Low Temperature Cross-sensitivity Research Articles

Ultrasensitive strain sensor of dual parallel FPIs utilizing femtosecond laser processing and Vernier effect

This study proposes and develops a highly sensitive fiber optic strain sensor utilizing femtosecond laser processing and the Vernier effect. The sensor features two parallel Fabry-Pérot interferometers (FPI), with each FPI consisting of an air bubble and a tapered fiber optic cascade. FPI 2 serves as the sensing cavity, while FPI 1 acts as the reference cavity. The strain sensing capability of a single FPI 2 is measured at 6.67 p.m./με, while the parallel configuration of FPI 2 with FPI 1 achieves a sensitivity of 68.3 p.m./με, resulting in an amplification of 10.24. Furthermore, the proposed sensor demonstrates excellent repeatability, low-temperature cross-sensitivity, simple fabrication, and cost-effectiveness, making it a promising tool for strain measurement applications.

MZI-based high intensity responsive sandwiched multi-layer fiber optic humidity sensor

This paper presents a multi-layer optical fiber humidity sensor based on dual-beam Mach-Zehnder interference (MZI) for optical intensity sensitivity. The sensor is fabricated using a large-scale axial offset fusion technique between a multimode fiber (MMF) and an offset hole two-core fiber (OHTC), making it suitable for agricultural and health monitoring applications. The 27 μm axial offset enables tilted light incidence, enhancing environmental sensitivity. With a single-cycle Graphene oxide (GO) coating of approximately 22 nm, the MMF-OHTC structure achieves high humidity sensitivity (0.39 dB/%RH) over the 25 %-80 % RH range with excellent linearity (R2≈0.99). The sensor exhibits stable, rapid response (2.52 s/0.46 s) and low temperature cross-sensitivity (0.06 %RH/°C). When combined with a hollow-core fiber Fabry-Perot interferometer (HCF-FPI), the system allows zero cross-talk simultaneous humidity and temperature measurements, expanding the potential applications of multi-layered structures in multifunctional detection systems.

High sensitivity Mach-Zehnder interferometric fiber-optic humidity sensor based on multimode interference enhancement

Variations in humidity can significantly disrupt the proper functioning of precision instruments and meters across various industrial, medical, and scientific applications, thereby necessitating real-time humidity monitoring. Here, a high sensitivity fiber-optic relative humidity (RH) sensor based on Mach-Zender interferometer (MZI) is proposed and demonstrated. The humidity sensor consists of single-mode fiber (SMF), peanut structure (PS), and multi-mode fiber (MMF). A multi-mode interference (MMI) enhanced fiber-optic MZI was constructed using a SMF-PS-SMF-MMF-SMF-PS-SMF cascade structure to detect small changes in the refractive index (RI) and expansion force of gelatin film caused by humidity variation. PS, as a beam splitter and combiner of MZI, provides the possibility for simple, economical, and efficient manufacturing of the fiber-based part of humidity sensors. The addition of MMF will excite higher-order core and cladding modes, laying the foundation for achieving high-sensitivity fiber-optic humidity sensors. The fiber-optic RH sensor prototype demonstrate a high sensitivity of about 0.633 nm/% RH in the RH range of 50 % RH to 80 % RH, and a low temperature cross-sensitivity of 0.165 % RH/°C. This sensor has the advantages of safe and easy manufacturing process, low manufacturing cost, and high sensitivity, providing continuous high accuracy humidity measurement results for humidity sensing in different fields.

High-sensitivity strain sensor based on an asymmetric tapered air microbubble Fabry-Pérot interferometer with an ultrathin wall

A Fabry-Pérot interferometer (FPI) with an asymmetric tapered structure and air microbubble with an ultrathin wall is designed for high-sensitivity strain measurement. The sensor contains an air microbubble formed by two single-mode fibers (SMF) prepared by fusion splicer arc discharge, and a taper is applied to one side of the air microbubble with a wall thickness of 3.6 µm. In this unique asymmetric structure, the microbubble is more easily deformed under stress, and the strain sensitivity of the sensor is up to 15.89 pm/µɛ as evidenced by experiments.The temperature sensitivity and cross-sensitivity of the sensor are 1.09 pm/°C and 0.069 µɛ/°C in the temperature range of 25-200°C, respectively, thus reducing the measurement error arising from temperature variations. The sensor has notable virtues such as high strain sensitivity, low-temperature sensitivity, low-temperature cross-sensitivity, simple and safe process preparation, and low cost. Experiments confirm that the sensor has good stability and repeatability, and it has high commercial potential, especially strain measurements in complex environments.

All-solid highly sensitive fiber-tip magnetic field sensor based on a Fabry-Perot interferometer with a breakpoint structure.

An all-solid fiber-tip Fabry-Perot interferometer (FPI) coated with a nickel film is proposed and experimentally verified for magnetic field sensing with high sensitivity. It is fabricated by splicing a segment of a thin-wall capillary tube to a standard single-mode fiber (SMF), then inserting a tiny segment of fiber with a smaller diameter into the capillary tube, and creating an ultra-narrow air-gap at the SMF end to form an FPI. When the device is exposed to magnetic field, the capillary tube is strained due to the magnetostrictive effect of the nickel film coated on its outer surface. In addition, owing to the unique breakpoint sensitivity-enhancement structure of the air-gap FPI, the elongation of the capillary tube whose length is over 100 times longer than the air-gap width is entirely transferred to the cavity length change of the FPI, and the sensor is extremely sensitive to the magnetic field as proved by our experiments, achieving a high sensitivity of up to 2.236 nm/mT for a linear magnetic field range from 40 to 60 mT, as well as a low-temperature cross-sensitivity of 56µT/°C. The all-solid stable structure, compact size (total length of ∼3.0 mm), and reflective working mode with high magnetic field sensitivity indicate that this sensor has good application prospects.

Highly Sensitive Strain Sensor Based on the Vernier Effect With High Extinction Ratio and Low-Temperature Cross-Sensitivity by Compact Double FPI

Highly Sensitive Strain Sensor Based on the Vernier Effect With High Extinction Ratio and Low-Temperature Cross-Sensitivity by Compact Double FPI

Long-Period Grating with Asymmetrical Modulation for Curvature Sensing

We propose and demonstrate a curvature sensor based on long-period fiber grating (LPFG) with asymmetric index modulation. The LPFG is fabricated in single-mode fiber with femtosecond laser micromachining. The grating structure is not introduced in the central fiber core, but is located off-axis with a distance of a few micrometers. Experimental results indicate that the offset distance has direct influence on the grating spectra. By utilizing such an asymmetric structure, two-dimensional vector curvature sensing can be realized. For an LPFG with an offset distance of 6 μm, the curvature sensitivity is around 29 nm/m−1 in the 0° and 180° direction and about 20 nm/m−1 in the 90° and 270° direction. The difference in curvature sensitivity in different bending directions makes the sensor capable of distinguishing the curvature orientation. The temperature response of the sensor is also experimentally investigated, and results indicate that the sensor has a very low temperature cross-sensitivity of 0.003 m−1/°C. The characteristics of high curvature sensitivity, two-dimensional bending direction identification, and compact structure make the device an ideal candidate to be applied in the field of power grid health monitoring and intelligent robotics.

Optical fiber Fabry–Perot sensor based on BPQDs-PVA compound for humidity measurement

A high-sensitivity fiber humidity sensor based on Fabry–Perot interferometer is experimentally proposed. The sensor is fabricated by coating black phosphorus quantum dots (BPQDs)-polyvinyl alcohol (PVA) film on the end face of single mode fiber via controllable dipping method. Benefit from the BPQDs-PVA based cavity, the length and the refractive index of the BPQDs-PVA compound changes with ambient humidity, evidently modulating the interference spectrum. Experimental results show that the sensor achieves a maximum humidity sensitivity of 4.844 nm/%RH in the humidity range of 33%–56%RH with a low temperature cross sensitivity of 0.017%RH °C−1. Meanwhile, stability test exhibits a low instability of 0.016%RH. Together with the advantages of simple manufacture, small size and fast response, the sensor has broad practical application prospect.

High sensitivity twist sensor based on suspended core fiber Sagnac interferometer with temperature calibration.

A high sensitivity optical fiber twist sensor based on Suspend Core Fiber Sagnac Interference (SCFSI) is proposed and experimentally demonstrated. By filling the air hole of the Suspend Core Fiber (SCF) with alcohol, the twist sensitivity of the twist sensor is greatly improved to 8.37 nm/°. Moreover, the valid angle measurement range of the sensor can be expanded by utilizing the combination of intensity demodulation and wavelength demodulation. The sensor not only has high twist angle sensitivity but also exhibits a capability of temperature calibration. Since the wavelength shifts of the interference fringes of Mach-Zehnder Interferometer (MZI) formed in the suspend core of SCF appears insensitive to twist angle, the parasitic interference formed by MZI can be used for temperature calibration. Due to compact structure, easy fabrication and low temperature cross sensitivity, the proposed sensor has a great potential for structural health monitoring, such as buildings, towers, bridges, and many other infrastructures.

Rapid and noninvasive cell assay by microfluidic-integrated intracavity evanescent field absorption in a fiber ring laser

BackgroundHighly sensitive and rapid detection of cell concentration and interfacial molecular events is of great value for biological, biomedical, and chemical research. Most traditional biosensors require large sample volumes and complicated functional modifications of the surface. It is of great significance to develop label-free biosensor platforms with minimal sample consumption for studying cell concentration changes and interfacial molecular events without labor-intensive procedures. ResultsHere, a fiber-optic biosensor based on intracavity evanescent field absorption sensing is designed for sensitive and label-free cell assays for the first time. The interaction between the cells and the evanescent field is enhanced by introducing microfluidic-integrated intracavity absorption in a fiber ring laser. This strategy extends the range of targeted analytes to include quantification of a large number of targets on a surface and improves the detection sensitivity of the fiber-optic biosensor. The level of sensing resolution could be improved from 10−4 RIU to 10−7 RIU using this strategy. The stem cells were studied over a wide concentration range (from 500 to 1.2 × 105 cells/ml) and were measured sequentially. By measuring the output power of the intracavity absorption sensing system, the cell concentration can be directly determined in a label-free manner. The results show that dozens of stem cells can be sensitively detected with a sample consumption of 72 μL. The response was fast (15 s) with a low temperature cross-sensitivity of 0.031 cells·ml−1/°C. SignificanceThe proposed method suggests its capacity for true label-free and noninvasive cell assays with a low limit of detection and small sample consumption. This has the potential to be used as a universal tool for quantitative and qualitative characterization of various cells and other biochemical analytes.

In-line open-cavity Mach-Zehnder interferometric refractive-index sensors based on inter-mode and inter-core-mode interferences

We experimentally demonstrated in-line Mach-Zehnder interferometric sensors based on inter-mode interference (IMI) and inter-core-mode interference (ICMI) for refractive index (RI) measurements. The structure of the IMI-based sensor consisted of a C-shaped fiber spliced between two multi-mode fibers (MMF) with the opposite ends spliced to lead-in and lead-out single-mode optical fibers. The ICMI-based sensor has a similar structure as the IMI-based sensor but with an additional in-house fabricated three-core multi-core fiber (MCF) spliced between the MMF and C-shaped fiber. The performance of the two RI sensors was evaluated and compared using water-glycerol solutions with RI values ranging from 1.333 to 1.338. The RI sensitivity, as well as other characteristic properties such as free spectral range and extinction ratio in relation to three lengths of the C-shaped fiber of 2, 2.5 and 3 mm, were investigated in both structures. The open-cavity characteristic of the C-shaped fiber allows the solution to interact with the optical path of the sensors, hence improving their RI measurement sensitivities. Both types of sensors exhibit linear wavelength responses with variation in RI. The RI sensitivity of the IMI-based sensor exhibits minor variations when the length of the C-shaped fiber is altered, with a maximum sensitivity of -7999.76 nm/RIU. In contrast, the RI sensitivity of ICMI-based sensor exhibits variability when changing the C-shaped fiber length, potentially due to variations in the RI distribution among the cores within the MCF, with its maximum sensitivity being -6211.98 nm/RIU. The temperature sensitivity of all the tested sensors was around 0.7 nm/ °C, denoting a low temperature cross-sensitivity of -7.427 × 10−5 RIU/ °C. This work experimentally exhibits a comparison of two sensing effects in the same configuration. The long length (longer than 2 mm) of the sensing element facilitates the fabrication process, and the open-cavity present in the proposed sensors yields a high sensitivity, which is in the same order of magnitude as the IMI-based sensors and is one order of magnitude higher than the ICMI-based sensors, emphasizing their promising candidacy for label-free optical sensing of chemical and biological samples.

Low-temperature cross-sensitivity strain sensor based on a microbubble Fabry-Pérot interferometer with a thin wall

A Fabry-Pérot interferometer (FPI) strain sensor with low-temperature cross-sensitivity is prepared by a simple, cost-effective, and safe process. The sensor is fabricated by splicing two segments of the single-mode fiber and several arc discharges to form a unique circular bubble and a thin wall. The thin air-bubble wall deforms more easily under stress due to the good stress concentration ability of the tapered structure, resulting in a high strain sensitivity of 10.78 pm/µε. The temperature sensitivity of the sensor is 1.24 pm/°C and the temperature cross-sensitivity is 0.115 με/°C in the temperature range of 25 to 200 °C, which reduces the temperature-induced measurement error. The experimental results show that the sensor with good stability and repeatability has large commercial potential in measuring strain in complex environments.

An Ultrasensitive Gas Pressure Sensor Based on Single-Core Side-Hole Fiber With Optical Vernier Effect

An ultrasensitive optical fiber gas pressure sensor with the Optical Vernier effect (OVE) is proposed based on fusion splicing a piece of the Single-core and Side hole (SCSHF) fiber between the No-core fiber (NCF) and the hollow core fiber (HCF). The fabrication process only refers to the operation of fusion splicing, which is suitable for mass production. The theoretical sensing principles of the proposed sensor are explicitly elucidated in this paper. The sensor can be used as a probe in a narrow place because it works based on the reflected mode. The experimental results demonstrated that the sensor could detect the gas pressure change with a sensitivity of ∼183 nm/MPa in the range of 0∼100 kPa. Repeated experiments are also conducted to verify the sensor's excellent sensing repeatability and long-term stability. In addition, the sensor possesses a low-temperature cross-sensitivity of ∼2.4 kPa/℃. Thus, the proposed sensor is expected to leverage its practical functionalities in sensing areas requiring high sensitivity, a simple fabrication process, and long-term stability.

Highly Sensitive Physiological Sensor Based on Tapered Fiber-Optic Interferometer for Sweat pH Detection

The pH value of human sweat correlates with metabolic activities and diseases. Precise sweat pH sensing can serve as a health-profiling strategy. This study demonstrates a compact fiber-optic sweat pH sensor that contains an optimized conical structure coated with polyelectrolyte membrane as the pH sensitive layer. To functionalize the tapered sensing area for pH monitoring, chitosan/polyacrylic acid (PAA) polyelectrolytes are deposited as pH-sensitive membranes. As the swelling degree of the sensing film depends on the pH value, the tapered fiber-optic sensor based on the Mach-Zehnder interferometer (MZI) demonstrates the corresponding wavelength shift to various pH values in sweat. The fiber-optic sensor presents a high pH sensitivity of -27.06nm/pH in the physiological sweat pH value ranging from 5.0 to 7.5 with a low temperature cross-sensitivity of 0.012 nm/°C in the normal physiological range of 25-45°C. Further, the pH sensing range can be extended to pH 10 in aqueous solutions, and the sensitivity is 28.44nm/pH. Considering the advantages including wide sensing range, simple fabrication technique, and high sensitivity, the sensor can be potentially applied in many other liquid physiological detections for healthcare monitoring.

High-sensitive and temperature-immune curvature sensor based on bitaper sandwiching in SMS fiber structure

A high-sensitive and temperature-immune curvature sensor based on all fiber Mach–Zehnder interferometer (MZI) is proposed and experimentally investigated. The sensor is constructed by sandwiching a short section of multimode fiber (MMF) between two single-mode fibers (SMFs) with two bitapers. Experimental results show that the sensor can achieve a maximum sensitivity of -34.3935 nm/m−1 in the curvature range of 2.96-4.19 m−1 and a low temperature cross-sensitivity of -0.0003 m−1/∘C in the temperature range of 30-100 °C. The stability test reveals that the standard error of instability is 0.0004 m−1, indicating the sensor’s capacity of long-term operation. The sensor has advantages of low cost, easy fabrication, large curvature detection range, high curvature sensitivity and temperature immunity, makes it has broad application prospects in the curvature sensing field.

High Extinction Ratio and Low Temperature Cross-Sensitivity Strain Sensor Based on Vernier Effect Through Hybrid Cascaded Sagnac Loop

Low extinction ratio (ER) and high temperature cross-sensitivity are serious but common problems for most strain sensors based on Vernier effect. In our work, we propose a strain sensor based on Vernier effect. It consists of hybrid cascaded Sagnac loop, one part is a microfiber coupler Sagnac loop (MFC–SL), and the other is a Sagnac loop connected to a Section of polarization-maintaining photonic crystal fiber (PMPCF–SL). Different from the traditional Vernier effect, in the proposed sensor, the spectra of two interferometers shift to opposite directions when the strain changes, which belongs to the enhanced Vernier effect. When ambient temperature varies, the spectra of the two interferometers shift to the same direction, which belongs to the reduced Vernier effect. Thus, its strain sensitivity is improved and its temperature cross sensitivity is reduced. In addition, the MFC–SL and PMPCF–SL exhibit a high fringe ER due to their good transmission of two orthogonal modes. Experimental show that the sensitivity of strain is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$- 124.17\,\,\text {pm}/\mu \varepsilon $ </tex-math></inline-formula> , while the temperature sensitivity is only <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$6.13~\text {pm}/^{\circ }\text{C}$ </tex-math></inline-formula> , and the ER of envelope is about 9 dB. Its robustness can be enhanced by the method of packaging with silicone adhesive. It will have a potential application in the field of strain sensing.

A Low-Flow Fiber-Optic Flowmeter Based on Bending Measuring Using a Cladding Fiber Bragg Grating

A low-flow flowmeter based on the bending measurement of a cladding fiber Bragg grating (CLFBG) has been proposed and demonstrated. The CLFBG is fixed in a cantilever beam structure composed of a spring and a circular target. A femtosecond laser side-illumination technique was utilized to ensure that the grating inscription remains close to the core–cladding interface of the multi-cladding fiber (MCLF). The cladding resonance presents fiber bending dependence because of the asymmetrical distribution of the CLFBG along the fiber cross section. The force exerted on the circular target by the slow flow rate of the fluid on the order of mm/s bends the fiber, which changes the peak intensity of the cladding mode. The experimental results show that the sensor responds in the range of 0–87 mm/s and has extremely low-temperature cross-sensitivity. The proposed sensor has potential application prospects in high-precision, low-start flow measurement and can be applied to oil pipelines, downhole, and other fields.

High-sensitivity and high extinction ratio fiber strain sensor with temperature insensitivity by cascaded MZI and FPI.

Low extinction ratio (ER) and high temperature cross-sensitivity are serious but common problems for most strain sensors based on Vernier effect. In this study, a hybrid cascade strain sensor of a Mach-Zehnder interferometer (MZI) and a Fabry-Perot interferometer (FPI) with high sensitivity and high ER based on Vernier effect is proposed. The two interferometers are separated by a long single-mode fiber (SMF). The MZI is used as the reference arm, which can be flexibly embedded in the SMF. The FPI is used as the sensing arm and the hollow-core fiber (HCF) as the FP cavity to reduce optical loss. Simulation and experiments have proven that this method can significantly increase ER. At the same time, the second reflective face of the FP cavity is indirectly spliced to increase the active length to improve the strain sensitivity. Through the amplification of Vernier effect, the maximum strain sensitivity is -649.18p m/μ ε, and the temperature sensitivity is only 5.76pm/∘C. The magnetic field was measured by combining the sensor with a Terfenol-D (magneto-strictive material) slab to verify the strain performance, and the magnetic field sensitivity is -7.53nm/mT. The sensor has many advantages and has potential applications in the field of strain sensing.

Optical Vernier sensor based on a cascaded tapered thin-core microfiber for highly sensitive refractive index sensing.

This study proposes a refractive index (RI) sensor using a cascaded tapered thin-core microfiber (TTCMF) based on the Vernier effect. The thin-core fiber was made into a TTCMF by arc discharging and flame heating and then sandwiched between two single-mode fibers (SMFs). The two structures with the same SMF-TTCMF-SMF but slightly different free spectral ranges (FSRs) were cascaded to generate the Vernier effect. The FSR varied with the taper parameters of TTCMF. The RI sensitivities of a single TTCMF sensor, series SMF-TTCMF-SMF sensor, and parallel SMF-TTCMF-SMF sensor were compared and analyzed. Using the Vernier effect in the RI measurement range from 1.3313 to 1.3392, a very high RI sensitivity of -15,053.411n m/R I U was obtained using the series SMF-TTCMF-SMF structure, and -16,723.243n m/R I U using the parallel structure, which were basically consistent with the simulation results. Compared with the RI sensitivity of the single TTCMF sensor, the RI sensitivities of series and parallel sensors were increased by 4.65 times and 5.16 times, respectively. In addition, in the temperature range from 35°C to 65°C, temperature sensitivities of -0.196n m/ ∘ C and -0.0489n m/ ∘ C were obtained using series and parallel structures, respectively; the corresponding temperature cross errors were 1.302×10-5 R I U/ ∘ C and 2.92×10-6 R I U/ ∘ C, respectively. Based on the advantages of high RI sensitivity, simple structure, low-temperature cross sensitivity, and convenient fabrication, the proposed sensors have great potential in biosensing fields.

High-sensitivity gas pressure sensor with low temperature cross-talk based on Vernier effect of cascaded Fabry-Perot interferometers

A gas pressure sensor composed of cascaded Fabry-Perot interferometers (FPIs) based on Vernier effect was demonstrated. Two hollow silica tubes (HSTs) with different inner diameters were used to construct the FP cavities, and spliced with single-mode fiber (SMF) to make a gas pressure sensor with a closed cavity FPI and an open cavity FPI in series. The experimental results show that the proposed sensor achieves a high gas pressure sensitivity of −44.31 nm/MPa in the range of 0–2.2 MPa due to the Vernier amplification effect and a very low temperature cross-sensitivity of −0.6 KPa/℃. This sensor has high sensitivity, wide measurement range, small temperature crosstalk, good robustness and low production cost in gas pressure measurement.