This PDF file contains the front matter associated with SPIE Proceedings Volume 12771, including the Title Page, Copyright information, Table of Contents and Conference Committee lists.

Lossy mode resonance (LMR) is a phenomenon that is observed in the optical spectrum when a mode that progresses through a waveguide starts to be guided in a thin film deposited on this waveguide under certain conditions, mainly related to materials and angles of incidence. An important property that LMRs have is that they can be guided into the thin film with both magnetic (TM) and electrical (TE) polarized light, unlike the other two main types of optical resonances with the same modus operandi that complete this trilogy, surface plasmon resonances (SPRs) and surface exciton plasmon resonances (SEPR). Regarding the potential materials that make up thin films, they include dielectrics suchs as metal oxides such as titanium dioxide (TiO2), zinc oxide (ZnO), tin oxide (SnO2) or polymers. In all cases it must be fulfilled that the real part of the refractive index must be greater than its imaginary part, unlike also the SPR and SEPR. As for the angles of incidence, they must be close to 90º, which explains the success of deposition of thin films around an optical fiber to obtain sensors based on LMR, although interesting results have recently been obtained using planar waveguides. This work will present the main milestones obtained during more than a decade using LMR-based sensors for the detection of multiple parameters. Among these interesting aspects, we can mention the sensitivity records achieved, hybridization with other sensing technologies or the possibility of multiplexing multiple sensors on the same substrate, just to mention a few.

DNA detection based on DNA hybridization has wide applications in various fields, such as clinical diagnostics, food safety, and environmental monitoring, etc. At present, common DNA detection approaches include fluorescence-based microarray technique, electrochemical method and surface plasmon resonance (SPR), etc. However, these methods require additional precise equipments and relatively complicated detection process. In this work, we developed a biosensor platform based on a liquid crystal (LC)-amplified optofluidic whispering gallery mode (WGM) resonator to achieve ultra-sensitive, label-free, and quick-response DNA hybridization detection. Liquid crystal is a material with high sensitivity, rapid response, and low cost. It exhibits significant directional and positional ordering, and it is sensitive and responsive to external stimuli. LC molecules exhibit a uniform orientation on the surface of the resonator, when the surface is covered with an appropriate amount of ssDNA. Once complementary DNA and ssDNA hybridize on the surface, the homotropic orientation of the LCs will be destroyed. Due to the simultaneous interaction of the WGM and the LCs in the optofluidic resonator, changes caused by the DNA hybridization can be amplified, resulting in a shift in the resonance wavelength. In this experiment, we used the spectral wavelength shift as a sensing parameter to achieve the detection of target DNA, and a lower detection limit compared to traditional DNA detection methods was obtained. At the same time, this biosensor platform also shows good selectivity. Our research results suggest that the LC based WGM optical microcavity sensing platform can provide an ultra-sensitive, label free solution for DNA detection.

The present paper proposes a Surface Plasmon Resonance (SPR) sensor utilizing Twisted Bilayer Graphene (TBG)- Hexagonal Boron Nitride (hBN) heterostructures to enhance the Goos-Hänchen (GH) shift. The study presents a theoretical demonstration of the adjustability of GH shift via tuning the TBG twist angle, the number of TBG layers, and the thickness of hBN, respectively. The two-dimensional twisted bilayer graphene twist angle effectively promotes the sensitivity of the sensor. With a relative twist angle of 76.87°, the sensitivity of this sensor structure is remarkably enhanced to 1.8×10⁸ μm/RIU. The outcomes of this investigation offer a theoretical underpinning for the development of new high-sensitivity biosensors.

Based on the liquid lens focus mechanism, the optical tactile sensor and its signal acquisition system are designed. To balance the relationship the manufacturing complexity, perceptual precision and sensitivity, perceptual stability, and robustness of the tactile sensor, they are important components of smart devices. Some of them, such as robots, smart wearable devices, etc., because they can intuitively reflect the interaction state of the man-machine-environment. Firstly, according to the liquid-film lens structure, the sensing structure of the optical tactile sensor was designed, and its tactile perception mechanism was explored; secondly, the signal acquisition system for the sensor was designed to improve the integration of the tactile sensing system; finally, the optical tactile sensor performance test and application demonstration platform was designed-built to test-calibrate the sensor system performance, and explore its application potential in touch force detection. Experiments show that the sensor has an effective range from 0.01 to 1.2 N, a sensitivity of up to 0.1654 V/N (range from 0.01 to 0.4 N), and a linear goodness of fit of up to R2 = 0.996 (range from 0.01 to 0.4 N). Good perceptual stability and responsiveness (loading response time: 0.078 s), can achieve accurate measurement of the target mass, with an error of less than 0.02 N.

We presented a low-cost, easy-to-fabricate and high-sensitivity distributed refractive index (RI) fiber taper sensor in optical frequency domain reflectometry (OFDR). When the RI of external medium surrounding the tapered fiber changed, the optical frequency shifts of the local back-reflection spectra in OFDR could be measured. We first simulated and derived the relationship between the structure and sensitivity of tapered fiber. Shown the relationship between the radius of the cone waist and sensitivity. And the simulation results were verified through experiments. In the experiments, we demonstrated that high-sensitivity RI measurement of 2970 nm/RIU in the RI range from 1.3347 to 1.3349 for NaCl solutions could be achieved by OFDR system. The spatial resolution was up to 5 mm with the effective sensing region of 5 cm. This is currently the report with the highest sensitivity among distributed RI sensors.

Distributed acoustic sensing technology is gaining attention as a valuable tool for seismic monitoring, oil and gas pipeline imaging, and underwater cable observation. Phase-sensitive Optical Time Domain Reflectometry (see abstract for this (phi -OTDR) is a popular technique within this field, known for its ability to recover high signal-to-noise ratio acoustic vibration signals. However, traditional see abstract for this phi-OTDR systems have limitations in observing both local microseismic events and strong seismic motions simultaneously and requiring months of data analysis and processing time. To address this, a fast recovery system based on multi-sideband pulse modulation is proposed. This system allows for simultaneous detection of different strain ranges at a low sampling rate, effectively managing large data volumes and enabling fast demodulation. This work will bring some inspiration to engineering applications and instrumentalization.

Residual stress in a part is a key factor in its fatigue life, structural stability, corrosion resistance and other properties. Therefore, accurate measurement of residual stress is essential. The mechanical release method is widely used in residual stress measurement for its high accuracy and mature technology. However, the method assumes that the material is homogeneous, and its accuracy is greatly affected in non-uniform materials. Aiming at the problem of inaccurate residual stress measurement in non-uniform materials, a fiber-optic ring array residual stress measurement method based on high spatial resolution distributed optical fiber measurement technology is proposed. The drilling method is taken as an example. In this method, we arrange the fiber-optic ring array around the hole in the same center to measure the residual stress on the circumference with high spatial resolution. The finite element simulation is used to analyze the influence of the spatial resolution of the fiber sensor, the non-uniform size, position and Young's modulus on the strain distribution and residual stress measurement. It is proved that the method can accurately measure the residual stress and obtain the information of the non-uniform state of the material.

Measuring bite force gives important information about the performance of the jaw muscles. Enhancement of bite force measurements, particularly 2D force mapping, can be achieved through the application of advanced distributed fiber optics sensing techniques, which present the capability to acquire high resolution measurements. Optical Backscatter Reflectometry (OBR), based on the principles of Optical Frequency Domain Reflectometry (OFDR), is the key to achieve spatially distributed measurements and transform a single-mode fiber (SMF) into a cost-effective minimally invasive sensor. The OBR interrogator, manufactured by LUNA, catches the light backscattered by each section of the fiber and detects the spectral variation induced by the stain and pressure application. We have developed two methods to construct a two-dimensional map of the applied bite force. In the first a SMF has been bent into four lines and embedded into a silicone mouthguard. In the second approach we used massive Scattering‑Level Multiplexing (SLMux), which exploits fibers with different scattering levels: a standard low scattering SMF to enable the multiplexing, and a MgO‑nanoparticles doped fibers with a 40 dB higher scattering for sensing purposes. The setup with eight multiplexed fibers allows to obtain bite force mapping for each side of the jaw by embedding four fibers for left and right sides. The pressure sensitivity coefficient of the maps has been effectively calculated. The setup has been validated for 2D force mapping over the silicone mouthguard with millimeter resolution. The capability of optical fiber to be multiplexed and embedded in non-uniform geometries results in a high resolved and precise 2D bite force reconstruction.

Rapid and accurate detection of hydrogen in anaerobic condition is more and more important for the safe production and operation of hydrogen energy. A novel fiber hydrogen sensor based on Pd/Ta composite film coating on π-shifted fiber Bragg grating is proposed and its sensing performance is evaluated experimentally. Due to the narrow reflective peak of π-shifted fiber Bragg grating, the proposed sensor has, shown higher accuracy on wavelength shift tracing. A Pd/Ta composite film was deposited on the side of π-shifted fiber Bragg grating by magnetron sputtering process. Compared with the grating hydrogen sensors of other Pd-based composite films, the hydrogen sensitivity and response time of the Pd/Ta composite film coated sensors with different thickness of sensing layer have been improved greatly. When the hydrogen concentration is 1%, The wavelength shifts of π-shifted fiber Bragg grating of Pd/Ta membrane with thicknesses of 100 nm, 200 nm, 300 nm and 800 nm are 16, 31, 50 and 115 pm, respectively, and the response time are 63, 71, 109 and 150 s, respectively. A longtime testing of the sensor proves that the sensor has good long-term stability and repeatability. The proposed hydrogen sensor with Pd/Ta composite film has great potential in the quasi-distributed monitoring of hydrogen leakage in hydrogen generation, refueling stations, and so on.

In this paper, we propose an optical fiber pH sensor with homemade poly (acrylic acid-co-acrylamide) hydrogel coating based on Mach-Zehnder interferometer (MZI), which is designed via concatenating no-core fiber, few-mode fiber and no-core fiber in sequence. The fiber surface of the sensing region is silanizated to enhance the adhesion of the hydrogel onto the fiber. When the pH value of the solution varied, the amount of free carboxylic acid ions in the hydrogel changes. This change causes the hydrogel to expand or contract, resulting in a change in refractive index. Such a refractive-index change of the hydrogel alters the effective refractive index of the cladding layer of the NFN MZI, leading to a spectral shift of the MZI. The sensor can be employed to measure a wide pH range of 2 to 12. According to the relationship between the pH values and spectral dips, the corresponding sensitivities are -0.346 nm/pH and -64 pm/pH at the pH ranges of 2-5 and 5-12, respectively. Meanwhile, the coefficients of determination of the fitting lines are both large than 0.973, which indicated that the sensor possesses an excellent linearity. Moreover, the response time of is 21.78 s. The results suggest that the proposed sensor exhibits a good repeatability and stability and holds considerable potential in fields such as environmental monitoring and chemical analysis.

In this paper, we proposed a highly sensitive temperature and humidity sensors based on two types of physically deformed long-period fiber gratings (LPFGs) which was fabricated by periodically twisting and micro-tapering single- mode fiber, also known as chiral long-period fiber grating (CLPG) and microtapered long-period fiber grating (MTLPFG), respectively. Meanwhile, the superior sensing characteristics of graphene-enhanced CLPG sensor for temperature and polyvinyl alcohol (PVA) coated MTLPFG sensor for humidity are successfully demonstrated. In the temperature experiment, due to the excellent thermal conductivity of graphene, the CLPG transmission spectrum has excellent responsiveness and linearity, and the measured temperature sensitivity is 115 pm/℃. Compared with the traditional fiber gratings, the temperature sensitivity has been significantly improved. Unfortunately, as a humidity measure, fiber grating is insensitive to humidity. Therefore, the MTLPG coated with PVA nanofilms is proposed to realize humidity sensing measurement. Due to the perfect combination of the special micro-nano structure and humidity sensitive film, this humidity sensor obtained excellent sensing sensitivity and linearity. Note that the humidity sensitivity of the PVA-coated MTLPFG has a humidity sensitivity of up to 13.27 pm/%RH. This physically deformed LPFGs are non-degradable, stable and higher sensitive, we believe that it will providing a key role in high-precision temperature and humidity sensing fields.

A highly sensitive integrated optical sensor based on cascaded polarization convertor and slot waveguide microring resonator is proposed. The structure consists of an edge coupler, slot waveguide microring resonators and a reflective polarization converter. One polarization input light of the through-port of the slot waveguide microring resonator is converted into the other polarization mode after passing through the reflective polarization converter, and is coupled into slot waveguide microring resonators again. The output light of the sensor is the cascaded through-port spectrum of two different polarizations. The power conversion efficiency of the reflective polarization converter is optimized to 80%. The free spectrum ranges of the two polarization modes are different to produce the Vernier effect. Thus, the sensitivity of the sensor increases dramatically. The theoretical sensitivity of our system is 21150 nm/ Refractive-Index-Unit (RIU) with the measurement range of 0.0007 RIU. This structure has great potential in biological recognition and chemical analysis.

Silicon photonics technology provides an attractive solution for compact, low-cost multi-array detection for chemical sensing. An on-chip multi-channel fluorescence gas sensor has been developed on an integrated photonic platform. By the simultaneous excitation and collection with the same waveguide, online detection of illicit drug simulants Methylphenethylamine (MPEA), nerve agent simulants Diethyl Chlorophosphate (DCP), and carcinogenic gas Aniline (AN) have been achieved by one source and detector on the same chip. Thanks to the compatibility with the complementary metal–oxide semiconductor (CMOS) fabrication processes, this technology paves the way for future development of wearable fluorescence-based gas sensors with high sensitivity, low cost, and compact size.

A fiber Fabry-Perot etalon interrogation system is proposed for dynamic signal measurement of fiber Bragg grating sensors. The system has a voltage feedback device and is designed to perform temperature control of the fiber Fabry-Perot etalon to increase the amplitude of the demodulated signal to a high level. Data acquisition system is used to form voltage feedback to control the temperature by adjusting the power of the thermoelectric cooler loaded under fiber Fabry-Perot etalon. By controlling the temperature change of the fiber Fabry-Perot etalon, it is possible to increase the amplitude of the demodulated signal and maintain it at a high threshold. The experimental results show that adaptive temperature control has good signal enhancement effect.

The evolution process of characteristic parameters of Fiber Bragg Grating (FBG) during the regeneration process was investigated, and a formation mechanism of regenerated FBG based on non-uniform crystallization in FBG was proposed. The experimental system was built for experimental verification, and the experimental results are consistent with the formation mechanism proposed. The regenerative phenomenon of fibers under different processes, structures, and stress conditions can be well explained with the proposed formation mechanism of regenerated FBG.

Among fiber optic acoustic sensors, Fabry-Perot sensors have attracted increasing research attention due to their high sensitivity, simple structure, and ease of preparation. The vibrating diaphragm of Fabry-Perot sensors is the key to achieve high performance. Compared to traditional diaphragms based on metal or polymer, 2D materials with the features of high Young's modulus and ultra-thinness are ideal diaphragm materials for significantly enhancing the acoustic sensitivity and broadening frequency range of the sensors. In this work, graphene oxide is used as the vibrating diaphragm due to the water solubility and the ease of thin-film preparation. In order to improve the stability of the system, we have developed a phase demodulation method using three orthogonal signals. The three-path phase-shifting algorithm can eliminate the effect of DC offset from the unstable optical signal. Our graphene oxide-based Fabry-Perot acoustic sensor can test and demodulate the vibration signal in real-time, demonstrating long-term stability and insensitivity to changes in light intensity.

We propose and demonstrate a high-sensitivity phase-measurement-based interrogation technique enabled by microwave photonics for optical fiber Fabry-Perot interferometer (FPI) sensors. A low-coherent broadband light source is employed as the light source, and the reflection spectrum of the FPI device is modulated by microwave signals output from Port 1 of a vector network analyzer via an electro-optic modulator. The intensity-modulated optical signal goes through a long length of dispersion compensating fiber and is then detected by a highspeed photodetector. The electrical signal is sent to Port 2 of the vector network analyzer for synchronized detection at each modulation frequency to obtain the frequency response of the FPI sensor. The frequency passband of the FPI sensor in the frequency response is then located and an operating frequency within the passband is designated. By monitoring changes in the phase information of the operating frequency, changes in the optical path difference of the FPI sensor can be determined with high sensitivity. As a proof of concept, an FPI device constructed by fusion splicing a section of photonic crystal fiber (PCF) between two SMFs is employed for tensile strain sensing. Due to the low Young’s modulus of the PCF and the high sensitivity provided by the proposed phase-measurement-based interrogation approach, a high-performance strain sensor with a resolution of 60 nanostrains is revealed

The main approaches to the construction of optical gyroscopes based on passive ring resonators are analyzed and reviewed. The history of the development of the first prototypes of such gyroscopes, which were in the focus of attention in the late 70s as a part of trials of vanishing the lock-in zone of laser gyroscopes, is considered. Modern studies of optical gyroscopes based on passive ring resonators are performed due to the trend towards the miniaturization of optical gyroscopes. The main disadvantages and problems of such gyroscopes as well as possible solutions are described. In particular, the results of research performed by the authors of the article are presented. The main types of gyroscope sensitive elements and their achievable characteristics are considered

An inline fiber Mach-Zehnder interferometer (MZI) that consisted of a standard single-mode fiber sandwiched between a core-offset and a peanut-shape joints is fabricated for refractive Index (RI) sensing purpose. It is found that the straight inline MZI was not sensitive enough with -27.6 nm/RIU for RI measurement. After bending it, the sensitivity was improved markedly, and the maximum RI sensitivity of -153.11 nm/RIU was achieved within the measured RI range of 1.333–1.373. The result showed that bending the inline MZI could enhance its RI sensing performance, and as the curvature radius deceased, the RI sensitivity increased.

The bonded composite laser crystals are common solid active media in the field of high-power lasers and amplifiers, used to improve thermal effect and laser output. There is a potential risk of affecting optical transmission due to the imperfect bonding interface. To characterize the interface for crystal fabrication and laser design, we propose a distributed interferometer-based interfacial loss characterizing method. Based on the stable broad-band light source and photodetector, our Optical Low-Coherence Reflectometry (OLCR) is easier to obtain the low noise background and realize the high-accuracy measurement, which is important for detecting the weak reflection of the interface. Using a fiber probe, any local area on the interface can be detected individually. With scanning the optical delay line, the surficial reflection and interfacial reflection of one light spot can be monitored separately to enable the lateral and vertical distributed measurement. Specifically, a Yb:YAG/YAG crystal was measured and demonstrated as an example. To avoid absorption and realize high spatial resolution(40μm), a source located at 1535nm with a bandwidth of 58nm is used. Meanwhile, the reflection coupling loss (RCL) of the fiber probe is self-calibrated and eliminated by the simulation and calculation model. Moving the fiber probe, the vertical and lateral interfaces are scanned. The point cloud distribution testing of vertical bonding interfaces is accomplished, and a total internal reflection method is employed to detect the lateral interface. At last, the resolution (~0.5μm) of the probe scanning is validated and discussed using a photomask.

A spatial light dispersion-based spectrometer is experimentally demonstrated for broadband gas absorption spectroscopy with time resolving capability. A diffraction grating combining with a plane-array camera serves as the spatial dispersion system operated in the near-infrared (NIR) waveband. Spectrum with a bandwidth of 40 nm can be acquired via a single exposure of the camera running in the line mode with 640 ×4 pixels. With the data stream transmitted to the computer and stored in real time, the time resolution for continuous spectrum measurement reaches 200 μs, which is validated by monitoring the spectral evolution of a broadband amplified spontaneous emission (ASE) under impulsive intensity modulation. Adopting the ASE light source and a multi-pass gas cell, carbon monoxide (CO) absorption is spectrally characterized with a 16-nm bandwidth, and the gas concentration retrieval based on the entire obtained absorption spectrum is performed, in which an averaged mean absolute percentage error (MAPE) of 5.6% is achieved.

A new design and detection method is proposed of image sensor based LED light marker with pulse modulation using under sampling. We use general image camera to detect different LED light makers with pulse modulation frequency more than 60 Hz. This is according to the under sampling analysis that converts signal with high frequency more than 60 Hz to lower frequency signals using image camera with low frame rate frequency less than 60 Hz. Experimental results are shown with different pulse modulation frequency.

At present，China has made a great progress in single photon laser imaging, especially in algorithms development for software. Xu’s work in University of Science and Technology of China has achieved remarkable reputation in single photon laser imaging technology. His team demonstrated the capabilities of successful single photon laser imaging at a distance of about 200 kilometers. Authors of this paper have some collaborations with Xu’s task team. In order to realize single photon laser imaging in space, a possible orbit solution for space-based single photon laser imaging mission is put forward in this paper. Low Earth orbit with an inclination of 65 deg and with zero eccentricity and with an altitude of less than 350 kilometers is proposed to satisfy the single photon laser imaging mission requirements. The orbit evolution,space radiation situation, orbit altitude keeping are studied too. Based on such a scenario of orbit, a space mission to carry out single photon laser imaging task is feasible and reliable.

A new method is reported for determining the concentration of dimethyl methylphosphonate (DMMP) vapor based on the capture of laser speckles from a liquid crystal (LC) sensor. By calculating the change of the correlation coefficients of speckle images, DMMP vapor is selectively detected over a 200 ppb to 5 ppm concentration range, demonstrating the feasibility of the method for determining vapor phase concentrations of Organophosphorus compounds (OPs).

Creatinine is a metabolite of human muscles that can be used to infer whether part of the kidneys is functioning properly. In this paper, a taper-in-taper fiber (TITF) based fiber optic biosensor is proposed for detecting the concentrations of creatinine solution and helps to diagnosis of kidney failure. The TITF structure used in this study is formed by fabricating a taper shape again in the tapered region of a normal taper optical fiber. This allows the fiber to produce more higher-order modes and thus improve its ability to sense changes in refractive index of the external environment. The sensitivity of the sensor was increased by coating the tapered region of the TITF fiber with a gold nanoparticles and zinc oxide nanomaterials. Creatinase enzyme is used to increase the specificity of the sensor for creatinine. The experimental results show that the sensor can detect the creatinine solution with the concentrations of 300-2000 μM and the sensitivity is 0.17 a.u./μM. It provides a good choice for biomedical applications.

Hydrogen energy is widely used as one of the cleanest energy sources. However, due to its explosive nature, monitoring the early stages of hydrogen leakage has become a current research focus. This study presents a novel hydrogen sensor suitable for low concentration hydrogen sensing, which combines a planar polymer Bragg grating (PPBG) with a porous Pt/WO3 hydrogen sensitive material modified by MIL-100 (Fe) as a template. PPBG are based on planar cyclo-olefin copolymer substrates with integrated waveguides and Bragg gratings. Due to the polymer's inherently increased coefficient of thermal expansion, the concept provides superior thermal sensitivity. Porous Pt/WO3 has a larger specific surface area and better response characteristics than the original Pt/WO3. By utilizing the thermal effect of the reaction between tungsten trioxide and hydrogen gas, the wavelength shift of PPBG is caused by temperature transformation. A series of experimental results tested at different hydrogen concentrations indicates the sensitivity of the sensor is 575 pm/%. The combination of materials and devices with superior performance enhances the sensitivity of the sensor to hydrogen at low concentrations compared with fiber Bragg grating (FBG) hydrogen sensors. The hydrogen sensor has the advantages of high sensitivity at low concentrations and compact structure, which will have great potential in industrial applications.

Thin films are used in many areas for protecting the base, isolating electrons, and so on. Based on the thin-film interference principle and the characteristics of Y-shaped fiber, this paper proposed a reflection objective with annular reflection. With the objective, incident light from the central fiber can be well received for high signal-to-noise ratio. By comparing with the prior-known spectral reflectance of the standard mirror, the absolute spectral reflectance of the thin-film can be achieved. Furthermore, according to the spectral reflectance of the thin-film, film thickness is acquired by the evolutionary genetic algorithm. In the algorithm, the thickness is taken as the independent variable, and the squared difference between the measured spectral reflectance and the preset theoretical spectral reflectance is set as the goal to carry out the global optimization. By testing different silicon-based silica films, the measurement results proved high feasibility and good global fitness. And the total measurement time of a single point is less than 1s. In conclusion, the thin-film measurement system based on the proposed reflection objective and inversion algorithm own excellent performance, which is hopeful for broad usage.

Sensitivity optimization in acoustic sensors necessitates the development of acoustic-sensitive elements. Among various designs, the cantilever structure has emerged as a highly sensitive choice. In this study, we introduce a novel Fabry-Perot (F-P) interferometric fiber-optic acoustic sensor (FOAS) that integrates a hollow cantilever structure. The proposed structure enhances vibration flexibility by reducing both the effective mass and spring constant of the cantilever. Experimental results demonstrate that the proposed structure outperforms the original design in terms of sensitivity. Notably, at 1.7 kHz, the sensitivity and minimum detectable acoustic pressure level (MDP) reach 91.4 mV/Pa and 6.2 µPa/√ Hz, respectively. The proposed hollow cantilever establishes an optimization framework for highly sensitive acoustic sensors.

The condition and concept of confocality for ring resonators is considered. The results of the development of a compact (meridional cross-sectional area less than 2 cm2) monoblock ring resonator designed to operate as a sensitive element of an optical gyroscope are presented. A prototype batch of these resonators were studied experimentally. In particular, the shape of the reflecting surfaces of ring confocal resonators and their mutual orientation were measured. The problem of stable input and output of light from monoblock ring confocal resonators and the possibility of their integration into the design of an optical resonator gyroscope are also considered.

Fluorescence spectroscopy has been successfully applied to the identification of colorants in oil paintings and murals due to the advantage of non-destructive detection, but the traditional equipment uses xenon lamp as the excitation light source, which is energy inefficient in the process of using, and the monochromatic light output power is weak and unstable. This paper proposes the use of ultra-continuous spectrum broad-band tunable laser with fiber optic sensors to achieve effective detection of weak fluorescence signals of silk dyes, compared with the traditional 3D-fluorescence equipment, single-wavelength excitation light output power is greatly increased, higher monochromaticity and energy density to achieve better spatial resolution and faster detection efficiency, while adding fiber optic sensors can be done to achieve the surface of the painted artifacts in situ non-destructive detection. The proposed method can detect more weak fluorescence information under the comparison results of multiple dyed silk samples with different fluorescence emission effects. Acknowledgement: This project is funded by National Key R&D Program of China No.2020YFE0204600.

Aiming at the existing density detection technologies, this paper proposes and designs a liquid refractive index (RI) sensor based on an all-fiber online Fabry-Perot interference cavity (FPI). We designed a real-time monitoring system by LabVIEW and performed density calibration, bubble and impurity interference and stability tests. The experimental results show that the effective cavity length of the FP cavity and the density have a good linear relationship, the fitting coefficient of the curve is as high as 0.998. Excluding the influence factors such as impurity，air bubbles and high temperature, the maximum offset between the measured value after temperature compensation of liquid density and the real value is within ±0.0024g/cm³.

A tapered plastic optical fiber (POF) assisted by a long period grating (LPG) is proposed as a refractive index (RI) sensor. A tapered POF assisted by a corrugated surface LPG is fabricated by the heating and drawing method and simple mechanical die press print method. By changing the structural parameters of the tapered POFs with LPG, the RI sensing performance of the obtained sensor are evaluated. It shows that the structure parameters of sensor affect the RI sensing performance, and the sensor with a smaller diameter of the tapered POF and a deeper groove depth of the LPG exhibits better RI sensing performance. When the POF with a diameter of 1mm is heated and drawn to form a tapered POF with a diameter of 600μm, then an LPG with a period of 300µm, groove depth of 150µm is fabricated on it, the RI sensitivity of 590.7%/RIU is obtained in the measured RI range

An ultra-long adiabatic tapered single-mode fiber is proposed to enhance the received scattered light in the fiber without additional loss. By controlling the radius and refractive index distribution on the length of the fiber, the backscattering coefficient of the scattered light is increased, and the signal-to-noise ratio of the scattered light is improved while the long distance sensing length is maintained. By analyzing and optimizing the length, radius and refractive index distribution of tapered optical fibers, scattering enhancement of 7dB can be achieved at 45km. Ultra-long adiabatic tapered single-mode fiber can realize Rayleigh backscattered light enhancement of equal signal-to-noise ratio at each position, which can be used to improve the signal-to-noise ratio and measurement range of fiber sensing system based on Rayleigh backscattered, and further promote the application of fiber sensing technology for ultra-long distance sensing.

We present a low-loss Er 3+ -doped silicate microsphere laser, exploring its temperature sensing characteristics. We measure the threshold power of the Er 3+ -doped silicate glass microsphere laser under 980 nm pumping, achieving an impressively low value of 3.14 mW. Subsequently, in order to study the effect of temperature on the laser, the microsphere laser is placed into a temperature controller, and high-temperature sensitivity of 17.56 pm/℃ is obtained in the experiment. The proposed microsphere is significant for applications in lasing and temperature sensing, offering substantial advantages in terms of lower threshold power and heightened temperature sensitivity.

In order to better monitor and identify PCCP pipeline wire broken and obtain wire broken signal characteristics, this paper adopts φ-OTDR system to monitor pipeline wire broken signal and use wavelet packet decomposition to study the energy distribution of wire broken signal in different frequency bands. Firstly, the wire broken signal is collected by φ-OTDR system, and secondly, the wavelet packet decomposition is performed on the wire broken signal collected by the system to obtain the energy distribution of the wire broken vibration signal in different frequency bands. The results show that at 10kHz sampling rate, the frequency band energy (FBE) distribution of the wire broken signal is characterized as follows: the energy of the broken wire signal is higher in the high frequency band (2500-3125Hz, 3437.5-3750Hz, 4375-4687.5Hz) and lower in the low frequency range, and this feature is obvious compared with the non-broken wire case. This method provides a new idea for identifying wire broken events.

Since gravitational waves are extremely weak, the frequency noise of the lasers in space-based gravitational wave detectors need to be substantially reduced. One of the key technologies for this laser frequency reduction is arm locking. Currently, all the reported controllers in arm locking have an inherent tradeoff between the frequency noise suppression ratio and feedback stability. In this paper, for the first time the inverse design idea is introduced into single arm locking controller. By utilizing a multi-parameter co-optimization scheme, the frequency noise suppression ratio and the feedback stability can be optimized simultaneously. This work can be generalized to different kinds of arm locking controller design and will effectively improve the sensitivity of the space-based gravitational wave detector.

The article presents a unique design for a hollow-core anti-resonant fiber, which utilizes a single-layer semi-circular Anti resonant unit positioned around the central core. The simulation analysis reveals this design can achieve transmission over a wide bandwidth of 500-1300nm, while exhibiting relatively low losses at wavelengths near 700-800nm and 900- 1100nm. The anti-resonant fiber with this structure boasts strong wavelength limitation capability for light transmission, along with high transmission stability in a low-loss mode. Based on the simulation analysis results of the changes in loss characteristics, the hollow core anti-resonant fiber we designed has a mode field diameter of approximately 30 microns, and the wall thickness of the core anti-resonant structure is approximately 0.4 microns. By optimizing the process, such as through preform shaping and wire tension adjustment, a 5km long span with high uniformity was achieved for the hollow core anti-resonant fiber, and corresponding tests were carried out. The test results indicate that the total loss value of the hollow anti-resonant fiber, prepared under optimized process conditions, can be managed at a reduced level, with an attenuation of less than 0.1dB/m at 1064nm. Due to the outstanding pure single mode characteristics of optical fibers, there is no obvious mode coupling between the optical signal and the quartz cladding, neither the inner quartz wall in the transmission channel. In devices utilizing fiber enhanced Raman spectroscopy (FERS), this particular fiber can achieve highly sensitive sensing and precise analysis of Raman spectroscopy. Due to its low attenuation characteristics, the system achieves excellent sensing performance in fiber-enhanced Raman spectroscopy systems, enabling precise detection of a 450ppm CO2 concentration. Based on simulation analysis and testing, this article confirms the benefits of this novel fiber in fiber-enhanced Raman spectroscopy systems, presenting a fresh option for researching fiber Raman gas measurement technology.

This paper designs an all fiber-optics disc accelerometer, which uses an online prepared weak fiber Bragg grating (WFBG) array to form a matching interferometer. By utilizing the linear relationship between acceleration and adjacent grating phase changes, 3 × 3 demodulation technology is used to demodulate the phase signal. Through finite element simulation, theoretical analysis is applied on sensitivity and resonance frequency of the accelerometer. Through the tests on vibration exciter, the frequency response range of the sensor array can reach 5Hz-1kHz. The experimental results confirm that the accelerometer features a wide frequency response and excellent linearity.

The depolarized fiber optic gyroscope (DFOG) uses single-mode fibers (SMFs) to fabricate a depolarized fiber optic coil, which has obvious cost advantages in high sensitivity and precision rotation measurement applications, such as fiber optic rotary seismometers, etc. The depolarized fiber coil assembly (DFCA) is the key component of DFOG, consisting of a SMF coil and at least two polarization maintaining fibers (PMFs) depolarizers. To control the quality of the DFCA, this paper proposes an online evaluation technology for it. A more accurate broad-spectrum light source spectral modulation model is established based on polarization mode interference in the DFCA. The model simulation and experimental testing indicate that the modulated spectrum is a multi-period deep modulated spectrum, and the envelope is the same as the input spectrum. Moreover, the self-coherence curve of the modulated spectrum was calculated, and it contains multiple coherent peaks. It was found that the position and amplitude of self-coherence peaks are determined by the length of PMFs and polarization angular deviation at joint points, respectively. A spectrometer was used to replace the detector of the DFOG and an online testing system for the quality of the DFCA was built and an online evaluation process and algorithm is developed. The exact length of PMFs and angular deviation of depolarizers are obtained. The proposed technology has the advantages of easy implementation and high sensitivity, making it well-suited for monitoring and evaluating the quality of DFCA for an assembled high-precision depolarized FOG.

Strain monitoring for components under low-temperature environment is used in a variety of fields, and Fiber Bragg grating (FBG) is ideally suited for cryogenic sensing measurements due to its unique properties. In this paper, a simulation model of surface-adhesive Fiber Bragg grating with the substrate is established for low-temperature strain precise measurement. The strain distribution on the fiber when subjected to thermal strain is explored. Liquid nitrogen impact experiments are conducted with groups of encapsulation parameters, and it is found that the paste length and paste width affect the spectral chirp rate to a different extent. Furthermore, the suitable encapsulation parameters are selected, and temperature compensation experiments are carried out at 77 K to 293 K for surface-adhesive FBG using 304 stainless steel as the substrate material. The temperature compensation curves are derived from the experiments, which provides an accurate temperature compensation scheme for FBG strain measurement.

In this paper, in order to improve the sensitivity of tactile sensor, we proposed and demonstrated a fiber in-line Mach-Zehnder interferometer (MZI) sensor for the measurement of grasp force. The sensor was fabricated by splicing a section of few-mode fiber (FMF) between two single mode fibers (SMFs). The simulation using Rsoft software shows that the sensor has the best performance with the FMF length is 10mm and the core offset is 5um. The mechanical strength of the sensor is improved by using heat shrink tube at the point of optical fiber fusion splicing. The MZI sensor is embedded in the gripper and the force calibration is carried out by using the piezoresistive sensor. The experiment shows that the sensitivity of the sensor is about 6nm/N. The sensor has many advantages including simple fabrication, low cost, and high sensitivity, demonstrating its great application potential in the fields of Human Machine Interface.

We demonstrate a high-sensitivity curvature sensor based on the negative curvature hollow core fiber (NCHCF). The graded index fiber (GIF) is introduced between single mode fiber (SMF) and NCHCF for the sensor of cascaded structure of SMF-GIF-NCHCF-GIF-SMF. The coupling efficiency can be improved by cascading the GIF. The experimental results show that there are two coexisted mechanisms in the cascaded structure, where multi-mode interference (MMI) is sensitive to curvature change and anti-resonance (AR) is insensitive to curvature change. The obtained curvature sensitivity based on MMI is up to -4.97 nm/m-1 . With advantages of compactness, flexibility, high sensitivity and so on, the curvature sensor shows prospects in structural health monitoring, medical applications, energy exploration and other fields.

The measurement range of ceilometer depends both on its optical system characteristics and on the properties of its electronic scheme, including the detector. The optical system of the ceilometer is required to have the lowest possible number of elements and small size. Thus, it appears to be promising to optimize the device characteristics by selecting the detector and the laser characteristics. Signals received in the scheme were compared for two types of photodetector: the first option was the conventional APD detector, and the second one was the HD-SiPM detector. The objective was to study the analog signals for these detectors in a ceilometer that registers the signal backscattered by aerosol. The shape and the features of the signal and the shape of the wavefront registered by these two photodetectors in the scheme are discussed.

Self-mixing interferometry (SMI) is a new sensing technology. However, the Self-Mixing Signal (SMS) used for measurement is usually subject to noise or interference, resulting in poor measurement accuracy. To get around this problem, in this paper, we propose a novel method which can remove the frequency distortion and noises simultaneously using variational mode decomposition (VMD). Proper choices of the VMD parameters are made based on the characteristics of the SMSs and an effective algorithm is designed for retrieving the distance and velocity of the target simultaneously. The proposed method has been verified by both simulation and experimental data showing the measured distance and velocity are more accurate than those using filtering methods.

In this work, a new scheme based on a Si₃N₄ microresonator for generating parallel pulsed chaos is proposed, and the performances of the parallel pulsed chaos and its application in imaging are experimentally investigated. Under optical injection with suitable injection parameters, the Si₃N₄ microresonator can output a continuous wave (CW) chaotic microcomb including nearly 100 comb lines. After passing through an acousto-optic modulator, the CW chaotic microcomb can be transferred into pulsed chaotic microcomb, in which each comb line provides a pulsed chaos. Therefore, parallel pulsed chaos signal can be generated. Taken the parallel pulsed chaos signal as the emitting resource of lidar, the quality of imaging has been analyzed. The experimental results show that clear target imaging can be achieved.

Marine instruments deployed in seawater inevitably experience biofouling, which severely reduces their service life and hinders ocean monitoring. Marine biofouling greatly affects the service life of marine optical instruments and thus has a detrimental impact on ocean monitoring. The fouling community exhibits an attachment succession phenomenon. Macroscopic fouling organisms have adherent and stubborn attachments, whereas microorganisms during early fouling stages are easy to remove, but excessive cleaning also greatly increases energy consumption. Therefore, monitoring biofouling and selecting appropriate removal timing is critical. Due to the complex and dynamic nature of the marine environment, in-situ detection of microbial fouling on optical window of marine optical instrument is challenging because of many factors such as target characteristics, seawater turbidity, light refraction and scattering. Currently, there are no mature technologies available for in-situ fouling detection so as to remove timely micro fouling. To solve this problem, this study deployed thin poly methyl methacrylate (PMMA) coupons within the coastal seawaters of Qingdao, followed by in-situ mapping of photoacoustic signals using a self-built excitation and detection platform, along with along with of transmittance spectrum analysis on fouled PMMA thin films using PerkinElmer LAMBDA750. By combining results from both techniques with microscopic morphology analysis, we explored the relationship between microbial fouling and photoacoustic signal. The research results will provide a novel approach and technical basis for in-situ detection and timely clearance of microbial fouling on optical windows of marine optical instruments.

Partial discharge often occurs in power equipment, which erodes insulation and eventually results in insulation failure. As partial discharge will generate sound such as acoustic or ultrasonic waves, a phase-sensitive optical time-domain reflectometry (φ-OTDR) is used to measure these signals. Then the number and locations of sound sources can be calculated using array signal processing methods such as Gerschgorin circle theorem and MUSIC (multiple signal classification) algorithm. In the experiment, several sound sources with frequencies below 600 kHz are used to simulate the partial discharges. Our system shows a high sensitivity and strong noise resistance performance. Accurate number estimation is realized with positioning errors less than 3 cm

Brillouin optical correlation domain analysis (BOCDA) offers high spatial resolution and multi-point measurement capabilities which are ideal for transformer winding temperature measurements. In this article, a BOCDA prototype is developed by changing many equipment to modules, and using field-programmable gate arrays (FPGA) for the control, acquisition, and processing. The system achieves a desired performance with a spatial resolution of 10 cm, a Brillouin frequency shift (BFS) accuracy of about 0.9 MHz, and a measurement speed of 500 points/second.

Microdisks and micro-rings are commonly used micro-optical devices that greatly enhance the interaction between light and matter within a cavity due to their high-quality factor and small mode volume, making them widely used in microcavity optical sensing. By introducing parity-time (PT) symmetric structures into the microcavity, the coupling efficiency of the optical field inside the cavity can be improved, which is conducive to obtaining higher sensing sensitivity. We theoretically verify the feasibility of using a PT-symmetric micro-ring coupled microdisk composite cavity as an active sensor based on the characteristics of exceptional point (EP) enhanced sensing in PT-symmetric systems. Gain is introduced to the microdisk cavity by injecting current until the system undergoes PT symmetry breaking, i.e., when the sensor is at the EP, the transmission of light will exhibit a nonlinear enhancement effect due to the degeneracy of eigenvalues and corresponding eigenvectors of the system, making the signal more sensitive to changes in the sensing medium. The results obtained through the finite difference time domain method show that the intensity sensitivity of the PT-symmetric microcavity at the EP is improved by about 11.8 times compared with the conventional microcavity when the working wavelength is in the range of communication band and different concentrations of the same gas are injected into the air, which is expected to provide reference and insight for the further development of microcavities for refractive index sensing.

A novel multi-parameter sensor has been proposed based on the W-type leaky fibre grating. The resonant wavelength for different mode in the reflection spectra has the same temperature sensitivity of ~9.13 pm/oC, but the reflection intensity has no dependence upon the temperature. The resonant wavelength of different mode linearly varies a little with refractive index increasing, however, its reflection intensity follows the negative exponential decay. The resonant wavelength for each mode almost linearly responds to the displacement, giving out a displacement sensitivity of 99.94 pm/mm. However, the reflection response to the displacement varies a lot. Based on these results, simple and compact multi-parameter fibre sensors are proposed based on W fibre grating.

Efficient and ultra-sensitive sensing of absolute angular position are critical technological requirements in various fields. Traditional angle measurement methods have disadvantages in terms of accuracy and device complexity, at the same time, the absolute angular position information cannot be obtained. Here, we report an laser angular position approach based on pixelated metasurfaces. By harnessing the interaction between a C-band Optical Frequency Comb (OFC) laser source and a plasmonic metasurface, we achieve precise angle encoding, enabling each individual meta-pixel within the pixelated metasurface to exhibit ultra-sensitive responses to angle variations. Experimental results demonstrate an angle resolution as low as µrad-level. Furthermore, by controlling the dimensions of meta-atom structures and designing specific combinations of meta-pixels, we manipulate the unique angular responses of individual pixels, effectively creating visualized unique scattering pattern. This technology efficiently captures angle information, and based on this, we demonstrate absolute angular position sensing.

Helical-core fibers have been widely utilized for various applications such as orbital angular momentum manipulation, optical filtering, and sensing. In this work, we demonstrate the sensitivity enhancement of a helical-core fiber by tailoring the resonance wavelength towards the dispersion turning point (DTP) utilizing a post chemical etching process. When the sensor works at different dispersion points, the responses of the device on external parameters, including mechanical torsion, temperature, strain, and external refractive index are investigated and compared. Tailoring the resonance wavelength of the helical-core fiber towards the dispersion turning point in the post chemical etching process significantly enhances the sensitivity to various kinds of parameters, which provides an avenue for modifying and optimizing performances of optical devices.

This paper investigates the performance of 2D-materials (such as Graphene and Maxine) assisted paper substrate-based electrodes for electrocardiogram (ECG) monitoring in personalized and ubiquitous healthcare. As a cost-effective rapid prototyping, the paper substrate-based technique allows the personalization of electrode sizes and shapes. It can be used with a range of different inks to provide better skin contact, which provides a very efficient and accurate measurement of the ECG signal. We compare the performance of graphene and MXene ink-based electrodes, which are printed directly on a paper substrate using graphite pencil and drop cast methods. The time-frequency decomposition of the collected signals is explored and reported. The results demonstrate that both electrodes can record ECG information with comparable performance to conventional Ag/AgCl electrodes.

In this paper, we demonstrate the operation of flexible and highly sensitive MXENE-based capacitive strain sensor fabricated on a paper substrate. The use of common printing paper as a substrate not only makes the fabrication facile and inexpensive but also makes the sensor eco-friendly and biodegradable. The presented sensor has been mounted on the human body to monitor the movements of various joints like the wrist and the elbow. Upon application of strain, a noticeable change of more than 30% in the total capacitance of the sensor is observed. By measuring the change in the capacitance of the sensor, the movements of human joints can be monitored and used for applications like exercise and therapy.