.

Konin Y.A., Scherbakova V.A., Bulatov M.I., Malkov N.A., Lucenko A.S., Starikov S.S., Grachev N.A., Perminov A.V., Petrov A.A.

We discuss the internal structure of microcavities in single-mode and multi-mode acrylate- and polyimide-clad optical fibers due to fusion occurring as a plasma spark propagates through the core. The calculated plasma propagation speed was 61±2cm/s. The structure of the microcavities was studied from the end and side surfaces of the fibers, and the microcavities had sizes of 2.7±0.5µm and 5.6±0.7µm, respectively, when viewed from each of these two directions. The ultimate strengths of damaged and undamaged single-mode fibers were determined via two-point bending and the axial tension method. It was experimentally determined that following the damage to the core, the flexure strength of fibers with each type of protective coating decreased by 5%–8%, while the tensile strength of the fibers with polyimide coating decreased by 72%–83%, and the tensile strength of the fibers with acrylate coating decreased by 26%–30%.

Guo M., Chen K., Yang B., Li C., Zhang B., Yang Y., Wang Y., Li C., Gong Z., Ma F., Yu Q.

Detection of weak acoustic signals is of great significance. To achieve ultrahigh sensitivity acoustic detection, a silicon cantilever-based fiber-optic acoustic sensor (FOAS) formed by a Fabry–Perot interferometric structure is proposed in this work. Theoretical analysis and finite element analysis are used to assist the sensor design. The cantilever is fabricated by the microelectro-mechanical system (MEMS) processing technology on a silicon-on-insulator (SOI) wafer. A white light interference (WLI) demodulation system based on an amplified spontaneous emission (ASE) source is used to demodulate the cavity length of the sensor. The acoustic pressure sensitivity of the sensor was measured to be $1.753~\mu \text{m}$ /Pa at a frequency of 1 kHz and $28.75~\mu \text{m}$ /Pa at the resonance frequency of the cantilever. Experimental results indicated that the minimum detectable pressure (MDP) level of the fabricated sensor was $0.21~\mu $ Pa/Hz 1/2 at 1 kHz, which is the lowest reported value. The silicon-based FOAS proposed in this article demonstrates its ability to detect ultraweak acoustic signals due to its extremely high sensitivity.

Wei X., Song X., Li C., Hou L., Li Z., Li Y., Ran L.

We experimentally demonstrated a high sensitive gas pressure sensor, which also has more concise processing steps and lower cost. The sensor was fabricated by splicing the single-mode fiber (SMF) and a short section of hollow-core fiber (HCF) which was filled with a Polydimethylsiloxane (PDMS) film to form an enclosed air microcavity with the length of 55 μm. The thin PDMS film also plays a role in reflecting the beam. The thermo-optical effect and the thickness of PDMS film can affect the cavity that is sensitive to external pressure. When the thickness of the PDMS membrane is 3 μm, the measured gas pressure sensitivity reaches to 52.143 nm/Mpa, the linearity is 99.86%, and the cross-sensitivity of the gas pressure over temperature is lower than 2.51 × 10 −3 Mpa/°C. Moreover, the compact structure, high sensitivity, high mechanical strength, and convenient fabrication make it suitable for measuring gas pressure in harsh conditions.

Cheng X., Dash J., Gunawardena D., Htein L., Tam H.

A simple, compact, and highly sensitive gas pressure sensor based on a Fabry–Perot interferometer (FPI) with a silicone rubber (SR) diaphragm is demonstrated. The SR diaphragm is fabricated on the tip of a silica tube using capillary action followed by spin coating. This process ensures uniformity of its inner surface along with reproducibility. A segment of single mode fiber (SMF) inserted into this tube forms the FPI which produces an interference pattern with good contrast. The sensor exhibits a high gas pressure sensitivity of −0.68 nm/kPa along with a low temperature cross-sensitivity of ≈ 1.1 kPa/°C.

Fan P., Yan W., Lu P., Zhang W., Zhang W., Fu X., Zhang J.

A Michelson interferometric fiber-optic acoustic sensor based on a large-area gold diaphragm is proposed in this paper. The Michelson interferometer (MI) based on 3×3 coupler is comprised of two beams that reflected from the gold diaphragm and a cleaved fiber end face. Thickness and diameter of the gold diaphragm are 300 nm and 2.5 mm, respectively. Based on the phase difference between each output port of the 3×3 fiber coupler, an ellipse fitting differential cross multiplication (EF-DCM) interrogation process is induced for phase demodulation, which can overcome the phase distortion caused by property degradation of 3×3 coupler. Experimental results show that the sensor has a phase sensitivity of about -130.6 dB re 1 rad/μPa@100 Hz. A flat response range between 0.8 to 250 Hz is realized with the sensitivity fluctuation below 0.7 dB. Besides, the signal-to-noise ratio (SNR) and minimal detectable pressure (MDP) of the sensor are 57.9 dB and 10.2 mPa/Hz1/2 at 5 Hz. The proposed sensor exhibits superiorities of compact size, high sensitivity, flat low-frequency response and ease of mass production, which gives the sensor great potential for low-frequency acoustic sensing and photo-acoustic spectroscopy.

Liu B., Zheng G., Wang A., Gui C., Yu H., Shan M., Jin P., Zhong Z.

In this article, we fabricated and tested a series of extrinsic fiber Fabry-Perot interferometric acoustic sensors based on the corrugated silver diaphragms. These diaphragms were fabricated through a simple and low-cost method. The corrugations were first etched on the silicon surface permanently and then transferred to the silver diaphragm with the help of photoresist as the sacrificial layer. The sensors with different corrugation depths were fabricated. The experimental results showed the same trend with the theoretical results on the mechanical sensitivities of the fabricated corrugated diaphragms, which first increased and then decreased with the increase in the corruption's depth. Other characteristics of the fabricated optical acoustic sensors based on the corrugated silver diaphragms were also tested in detail. The proposed fabrication method of the corrugated diaphragm showed the advantages of simplicity and low cost, and the acoustic sensors showed high sensitivity, which was suitable for weak acoustic's sensing.

Xiao Q., Tian J., Yan P., Li D., Gong M.

We report an investigation of conditions for the initiation of fiber fuse (IFF), a kind of catastrophic damage that troubles all kinds of optical fibers, in silica-based optical fibers. The fibers of different chemical compositions were processed and tested in controlled conditions without mechanical damages before the IFF. For all the fibers of IFF, the same correlation between the critical temperatures and the optical powers transmitted therein was revealed for the first time. The fibers of different chemical compositions exhibited different resistances to the IFF under the threshold powers for propagation of fiber fuses. The results offered promise for predicting fiber fuses in optical fiber systems, which could facilitate avoiding catastrophic losses. They could direct the optimization of fiber production technologies for suppressing the damages, as well as open a new path towards controlled utilization of fiber fuse in in-fiber microstructure fabrication.

Cui Q., Thakur P., Rablau C., Avrutsky I., Cheng M.M.

This paper presents an integrated microcavity optical fiber pressure sensor using exfoliated ultrathin graphene atomic layers as a reflective surface to form a Fabry-Perot (F-P) interferometric structure. The fabrication was based on the focused-ion-beam (FIB) micromachining and dry exfoliated graphene transfer method, where the sensor was fabricated directly on a facet of a standard SMF-28 single-mode fiber with the core diameter of 8 μm and the cladding diameter of 125 μm. The air leakage from the cavity was negligibly small during the 30-min measurement time. The responsivity of pressure sensors, defined as the spectral shift of F-P fringes in response to the air pressure change, was achieved as large as 1.28 nm/mmHg at 1300-nm wavelength when using a diaphragm diameter of 20 μm. Demonstrated responsivity and compactness makes the sensor suitable for medical, environmental, and other applications. An analytical model was presented describing responsivity of an F-P pressure sensor with a thin diaphragm. It has predicted that reducing the diaphragm thickness below some critical value leads to the highest possible responsivity that no longer depends on the elastic properties or thickness of the diaphragm. This upper limit for the responsivity can be increased using the cavity shape other than a simple cylinder, while residual strength of the diaphragm results in reduced responsivity.

Martins J., Diaz C.A., Domingues M.F., Ferreira R.A., Antunes P., Andre P.S.

Liquid level sensing is nowadays a relevant issue in a broad range of applications, forcing the sensors performance and cost to be evaluated in parallel. This paper proposes a fiber optic-based liquid level sensor system using a fiber Fabry-Pérot interferometer (FPI) embedded into a polyurethane resin diaphragm. The FPI is based on microcavities generated upon catastrophic fuse effect, enabling the fiber recycling and sensors fabrication in a cost-effective way, compared to traditional methods. To enable the simultaneous temperature control, a fiber Bragg grating was used as thermal reference sensor to compensate the temperature cross-sensitivity. The sensor prototype was tested in a field application, using two different configurations, an open chamber configuration, where the diaphragm is in contact with the atmosphere, and a closed chamber configuration revealing the sensitivities of 4.4 ± 0.1 pm/mm and 1.57 ± 0.04 pm/mm, respectively. These sensitivity values are within the figures of merit for diaphragm-based sensors as reported recently.

Tian Y., Xu B., Chen Y., Duan C., Tan T., Chai Q., Carvajal Marti J.J., Zhang J., Yang J., Yuan L.

A novel side-hole fiber Bragg grating (SHFBG)-based sensor is proposed and realized surface tension, refractive index (RI), and temperature of liquid monitoring simultaneously. A segment of side-hole fiber (SHF) is spliced of a single mode fiber (SMF), and a 10 mm long fiber Bragg grating (FBG) is inscribed within the core of the open end of the SHF. Due to the capillary effect, the liquid level in the SHF changes as a function of the surface tension of the liquid, which can be measured by monitoring the liquid level in the hole. The reflectivity of the FBG is affected by the fraction length immersed in the liquid, while the Bragg wavelength is affected by the refractive index and the temperature of the liquid. The absolute shift and relative shift of the Bragg wavelength can be used to measure the temperature and the refractive index of the liquid, respectively. The sensor described in this paper can measure the surface tension and the refractive index easily taking advantage of the simple structure and bring, an additional benefit of the “internal thermometer” provided by the absolute value of the Bragg wavelength.

Shagvaliev R.M., Morozov O.G., Grabovetcky D.S., Shagvaliev T.R., Sarvarova L.M.

Morozov O.G., Shagvaliev R.M., Grabovetcky D.S., Shagvaliev T.R., Sarvarova L.M.