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Optical fiber microresonators have attracted considerable interest for acoustic detection because of their compact size and high optical quality. Here, we have proposed, designed, and fabricated a spring-based Fabry–Pérot cavity microresonator for highly sensitive acoustic detection. We observed two resonator vibration modes: one relating to the spring vibration state and the other determined by the point-clamped circular plate vibration mode. We found that the vibration modes can be coupled and optimized by changing the structure size. The proposed resonator is directly 3D printed on an optical fiber tip through two-photon polymerization and is used for acoustic detection and imaging. The experiments show that the device exhibits a high sensitivity and low noise equivalent acoustic signal level of 2.39 mPa/Hz1/2 at 75 kHz that can detect weak acoustic waves, which can be used for underwater object imaging. The results demonstrate that the proposed work has great potential in acoustic detection and biomedical imaging applications.

The all-optical fiber-based intelligent sensing system is one key technology for acoustic/ultrasonic structural health monitoring. Damages such as cracking or impact loading in civil, aerospace, and mechanical structures can generate transient ultrasonic waves, which can reveal the structural health condition. Hence, there is a great need to develop a high precision adaptive sensor for large-value strain signals with large frequency range that can extent to several hundred kilohertz in ultrasonic/acoustic sensing. In this work, we explore an intelligent system based on a fiber Bragg grating (FBG) and an erbium-doped fiber amplifier (EDFA), composing as a fiber cavity that offers significant advantages and higher performance in ultrasonic/acoustic sensing applications. The ASE light emitted from the EDFA and reflected by a FBG is amplified in the fiber cavity and coupled out by a 90:10 coupler, which is demodulated by an unbalanced Mach-Zehnder interferometer (MZI) composed by a 2×2 coupler and a 3×3 coupler. As the reflective spectrum of the FBG sensor changes due to excited acoustic waves, the shift of the laser output wavelength is subsequently converted into a corresponding phase change. We theoretically and experimentally calculate the three output signals using a differential cross-multiplication (DCM) algorithm to directly demodulate the wavelength shift of the FBG sensor. The experimental results demonstrate that the proposed FBG acoustic sensing system has high sensitivity and can respond the ultrasonic waves into the hundreds of kilohertz frequency range, which shows a potential for acoustic emission detection in practical applications.

An optofluidic polymer Mach-Zehnder interferometer (MZI) fabricated via direct laser writing (DLW) on a fiber tip is proposed and experimentally demonstrated as a refractometer where one of the interference arms is made of the polymer waveguide while the other arm is a hollow cavity with a 1- $\mu \text{m}$ wall thickness and containing two open channels. The analytes can be filled through the channels, forming an optofluidic component of the interferometer. In this letter, two MZI microstructures with different hollow lengths are described and characterized. The experimental results of the two MZIs in air and water show that the MZI with a longer hollow cavity has higher sensitivity. Analytes such as de-ionized (DI) water and glucose solutions with different concentrations have been measured in the MZI with the long hollow cavity. The results show that this kind of sensing device provides a promising platform for refractive index measurements with a sensitivity of 593.75 nm/RIU based on the FSR calculation method.

Optical vortex is a type of structured beam with helical phase wavefronts that carry orbital-angular momentum (OAM) with different topological charge numbers. Optical vortex beams are of use in a wide range of areas ranging from communications to biomedical applications. In this letter, we demonstrate that microscale polymer spiral phase plates (SPPs) with different topological charges can be fabricated through two-photon polymerization by femtosecond 3D direct laser writing. These micro-optical SPPs can modify input light beams carrying no OAM into optical vortex beams carrying an OAM of $\text{l}\boldsymbol \hbar $ per photon. Intensity patterns and interferograms of the vortex beams are experimentally obtained, showing good agreement with numerical simulations. The two-photon direct laser writing technique with a high resolution of 100 nm enables easy fabrication of micro-optic devices such as SPPs with broad applications in mesoscale photonic devices.

Aim at nondestructively characterizing the interfacial roughness of coatings, ultrasonic reflection coefficient phase spectrum (URCPS) as a function of interfacial roughness is derived based on the phase screen approximation theory [ 1 ]. For inhomogeneous coatings, the attenuation coefficient α(f) shows a non-negligible effect on the URCPS. The relationship of α(f) on frequency f is used to decouple the URCPS. The constructed URCPS is used to determine the interfacial roughness and thickness of specimens through a two-parameter inversion utilizing the cross-correlation algorithm. The effects of the coating inhomogeneity (such as porosity) on the roughness measurement are analyzed through numerical calculation. A series of simulations with interfacial roughness from 6.2 to 12.7 μm indicate that measurement errors of the thickness are all less than 8.0%. The relative errors of the measured roughness of models without porosity and models with the porosity of 3% are less than 11.8%. For the models with porosity 5%, when the roughness is larger than 10.3 μm, the relative error is still larger than 11.7%. Ultrasonic experiments were carried out on a tungsten carbide (WC Ni) coating utilizing water immersion, flat transducer. The WC-Ni coating, with unknown interfacial roughness, was sprayed on a stainless steel using the high-velocity oxygen fuel (HVOF) method. Experimental results show that the interfacial roughness of specimen obtained by the proposed ultrasonic measurement are in good agreement with that of SEM observations, the absolute error of the measured roughness is less than 1.4 μm and the relative error was less than 11.0%.

This paper describes a self-regulating system that combines wrinkle-patterned hydrogels with plasmonic nanoparticle (NP) lattices. In the feedback loop, the wrinkle patterns flatten in response to moisture, which then allows light to reach the NP lattice on the bottom layer. Upon light absorption, the NP lattice produces a photothermal effect that dries the hydrogel, and the system then returns to the initial wrinkled configuration. The timescale of this regulatory cycle can be programmed by tuning the degree of photothermal heating by NP size and substrate material. Time-dependent finite element analysis reveals the thermal and mechanical mechanisms of wrinkle formation. This self-regulating system couples morphological, optical, and thermo-mechanical properties of different materials components and offers promising design principles for future smart systems.

A distributed feedback (DFB) fiber laser and fiber Bragg gratings (FBGs) are configured to demodulate the wavelength shifts of FBG dynamic strain sensors. The FBG sensors act as sensing units to detect the dynamic strain and the demodulators while the DFB fiber laser only acts as a narrow-linewidth light source. As the reflective spectrum of the FBG sensor changes due to dynamic strains, the output is subsequently converted into a corresponding intensity change and detected directly by a photodetector. The 0.2 nm linewidth FBG sensor can detect the impact signal with a frequency of up to 300 kHz with a maximum of 29.17 µɛ, which is comparable with the detecting result of the piezoelectric transducer sensor. Moreover, the directional response of the FBG sensor is maximized when the direction of acoustic wave propagation is parallel to the optical fiber. The relation between the sensitivity and the FBG spectrum linewidth is presented, and the detectable strain range versus different FBG linewidths is also discussed.

An inverse-designed metalens is proposed, designed, and fabricated on an optical fiber tip via a 3D direct laser-writing technique through two-photon polymerization. A computational inverse-design method based on an objective-first algorithm was used to design a thin circular grating-like structure to transform the parallel wavefront into a spherical wavefront at the near-infrared range. With a focal length about 8 μm at an operating wavelength of 980 nm and an optimized focal spot at the scale of 100 nm, our proposed metalens platform is suitable for two-photon direct laser lithography. We demonstrate the use of the fabricated metalens in a direct laser lithography system. The proposed platform, which combines the 3D printing technique and the computational inverse-design method, shows great promise for the fabrication and integration of multiscale and multiple photonic devices with complex functionalities.

Optical fiber-based smart sensing is a key technology for ultrasound sensing and monitoring applications. It plays a vital role in areas from laboratorial scientific research to the field non-destructive testing. However, the sensitivity of the current optical fiber acoustic sensor is limited. Hence, it is necessary to develop highly sensitive fiber-based sensors for ultrasonic/acoustic sensing. Here, we present a photonic crystal fiber-based Bragg grating sensor, which offers significant advantages and has higher performance for ultrasonic/acoustic sensing applications. In this research, the theoretical investigations of the proposed sensor are presented. The polymer material is utilized for filling into the fiber air hole structure to enhance the sensitivity. The design of the proposed device has been optimized to provide high optical quality factor to ensure high detection sensitivity, which can be used for high sensitive ultrasonic/acoustic sensing applications.

A three-dimensional notched-elliptical microdisk with a wavelength-size notch on the boundary is proposed as a multi-wavelength and unidirectional emission lasing source. The device contains multiple properly designed two-dimensional whispering gallery mode-based polymer notched microdisks with different dimensions for use as a multi-wavelength source. It can have a relatively high optical quality factor of 4000, unidirectional emission with low far-field divergence ∼4°, and the efficiency of emission is as high as 84.2%. The effect of the notch size on the far-field divergence is analyzed, and the multi-wavelength lasing performance is characterized, demonstrating that the resonator is robust and reliable. This work paves a unique but generic way for the design of compact multi-wavelength microlasers.

We demonstrate a three-dimensional (3D)-printed miniature optical fiber-based polymer Fabry–Perot (FP) interferometric pressure sensor based on direct femtosecond laser writing through two-photon polymerization. An unsealed cylinder column with a suspended polymer diaphragm is directly printed on a single-mode fiber tip to form an FP cavity. Here, two FP cavities with different lengths and the same diaphragm thickness (5 µm) are presented. The fabricated FP interferometer has a fringe contrast larger than 15 dB. The experimental results show that the fabricated device with a 140 µm cavity length has a linear response to the change of pressure with a sensitivity of 3.959 nm/MPa in a range of 0–1100 kPa, and the device with a 90 µm cavity length has a linear pressure sensitivity of 4.097 nm/MPa. The temperature sensitivity is measured to be about 160.2 pm/°C and 156.8 pm/°C, respectively, within the range from 20 to 70°C. The results demonstrate that 3D-printing techniques can be used for directly fabricating FP cavities on optical fiber tips for sensing applications.

A laser-ultrasonic system was established combining an 8 ns width pulsed laser for ultrasound generation and a two-wave mixing (TWM) interferometer for detection. Several Ti-6Al-4V samples were heat treated variously to get different grain sizes. Longitudinal wave back wall echoes were used to calculate the frequency dependent attenuation of ultrasound after the signals were denoised using wavelet transform. A model was built to calculate the log-normal distribution of grain size based on the correlation between the frequency dependent attenuation and the volumetric grain size distribution derived from electron backscattering diffraction data (EBSD) using Schwartz–Saltykov method.

In this study, laser-generated surface acoustic wave (SAW) interaction with surface-breaking cracks is numerically investigated to identify the relationship between crack depth and features of SAWs. The Scanning Laser Source (SLS) technique is utilized to detect and quantify cracks by monitoring the changes of the SAWs as a laser source scans over the uniform and detective area. The simulation results show that crack depth can be described by several important features of the transmitted waves and reflected waves. These features are used as inputs to a quantitative machine learning approach for crack-depth evaluation based on a Neural Network (NN) optimized with Particle Swarm Optimization (PSO) algorithm. The results show the feasibility of the proposed machine learning method to estimate the crack depth rapidly and accurately using SLS data.

The surface roughness of a machined metal surface is crucial to its appearance and performance. There are hardly any in-situ methods for surface roughness measurement of moving surfaces. A study of the digital speckle patterns generated by rough surfaces illuminated by a laser is performed experimentally. Laser speckle phenomenon can be used to monitor the surface roughness in a non-contact way. By investigating the effect of the surface roughness on the statistical and fractal parameters of the laser speckle pattern, an assessment method for evaluating surface roughness is discussed. The results show that some of the proposed statistical parameters have definite relationships with the surface roughness and can be explored to evaluate the surface roughness. Furthermore, the fractal parameters of the speckle pattern are sensitive to the type of machining process and therefore they can be used to classify the machined surface. The method can be a practical tool to achieve in-situ surface roughness measurement of moving surfaces.

In this paper, a non-destructive method for determining the thicknesses of dual-layer coatings is developed using an ultrasonic spectral filtering technique based on the theoretical investigation of wave propagation in specimens with two layers and three interfaces. The acoustic reflection coefficient of dual-layer coatings was separated from the measured signal through Fourier transformation and Wiener deconvolution. Utilising the multi-frequency windows autoregressive spectral extrapolation (MARSE) technique, the cosine signals (sine signals), including the time-of-flight (TOF) of each layer, were split from the real part (imaginary part) of the acoustic reflection coefficient. The thicknesses of the dual-layer coatings were calculated utilising the periods of the cosine signals (sine signals) and the longitudinal wave velocities of each layer. Ultrasonic experiments were carried out on the dual-layer coatings, where the thicknesses of layer 1, layer 2 and the aluminium substrate were 230 μm, 450 μm and 5 mm, respectively. The thicknesses measured by the proposed ultrasonic method were in good agreement with those observed by optical microscopy (OM) and the relative errors of layers 1 and 2 were both less than 5.0%.

Whispering gallery mode (WGM) microresonators are widely exploited for highly sensitive sensing applications due to their ultrahigh quality $(Q)$ factor enabling single-particle detection. The WGM surface-confined resonant light recirculates around the resonator, allowing the light to interact with analytes over several round trips, which enhances the sensitivity of detection. However, the stability of efficient coupling of WGMs to ingress-egress waveguides for lab-on-chip has been problematic. The typical way to achieve WGM coupling is to use a tapered optical fiber or a prism, which makes the device either fragile or bulky, and is not conducive to sensor integration that can lead to robust lab-on-chip devices. In this letter, we demonstrate three-dimensional (3D) fabricated polymer waveguides that can be integrated and provide evanescent coupling to silica WGM microspheres without sacrificing $Q$ factor. By mechanically connecting the waveguides to the microcavity, we can control the coupling to the resonator and the mechanical stability of the 3D lab-on-chip sensor is greatly improved. This provides a pathway to enhance the stability of WGM coupling for sensor integration.

a nondestructive method for characterization of thermally sprayed coatings is developed using an ultrasonic small angle incidence scheme in coatings of four layers and three interfaces. Through wave mode conversion analysis, the longitudinal and the transverse waves at the interfaces are simultaneously derived at an incident angle of 4.1°. The ultrasonic reflection coefficient amplitude spectrum (URCAS) is used to determine the thickness and longitudinal wave velocity of specimens through a two-parameter inversion utilizing the cross-correlation algorithm. The elastic modulus and Poisson's ratio of coatings were calculated utilizing the inversion results and modified density. Ultrasonic experiments were carried out on four WC-Ni specimens sprayed using the high velocity oxygen fuel (HVOF) method at spray angles of 30°, 45°, 60°, and 90°. The thicknesses measured by the ultrasonic method were in good agreement with those observed by optical microscopy with less than 10% error. The porosities determined from cross-section SEM photographs were 4.67%, 1.76%, 0.92%, and 0.19%, respectively with increasing spray angle. The elastic moduli of the specimens measured by the ultrasonic method were in the range of 315 GPa–351 GPa, and the Poisson's ratios were during 0.221–0.245. Metallurgical analysis indicated that increasing the spray angle increases both the density and the bond strength between particles which leads to an enhancement of the elastic modulus of the coatings. The proposed ultrasonic method is valid for nondestructive characterization of the elastic modulus and Poisson's ratio of the coatings.

An approach of continuously tunable unidirectional emission through bending a notched-elliptical polymer microdisk is proposed. The characteristics of the bending-dependent action are carefully analyzed, and the resonance wavelength for unidirectional emission can be tuned continuously through bending the device. Such a whispering-gallery-mode microresonator enables unidirectional emission with ultra-low divergence, of which the emission efficiency and Q factor are stabilized, demonstrating the whole structure is robust and relatively insensitive within a certain bending angle range. A maximum resonance wavelength shift of ∼100 nm and Q factor of 1500 can be achieved with the total size of the microdisk less than 10 μm. This kind of microresonator is promising for applications in multilevel integrated photonics circuits and may open the door to new functionalities of resonator devices, from sensing to optical amplification.

A high-birefringence spiral Sagnac waveguide (SSW) device fabricated via direct laser writing (DLW) using a two-photon polymerization (2PP) technique is proposed, designed, and experimentally demonstrated as an ultrahigh magnetic field sensor. The sensor comprises a Y-style tapered waveguide and an SSW containing two microfluidic channels. The SSW has a total length of ∼ 2.4 mm and a spiral radius of ∼ 200 μm . Due to the asymmetric structure, the SSW has a high birefringence of 0.016, which can be designed as a magnetic field sensor, as a magnetic fluid can be filled into the microfluidic channel changing the guiding mode and the birefringence and consequently leading to a change in phase of the interferometer when the applied magnetic field changes. The experimental results show that the proposed photonic device has a sensitivity to magnetic fields as high as 0.48 nm/Oe within a range from 10 to 100 Oe. The proposed device is very stable and easy to fabricate, and it can therefore be used for weak magnetic field detection.

A single average grain size has been used to characterize all grain sizes of polycrystalline materials. However, the grain size distribution also affects performance and quality. A grain size distribution characterization method was investigated based on a machine learning approach using a laser-induced ultrasonic technology. A pulsed laser was used to generate ultrasound inside the specimens and the ultrasonic signals were detected using a two-wave-mixing interferometer. The grain size distribution was quantified using the expectation and standard deviation of the logarithmic normal distribution function. The attenuation coefficients in different frequencies of ultrasonic signals were set as inputs and the expectation and standard deviation of grain size distribution were set as outputs. The grain size distribution prediction model was built with a neural network optimized by the particle swarm optimization algorithm. 90 data samples were selected as training data (75%) to train the characterization model and 30 data samples were set as test data (25%). The method does not require a physical model and avoids the problem of choosing different scattering attenuation mechanisms for grain size distribution. The results show the machine learning method has the feasibility to characterize the grain size distribution.

Wiener processes have received considerable attention in degradation modeling over the last two decades. In this paper, we propose a generalized Wiener process degradation model that takes unit-to-unit variation, time-correlated structure and measurement error into considerations simultaneously. The constructed methodology subsumes a series of models studied in the literature as limiting cases. A simple method is given to determine the transformed time scale forms of the Wiener process degradation model. Then model parameters can be estimated based on a maximum likelihood estimation (MLE) method. The cumulative distribution function (CDF) and the probability distribution function (PDF) of the Wiener process with measurement errors are given based on the concept of the first hitting time (FHT). The percentiles of performance degradation (PD) and failure time distribution (FTD) are also obtained. Finally, a comprehensive simulation study is accomplished to demonstrate the necessity of incorporating measurement errors in the degradation model and the efficiency of the proposed model. Two illustrative real applications involving the degradation of carbon-film resistors and the wear of sliding metal are given. The comparative results show that the constructed approach can derive a reasonable result and an enhanced inference precision.

A miniature resonator sensor based on a hybrid plasmonic nanoring with a gold layer coated uniformly on the outer boundary is described and investigated. By using the Lumerical finite-difference-time-domain (FDTD) method, the optimized sizes of the plasmonic layer thickness and the central hole are given and insight into the dependence of spectral displacements, Q factors, sensitivity and detection limits on the ambient refractive index is presented. Simulation results reveal that the miniature resonator sensor featuring high sensitivity of 339.8 nm/RIU can be realized. The highest Q factor can reach ∼60,000 with this nanoring and the minimum detection limit is as low as 1.5 × 10−4 RIU. The effects on the resonance shifts and Q factors due to geometric shapes of the inner boundary of the nanoring are discussed as well. This miniature resonator sensor has good potential for highly sensitive ultracompact sensing applications.

We demonstrate a phase-shifted Bragg grating waveguide (PS-BGW) fabricated by a direct laser writing technique via two-photon polymerization as a high-frequency ultrasonic sensor. The PS-BGW device has a cross-sectional area of 1.5 μm×2 μm, and the grating length is about 100 μm. The optical resonant spectrum is investigated numerically and experimentally. The result shows that the fabricated device with a grating depth of 250 nm and a quarter wave phase lead to a 20-nm stopband and a 0.085-nm full-linewidth of the transmission notch, making it suitable for photonic sensing applications. Here, the PS-BGW device is demonstrated for the detection of acoustic waves, which can cause strain or deformation of the device and lead to a shift in the resonance wavelength. The sensitivity and the limitations of the device for ultrasound detection are also investigated.

A novel special waveguide sensor, which is very sensitive to magnetic field, is proposed. This novel special wave guide is combined by two kinds material, YIG polymer and Bi:YIG polymer. The two beam lights counter-propagating in this kind of waveguide can generate large nonreciprocal phase shift. We designed the sensor structure, simulated the propagated characteristic of the structure. Based on the simulations, we concluded that waveguide with the core divided by two half circles can improve the sensitive to the magnetic field which is parallel to the interface of the two semicircles. We design a Sagnac interference demonstration with the light wavelength of 1550 nm, two difference-resonant loops. This sensor with 0.51pTesla-level sensitivity can be used in ultra-low magnetic field detection.

Photonic Floquet topological insulators (PFTI) allow scatter-free propagation of light along its edges. The PFTI of interest consists of helical waveguides arranged in a honeycomb lattice. When irradiated with an input beam on the edge of the PFTI, light propagates from one end of the waveguide-system to the other along the edges. The intensity and the final position of light is theoretically found to be dependent on the difference in the refractive indices of the core and cladding of the waveguides. For a system of helical-waveguides filled with a solvent, the effective refractive index of the system varies with the concentration of the analyte in the solvent and this can be measured by monitoring the position and intensity of the output-light. This paper discusses the design, principle, simulation and fabrication of such a PFTI based biosensor.

With the development of micro/nano-scale fabrication technologies, smart active/passive photonic devices have been fabricated by using silicon/polymer materials, which show great potential applications in photonics and optoelectronics. The current fabrication techniques such as electron-beam lithography give a high resolution, but they are expensive and time-consuming. Here, we present some polymer-based photonic devices fabricated by 3D femtosecond laser writing through two-photon polymerization. The resolution can reach up to ~100 nm, which is less than 1/10 wavelength within the C-band. Hence, the fabricated photonic devices can be used for micro lasing and sensing application. In this research, we show the spectral characteristics of several photonic devices such as phase-shifted Bragg grating waveguides. Due to the properties of polymer materials, the devices have a higher sensitivity on acoustic waves that can modify the geometry of the waveguide and thus induce a change in the effective index of the mode, which can be utilized for designing ultrasonic sensors. Although the fabricated quality is lower than that of photonic devices fabricated by the electron-beam lithography, the results show our fabricated devices can be useful for inexpensive sensors for ultrasound detection, demonstrating the usability of the femtosecond laser writing technique for photonic applications.

Degradation analysis can be used to assess the reliability of complex systems and highly reliable products because few or even no failures are expected during their life span. To further previous studies on degradation analysis, an independent increment random process method with linear mean and standard deviation functions is presented to model the degradation procedure. It is essentially a Wiener process method with two different transformed time scales. A one-stage maximum likelihood estimation approach is constructed, and the closed form of the product’s median life and the percentile of the failure time distribution (FTD) are also derived. The proposed method is illustrated and verified in a simulation study and practical degradation analysis for IRLED degradation. The Wiener process model with mixed effects is considered as a reference method. Comparisons show that the difference between the two methods regarding the degradation path is not obvious. However, the estimation accuracy of FTD percentile is significantly enhanced by the proposed model, because a more accurate dispersion can be obtained.

In the present study, we report on an efficient method for large-area photoreduction of graphene oxide flexible films. The laser-based reduction can be carried out in situ and can be tuned to attain the properties required. A systematic study has been conducted to evaluate the variation of the degree of reduction with the actual reduction temperature, which is measured using an infrared thermal camera. Local reduction temperature is varied up to 350&#176C, and the degree of reduction is measured using the C/O ratio. The C/O ratio is increased from 2:1 for graphene oxide to 10:1 for reduced graphene oxide. This high degree of reduction is observed at low temperatures, and also in a short period of time. Thermal conductivity properties calculated using the temperature distribution shows the in-plane thermal conductivities of graphene oxide and reduced graphene oxide are a few orders of magnitude lower than single layer graphene. This can be attributed to oxygen-defect scattering, and also due to the heat conduction through the thickness of the sample by way of contact between adjacent flakes. This photoreduction method provides a way for roll-to-roll scalable production of graphene-based flexible films.

Ultrasonic pulse-echo methods for flaw detection have been widely employed as an effective strategy for nondestructive evaluation, and flaw detection plays an important role due to its ability to detect localized damage in structures. In practice, flaw damage typically occurs in a few areas in the material, resulting in only a few echoes that exist in a received signal, which motivates us to detect flaws using sparse representation methods. In this study, the noisy signal is modelled by a linear combination of modulated Gaussian pulses, which form an over-complete dictionary. The over-complete dictionary is designed such that the sparseness of the representation is expected. A robust sparse Bayesian learning framework is employed with the goal of enforcing model sparseness and reducing the source of ill-conditioning in the inversion problem for flaw detection. Useful information, including the range of frequency and bandwidth parameters of the flaw echoes, is also estimated. Based on this information, we propose a post-processing scheme for structure noise elimination and flaw detection. The capability of the proposed method is quantitatively evaluated by simulation studies and is further validated by the experimental data.

A sensitive refractometric polymer micro-resonator sensor is fabricated using direct laser writing. The design consists of two tapered waveguides that can be directly coupled by single mode fibers, two “Y” splitting waveguides that are combined in a Mach–Zehnder interferometer (MZI) configuration and a micro-cylinder that evanescently couples with the arms of the MZI. The resonant wavelength shifts in response to the refractive index change in the surrounding medium of the micro-cylinder. The strong interaction between the medium and the whispering gallery mode results in a sensitivity of 154.84 nm/RIU (refractive index units), which demonstrates that such micro-resonators can be used for measurements of refractive index of liquids.

Guided waves have been used for structural health monitoring to detect damage or defects in structures. However, guided wave signals often involve multiple modes and noise. Extracting meaningful damage information from the received guided wave signal becomes very challenging, especially when some of the modes overlap. The aim of this study is to develop an effective way to deal with noisy guided-wave signals for damage detection as well as for de-noising. To achieve this goal, a robust sparse Bayesian learning algorithm is adopted. One of the many merits of this technique is its good performance against noise. First, a Gabor dictionary is designed based on the information of the noisy signal. Each atom of this dictionary is a modulated Gaussian pulse. Then the robust sparse Bayesian learning technique is used to efficiently decompose the guided wave signal. After signal decomposition, a two-step matching scheme is proposed to extract meaningful waveforms for damage detection and localization. Results from numerical simulations and experiments on isotropic aluminum plate structures are presented to verify the effectiveness of the proposed approach in mode identification and signal de-noising for damage detection.

Degradation analysis is very useful in reliability assessment for complex systems and highly reliable products, because few or even no failures are expected in a reasonable life test span for them. In order to further our study on degradation analysis, a novel Wiener process degradation model subject to measurement errors is proposed. Two transformed time scales are involved to depict the statistical property evolution over time. A situation where one transformed time scale illustrates a linear form for the degradation trend and the other transformed time scale shows a generalized quadratic form for the degradation variance is discussed particularly. A one-stage maximum likelihood estimation of parameters is constructed. The statistical inferences of this model are further discussed. The proposed method is illustrated and verified in a comprehensive simulation study and two real applications for indium tin oxide (ITO) conductive film and light emitting diode (LED). The Wiener process model with mixed effects is considered as a reference. Comparisons show that the proposed method is more general and flexible, and can provide reasonable results, even in considerably small sample size circumstance. Copyright © 2016 John Wiley & Sons, Ltd.

Ion thrusters play an important role in ensuring the success of future deep-space missions. Based on a ground-life test, a proper failure mechanism analysis was conducted, and the groove depth of the centerline accelerator grid aperture can be considered as the key lifetime metric. A nonstationary independent increment degradation model including both deterministic and stochastic parameters was proposed. The maximum likelihood estimation of the model parameters was derived by incorporating the product-level degradation data and the component-level degradation data. A Bayesian method to update the stochastic parameters describing the individual ion thruster characteristics based on product-level degradation data was developed. Then, the remaining useful life and the mean time-to-failure can be estimated and updated once a new online performance measurement is obtained. Finally, an illustrative example is given to demonstrate the effectiveness of the proposed model.

The extended Kalman filter (EKF), as a popular tool for optimally estimating system state from noisy measurement, has been used successfully in various areas over the past several decades. However, classical EKF has several limitations when applied to structural system identification; thus, researchers have proposed a number of variations for this method. The current study focuses on using EKF for real-time system identification and damage detection in civil structures. An improved EKF approach, called moving-window EKF (MWEKF), is proposed in this paper after a discussion on the problems associated with the application of classical EKF in time-variant systems. The proposed approach uses the moving-window technique to estimate several statistical properties. MWEKF is more robust and adaptive in structural damage detection compared with classical EKF because of the following reasons: (1) it is insensitive to the selection of the initial state vector; (2) it exhibits more accurate system parameter identification; and (3) it is immune to the inaccurate assumption of noise levels because measurement and process noise levels are estimated in this approach. The salient features of MWEKF are illustrated through numerical simulations of time-variant structural systems and an experiment on a three-story steel shear building model. Results demonstrate that MWEKF is a robust and effective tool for system identification and damage detection in civil structures.

Degradation testing is an important technique for assessing life time information of complex systems and highly reliable products. Motivated by fatigue crack growth (FCG) data and our previous study, this paper develops a novel degradation modeling approach, in which degradation is represented by an independent increment process with linear mean and general quadratic variance functions of test time or transformed test time if necessary. Based on the constructed degradation model, closed-form expressions of failure time distribution (FTD) and its percentiles can be straightforwardly derived and calculated. A one-stage method is developed to estimate model parameters and FTD. Simulation studies are conducted to validate the proposed approach, and the results illustrate that the approach can provide reasonable estimates even for small sample size situations. Finally, the method is verified by the FCG data set given as the motivating example, and the results show that it can be considered as an effective degradation modeling approach compared with the multivariate normal model and graphic approach.

A 3D printed castle style Fabry-Perot (FP) microcavity on optical fiber tip for humidity sensing is developed and demonstrated experimentally. The castle style FP microcavity, which has periodically arranged square holes on its side wall, is directly fabricated on an optical fiber tip through two-photon polymerization. Because of the existence of square holes in the castle style FP microcavity, Polyvinyl Alcohol (PVA) can be filled into the FP microcavity and a 360-degree humidity response sensitive area is formed. The contact area between water molecules and humidity sensitive materials is significantly increased, which will result in a high uniformity and a faster response of humidity sensing. Experimental results show that, the sensitivity of proposed castle style FP microcavity humidity sensor is up to 248.9 pm/%RH with relative humidity changing from 46 %RH to 75 %RH. In addition, repeated experiments indicate that the 3D printed castle style FP microcavity humidity sensors are repeatable and stable. Such excellent sensing performance with compact structure, highly sensitive, faster response and good stability makes the proposed sensing structure a highly promising candidate for humidity sensing applications.

In this article, we describe the use of a continuous-wave laser scanning method to generate non-dispersive surface acoustic wavepackets, which propagate along the sample surface without any waveform change. To achieve this goal, a coated linear elastic film on a non-linear substrate allows for careful balancing of dispersion and non-linearity effects. The scanning speed of the laser source and the thickness of the coated film were parametrically investigated to determine the optimal scanning speed for the generation of ultrasound for a given thickness of the coated film. In the first step, four different combinations of scanning speeds and the thickness of the coated film are presented to illustrate the generation of the narrowband ultrasound. The purpose of the scanning laser source is to effectively generate large amplitude ultrasound that takes the material into the nonlinear range. Further optimization through a careful matching combination of the scanning speed and the thickness of the coated film, whereby the dispersion effect was compensated entirely by the non-linearity effect, was used to generate non-dispersive ultrasonic wavepackets, which subsequently propagate with little distortion. The main findings of the simulations indicate that non-dispersive surface acoustic wavepackets for coated systems can be generated via the scanning laser source approach for specific values of scanning speed and thickness of the coated film.

Mechanical metamaterials inspired by the Japanese art of paper folding have gained considerable attention because of their potential to yield deployable and highly tunable assemblies. The inherent foldability of origami structures enlarges the material design space with remarkable properties such as auxeticity and high deformation recoverability and deployability, the latter being key in applications where spatial constraints are pivotal. This work integrates the results of the design, 3D direct laser writing fabrication, and in situ scanning electron microscopic mechanical characterization of microscale origami metamaterials, based on the multimodal assembly of Miura-Ori tubes. The origami-architected metamaterials, achieved by means of microfabrication, display remarkable mechanical properties: stiffness and Poisson's ratio tunable anisotropy, large degree of shape recoverability, multistability, and even reversible auxeticity whereby the metamaterial switches Poisson's ratio sign during deformation. The findings here reported underscore the scalable and multifunctional nature of origami designs, and pave the way toward harnessing the power of origami engineering at small scales.

In this paper, a polymer micro-Fabry–Perot interferometer (FPI) fabricated via direct laser writing using two-photon polymerization techniques on a single mode fiber tip is proposed, designed, simulated, and experimentally demonstrated as a magnetic field probe. The sensor comprises a tapered waveguide with a length of 100 µm and a diameter of 1 µm along the axis connecting the two reflecting surfaces of the micro-FPI open cavity. The cavity is filled with a magnetic fluid (MF) changing the evanescent coupling and consequently leading to a change in the phase of the FPI. To improve the signal reflection, a thin layer of Au is coated on the device before fabricating the probe sensor, which is then sealed in a MF-filled glass tube. The experimental results show that the proposed probe sensor has a low temperature sensitivity and a sensitivity to magnetic field as high as 1.54 nm/mT, and the magnetic field measurement accuracy is ∼649.4 μT within a range from 1 mT to 8 mT. The microsensor, which is very stable and easy to fabricate, can be used as a probe to detect weak magnetic fields.

Adhesive bonding is widely used in the structural assembly of aircraft, and assessing the curing state of the adhesive is important to ensure the safe operation of the assembly. In this paper, the curing process of epoxy resin is monitored by laser ultrasonics. The propagation of ultrasonic waves in a composite structure is discussed, and the curing of epoxy resin and its influence on ultrasonic characteristics are studied. In the fast-drying epoxy resin, the molecular cross-linking rate is fast, the difference of acoustic impedance between the epoxy resin and aluminum gradually decreases, so the transmission coefficient becomes larger and the reflection coefficient decreases. The calculation of absorption attenuation and dispersion should take this change into account. The relationship between amplitude and reflection coefficient is established by far-field ultrasound. The difference in molecular mobility causes the relaxation process, which results in absorption and dispersion of the ultrasonic waves. Making use of the broadband characteristic of laser ultrasonics, the transmission waves are analyzed. As the epoxy resin changes from liquid to solid, the relaxation time is gradually shortened, and the center frequency of the transmitted wave is gradually increased, the absorption attenuation is approximately linear with frequency. The results show that the curing process has a significant effect on ultrasonic characteristics, and laser ultrasonics could be used as an online monitoring method.

Rare earth-doped fiber lasers are interesting in the field of optical fiber lasing and sensing. One of the interesting topics is the tunable/switched multi-wavelength lasers. However, due to the homogeneous broadening gain, it is difficult to generate multiple wavelengths in the fiber lasers based on erbium-doped fibers. Here, we propose a tunable multiwavelength erbium-doped fiber ring laser based on an optical fiber tip Fabry-Perot (FP) interferometer, which acts a wavelength filter and a reflector of the fiber ring laser. With the purpose to propose a method for switch multiwavelength spectra, the strain and thermal variations around the modal interferometers are investigated. The spectra are symmetric with a maximal power difference about 25 dB between the lasing modes and the average of the side mode suppression ratio, which is tuned into the C-band with a resolution of 0.02 nm. This laser offers low wavelength drift, good signal to noise ratio and high-power stability, and can therefore be used for sensing applications.

High-frequency ultrasonic sensors are an important sensing technology in structural health monitoring applications. Compared with the traditional PZT transducer as ultrasonic sensors, novel ultrasonic sensors based on optical methods such as micro-ring resonators have gained increased attention. These micro-rings can be as small as a few microns in diameter, which improves their sensitivity to high-frequency ultrasound. In principle, acoustic waves irradiating the micro-ring induce strain, changing the dimensions and refractive index of the waveguides via the elasto-optic effect. This leads to a change of the guided whispering gallery modes (WGMs), which are extremely sensitive to change in the ring radius induced by the ultrasound strain field. Based on our prior research, here we present an integrated high-frequency ultrasonic sensor array based on optical micro-ring resonator array fabricated by direct laser writing. The fabrication has been optimized to provide high optical quality factor to ensure high detection sensitivity. The experiments demonstrate the potential of the polymer micro-ring resonator working as a high-performance ultrasonic sensor. Applications of the integrated ultrasonic sensor array for acoustic-emission ultrasound detection are shown.

Damage in civil, aerospace, and mechanical structures caused by crack growth and impact loading generate transient ultrasonic waves whose frequency and amplitude can reveal the underlying structural health condition. Hence, it is necessary to find a useful tool based on ultrasonic detection for structural health monitoring. Recently, smart sensors based on gratings such as fiber Bragg gratings (FBGs) have been shown to be suitable to detect such acoustic waves in structural health monitoring applications. However, the fiber-based gratings as the ultrasonic sensor has limited sensitivity to high frequency ultrasound detection due to a specific grating length and a finite spectrum width. To eliminate this limitation, one improvement has been made by using phase shift FBGs due to their special filtering characteristics. The phase shift FBGs can have a narrower spectral width, which will significantly improve the detection sensitivity. Another big improvement, for example Bragg grating waveguide (BGW) sensor, is to optimize the grating structure using different materials. In this work, we describe a 3D printed-polymer BGW sensor for ultrasound detection fabricated through a two-photon polymerization process. The design and fabrication have been optimized for high detection sensitivity. The results demonstrate the potential application of BGW devices for high-sensitivity ultrasound detection.

In this paper, we demonstrate the fabrication of a chemical sensor for 2,4-dinitrotoluene (DNT), based on an opticalfiber- microsphere coated with upconversion nanocrystals functionalized with layers of polyelectrolytes - poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH). The design consists of a microsphere, which supports whispering-gallery-modes (WGM), coupled to an optical fiber. The NaYF4-Yb3+,Er3+ nanocrystals have a bright fluorescence around 550 nm and 650 nm when irradiated with 980 nm, which is enhanced by the WGM. When functionalized with PAA/PAH layers, these nanocrystals can be coated on the microsphere with control over layer thickness. The presence of DNT on the surface of the microsphere quenches the fluorescence as the absorption spectrum of DNT has peaks in 500 - 600 nm. The effect of concentration of the analyte on the magnitude of quenching has been studied. The paper discusses the design, fabrication and characterization of the chemical sensor.