All-fiber Mach-Zehnder interferometers for sensing applications

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.

Investigation of the molecular switching process between spin crossover states of triazole complexes as basis for optical sensing applications

With the advent of the first laser sources and suitable detectors, optical sensor applications immediately also came into focus. During the last decades, a huge variety of optical sensor concepts were developed, yet the forecast for the future application potential appears even larger. In this context, the development of new sensor probes at different scales down to the atomic or molecular level open new avenues for research and development. We investigated an iron based triazole molecular spin-crossover complex changing its absorption characteristics significantly by varying environmental parameters such as humidity, temperature, magnetic or electric field, respectively, with respect to its suitability for a new class of versatile molecular sensor probes. Hereby, besides the investigation of synthesized pure bulk material using different analyzing methods, we also studied amorphous micro particles which were applied in or onto optical waveguide structures. We found that significant changes of the reflection spectra can also be obtained after combining the particles with different types of optical waveguides.The obtained results demonstrate the suitability of the material complex for a broad field of future sensor applications.

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.

A Fiber-Optic Sensor-Embedded and Machine Learning Assisted Smart Helmet for Multi-Variable Blunt Force Impact Sensing in Real Time

Early on-site diagnosis of mild traumatic brain injury (mTBI) will provide the best guidance for clinical practice. However, existing methods and sensors cannot provide sufficiently detailed physical information related to the blunt force impact. In the present work, a smart helmet with a single embedded fiber Bragg grating (FBG) sensor is developed, which can monitor complex blunt force impact events in real time under both wired and wireless modes. The transient oscillatory signal "fingerprint" can specifically reflect the impact-caused physical deformation of the local helmet structure. By combination with machine learning algorithms, the unknown transient impact can be recognized quickly and accurately in terms of impact magnitude, direction, and latitude. Optimization of the training dataset was also validated, and the boosted ML models, such as the S-SVM+ and S-IBK+, are able to predict accurately with complex databases. Thus, the ML-FBG smart helmet system developed by this work may become a crucial intervention alternative during a traumatic brain injury event.

Modal interference discrepancy and its application to a modified fiber Mach-Zehnder Vernier interferometer

In this paper, modal interference discrepancy in an all-fiber MZI is theoretically analyzed and experimentally verified. Theoretical analysis demonstrates that ambient refractive index (RI) response of core-cladding modal interference in an all-fiber MZI is blue-shift, while that of cladding-cladding modal interference is red-shift. Temperature response trends of the two kinds of modal interference are uniformly red-shift. The discrepancy is used to fabricate an improved Vernier sensor which is cascaded by two unit MZIs. One MZI is slightly core-offset fused to obtain core-cladding modal interference, and the other is obviously offset fused to get cladding-cladding modal interference. Ambient RI sensitivity of the cascaded sensor is improved with temperature cross-talk restrained. Ambient RI responses of the two unit MZIs are measured to be opposite, which are -54.009 nm/RIU (within RI range of 1.3362∼1.3811) for the slight and 142.581 nm/RIU for the obvious offset unit MZI. While, temperature response trends of them are consistent, which are 0.042 nm/°C for the slight and 0.025 nm/°C for the obvious offset unit MZI, respectively. For the cascaded Vernier sensor ambient RI sensitivity reaches -1788.160 nm/RIU, which is 33.1 and 12.5 folds improved over the two unit MZIs, respectively. Temperature sensitivity of the cascaded sensor is as low as 0.167 nm/°C and only causes a slight RI error of 9.339 × 10^-5 RIU/°C. Due to the simple structure, ease of fabrication, and low temperature cross-talk, the modal interference discrepancy-based Vernier sensor is believed to have potential application prospects in biochemical sensing fields.

Simultaneous Measurement of Refractive Index and Temperature Based on SMF-HCF-FCF-HCF-SMF Fiber Structure

In this research, we proposed and experimentally verified a compact all-fiber sensor that can measure refractive index (RI) and temperature simultaneously. Two segments of hollow-core fiber (HCF) are connected to the two ends of the four-core fiber (FCF) as a beam splitter and a coupler, and then spliced with two sections of single-mode fibers (lead-in and lead-out SMF), respectively. The two hollow-core fibers can excite the higher-order modes of the four-core fiber and recouple the core modes and higher-order modes into the outgoing single-mode fiber, thereby forming inter-mode interference. The different response sensitivities of two interference dips to RI and temperature manifest that the proposed structure can achieve simultaneous measurement. From the experimental results, it can be seen that the maximum sensitivity of the sensor to RI and temperature is 275.30 nm/RIU and 94.4 pm/°C, respectively. When the wavelength resolution is 0.02 nm, the RI and temperature resolutions of the sensor are 7.74 × 10^-5 RIU and 0.335 °C. The proposed dual-parameter optical sensor has the advantages of high sensitivities, good repeatability, simple fabrication, and structure. In addition, it has potential application value in multi-parameter simultaneous measurement.

Highly sensitive soft optical fiber tactile sensor

A soft highly sensitive tactile sensor based on an in-fiber interferometer embedded in polydimethylsiloxane (PDMS) structure is studied. Theoretical simulation obtains that the high order sensing modes and PDMS can improve the sensitivity. Experiments show that different order sensing modes, derived by fast Fourier transform (FFT) and inverse FFT methods, present different sensing performance. Corresponding to high order mode, 1.3593 nm/kPa sensitivity and 37 Pa (0.015 N) detection limit is obtained. Meanwhile, it also shows very good stability, reproducibility, and response time. This study not only demonstrates a tactile sensor with high sensitivity but also provides a novel sensing modes analysis method.

Simulation Study of High Sensitivity Fiber SPR Temperature Sensor with Liquid Filling

In this paper, a high sensitivity fiber temperature sensor based on surface plasmon resonance is designed and studied. In the simulation, the single mode fiber is polished to remove most of the cladding, and then gold and silver films are added. Finally, it is embedded in the heat shrinkable tube filled with a thermo-optic coefficient liquid for curing. The numerical simulation results show that the sensing characteristics are sensitive to the remaining cladding thickness of the fiber, the thickness of the gold film and the thickness of the silver film. When the thermo-optic coefficient of the filling liquid is -2.8 × 10^-4/°C, the thickness of the gold film, the thickness of the silver film and the thickness of the remaining cladding of the fiber are 30 nm, 20 nm and 1 μm, respectively. The sensitivity of the sensor designed in this paper can reach -6 nm/°C; this result is slightly higher than that of similar research in recent years. It will have a promising application prospect in flexible wearable temperature sensors, smart cities and other fields.

Simultaneous measurement of axial strain and lateral stress based on cascaded interference structure

To solve the cross-sensitivity problem in the dual-parameter optical fiber system, a new type of sensor based on cascaded interference structure is proposed without cross-sensitivity. The design consists of a Michelson interferometer and a Sagnac interferometer based on a high-birefringence suspended core fiber segment. After calculating by the analogous Fast Fourier Transformation (FFT) and filtering by FFT filter, the spectrum of the sensor responds linearly to the change of axial strain and lateral stress. The sensitivity to lateral stress is 3.13 nm/(kPa) in the range from 0 to 1200 Pa and the axial strain is 1.846e^-4 (nm·µɛ)^-1 from 0 to 4000 µɛ. The capability of the proposed sensor for dual-parameter sensing is also experimentally demonstrated. The precision rate for dual-parameter sensing is as high as 66.7%, reflecting the sensor's usability for simultaneous measurement of axial strain and lateral stress.

Cost-effective improvement of the performance of AWG-based FBG wavelength interrogation via a cascaded neural network

Fiber Bragg grating (FBG) sensors have been widely applied in various applications, especially for structural health monitoring. Low cost, wide range, and low error are necessary for an excellent performance FBG sensor signal demodulation system. Yet the improvement of performance is commonly accompanied by costly and complex systems. A high-performance, low-cost wavelength interrogation method for FBG sensors was introduced in this paper. The information from the FBG sensor signal was extracted by the array waveguide grating (AWG) and fed into the proposed cascaded neural network. The proposed network was constructed by cascading a convolutional neural network and a residual backpropagation neural network. We demonstrate that our network yields a vastly significant performance improvement in AWG-based wavelength interrogation over that given by other machine learning models and validate it in experiments. The proposed network cost-effectively widens the wavelength interrogation range of the demodulation system and optimizes the wavelength interrogation error substantially, also making the system scalable.

Alkali etched fiber Mach-Zehnder interferometer with compact sensor head

We demonstrate a scheme for fabricating compact fiber Mach-Zehnder interferometer (MZI). A section of Ge-doped fiber (GDF) is sandwiched between two single-mode fibers. The sandwich structure is side polished to make the core of GDF exposed to the surroundings. Alkali solution is utilized to etch the core of GDF. A compact fiber MZI is achieved when about half of the core is etched. Compared with the traditional ones, our scheme for fabricating fiber MZI has the characteristics of low cost, environmentally friendly, and regular transmission spectrum. This fiber MZI not only reduces the consumption of the sample, but also brings forth a good potential for micro-scale detection of refractive index.

In-Fiber Interferometric-Based Sensors: Overview and Recent Advances

In-fiber interferometric-based sensors are a rapidly growing field, as these sensors exhibit many desirable characteristics compared to their regular fiber-optic counterparts and are being implemented in many promising devices. These sensors have the capability to make extremely accurate measurements on a variety of physical or chemical quantities such as refractive index, temperature, pressure, curvature, concentration, etc. This article is a comprehensive overview of the different types of in-fiber interferometric sensors that presents and discusses recent developments in the field. Basic configurations, a brief approach of the operating principle and recent applications are introduced for each interferometric architecture, making it easy to compare them and select the most appropriate one for the application at hand.

Simultaneous measurement of refractive index and temperature or temperature and axial strain based on an inline Mach-Zehnder interferometer with TCF-TF-TCF structure

A refractive index (RI) and temperature or a temperature and axial strain sensor based on an inline Mach-Zehnder interferometer with thin core fiber (TCF)-thin fiber (TF)-TCF structure is proposed and experimentally demonstrated, requiring only the cleaving and fusion splicing methods. The operation principle depends on the effect that the TF cladding modes interfere with the core mode as an optical coupler. The RI, temperature, or axial strain variations can lead to resonance dip variations in the interferometer spectra, and the RI, temperature, or axial strain sensitivity can be measured by monitoring the wavelength shifts of resonance dips. Then we can measure both RI and temperature, or temperature and axial strain through the demodulation matrix. Four sensors with different TF lengths are fabricated based on numerical simulation. A 15 mm long TF sensor displays an RI sensitivity as high as -174.357nm/RIU, temperature sensitivities in the glycerin solution and the air of 12.47 and 26.19 pm/°C, and axial strain sensitivity of -3.43×10^-4nm/µε. Moreover, due to its simple manufacture, high cost-effectiveness and compactness, the proposed sensor has a broad application prospect in physical, chemical, and biological sensing.

Optimization, fabrication, and performance analysis of SMF-/MMF-based microfiber ball structure

In this paper, four kinds of microfiber ball structure of different sizes, such as 330, 340, 350, and 360 µm were fabricated by single-mode fiber and multimode fiber, respectively. To test its performance, the reflected intensity was measured, and the results showed that the microfiber ball with the largest diameter of 360 µm had the maximum reflected intensity. The 360 µm microfiber ball was characterized by scanning electron microscope. This indicates that compared with a small-diameter microfiber ball, large-diameter microspheres are more suitable to be used as follow-up experimental objects, with the diameter of the fiber ball ranging from 330 to 360 µm.

Temperature-insensitive hybrid interferometric liquid refractive index sensor

A temperature-insensitive all fiber Fabry-Pérot (F-P) and Mach-Zehnder (M-Z) hybrid compact structure and its sensing characteristics are proposed and theoretically and experimentally demonstrated. In one sensor, two kinds of sensing principles are existing, which is shown in the sensing process that with the increase in the refractive index (RI) of the liquid, the dip wavelengths coming from the F-P interference do red-drift, and the dips from the M-Z interference do blue-drift, respectively. Due to the opposite shift and almost the same temperature sensitivities, the dip difference between M-Z and F-P refractometers is used to eliminate temperature cross-sensitivity and improve the RI sensitivity of the sensor. In our experiments, the liquid RI sensitivity is 134.383 nm/RIU, and temperature cross-sensitivity is effectively eliminated within the ±21.74 °C change range at room temperature. This sensing structure also has advantages of simple structure, good integration, and low power loss.

Backpropagation neural network assisted concentration prediction of biconical microfiber sensors

The response of the optical microfiber sensor has a big difference due to the slight change in fiber structure, which greatly reduces the reliability of microfiber sensors and limits its practical applications. To avoid the nonlinear influences of microfiber deformation and individual differences on sensing performance, a backpropagation neural network (BPNN) is proposed for concentration prediction based on biconical microfiber (BMF) sensors. Microfiber diameter, cone angle, and relative intensity are the key input parameters for detecting the concentration of chlorophyll-a (from ∼0.03 mg/g to ∼0.10 mg/g). Hundreds of relative intensity-concentration data pairs acquired from 32 BMF sensors are used for the network training. The prediction ability of the model is evaluated by the root-mean-square error (RMSE) and the fitness value (F). The prediction performance of BPNN is compared with the traditional linear-fitting line method. After training, BPNN could adapt to the BMF sensors with different structural parameters and predict the nonlinear response caused by the small structural changes of microfiber. The concentration prediction given by BPNN is much closer to the actual measured value than the one obtained by the linear fitting curve (RMSE 1.84×10^-3 mg/g vs. 4.6×10^-3 mg/g). The numbers of training data and hidden layers of the BPNN are discussed respectively. The prediction results indicate that the one-hidden-layer network trained by more training data provides the best performance (RMSE and fitness values are 1.63×10^-3 mg/g and 97.91%, respectively) in our experiments. With the help of BPNN, the performance of the BMF sensor is acceptable to the geometric deformation and fabrication error of microfiber, which provides an opportunity for the practical application of sensors based on micro/nanofibers.

Simultaneous measurement of temperature and strain based on a hollow core Bragg fiber

A novel, to the best of our knowledge, reflective sensor fabricated by simply sandwiching a homemade hollow core Bragg fiber (HCBF) between two single-mode fibers is proposed and demonstrated for the simultaneous measurement of the temperature and the strain. Different from traditional Fabry-Perot interferometer (FPI) sensors that can achieve only one-parameter sensing with inevitable cross-correspondence to other parameters, the proposed sensor based on the HCBF, which functions as an FPI-inducing FPI spectrum pattern and a weak waveguide confining light-inducing periodic envelope in reflection spectrum, ensures double-parameter sensing. For the HCBF-based reflective sensor, different sensing mechanisms lead to the various sensitivity values of temperature and strain (2.98 pm/°C, 19.4 pm/°C, 2.02 pm/µε, -0.36pm/µε), resulting in a different shift of the confining spectrum envelope and the FPI spectrum fringe. Experimental results indicate that our proposed sensor can measure temperature and strain simultaneously by utilizing a 2×2 matrix. Taking advantage of the compact size, easy fabrication, and low cost, this sensor has an applicable value in harsh environment for simultaneous strain and temperature sensing.

A Refractive Index Sensitive Liquid Level Monitoring Sensor Based on Multimode Interference

According to the beam propagation method, a fiber refractive index-sensitive multimode interference (MMI) structure fabricated by splicing a self-made silica glass rod between two single mode fibers (SMF–NCF (no core fiber)–SMF structure) is proposed for liquid level monitoring. Theoretical and experimental investigation was carried out meticulously using a 4.5 cm and a 9.5 cm long silica glass rod. It is proved that the simple and economical sensor with the shorter length has high sensitivity, satisfactory repeatability, and favorable stability. The sensitivity climbs with the increase in refractive index of the measured liquid, which is 204 pm/mm for pure water, 265.8 pm/mm for 10% glycerin solution, and 352.5 pm/mm for 25% glycerin solution. The proposed sensor can be standardized in certain application circumstances to achieve accurate liquid level monitoring.

Simultaneous Measurement of Temperature and Mechanical Strain Using a Fiber Bragg Grating Sensor

Based on the shift of the Bragg wavelength, fiber Bragg grating (FBG) sensors have been employed to measure a variety of physical parameters such as stress, strain, displacement, temperature, vibration and pressure. In this work, a simple and easy way to be implemented FBG sensing methodology was proposed to measure the temperature and strain simultaneously. Half of the FBG was bonded on the host structure, while the other half of the FBG was left free. The host structure was an aluminum test specimen with dimensions of 20 × 3.8 × 0.5 cm3. As the host structure subjected to mechanical and thermal loadings, the Bragg wavelengths reflected from the bonded and unbonded FBGs are different. Theoretical predictions of the Bragg wavelength shifts of the bonded and unbonded FBGs were presented. Utilizing the Bragg wavelength shift of unbonded FBG, the temperature can be determined and is independent of mechanical strain. The Bragg wavelength shift of the bonded FBG allows the determination of the mechanical strain. The temperature measured by FBG sensor was compared with the result from a thermocouple, while the mechanical strain was validated with the theoretical prediction. Good agreement between the experimental measurement and theoretical prediction demonstrates that temperature-strain discrimination can be realized using the proposed method with one single FBG sensor.

Strain sensitivity enhancement based on periodic deformation in hollow core fiber

Recent progress in optical fiber Mach-Zehnder interferometers (MZIs) has gained many achievements in sensing application. However, the strain sensitivity of optical fiber MZIs is low due to the low elasto-optical coefficient of silica. In this Letter, we propose and demonstrate a method to modulate the guided modes in an MZI based on a special hollow core microstructured optical fiber (HCMOF) by fabricating periodical deformations. Specifically, periodical deformations reduce the extinction ratio of the transmission spectrum. Furthermore, the axial tension modulates these periodical deformations, leading to the enhanced strain sensitivity in comparison to the configuration without deformations. In our experiment, the strain response from 0 to 1000µε is obtained with a sensitivity of 0.00359dB/µε corresponding to an improvement of 13 times compared with a sensor based on same HCMOF without deformations.

Long-Period Gratings and Microcavity In-Line Mach Zehnder Interferometers as Highly Sensitive Optical Fiber Platforms for Bacteria Sensing

Selected optical fiber sensors offer extraordinary sensitivity to changes in external refractive (RI), which make them promising for label-free biosensing. In this work the most sensitive ones, namely long-period gratings working at (DTP-LPG) and micro-cavity in-line Mach-Zehnder interferometers (µIMZI) are discussed for application in bacteria sensing. We describe their working principles and RI sensitivity when operating in water environments, which is as high as 20,000 nm/RIU (Refractive index unit) for DTP-LPGs and 27,000 nm/RIU for µIMZIs. Special attention is paid to the methods to enhance the sensitivity by etching and nano-coatings. While the DTP-LPGs offer a greater interaction length and sensitivity to changes taking place at their surface, the µIMZIs are best suited for investigations of sub-nanoliter and picoliter volumes. The capabilities of both the platforms for bacteria sensing are presented and compared for strains of Escherichia coli, lipopolysaccharide E. coli, outer membrane proteins of E. coli, and Staphylococcus aureus. While DTP-LPGs have been more explored for bacteria detection in 10²–10⁶ Colony Forming Unit (CFU)/mL for S. aureus and 10³–10⁹ CFU/mL for E. coli, the µIMZIs reached 10²–10⁸ CFU/mL for E. coli and have a potential for becoming picoliter bacteria sensors.

Hollow-Core Photonic Crystal Fiber Mach-Zehnder Interferometer for Gas Sensing

A novel and compact interferometric refractive index (RI) point sensor is developed using hollow-core photonic crystal fiber (HC-PCF) and experimentally demonstrated for high sensitivity detection and measurement of pure gases. To construct the device, the sensing element fiber (HC-PCF) was placed between two single-mode fibers with airgaps at each side. Great measurement repeatability was shown in the cyclic test for the detection of various gases. The RI sensitivity of 4629 nm/RIU was demonstrated in the RI range of 1.0000347-1.000436 for the sensor with an HC-PCF length of 3.3 mm. The sensitivity of the proposed Mach-Zehnder interferometer (MZI) sensor increases when the length of the sensing element decreases. It is shown that response and recovery times of the proposed sensor inversely change with the length of HC-PCF. Besides, spatial frequency analysis for a wide range of air-gaps revealed information on the number and power distribution of modes. It is shown that the power is mainly carried by two dominant modes in the proposed structure. The proposed sensors have the potential to improve current technology's ability to detect and quantify pure gases.

Radiation Effects on Long Period Fiber Gratings: A Review

Over the last years, fiber optic sensors have been increasingly applied for applications in environments with a high level of radiation as an alternative to electrical sensors, due to their: high immunity, high multiplexing and long-distance monitoring capability. In order to assess the feasibility of their use, investigations on optical materials and fiber optic sensors have been focusing on their response depending on radiation type, absorbed dose, dose rate, temperature and so on. In this context, this paper presents a comprehensive review of the results achieved over the last twenty years concerning the irradiation of in-fiber Long Period Gratings (LPGs). The topic is approached from the point of view of the optical engineers engaged in the design, development and testing of these devices, by focusing the attention on the fiber type, grating fabrication technique and properties, irradiation parameters and performed analysis. The aim is to provide a detailed review concerning the state of the art and to outline the future research trends.

Thin fiber-based Mach-Zehnder interferometric sensor for measurement of liquid level, refractive index, temperature, and axial strain

An all-fiber Mach-Zehnder interferometric sensor capable of measuring liquid level, refractive index (RI), temperature, and axial strain is proposed and experimentally demonstrated. The proposed sensor is based on a fiber ball-thin fiber (TF)-core-offset structure sandwiched between two standard single-mode fibers. The variations of ambient liquid level, RI, temperature, and axial strain cause the change of phase difference between the cladding modes and the core mode, which leads to the shift of interference spectrum. The wavelength shifts of three resonant dips in the transmission spectrum are used to investigate the sensing characteristics of the sensor. Experimental results show that the sensor with TF length of 20 mm exhibits high RI and liquid-level sensitivities of $ - {131.7092}\;{\rm nm/RIU}$-131.7092nm/RIU and $ - {120.7}\;{\rm pm/mm}$-120.7pm/mm at a wavelength of 1589.5 nm. Meanwhile, the sensor is insensitive to temperature and axial strain, and the maximum sensitivities are 0.0390 nm/°C and $ - {4}.{84}\; \times \;{{10}^{ - 4}}\;{\rm nm}/\unicode{x00B5} \varepsilon $-4.84×10^-4nm/µε, respectively. In addition, the sensor shows superiority in measuring multiple parameters simultaneously.

Investigation of a Bragg Grating-Based Fabry–Perot Structure Inscribed Using Femtosecond Laser Micromachining in an Adiabatic Fiber Taper

This paper presents the fabrication of a fiber Bragg grating (FBG)-based Fabry–Perot (FP) structure (7 mm total length) in an adiabatic fiber taper, investigates its strain and temperature characteristics, and compares the sensing characteristics with a standard polyimide coated FBG sensor. Firstly, a simulation of the said structure is presented, followed by the fabrication of an adiabatic fiber taper having the outer diameter reduced to 70 μ m (core diameter to 4.7 μ m). Next, the sensing structure, composed of two identical uniform FBG spaced apart by a small gap, is directly inscribed point-by-point using infrared femtosecond laser (fs-laser) micromachining. Lastly, the strain and temperature behavior for a range up to 3400 μ ε and 225 ° C, respectively, are investigated for the fabricated sensor and the FBG, and compared. The fabricated sensor attains a higher strain sensitivity (2.32 pm/ μ ε ) than the FBG (0.73 pm/ μ ε ), while both the sensors experience similar sensitivity to temperature (8.85 pm/ ° C). The potential applications of such sensors include continuous health monitoring where precise strain detection is required.

High-performance fiber sensor via Mach-Zehnder interferometer based on immersing exposed-core microstructure fiber in oriented liquid crystals

Rapid technology development and various applications show great demands for high-quality temperature sensors with super-sensitivity, broad working temperature ranges, excellent linearity and high stability. Although tremendous efforts have been dedicated towards developing fiber sensors with high performance, challenges still remain in achieving all of the four parameters. Herein, we fabricate a fiber sensor via a Mach-Zehnder interferometer (MZI) combined with a liquid crystal (LC)-filled microtube, where the LC in the microtube is uniformly orientated. The LCs with uniform orientation treatment play a vital role in the fiber sensor. The feasibility of this sensor was verified by theoretical simulation and demonstrated through experiments. The fabricated LC fiber sensor has super temperature sensitivity of -21.6 nm/°C with a good linearity of 0.976 from 22°C to 31°C, -558.5 nm/°C from 31°C to 32°C, -37.3 nm/°C with a good linearity of 0.999 from 32°C to 34°C and -6.7 nm/°C with a good linearity of 0.999 from 34°C to 110°C, respectively. The sensitivity of the fiber sensor is increased by up to 155 times, compared to the previously reported fiber sensor filled with LC based on the MZI without LC orientation treatment. The fiber sensor with super-sensitivity, broad working temperature range, excellent linearity and high stability provides great potential applications in such as environment monitoring, food detection, medicine, and chemical industry.

All-optical intensity fluctuation magnification using Kerr effect

We present a new all-optical method for the magnification of small-intensity fluctuations using the nonlinear Kerr effect. A fluctuation of interest is impressed onto a sinusoidally modulated optical signals (SMOS) and spectral sidebands are generated as the SMOS experiences self-phase modulation in a nonlinear medium. Magnification of these temporal variation is obtained by filtering one of the sidebands. For small fluctuations, the amount of magnification obtained is proportional to (2m + 1), with m being the sideband order. This technique enhances fiber-based point sensor capabilities by bringing signals originally too small to be detected into the detection range of photodetectors.

Ultrawide temperature range operation of SPR sensor utilizing a depressed double cladding fiber coated with Au-Polydimethylsiloxane

A surface plasmon resonance (SPR) temperature sensor on the basis of depressed double cladding fiber (DDCF) is theoretically proposed and experimentally demonstrated for the first time. Simulation analysis implies that the SPR fiber optic structure consisting of a multimode fiber (MMF) inserted into an 8 mm long DDCF is highly sensitive to the refractive index (RI) of the surrounding environment, owing to their mismatched cores, large discrepancy in cladding diameters, and the depressed inner cladding in DDCF. The experimental results further verify that the highest RI sensitivity is 7002 nm/RIU established with a 50nm Au coated DDCF-SPR sensor. Additionally, the temperature sensitivity reaches up to -2.27 nm/^°C within a wide working temperature range of -30 to 330 ^°C by combining polydimethylsiloxane (PDMS) film as the temperature sensitive material with DDCF-Au architecture. The integrated PDMS, Au and DDCF temperature sensor possesses high performance in terms of sensing capability and physical construction, opening a route to their potential applications in other types of sensors.

Optical fiber temperature sensor based on a Mach-Zehnder interferometer with single-mode-thin-core-single-mode fiber structure

A Mach-Zehnder interferometer for measurement of temperature is proposed and experimentally demonstrated, which consists of two sections of single mode fiber (SMF) and a section of thin core fiber spliced between the two SMFs. The two welding areas are heated and stretched to improve the split and recombination of light. The wavelength of the resonant dip will shift when temperature varies due to the thermo-optic and thermal expansion effect. The experimental results show that a temperature sensitivity of 65 pm/°C with a linear correlation coefficient of 0.996 can be achieved in a temperature range from 25 °C to 80 °C. Due to its ease of manufacture, low cost, and high sensitivity, the fiber optic temperature sensor is suitable for temperature measurement applications.

Optical spectral intensity-based interrogation technique for liquid-level interferometric fiber sensors

In this paper, we propose a new, to the best of our knowledge, technique based on the measurement and analysis of the intensity of the interference pattern as an alternative approach for interrogating liquid-level interferometric fiber sensors. This interrogation is based on calculations that can take into account a vast number of peaks and dips of an interferometric spectrum, allowing the use of such devices as distributed sensors capable of measuring longer-level ranges. Here, liquid-level measurements of up to 120 mm were experimentally obtained with high linearity and a sensitivity of $ - {0.042}\;{\rm dB/mm}$-0.042dB/mm.

Temperature Self-Compensated Refractive Index Sensor Based on Fiber Bragg Grating and the Ellipsoid Structure

In this paper, a temperature self-compensated refractive index sensor based on fiber Bragg grating (FBG) and the ellipsoid structure is demonstrated. The ellipsoid can excite the cladding modes and recouple them into the fiber core. Two well-defined wavelength bands are observed in the reflection spectrum of the proposed sensor, i.e., the Bragg resonant peak and the cladding resonant peaks. By measuring the wavelength shift of the cladding resonant peak, the surrounding refractive index (SRI) can be determined, and the wavelength shift of the Bragg resonant peak can be used as a reliable reference to self-compensate the temperature variation (temperature sensitivity of 10.76 pm/°C). When the SRI changes from 1.3352 to 1.3722, the cladding resonant peak redshifts linearly with an average sensitivity of 352.6 pm/RIU (refractive index unit). When the SRI changes from 1.3722 to 1.4426, an exponential redshift is observed with a maximum sensitivity of 4182.2 pm/RIU. Especially, the sensing performance is not very reliant on the distance between the FBG and the ellipsoid, greatly improving the ease of the fabrication.

High sensitivity micro-fiber Mach-Zehnder interferometric temperature sensors with a high index ring layer

The influence of the high index ring layer (HIRL) in a tapered fiber Mach-Zehnder interferometer (MZI) on the interference observed, and thus on its potential applications in temperature sensing, has been investigated. The MZI was comprised of a tapered Ring Core Fiber (RCF), spliced between two single mode fibers (SMF). Since part of core mode from the SMF was converted into cladding modes in the RCF, due to the mismatch in the cores between the RCF and SMF, the residual power enters and then propagates along the center of the RCF (silica). The difference in phase between the radiation travelling along these different paths is separated by the HIRL to generate an interference effect. Compared with fiber interferometers based on core and cladding mode interference, the thin fiber HIRL is capable of separating the high order cladding modes and the silica core mode, under grazing incident conditions. Therefore, the optical path difference (OPD) and the sensitivity are both substantially improved over what is seen in conventional devices, showing their potential for interferometric temperature sensor applications. The optimum temperature sensitivity obtained was 186.6 pm/°C, which is ∼ 11.7 times higher than has been reported previously.

Simultaneous measurement of refractive index, strain, and temperature based on a Mach-Zehnder interferometer with hybrid structure optical fiber

An optical fiber sensor based on a Mach-Zehnder interferometer with hybrid structure optical fiber for simultaneous measurement of refractive index (RI), strain, and temperature is proposed and demonstrated. The proposed structure is a hybrid structure based on a non-core fiber combined with few-mode fiber. The possibility of simultaneously measuring RI, strain, and temperature relies on the different sensitivity responses of three resonance peaks in the transmission spectrum. Thus, simultaneous measurement of RI, strain, and temperature is realized by calculating the wavelength shift of the three resonance peaks. The experimental results show that the sensitivities of RI are 22.9 pm/RIU, 24.6 pm/RIU, and 97 pm/RIU when RI changes from 1.3707 to 1.39809, respectively. The sensitivities of strain are $ - {3.5}\,\, \text{pm}/ \unicode{x00B5}\unicode{x03B5}$-3.5pm/µε, $ - {1.9}\,\, \text{pm}/ \unicode{x00B5} \unicode{x03B5}$-1.9pm/µε, and $ - {4.1}\,\, \text{pm}/ \unicode{x00B5} \unicode{x03B5}$-4.1pm/µε in the range from 0 to 1400 pm/µ$\unicode{x03B5}$ε. The sensitivities of temperature ranging from 35°C to 55°C are 162 pm/°C, 194 pm/°C, and 162 pm/°C, respectively. The proposed sensor, with advantages of simple configuration, compact structure, and high sensitivity, exhibits great potential in fields of multi-parameter measurement.

Speckle-based strain sensing in multimode fiber

The diversity of spatial modes present within a multimode fiber has been exploited for a wide variety of imaging and sensing applications. Here, we show that this diversity of modes can also be used to perform quantitative strain sensing by measuring the amplitude of the Rayleigh backscattered speckle pattern in a multimode fiber. While most Rayleigh based fiber sensors use single mode fiber, multimode fiber has the potential to provide lower noise due to the higher capture fraction of Rayleigh scattered light, higher non-linear thresholds, and the ability to avoid signal fading by measuring many spatial modes simultaneously. Moreover, while amplitude measuring single mode fiber based Rayleigh sensors cannot provide quantitative strain information, the backscattered speckle pattern formed in a multimode fiber contains enough information to extract a linear strain response. Here, we show that by tracking the evolution of the backscattered speckle pattern, the sensor provides a linear strain response and is immune to signal fading. The sensor has a noise floor of 2.9 pɛ/√Hz, a dynamic range of 74 dB at 1 kHz, and bandwidth of 20 kHz. This work paves the way for a new class of fiber optic sensors with a simplified design and enhanced performance.

Ultra-high sensitivity of dual dispersion turning point taper-based Mach-Zehnder interferometer

We present here a detailed investigation into the sensitivity of the taper-based Mach-Zehnder interferometer as a function of external refractive index, with particular attention to the dispersion turning point (DTP) and possibilities for ultra-sensitive sensors. Our numerical simulation revealed that two DTPs exist with a decrease in the microfiber waist diameter; given this relationship, it is possible to obtain an ultra-sensitive operation. We then conducted experiments with fabricated devices with different waist diameters to achieve both positive and negative sensitivities at two DTPs. In particular, we achieved an ultrahigh refractive index sensitivity of approximately 95,832 nm/RIU at the second DTP when working with a diameter of 1.87 µm around the RI of air. These results show its potential for use in acoustic sensing and biochemical detection.

Monitoring of Carbon Dioxide Using Hollow-Core Photonic Crystal Fiber Mach-Zehnder Interferometer

Monitoring of greenhouse gases is essential to understand the present state and predict the future behavior of greenhouse gas emissions. Carbon dioxide (CO₂) is the greenhouse gas of most immediate concern, because of its high atmospheric concentration and long lifetime. A fiber-optic Mach–Zehnder interferometer (MZI) is proposed and demonstrated for the laboratory-scale monitoring of carbon dioxide concentration. The interferometric sensor was constructed using a small stub of hollow-core photonic crystal fiber between a lead-in and lead-out standard single mode fiber, with air-gaps at both interfaces. At room temperature and atmospheric pressure, the sensor shows the sensitivity of 4.3 pm/% CO₂. The device was packaged to demonstrate the laboratory-scale leakage detection and measurement of CO₂ concentration in both subsurface and aqueous environments. The experimental study of this work reveals the great potential of the fiber-optic approach for environmental monitoring of CO₂.

Multiple hollow-core anti-resonant fiber as a supermodal fiber interferometer

Hollow-core anti-resonant fiber technology has made a rapid progress in low loss broadband transmission, enabled by its much reduced light-material overlap. This unique characteristic has driven emerging of new applications spanning from extreme wavelength generation to beam delivery. The successful demonstrations appear to suggest progression of the technology toward device level development and all-fiberized systems. We investigate this opportunity and report an in-fiber interferometer built in a dual hollow-core anti-resonant fiber. By placing multiple air cores in a single fiber, coherently interacting transverse modes are excited, which becomes a basis of an interferometer. We use this hollow core based inherent supermodal interaction to demonstrate highly sensitive in-fiber interferometer. Unique combination of the air guidance and the supermodal interaction offers robust, simple yet highly sensitive interferometer with suppressed temperature cross-talk that has been an enduring problem in fiber strain sensing applications. The in-fiber interferometer is further investigated as a sensing element for pressure measurement based on an interferometric phase change upon external strain. The interferometer features 39.3 nm/MPa of ultrahigh sensitivity with 0.14 KPa/°C of negligible gas pressure temperature crosstalk. The performance, which is much improved from prior fiber sensors, testifies advances of hollow core fiber technology toward a device level.

High Temperature (Up to 950 °C) Sensor Based on Micro Taper In-Line Fiber Mach–Zehnder Interferometer

A high temperature (up to 950 °C) sensor was proposed and demonstrated based on a micro taper in-line fiber Mach–Zehnder interferometer (MZI) structure. The fiber MZI structure comprises a single mode fiber (SMF) with two micro tapers along its longitudinal direction. An annealing at 1000 °C was applied to the fiber sensor to stabilize the temperature measurement. The experimental results showed that the sensitivity was 0.114 nm/°C and 0.116 nm/°C for the heating and cooling cycles, respectively, and, after two days, the sensor still had a sensitivity of 0.11 nm/°C, showing a good stability of the sensor. A probe-type fiber MZI was designed by cutting the sandwiched SMF, which has good linear temperature responses of 0.113 nm/°C over a large temperature range from 89 to 950 °C. The probe-type fiber MZI temperature sensor was independent to the surrounding refractive index (RI) and immune to strain. The developed sensor has a wide application prospect in the fields of high temperature hot gas flow, as well as oil and gas field development.

Preparation and Characterization of a New Low Refractive Index Ferrofluid

In this research, a new low refractive index ferrofluid is proposed by coating magnetic nanoparticles with a layer of silver, applying the method of modified chemical co-precipitation. This preparation method is green and environmentally friendly without toxic gases being released. Coated nanoparticles are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray spectrometry (EDS), X-ray photoelectron spectroscopy (XPS), and vibration sample magnetometery (VSM). These characterizations show that the silver nanoparticles grow on the surface of magnetic nanoparticles in this new ferrofluid. The hysteresis loop of this new ferrofluid demonstrates that it maintains superparamagnetic properties. A new method of refractive index measurement is applied in this research by employing a long-period grating (LPG) optical fiber sensor. The change value in the refractive index per unit concentration reduces by 16.46% compared to the conventional ferrofluid.

In-Fiber Closed Cavity Interferometric High-Resolution Aqueous Solution and Alcohol Gas Refractometer

An optical fiber interferometric refractometer for alcohol gas concentration and low refractive index (RI) solution (with 1.33–1.38 RI range) measurement is theoretically and experimentally demonstrated. The refractometer is based on a single-mode thin-core single-mode (STS) interferometric structure. By embedding a suitably sized air cavity at the splicing point, high-order cladding modes are successfully excited, which makes the sensor more suitable for low RI solution measurement. The effect of the air cavity’s diameter on the sensitivity of alcohol gas concentration was analyzed experimentally, which proved that RI sensitivity will increase with an enlarged diameter of the air cavity. On this basis, the air cavity is filled with graphene in order to improve the sensitivity of the sensor; and the measured sensitivity of the alcohol gas concentration is −1206.1 pm/%. Finally, the characteristics of the single-cavity structure, graphene-filled structure and double-cavity structure sensors are demonstrated, and the linear RI sensitivities are −54.593 nm/RIU (refractive index unit), −85.561 nm/RIU and 359.77 nm/RIU, respectively. Moreover, these sensor structures have the advantages of being compact and easily prepared.

High Sensitivity Refractometer Based on a Tapered-Single Mode-No Core-Single Mode Fiber Structure

We have proposed a novel tapered-single mode-no core-single mode (TSNS) fiber refractometer based on multimode interference. The TSNS structure exhibits a high contrast ratio (>15 dB) and a uniform interference fringe. The influence of different lengths and diameters of the TSNS on the refractive index unit (RIU) sensitivity was investigated. The experimental investigations indicated a maximum sensitivity of 1517.28 nm/RIU for a refractive index of 1.417 and low-temperature sensitivity (<10 pm/°C). The experimental and simulation results are also in good agreement.

Hydrogen sulfide gas sensor based on copper/graphene oxide coated multi-node thin-core fiber interferometer

A hydrogen sulfide gas sensor based on a copper/graphene oxide (Cu/GO) coated multi-point thin-core fiber Mach-Zehnder interferometer is proposed and experimentally demonstrated. The single-mode fiber (SMF) is sandwiched between the thin-core-fiber-1 (TCF-1) and thin-core-fiber-2 (TCF-2), and the SMF-TCF-1-SMF-TCF-2-SMF Mach-Zehnder interferometer is obtained. In order to detect the concentration of hydrogen sulfide, Cu/GO composite sensitive film was coated on the outside surface of two thin-core fibers. When the composite film absorbs the gases, it leads to a change of the effective refractive index of the cladding and causes the regular shift of dip wavelength. The result indicates that the thickness of the sensitive film is 1.6 μm. With the increase of concentration of hydrogen sulfide, the transmission spectra appear blueshift in the range of 0-60 ppm H₂S. The linearity of 0.9884 and sensitivity of 4.83 pm/ppm are achieved. In addition, the dynamic response time and recovery time of the hydrogen sulfide sensor are about 32 s and 52 s, respectively. This sensor has the advantages of the small size, simple structure, and easy manufacture, and it is suitable for the detection of trace hydrogen sulfide.

Polarization dependence of a graphene-optical fiber hybrid Mach-Zehnder interferometer

Graphene exhibits a particular optical polarization dependence property, which is that supporting optical polarization states of graphene can be readily altered through tuning the polarity of the imaginary part of its conductivity. The in-fiber Mach-Zehnder interferometer (MZI) possesses extremely high sensitivity to the surrounding refractive index through cladding modes. Combining graphene and the in-fiber interferometer, a graphene-optical fiber hybrid MZI is constructed. Depending on the graphene polarization dependence property, the interference wavelength of the graphene-optical fiber hybrid MZI expresses periodic drift with the in-fiber light linear polarization angle adjusting within 180°. Meanwhile, drift periods corresponding to different interference wavelengths are slightly different, which is primarily due to the superposition of the polarization dependence behaviors of different cladding modes. For different light polarization states, with the in-fiber optical power increasing, the interference wavelengths and contrast intensities of the hybrid MZI transmission spectrum show a polarization independent linear blue shift and a nonlinear decrease, respectively.

Automated Chemical Sensing Unit Integration for Parallel Optical Interrogation

We report the integration of an automated chemical optical sensing unit for the parallel interrogation of 12 BICELLs in a sensing chip. The work was accomplished under the European Project Enviguard (FP7-OCEAN-2013-614057) with the aim of demonstrating an optical nano-biosensing unit for the in-situ detection of various chemical pollutants simultaneously in oceanic waters. In this context, we designed an optical sensing chip based on resonant nanopillars (R-NPs) transducers organized in a layout of twelve biophotonic sensing cells (BICELLs). The sensing chip is interrogated in reflection with a 12-channels optical spectrometer equipped with an embedded computer-on-chip performing image processing for the simultaneous acquisition and analysis (resonant mode fitting) of the 12 spectra. A microfluidic chip and an automated flow control system composed of four pumps and a multi-path micro-valve makes it possible to drive different complex protocols. A rack was designed ad-hoc for the integration of all the modules. As a proof of concept, fluids of different refractive index (RI) were flowed in the system in order to measure the time response (sensogram) of the R-NPs under optical reflectance, and assess the sensors’ bulk sensitivity (285.9 ± 16.4 nm/RIU) and Limit of Detection (LoD) (2.95 × 10⁻⁶ RIUS). The real-time response under continuous flow of a sensor chip based on R-NP is showed for the first time, obtaining 12 sensograms simultaneously, featuring the unit as a potential excellent multiplexed detection system. These results indicate the high potential of the developed chemical sensing unit to be used for in-situ, multiplex and automatic optical biosensing.

Compact gas refractometer based on a tapered four-core fiber

A compact in-line interferometer is proposed and experimentally demonstrated for gas refractive index (GRI) measurement. The sensor comprises a tapered four-core fiber (TFCF) sandwiched between two single-mode fibers (SMFs), forming an in-line SMF-TFCF-SMF structure. The fiber taper acts as a bridge between the external GRI variation and the multimode interference within the TFCF segment. A high sensitivity of 1280.94 dB/refractive index unit is obtained in GRI measurement around 1.0. Temperature change only shifts the interference wavelength, and the cross-sensitivity of temperature can be ignored by intensity demodulation. The proposed gas refractometer, with its improved performance, can be a good candidate for chemical sensing or bio-sensing.

Hydrogen sulfide sensor based on tapered fiber sandwiched between two molybdenum disulfide/citric acid composite membrane coated long-period fiber gratings

In this work, a novel hydrogen sulfide detection scheme based on tapered fiber seeded in two long-period fiber gratings (LPGs) coated by a molybdenum disulfide/citric acid composite membrane is proposed and fabricated. The input light of a broadband source is coupled twice by passing through two LPGs with identical parameters, from which a Mach-Zehnder interferometer can be formed. The composite sensitive membrane was prepared with molybdenum disulfide and citric acid, which was coated on the surface of the two LPGs. The experimental results show that in the range of 0-70 ppm of hydrogen sulfide, with the increase of gas concentrations the interference spectra appear to blueshift. In addition, a high sensitivity of 16.65 pm/ppm, an excellent linear relationship (R²=0.97721), and high selectivity for hydrogen sulfide are achieved. The effect of temperature is also discussed. The sensor has the advantages of low cost and small volume, and can be used for detection applications at sites where hydrogen sulfide is produced, such as natural gas plants, areas of magmatic activity, coal mines, etc.

Dual-wavelength intensity-modulated Fabry-Perot refractive index sensor driven by temperature fluctuation

A refractive index sensor based on an in-line Fabry-Perot interferometer is proposed and experimentally demonstrated. Two lasers are combined and injected into the sensor head. The power responses of two wavelengths are measured by a dual-channel optical power meter simultaneously. The two reflected power signals distribute along an ellipse. The refractive index of the liquid is calculated from the half length of the longer axes of the fitted ellipse. The refractive index sensing system is demonstrated to measure the refractive index of the salt solutions with different concentrations. The demodulated results matched well with the refractive index measured by the Abbe refractometer, and a resolution of 0.0017 was obtained. Since the temperature is eliminated during the ellipse fitting, the measuring result is insensitive to the temperature fluctuation. The proposed refractive index sensing sensor has outstanding advantages, such as low demodulation cost, simple fabrication, easy cleaning, and good mechanical strength, and will be of importance in biological detection, chemical analysis, and water pollution monitoring.

Liquid-level sensing based on a hollow core Bragg fiber

A novel optical fiber liquid level sensor based on a hollow core Bragg fiber (HCBF) was proposed and demonstrated. The HCBF was first designed and successfully fabricated with periodic transmission band in the spectrum and a transmission loss of ~3.48 dB/cm. An inline optical fiber liquid-level sensor was fabricated by simply sandwiching a piece of HCBF between two single mode fibers. The sensing performance was experimentally tested. A linear liquid-level sensitivity of ~1.1 dB/mm, and fast response time less than 3s was obtained by the intensity demodulation measurement. The temperature and refractive index cross-sensitivities were also investigated. The experimental results indicate that our proposed structure has tiny temperature and RI dependence, which makes it a promising liquid level sensing platform for different liquids.