All-fiber Sensor Research Articles

Numerical investigation of a real-time temperature sensor based on high-order soliton compression

Temperature sensors based on photonic crystal fibers (PCFs) have attracted considerable attentions due to their desirable advantages. However, the real-time temperature sensing in the temporal region is rarely studied. Here, an all-fiber real-time high-sensitivity temperature sensor is fabricated based on the high-order soliton compression process. A 1560 nm femtosecond fiber laser is used as the injected pulse source and the alcohol-filled silica PCF is adopted as the temperature sensitive device. Temperature sensing can be realized by detecting the peak values of temporal profiles with an oscilloscope at the change of temperature. The oscilloscope possesses faster response rate than the optical spectrum analyzer and can record the variation of the single pulse. Through numerical simulations, a real-time temperature sensor with the sensitivity of 4.91 W °C−1 is achieved at the fiber length of 21 cm. Our simulated results show that the designed temperature sensors with low cost, compact all-fiber structure and real-time response are competitive for application in temperature measurement devices.

All-fiber sensors for salinity and temperature simultaneous measurement

In this work, simple all-fiber sensors are developed based on a balloon shaped single mode fiber (SMF) structure together with core diameter mismatch (CDM) structures to measure salinity and temperature. The balloon shaped SMF structure as well as the CDM structures are based on the Mach–Zehnder interferometer. Experimental results show that the maximum sensitivity of salinity is 168.35 pm/% of NaCl in saline solution in the range from 5 to 35% and the maximum sensitivity of temperature ranging from 21 to 42 °C is 735.6 pm/°C. The feasibility of simultaneously measuring temperature and salinity with the present sensors has been successfully demonstrated.

All-fiber temperature and refractive index sensor based on cascaded tilted Bragg grating and Bragg grating

All-fiber temperature and refractive index sensor based on cascaded tilted Bragg grating and Bragg grating

A highly sensitive all-fiber temperature sensor based on the enhanced green upconversion luminescence in Lu2MoO6:Er3+/Yb3+ phosphors by co-doping Li+ ions

Great improvement of upconversion luminescence in Lu2MoO6:Er3+/Yb3+ phosphor is observed due to the introduction of Li+ ions and a highly sensitive all-fiber optical temperature measurement platform is established. X-ray diffraction (XRD) results show that pure Lu2MoO6:Er3+/Yb3+ phase has been prepared and Li2CO3 dramatically promotes the growth of Lu2MoO6:Er3+/Yb3+ phase in the scanning electron microscope(SEM) images. In comparison to the sample without Li+ ions, green upconversion luminescence intensity was increased by 516 times. The maximum absolute sensitivity of 0.015 K−1 is achieved at 423 K and temperature measurement error is ±0.8 K. The experimental results lay the foundation for the practical application of the ratiometric all-fiber optical temperature sensor.

All fiber, highly sensitive sensor based on gold nanoparticle-coated macrobent single mode fiber for human temperature monitoring

A compact all-fiber temperature sensor based on a gold nanoparticle (GNP)-coated macrobent standard single-mode fiber (SMF) has been proposed and experimentally demonstrated. It can be easily constructed by just bending an SMF into a suitable bending radius to constitute a Mach–Zehnder modal interferometer. The sensing area of the bent SMF was coated with GNP utilizing the magnetron sputtering technique. Different tuned GNP thicknesses of ∼10, 20, and 30 nm were deposited on different bent fibers and the temperature-sensing performance was examined experimentally. Throughout the experiments, the wavelengths of the monitored interference dips decreased gradually and were blueshifted with an increase in temperature in the range of 35°C to 47°C. Among the three coated sensing heads, the proposed sensor coated by an ∼20-nm layer thickness of GNP showed the best performance with excellent sensitivity, fast rise time, and an excellent resolution of −2.56 nm / °C, 1.73 ms, and 1.82 × 10 − 4°C, respectively. Benefiting from its excellent advantages of simple configuration, easy fabrication, and high mechanical strength, this high-sensitivity temperature sensor could be a competitive candidate for precise temperature measurement of the human body.

All-fiber humidity sensor based on Michelson interferometer with hyaluronic acid and polyvinyl alcohol composite film

Abstract All-fiber humidity sensor based on Michelson interferometer with hyaluronic acid (HA) and polyvinyl alcohol (PVA) composite film is investigated. The interferometer is fabricated by fusing 691 μm thin-core fiber (TCF) with single mode fiber (SMF) together and melting the other end of the TCF into a circular shape. The outer of the interferometer is coated with HA and PVA composite film. As far as we know, hyaluronic acid is used in relative humidity (RH) measurement for the first time. Through wavelength and intensity demodulation, the proposed sensor’s maximum sensitivity is −0.1679 nm/%RH and −0.148 dB/%RH, respectively. Besides, the sensor’s cross-sensitivity is low (0.0727%RH/°C). Therefore, the proposed sensor with hyaluronic acid has great potential in humidity measurement.

High Sensitivity Flow Velocity Sensor Based on All-Fiber Target-Type Structure

An all-fiber target-type flow velocity sensor was proposed and experimentally demonstrated based on the Michelson interferometer (MI). Commercially available single mode fiber (SMF) was used as the basic material and assisted with the arc discharge technology the sensing structure can be fabricated with high efficiency and quality. The designed sensor structure contains a peanut-shape structure that acts as a coupler/splitter, a SMF that acts as a cantilever beam, and a spherical tip that acts as a target, which all can be realized by arc discharge technology. The simulation verifies the effectiveness and high sensitivity of the peanut-shape structure for flow velocity measurement, and the key parameters were optimized. A comprehensive experiment was carried out on the flow velocity experiment. The experimental results show that the sensor has high sensitivity of -1.30 nm/(cm/s), precision of 1.4% and detection limit (DL) of 0.015 cm/s in the range of 0-11 cm/s. In addition, the temperature sensitivity is 58.5 pm/°C and the flow velocity-temperature cross-sensitivity is less than 0.05 (cm/s)/°C. Such a high-sensitivity, low cross-sensitivity and low-cost sensor, is very promising for the applications on high-precision liquid and abysmal sea flow velocity measurement.

All-fiber online Raman sensor with enhancement via a Fabry-Perot cavity.

In this Letter, a novel all-fiber online Raman sensor with significant signal enhancement via a Fabry-Perot (FP) cavity is proposed and demonstrated. The FP cavity structure is formed by inserting a long-pass coated fiber and a gold-plated capillary into a silver-lined capillary with a gap. A corroded single-mode fiber is inserted into the gold-plated capillary to guide the excitation light into the FP cavity. The multiple reflections of excitation light in the FP cavity have significantly increased the interaction volume between the light and the sample. Experiment results have demonstrated an enhancement factor of 5 times in the detected Raman signal for ethanol compared to that measured using the silver-lined hollow-core fiber-based Raman cell without FP cavity, or 86 times compared with direct detection using a bare fiber tip. The measurement results are in good agreement with theoretical analyses. This Raman sensor with signal enhancement via the FP cavity has the potential to realize rapid sample replacement and online detection with high sensitivity and high accuracy for biochemical applications.

All-Fiber Laser-Self-Mixing Sensor for Acoustic Emission Measurement

Self-mixing interferometry (SMI) is a promising non-destructive sensing technology with advantages of simplicity in system structure, low cost in implementation, ease in optical alignment, and high resolution in measurement. It consists of a laser diode, a photodiode packaged at the rear of the laser diode, a lens, and a target to be measured. Typically, SMI-based sensing is to measure the phase change in an SMI signal with a fringe pattern as sinusoidal or saw-tooth like. The phase is called optical phase in external cavity that is linked to the quantity to be measured, e.g., displacement, velocity, and vibration. Each SMI fringe maps to a half-laser-wavelength displacement of the target. With increasing feedback level in an SMI system, some fringes may lost or even totally disappear. In this case, the laser output from an SMI system can trace the optical phase to achieve a linear sensing relationship. In this article, we design an all-fiber SMI linear sensor for real-time and fast tracking acoustic emission (AE) signals. Firstly, a system design in terms of selection of operating parameters is presented. Then, a proof of concept experimental system was built for detecting AE signals. The results show that the proposed SMI sensor has comparable AE detection capability to piezoelectric sensor, providing a compact and cost-effective optical AE sensing solution without need of extra signal processing, which can be considered as a viable alternative to the well-established piezoelectric AE sensor.

Temperature-Compensated Interferometric Torque Sensor With Bi-Directional Coiling

Most fiber-optic torque sensors lack the accuracy to resolve small changes in torque when coupled with thick metal shafts in heavy industry. We present a new type of fiber-optic torque sensor based on the stress-optic effect with a high accuracy and a potentially wide dynamic-range. It is the first interferometric torque sensor based on the 3 × 3 coupler demodulation method, and being an all-fiber sensor head paves the way towards system-level immunity against electromagnetic interference. The optical fibers are located on the external surface of the shaft, which allows numerous possibilities for the internal design. We also studied the feasibility of three temperature-compensation schemes and two signal processing methods for different applications. For the dual-coil configuration, the instantaneous method yields a sensitivity, detection limit and response/recovery times of 6.9 mrad/(N.m), 0.79 N.m, >274 ms and >274 ms, respectively. The cumulative method produces 32.3 mrad/(N.m), 0.08 N.m, >95 s and >274 ms respectively. Torque up to ~50 N.m has been tested.

All-Fiber Vector Bending Sensor Based on a Multicore Fiber With Asymmetric Air-Hole Structure

In this article we present an all-fiber vector bend sensor by means of a self-fabricated micro-structured multicore optical fiber. The reported solution is based on differential intensity variations of the light transmitted along the cores whose changes are influenced by the bending angle and orientation. The unique asymmetric structure of the air-holes in the optical fiber provides each core with different confinement losses of the fundamental mode depending on the bending radius and orientation, making each of the cores bend-sensitive in a range of at least 80°. It has been experimentally demonstrated that the reported sensor enables the bending angle and orientation to be detected in a full range of 360° without any dead-zones, and the possibility of end point detection with millimeter precision. Additionally, a reconstruction of the bending vector has been carried out theoretically, and a good match can be observed between the experimental and theoretical data.

Investigation of the Properties of an All-Fiber Temperature Sensor Created Using the Melting Effect

The temperature sensitivity of an all-fiber temperature sensor created using the melting effect is investigated. In order to study the dependence, a special model was developed and assembled to check the sensitivity of the sensors to temperature changes. During the experiments, the temperature dependences of the spectral shift for the sensor were obtained in the 30–90°С and 20–320°С temperature ranges. During the study, a graph of the dependence of the spectrum on temperature was obtained and a calibration graph was constructed. Thus, the temperature sensitivity of the sensor was determined, which was ≈16 mln–1/°C. The results of the fiber strength in an acrylate coating were obtained before and after tests using the axial tension method. It was found that the ultimate strength of the fiber decreased by more than 2 times when exposed to high temperature.

All-Fiber Liquid-Level Sensor Based on In-Line MSM Fiber Structure

We propose and demonstrate an all-fiber liquid-level sensor using an in-line multimode-single-mode-multimode (MSM) fiber structure. A piece of single-mode fiber (SMF) is spliced to two sections of equivalent multimode fiber (MMF) which are used as both mode splitter and mode coupler. The cladding mode will be excited when the light propagates from MMF to SMF, and then it will be combined with fundamental mode to form a Mach-Zehnder interferometer (MZI) when the light propagates from SMF to the other MMF. The liquid level is detected by the selected resonant dips shift of the transmission spectrum. A sensing sensitivity of 264.6 pm/mm is achieved for the proposed sensor with an SMF length of 26mm. Due to its compact structure, easy fabrication, and high sensitivity, the proposed liquid-level sensor is attractive for practical applications in a variety of fields, such as marine detection and chemical processing.

Residual thickness enhanced core-removed D-shaped single-mode fiber and its application for VOC evaporation monitoring.

A core-removed D-shaped structure with different residual thickness (RT) was manufactured on a single mode silica fiber (SMF) to enhance the sensitivity by using of ultra-precise polishing technology. With six different RTs ranging from ∼55 µm to ∼28 µm, the RT enhancement effect in a D-shaped SMF was researched in detail. The influence of the RT on its transmission spectra was investigated both theoretically and experimentally. Considering a compromise between the multimode interference efficiency and optical power loss, an optimum RT value of 34.09 µm was achieved. The obtained refractive index (RI) sensitivity was 10243 nm/RIU in the RI range of 1.430-1.444, corresponding to a RI resolution of 1.9×10-6 RIU. A high-performance all-fiber sensor was developed to monitor the evaporation process volatile organic compounds (VOCs) based on the RT-enhanced D-shaped SMF. As proof of concept, a 2-hour continuous monitoring was carried to monitor the chloroform and alcohol mixture. As a result, the evaporation of alcohol and chloroform were clearly identified and monitored. The developed RT-enhanced D-shaped fiber sensor provides an alternative way for chemical process monitoring and industrial applications.

Label Free All-Fiber Static Pressure Sensor Based on Vernier Effect With Temperature Compensation

This paper reported a label free, all-fiber static pressure sensor constructed by cascading two single mode fiber(SMF) – few-mode fiber(FMF) — single mode fiber(SFS) critical wavelength structures. Vernier effect was adopted to magnify the static pressure wavelength sensitivity of the interference peaks and the superimposed transmission spectrum has an envelope which the free spectrum range varies with wavelength and reaches maximum when approaching the envelope critical wavelength (CWLE). Thus, the envelope peaks near the CWLE in the transmission spectrum with the modified Vernier effect is label free, which is easy to recognize and entirely different from the envelope peaks in a transmission spectrum with uniform free spectrum range. Both theoretical and experimental results show that the envelope peaks near the CWLE exhibit different static pressure and temperature wavelength sensitivities, and the wavelength sensitivity increase significantly when the envelope peak approaching the CWLE. The experimental results show that the left envelope peak 1 and the left envelope peak 2, which numbered from the peak on the left side and closest to the CWLE, have different static pressure sensitivities of 4.072 nm/MPa and 2.649 nm/MPa, respectively, and temperature sensitivities of 1.753 nm/°C and 1.045 nm/°C, respectively. The performances of this proposed approach can be further improved by optimizing the physical lengths of the FMF employed in the sensing and reference SFS structures, to satisfy the requirements in practical applications, especially the extreme conditions and two-parameter measurement of ultra-high pressure and high temperature.

Miniature Fiber-Optic Pitot Tube Sensor

This paper presents a miniature fiber-optic Pitot tube for gas flow rate measurements created at the tip of an optical fiber. The proposed all-fiber sensor employs two in-fiber Fabry-Perot cavities/chambers. The first chamber is arranged as a differential pressure sensor using a thin flexible silica diaphragm and two side holes for measurement of the difference between stagnation and static pressure, while the second chamber acts as a reference sensor for compensation of changes in the gas refractive index. The entire sensor is fusion-spliced, and is made out of sections of silica fibers and silica capillaries. It is appropriate for mid to high-velocity microfluidic gas flow sensing where miniature size is essential. Sensor operation is demonstrated in a flow velocity range between 0 to 260 m/s with a resolution better than 1.5 m/s. The sensor has a diameter of only $200~\mu $ m.

Photonic crystal all-fiber Interferometer for Temperature and Transverse Load Sensing based on phase demodulation

In this paper, a hybrid photonic crystal all-fiber sensor consisting of a fiber Mach–Zehnder interferometer (MZI) and a Fabry-Perot interferometer (FPI) was proposed and experimentally demonstrated, which spliced a section of photonic crystal fiber(PCF) between a microsphere cavity and single mode fiber (SMF) ball. Sensors' sensitivities to temperature and transverse load calculated through the phase shift were -0.032 rad/°C and -0.386 rad/N, which are more stable and accurate than conventional peak wavelength tracking due to the phase demodulation can avoid the effects of dip selection, wavelength sampling rate and spectral noise on the peak wavelength tracking.

Ultraviolet sensing characteristics of Ag-doped ZnO micro-nano fiber

Different concentrations of Ag-doped ZnO micro-nano fiber ultraviolet sensors were successfully prepared by hydrothermal method. The effects of different concentrations of Ag on the sensitivity of ultraviolet sensors are further investigated. The results show that the sensitivity of the sensor is improved after doping the Ag. And Ag-doped ZnO with 4% concentration ultraviolet sensor exhibits a higher sensitivity. Therefore, the effects of different tapered waist diameters on the sensitivity of Ag-doped ZnO with 4% ultraviolet sensors are further studied. The experimental results indicate that the sensitivity of the ultraviolet sensor increases as the taper waist diameter decreases from 6.92 μm to 3.06 μm. The 4% concentration of Ag-doped ZnO ultraviolet sensor whose tapered waist diameter of 3.06 μm is more sensitive. The work is meaningful, and the new all-fiber composite structure Ag-doped ZnO ultraviolet sensor proposed by us can provide a better solution for ultraviolet detection as well.

A high sensitivity optical fiber strain sensor based on hollow core tapering

Abstract An optical fiber strain sensor based on a Mach-Zehnder interferometer (MZI) is demonstrated experimentally. The sensor is fabricated by fusion splicing a tapered hollow core fiber (THCF) with a 500 µm taper region between two single mode fibers (SMFs). Tapering on the HCF increases the sensor’s strain sensitivity to 2.7 pm/µe in a measurement range of 0 µe to 2100 µe, 1.5 times larger than that of all-fiber MZI strain sensors without tapered areas (approximately 1.8 pm/µe). The experimental results show that the sensor has good stability and repeatability. This sensor has high strain sensitivity and low temperature sensitivity and can be used in environments with little temperature fluctuation.

Ultra-high-sensitivity refractive index sensor based on dual-microfiber coupler structure with the Vernier effect.

We demonstrate a novel, to the best of our knowledge, refractive index (RI) sensor based on the Vernier effect in dual-microfiber coupler (MFC) structures. The sensor sensitivity was studied both theoretically and experimentally. The numerical results show that by tracing the wavelength shifts of the envelope formed by the Vernier effect, the sensitivity can be improved by several times compared to that obtained for normal coupler-based sensors. In this Letter, two MFCs with a width and free spectral range (FSR) of $\sim{3.5}\;{\unicode{x00B5} \rm m}$∼3.5µm and 6 nm, respectively, were fabricated. Based on the sensitivity of 5820 nm/RIU for a single coupler, we experimentally achieved an ultra-high sensitivity of 126,540 nm/RIU using dual MFCs by the Vernier effect in the RI range of 1.3350 to 1.3455, which shows good agreement with numerical simulations. The proposed all-fiber RI sensor has the advantages of high sensitivity and low cost and can find applications in chemical and biological detection as well as electronic/magnetic field measurement.