Fiber End Face Research Articles

High-Temperature Fiber-Optic Fabry–Perot Vibration Sensor Based on Single-Crystal Sapphire

In this paper, a fiber-optic Fabry–Perot (F–P) vibration sensor that can work at 800 °C is proposed. The F–P interferometer is composed of an upper surface of inertial mass placed parallel to the end face of the optical fiber. The sensor was prepared by ultraviolet-laser ablation and three-layer direct-bonding technology. Theoretically, the sensor has a sensitivity of 0.883 nm/g and a resonant frequency of 20.911 kHz. The experimental results show that the sensitivity of the sensor is 0.876 nm/g in the range of 2 g to 20 g at an operating frequency of 200 Hz at 20 °C. The nonlinearity was evaluated from 20 °C to 800 °C with a nonlinear error of 0.87%. In addition, the z-axis sensitivity of the sensor was 25 times higher than that of the x-axis and y-axis. The vibration sensor will have wide high-temperature engineering-application prospects.

Ampere force fiber optic magnetic field sensor using a Fabry-Perot interferometer.

The paper presents a novel fiber-optic vector magnetic field sensor using a Fabry-Perot interferometer, which consists of an optical fiber end face and a graphene/Au membrane suspended on the ceramic ferrule end face. A pair of gold electrodes are fabricated on the ceramic ferrule by femtosecond laser to transmit electrical current to the membrane. Ampere force is generated when an electrical current flows through the membrane in a perpendicular magnetic field. The change in Ampere force causes a shift in the resonance wavelength in the spectrum. In the magnetic field intensity range of 0 ∼ 180 mT and 0 ∼ -180 mT, the as-fabricated sensor exhibits magnetic field sensitivity of 5.71 pm/mT and 8.07 pm/mT. The proposed sensor has great potential application in weak magnetic field measurements due to its compact structure, cost-effectiveness, ease to manufacture, and good sensing performance.

Self‐assembly of plasmonic nanoparticles on optical fiber end face

AbstractDue to low losses, optical fibers are excellent optical waveguides, but manipulating the wavefront below the diffraction limit while keeping fabrication costs down is a significant challenge. Top‐down lithographic methods can create arbitrary nanostructures on the fiber end face to manipulate the wavefront. Still, this method requires a flat fiber end face, which can only be made using elaborate preparation processes. We present a facile coating method in which we transfer a hexagonally packed monolayer of gold nanoparticles onto an untreated fiber end face. Using a poly(N‐isopropylacrylamide) particle coating, we could transfer the free‐floating monolayer from a water‐air interface to the fiber end face. Our self‐assembly method enables plasmonic gratings on rough surfaces and objects with large aspect ratios, which have been challenging for existing nanofabrication methods. Using electromagnetic simulation, we demonstrate the performance and utility of the concept as a refractive index sensor in which we consider different lattice constants. Our simulations cover possible analyses by calculating the structure under air, water, and polymer environments. Thus, we study the potential applications of low‐cost fiber‐based sensors with low optical losses.

Fiber-optic hydrophone for detection of high-intensity ultrasound waves.

Fiber-optic hydrophones (FOHs) are widely used to detect high-intensity focused ultrasound (HIFU) fields. The most common type consists of an uncoated single-mode fiber with a perpendicularly cleaved end face. The main disadvantage of these hydrophones is their low signal-to-noise ratio (SNR). To increase the SNR, signal averaging is performed, but the associated increased acquisition times hinder ultrasound field scans. In this study, with a view to increasing SNR while withstanding HIFU pressures, the bare FOH paradigm is extended to include a partially reflective coating on the fiber end face. Here, a numerical model based on the general transfer-matrix method was implemented. Based on the simulation results, a single-layer, 172 nm TiO2-coated FOH was fabricated. The frequency range of the hydrophone was verified from 1 to 30 MHz. The SNR of the acoustic measurement with the coated sensor was 21 dB higher than that of the uncoated one. The coated sensor successfully withstood a peak positive pressure of 35 MPa for 6000 pulses.

Ultracompact optical fiber high‐temperature sensor based on the angled fiber end face

AbstractAn ultracompact optical fiber Fabry–Perot interferometric sensor is proposed and demonstrated for high‐temperature measurement. The device is simply a section of single‐mode fiber with angled end face of 45°, the reflected light from the angled fiber end face is directed sequentially to the fiber core–cladding and cladding–air interfaces, respectively, and reflected back, and finally returned to the fiber core after being reflected by the angled fiber end face again. As a result, a Fabry–Perot interferometer is constructed. Because of the thermal‐expansion and thermal‐optical effect of the fiber material, the optical path difference between the core–cladding and cladding–air surfaces is changed with temperature, hence the device is well suitable for temperature measurement. In particular, the fiber device has good high‐temperature sustainability up to 1100°C, which makes it attractive for high‐temperature measurement in a narrow or hard‐to‐reach spatial environment.

A Fabry-Perot Fiber-Optic Interferometer for Highly Sensitive CO2 Detection Based on the Nanoparticles Self-Assembly Technology

The highly sensitive CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> detective instrument is intensively studied recently for the atmosphere protection. Fiber-optic interferometer is widely used as a gas sensor because its interference wavelength can be influenced by the environmental refractive index. In this work, by adopting the nanoparticles self-assembly technology, a spherical optical device with a primary pore width of 8.13 nm and a specific surface area as large as 74.58 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /g is fabricated. For gas detection, the larger the specific surface area of the device, the more gas molecules being adsorbed, resulting in a higher sensitivity. A Fabry-Perot fiber-optic interferometer is designed by mounting the self-assembled device on a fiber end face. In the experiment, it shows a fast response time of 21 s, a high wavelength sensitivity of 200 pm/%, and a wide detection range from 350 ppm to 36% for CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas. It also performs a high selectivity, good repeatability, and immunity to the gas flow rate. Such a Fabry-Perot interferometer, as a standard lab-on-fiber device, will have a significant application potential for trace CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> monitoring in the field of carbon capture, utilization, and storage (CCUS).

Miniaturized High-Sensitivity Temperature Sensor Based on Cascaded Fiber-Optic FPI

In letter, a cascaded fiber-optic Fabry-Perot interferometers temperature sensor with high-sensitivity and small-dimension is proposed and demonstrated. The sensor is fabricated by filling the polydimethylsiloxane (PDMS) and air between two single-mode fiber end-faces located in a section of silica capillary, thus a PDMS-based glue cavity, an air cavity and combined PDMS-air cavity are constructed. Due to the similar optical length between PDMS glue cavity and PDMS-air cavity, the conditions for Vernier effect can be easily satisfied in the sensor fabrication. The temperature sensitivity of proposed sensor is effectively improved in experiment. The research results show that the sensitivity of sensor is enhanced from 2.62 nm/°C to 23.22 nm/°C, which is about 8.86 times than that of the single PDMS-based glue cavity, and the linearity of temperature-dependent wavelength shifts is superior to 0.99 in temperature range of 32~50 °C. The sensor has advantages of high sensitivity, simplicity and easy to fabrication, and the small-dimension merit make it more suitable for point-temperature measurement.

Temperature-insensitive optical fiber strain sensor fabricated by two parallel connection Fabry–Perot interferometers with air-bubbles

A strain sensor formed by a parallel connection of two Fabry–Perot interferometers (FPI) is proposed. The femtosecond laser is used to process a micro groove on the end face of a single-mode fiber (SMF), and then, it is welded with another SMF to form a small air bubble at the fusion point, fabricating an FPI. When the axial strain acts on the air bubble, the transverse length of the air bubble will change, causing the air cavity of the FPI to be easily deformed, and FPI can obtain high strain sensitivity. Three FPIs were manufactured with the air bubble sizes of 63, 78, and 93 µm, respectively, and the strain sensitivities of the three FPIs are 2.9, 2.0, and 1.5 pm/µε, respectively. The experimental results show that the smaller the air bubble, the higher the strain sensitivity of FPI. Since the free spectral ranges of the three FPIs are relatively similar, we, respectively, paralleled them to form two Vernier effect strain sensors, and their sensitivities are −14.9 and −14.5 pm/µε, respectively. Their sensitivities are increased by 5.1 times and 7.3 times, respectively. In addition, because three FPIs are composed of air cavities, they have very low temperature sensitivities. When they are connected in parallel, their resonance peak wavelength moves in the same direction with an increase in temperature, forming a reduced Vernier effect, and the temperature sensitivity amplification is very small. Therefore, the temperature cross-sensitivity of the sensor is extremely low and can be ignored.

Flexible and Speedy Preparation of Large-Scale Polystyrene Monolayer through Hemispherical-Depression-Assisted Self-Assembling and Vertical Lifting

Liquid interfacial self-assembled two-dimensional nanomonolayers are promising as colloidal crystal templates or lithographic masks for the mass production of periodic nanoarrays. However, the interfacial self-assembled monolayers usually suffer various defects such as grain boundaries, line defects, vacancy defects, and multilayers due to imperfect process technology. Here, we demonstrated a series of optimized strategies including the hemispherical-depression-assisted self-assembly, dot coating approach, and vertical lifting transfer protocol to eliminate surface defects especially multilayers jointly. After stabilizing the hemispherical depression height to 1.5 mm with a 40° tilted silicon slide, the self-assembly time was significantly shortened to 10 min for a 1 × 2 cm2 area of PS monolayer. The series of strategies were successfully implemented not only on glass and silicon flat substrates but also on the side and end face of optical fiber, even on flexible substrates, while maintaining great morphologies. Besides, in association with nanospherical lens photolithography and nanosphere lithography technologies, various two-dimensional nanoarrays such as nanotriangles and nanoholes could be successfully obtained for subsequent optical properties research, which present broad application prospects in biochemical sensing, medicine, environmental protection, and other fields.

High speed and broadband fiber-integrated WS2/Bi2O2Se avalanche photodetector

Photodetectors based on two-dimensional(2D) materials have attracted tremendous attention in recent years because of their potential for development in compact and integrated devices. Owing to the high carrier mobility and air stability, the newly discovered 2D bismuth oxyselenide (Bi2O2Se) exhibits outstanding optoelectronic properties. However, the high dark current inevitably restricts the performance of the Bi2O2Se-based photodetectors. Here, we propose and demonstrate a fiber-integrated photodetector with a low dark current of ∼10 −10 A by assembling a WS2/Bi2O2Se Van der Waals heterostructure onto the end-face of a standard optical fiber. Benefiting from the narrow bandgap of the 2D materials, the fiber-integrated photodetector exhibits a broadband detection range from 650 to 1630 nm along with a responsivity of 1.13 AW−1. The fast rise time is measured as ∼410μs, while the fall time is ∼600μs. Taking advantage of the built-in potential in the WS2/Bi2O2Se heterostructure, the photodetector can work on self-powered mode. Applied a certainly high reverse voltage, the WS2/Bi2O2Se fiber-integrated photodetector can operate at avalanche breakdown region with an external quantum efficiency of 141% and an avalanche gain of 44 under the input power of 56.7μW. Finally, the fiber-integrated photodetector is applied to measure the wrist bending owing to its high photoresponsivity. These findings indicate that our proposed device has intriguing capabilities in terms of photodetection and on-line power monitoring, which enable our proposed device to have superb application potential in a fiber-integrated multi-function system.

Dual-parameter optical fiber sensor for temperature and humidity based on PMMA-microsphere and FBG composite structure

In this work, we proposed and experimentally demonstrated an optical fiber temperature and humidity sensor composed of a polymethyl methacrylate (PMMA)-microsphere and a fiber Bragg grating (FBG), and realize monitoring the environment temperature and humidity simultaneously. The experimental results show that when the relative humidity (RH) change from 35% to 85%, the response sensitivity of the Fabry-Pérot (F-P), which is formed between PMMA-microsphere and the fiber end-face, is about 127 pm/%RH. However, the FBG in the structure is not sensitive to humidity. When temperature changes from 33 °C to 58 °C, the response sensitivity of the F-P and FBG are about 45.68 pm/°C and 10 pm/°C, respectively. When temperature and humidity change simultaneously, these two-parameters can be monitored simultaneously by using the dual-parameter measurement matrix method. The optical fiber dual-parameter sensor offers numerous advantages, such as low cost, high sensitivity, and easy fabrication.

3D printed fiber-optic nanomechanical bioprobe

Ultrasensitive nanomechanical instruments, e.g. atomic force microscopy (AFM), can be used to perform delicate biomechanical measurements and reveal the complex mechanical environment of biological processes. However, these instruments are limited because of their size and complex feedback system. In this study, we demonstrate a miniature fiber optical nanomechanical probe (FONP) that can be used to detect the mechanical properties of single cells and in vivo tissue measurements. A FONP that can operate in air and in liquids was developed by programming a microcantilever probe on the end face of a single-mode fiber using femtosecond laser two-photon polymerization nanolithography. To realize stiffness matching of the FONP and sample, a strategy of customizing the microcantilever’s spring constant according to the sample was proposed based on structure-correlated mechanics. As a proof-of concept, three FONPs with spring constants varying from 0.421 N m−1 to 52.6 N m−1 by more than two orders of magnitude were prepared. The highest microforce sensitivity was 54.5 nm μN−1 and the detection limit was 2.1 nN. The Young’s modulus of heterogeneous soft materials, such as polydimethylsiloxane, muscle tissue of living mice, onion cells, and MCF-7 cells, were successfully measured, which validating the broad applicability of this method. Our strategy provides a universal protocol for directly programming fiber-optic AFMs. Moreover, this method has no special requirements for the size and shape of living biological samples, which is infeasible when using commercial AFMs. FONP has made substantial progress in realizing basic biological discoveries, which may create new biomedical applications that cannot be realized by current AFMs.

Multilayer all-polymer metasurface stacked on optical fiber via sequential micro-punching process

Abstract Metasurface technology is revolutionizing the field of optics and pursuing expanded functions via technical developments, such as the integration of multiple metasurfaces with optical fibers. Despite several attempts to realize metasurface-on-fiber platforms, negligible fiber-facet areas pose a serious obstacle to efficient and precise fabrication. Herein, we demonstrate a novel sequential micro-punching process that enables rapid and precise stacking of multiple polymer metasurfaces on the end face of a single-mode optical fiber. Mesh-type nanohole metasurfaces are fabricated on a 1.8-μm-thick polymethyl methacrylate (PMMA) layer via e-beam lithography, and the PMMA layer is separated from the substrate and prepared in the form of a membrane using the external frame. Furthermore, the PMMA metasurfaces are sequentially punched through the fiber and stacked on top. Employing a micro-punching process, we demonstrate highly efficient all-polymer metalenses and orbital angular momentum (OAM) metasurfaces coupled with single-mode fibers operating in the telecommunication band. A 1550 nm laser beam passing through three metalens layers stacked on the fiber is focused at a distance of 135 μm with 83% efficiency. In addition, the 1550 nm beam passing through three OAM metasurfaces on the fiber is converted into a perfect vortex beam with a topological charge of 3. We believe that our proposed micro-punching process will cause a breakthrough in the fabrication of metasurface-integrated optical fibers that will be utilized in a wide range of applications.

High-Performance Optical Fiber Fabry–Perot Sensor Based on a Sawtooth Spectrum and Its Application in Acoustic Sensing

An optical fiber sensor based on Fabry–Perot (F–P) resonance with sawtooth spectral shape can break the mutual constraint of sensitivity and response range through simultaneously demodulating intensity variation of lasers working on the two asymmetric spectral lines. Here, theoretical solution and practical approach for obtaining the F–P resonance with a sawtooth spectrum have been proposed. The linear range and slope of the asymmetric spectrum are first calculated based on the proposed theoretical model, which regulates the spectral shape variation. Then, the composite film on fiber end face has been solved to fulfill the model parameters required for the sawtooth spectrum through reversely utilizing an equivalent interface approach, which has been theoretically and experimentally demonstrated by using a composite film of platinum and germanium (Pt/Ge) as the first reflective mirror and gold film as the second mirror. Finally, an optical fiber F–P sensor with sawtooth spectral characteristics is prepared for underwater acoustic sensing application, which demonstrates an average of 10-dB improvement in dynamic range at the frequency range of 20–800 Hz.

Highly Sensitive Fiber-Optic Humidity-Sensing System by Using GO-FPI-Based Microwave Photonic Filter

In this article, a microwave photonic filter (MPF)-based fiber-optic humidity sensor is proposed and experimentally demonstrated. An Fabry-Pérot (FP) cavity using a single-mode fiber (SMF) end face and a graphene oxide (GO) film is fabricated as the sensing head, and by tracing the central frequency of the MPF’s passband, the humidity can be measured. The sensitivity of the sensor can be tailored by fabricating GO films with different thicknesses. The experimental results indicate that the thinner the GO film, the higher the sensitivity, and the sensitivity can reach 1.31 MHz/%RH when the thickness of the GO film is 116.3 nm. The response time is measured to be 40 ms, and the temperature cross-sensitivity can be as low as 0.0035%RH/°C. The proposed sensing system enjoys the advantages of high sensitivity, rapid response, good stability, low cross-sensitivity to temperature, low cost, and straightforward fabrication.

Temperature response of a fiber end-face integrated lithium niobate crystal probe based on pyroelectric and thermo-optic effects via polarization state modulation

This paper presents the temperature response in a fiber end-face integrated lithium niobate (LN) crystal probe. It is a reflective probe. The fiber end-face integrated with components such as a focusing lens, a quarter waveplate, a cube of LN crystal and a gold mirror. Thanks to the pyroelectric and thermo-optic effects of the LN, the temperature response with a steep change in a narrow range of temperature is observed and it might be exploited in temperature measurement applications. The ambient temperature is mapped to the output light via polarization state modulation (PSM) method. Experimentally, the change of exit beam polarization is measured as the variation of the S1. The experimental results show that the variation of the S1 per unit interaction length which is the sensitivity reaches 5.37 K−1cm−1 in the temperature range of 309.95 K to 316.15 K. It is 447 times higher than that in a polarization-maintaining fiber (PMF). The temperature accuracy is 0.86 K in the range of 311.15 K to 316.15 K which is estimated via the standard deviation between the experimental data and fitting cure. In terms of size, the sensitive material lithium niobate cube has a side length of 2 mm and the total length including all the assembled optical components is 1.7 cm. Due to the compact size and optic signal transmission, the probe might be used in biological domain such as measuring core temperature and in industrial fields such as remote monitoring.

Luminescent Polymer Composites for Optical Fiber Sensors.

Optical fiber sensors incorporating luminescent materials are useful for detecting physical parameters and biochemical species. Fluorescent materials integrated on the tips of optical fibers, for example, provide a means to perform fluorescence thermometry while monitoring the intensity or the spectral variations of the fluorescence signal. Similarly, certain molecules can be tracked by monitoring their characteristic emission in the UV wavelength range. A key element for these sensing approaches is the luminescent composite, which may be obtained upon allocating luminescent nanomaterials in glass or polymer hosts. In this work, we explore the fluorescence features of two composites incorporating lanthanide-doped fluorescent powders using polydimethylsiloxane (PDMS) as a host. The composites are obtained by a simple mixing procedure and can be subsequently deposited onto the end faces of optical fibers via dip coating or molding. Whereas one of the composites has shown to be useful for the fabrication of fiber optic temperature sensors, the other shows promising result for detection of UV radiation. The performance of both composites is first evaluated for the fabrication of membranes by examining features such as fluorescent stability. We further explore the influence of parameters such as particle concentration and density on the fluorescence features of the polymer blends. Finally, we demonstrate the incorporation of these PDMS fluorescent composites onto optical fibers and evaluate their sensing capabilities.

Constructing the Au nanoparticle multimer on optical fiber end face to enhance the signal of localized surface plasmon resonance biosensors: A case study for deoxynivalenol detection

Localized surface plasmon resonance (LSPR) based optical fiber end-face biosensors have the advantages of convenience, fast, accuracy, and high sensitivity. Because the sensor area (optical fiber end face)) is small, how to improve its signal strength is a challenge faced by researchers. In this study, an Au nanoparticle (AuNP) multimer structure was designed to enhance the LSPR signal by the hot spots effect produced by the AuNP multimers. The fiber end face was coupled with the first batch of AuNPs, on which the second batch of AuNPs was coupled using 3-aminopropyl triethoxysilane as a spacer, then deoxynivalenol (DON) aptamer was coupled on to complete the biosensor. Also, the preparation parameters were optimized. The results showed that this biosensor with the AuNP multimers had a higher signal strength, its LSPR absorbance was 2.1 fold of that of the biosensor with AuNPs. It had a high performance for DON detection, furthermore, the LSPR peak shift value and LOD of this developed biosensor was 3.4 fold and 1/8 of that of LSPR the biosensor with AuNPs. Therefore, it is a good strategy to enhance the signal of the optical fiber LSPR biosensors by constructing the AuNP multimers on the fiber end face.

Multiplexed optical fiber tip refractive index sensor interrogated by microwave photonics

A multiplexed and spatially-resolved optical fiber tip sensor is proposed and demonstrated for multi-point refractive index (RI) sensing based on a microwave-photonic interrogation technique. The RI sensor is simply based on an array of cleaved end facets of optical fibers with different lengths fabricated from off-the-shelf optical fiber couplers. An auxiliary optical fiber endface that serves as the reference reflector is also introduced into the system. As a result, each of the sensing reflectors from the sensor array, together with the reference reflector, forms a Michelson interferometer (MI). Interrogated by a microwave-photonic system operating at an incoherent regime, the interferogram for each of the MIs in the microwave domain can be unambiguously reconstructed. The spatial location and ambient RI of the sensing reflector can be determined based on the free spectral range and magnitude at the resonance of the microwave interferogram. An analytical model is first developed to understand the physics of the sensor, followed by a proof-of-concept demonstration. Two sensing reflectors and a reference reflector are multiplexed in the prototype system, showing high RI sensitivity and low temperature crosstalk. The unique features of the sensor, such as ease of fabrication, low cost, high sensitivity, low temperature dependence, multiplexing and spatially-resolved capabilities, make the sensor a strong candidate for biochemical and environmental sensing applications.

Thermal diffusion coupling mechanism and its application of discrete waveguide

For discrete optical systems integrated into optical fibers, the optical fields of the individual waveguides are coupled and correlated with each other. This paper studies how to adjust the refractive index of discrete waveguides by thermal diffusion, so as to enhance the coupling between discrete waveguides, and also constructs the discrete waveguide thermally diffused model and the thermally diffused coupling model of twin-core and three-core fibers. The multicore fiber is heated different times by a hydrogen-oxygen flame, and the outgoing light field at the end face of the optical fiber is monitored at the same time. Then, the three-dimensional refractive index measurement results of the thermally diffused multicore fiber verify the feasibility of thermal diffusion technology to change the refractive index of discrete waveguides for coupling. Thermal diffusion technology can be used to fabricate multicore fiber couplers. By combining multicore fiber and core-by-core inscribed fiber Bragg grating technology and by using thermal diffusion technology, the single-channel sensing measurement can be realized. The method of changing the refractive index of discrete waveguides through thermal diffusion has the advantages of high integration, high stability, and mass fabrication. The research on the thermal diffusion of discrete waveguides can improve the application potential of multicore fiber sensing systems, and promote the broad application of discrete waveguide structure optical fiber in the fields of optical communication, optical sensing, biomedicine, and artificial intelligence.