Cascaded multicore fiber interferometers for enhanced bending sensing based on the Vernier effect.

In this paper, cascaded modal interferometers constructed by strongly-coupled seven-core fiber (SC-SCF) with different lengths are demonstrated for enhanced bending sensing based on Vernier effect. The free spectral range (FSR) of a single SC-SCF interferometer is determined by the length of SC-SCF. Two SC-SCF interferometers with different FSRs are cascaded, in which, one functions as the sensor while the other functions as the reference. The wavelength shift of the envelope of the output spectrum is much larger than that of a single SC-SCF interferometer due to the Vernier effect. Therefore, enhanced sensing can be achieved. Experimental results show that the bending sensitivity of the proposed sensor is improved from -2.20 nm/m-1 (single SC-SCF interferometer) to 42.32 nm/m-1 (cascaded SC-SCF interferometers). The temperature response of the sensor is also investigated. Our proposed cascaded SC-SCF sensor has advantages of high sensitivity, ease of fabrication, and low cost. It is attractive for high precision bending sensing applications.

Operando Decoding of Surface Chemical and Thermal Events in Photoelectrocatalysis via a Lab-Around-Microfiber Sensor.

Operando decoding of the key parameters of photo-electric catalysis provides reliable information for catalytic effect evaluation and catalytic mechanism exploration. However, to capture the details of surface-localized and rapid chemical and thermal events at the nanoscale in real-time is highly challenging. A promising approach based on a lab-around-microfiber sensor capable of simulating photo-electric catalytic reactions on the surface of optical fibers as well as monitoring reactant concentration changes and catalytic heat generation processes is demonstrated. Due to the penetration depth of submicron size and the fast response ability of the evanescent field, the lab-around-microfiber sensor overcame the difficulty of reading instantaneous surface parameters in the submicron range. This sensor operando dismantled the changes in reactant concentration and temperature on the catalyst surface induced by light and voltage, respectively. It also decoded the impact of catalyst composition on the adsorption efficiency and catalytic efficiency across various wavelengths and determined the synchronized occurrence of pollutant degradation and catalytic thermal effects. Stable correlations between the real-time parameters and catalytic activities are obtained, helping to provide a basic understanding of the catalytic process and mechanism. This approach fills an important gap in the current monitoring methods of catalytic processes and heat production.

Glass Ceramic Fibers Containing PbS Quantum Dots for Fluorescent Temperature Sensing.

Glass ceramics (GCs) containing PbS quantum dots (QDs) are prepared for temperature sensing. Broadband emissions are detected in the GCs when PbS QDs are precipitated from the glasses, and emissions centers are modulated from 1250 nm to 1960 nm via heat treatments. The emission centers of GCs exhibit blue-shifts when environment temperatures increase from room temperature to 210 °C. Importantly, the shift values of emission centers increase linearly with the test temperature, which is beneficial for applications in temperature sensing. A temperature sensor based on PbS QDs GC is heat-treated at 500 °C for 10 h, possesses the highest sensitivity of 0.378 nm/°C, and exhibits excellent stability and repeatability at high temperatures (up to 210 °C). Moreover, GC fibers are fabricated by using the GCs as the fiber core. The sensitivity of the temperature-sensing sensor of the GC fibers is also demonstrated and the sensitivity is as high as 0.558 nm/°C. The designed PbS QDs GCs provide a significant materials base for the manufacturing of fluorescent temperature sensors and the GC fibers offer significant opportunities for temperature detection in complex, integrated and compact devices.

A capillary-aided microfiber Bragg grating pH sensor for hydrovoltaic technology.

Hydrovoltaic is an emerging technology that aims to harvest energy from water flow and evaporation, in which the plasmonic hydrogen ions are generated by the interaction between water and hydrovoltaic device. However, the volume of the water sample for the interaction is usually ultra-small due to the compact size of hydrovoltaic device, making the quantification and characterization of the hydrogen ions in such water sample an elusive goal. To address this issue, a miniature fiber-optic pH probe is proposed using a unilaterally tapered-microfiber Bragg grating. The microfiber Bragg grating has an intrinsic Bragg reflection signal with a narrow linewidth. The fiber probe is functionalized by coating the sodium alginate, which can respond to the variation of pH mediated by the alteration of the hydrophilicity. The rigidity and robustness of microfiber Bragg grating facilitates the encapsulation of the sensor into a sampling capillary, allowing for the detection of trace aqueous sample less than 2 µL. The pH sensitivity of the tapered-µFBG-based sensor is 62.8 p.m./pH (R2 = 0.995) with a limit resolution of 0.096 pH. The sensor performed a practical application in the monitoring and characterization of the hydrovoltaic microdevice, which can generate microcurrent as soaked in the water. This work demonstrates a promising technology in the fields of materials, energy, biology and medicine, in which the detection of the microsamples is inevitable.

Quantitative Assessment of Fungal Biomarkers in Clinical Samples via an Interface-Modulated Optical Fiber Biosensor.

Invasive fungal infections pose a significant public health threat. The lack of precise and timely diagnosis is a primary factor contributing to the significant increase in patient mortality rates. Here, an interface-modulated biosensor utilizing an optical fiber for quantitative analysis of fungal biomarkers at the early stage of point-of-care testing (POCT), is reported. By integrating surface refractive index (RI) modulation and plasmon enhancement, the sensor to achieve high sensitivity in a directional response to the target analytes, is successfully optimized. As a result, a compact fiber-optic sensor with rapid response time, cost-effectiveness, exceptional sensitivity, stability, and specificity, is developed. This sensor can successfully identify the biomarkers of specific pathogens from blood or other tissue specimens in animal models. It quantifies clinical blood samples with precision and effectively discriminates between negative and positive cases, thereby providing timely alerts to potential patients. It significantly reduces the detection time of fungal infection to only 30 min. Additionally, this approach exhibits remarkable stability and achieves a limit of detection (LOD) three orders of magnitude lower than existing methods. It overcomes the limitations of existing detection methods, including a high rate of misdiagnosis, prolonged detection time, elevated costs, and the requirement for stringent laboratory conditions.

Free-moving-state microscopic imaging of cerebral oxygenation and hemodynamics with a photoacoustic fiberscope.

We report the development of a head-mounted photoacoustic fiberscope for cerebral imaging in a freely behaving mouse. The 4.5-gram imaging probe has a 9-µm lateral resolution and 0.2-Hz frame rate over a 1.2-mm wide area. The probe can continuously monitor cerebral oxygenation and hemodynamic responses at single-vessel resolution, showing significantly different cerebrovascular responses to external stimuli under anesthesia and in the freely moving state. For example, when subjected to high-concentration CO2 respiration, enhanced oxygenation to compensate for hypercapnia can be visualized due to cerebral regulation in the freely moving state. Comparative studies exhibit significantly weakened compensation capabilities in obese rodents. This new imaging modality can be used for investigating both normal and pathological cerebrovascular functions and shows great promise for studying cerebral activity, disorders and their treatments.

Point-of-care diagnosis of pre-eclampsia based on microfiber Bragg grating biosensor.

Pre-eclampsia is a serious multi-organ complication that severely threatens the safety of pregnant women and infants. To accurate and timely diagnose pre-eclampsia, point-of-care (POC) biosensing of the specific biomarkers is urgently required. However, one of the key biomarkers of pre-eclampsia, placental growth factor (PlGF), has a reduced level of expression in patients, which challenges the quantification capability and Limit-of-detection (LOD) of biosensors. Herein, we reported a microfiber Bragg grating biosensor for the quantification of PlGF in clinical serum samples. The Bragg grating was inscribed in a unilateral tapered fiber to generate the segmented Fabry-Perot spectrum for improving the capability of detection. Furthermore, a temperature-calibrated Bragg grating was added to enable dual parametric detection of PlGF and temperature simultaneously for removing the crosstalk. Finally, the biosensor was envisaged to be perfectly compatible with microfluidic chips, and thus dramatically reducing the sample consumption to as small as 10 µL. The proposed biosensor can respond to PlGF with concentrations ranging from 5 to 120 pg mL-1, attaining a LOD of 5 pg mL-1 of clinical relevance. More importantly, the biosensor achieved micro volume detection of clinical serum samples from patients, and the ROC curve with an AUC of 0.977 confirmed the viability of the device. Our study paves the way to a new idea for cost-effective and high-precision screening of patients with pre-eclampsia, and hence envisages a promising prospect for point-of-care (POC) diagnosis of patients with pre-eclampsia.

Enabling Broadband Solar-Blind UV Photodetection by a Rare-Earth Doped Oxyfluoride Transparent Glass-Ceramic.

Oxyfluoride transparent glass-ceramics (GC) are widely used as the matrix for rare-earth (RE) ions due to their unique properties such as low phonon energy, high transmittance, and high solubility for RE ions. Tb3+ doped oxyfluoride glasses exhibit a large absorption cross section for ultraviolet (UV) excitation, high stability, high photoluminescence quantum efficiency, and sensitive spectral conversion characteristics, making them promising candidate materials for use as the spectral converter in UV photodetectors. Herein, a Tb3+ doped oxyfluoride GC is developed by using the melt-quenching method, and the microstructure and optical properties of the GC sample are carefully investigated. By combining with a Si-based photo-resistor,a solar-blind UV detector is fabricated, which exhibits a significant photoelectric response with a broad detection range from 188 to 400 nm. The results indicate that the designed UV photodetector is of great significance for the development of solar-blind UV detectors.

Ultrasensitive Photothermal Switching with Resonant Silicon Metasurfaces at Visible Bands.

Dynamic access to quasi-bound states in the continuum (q-BICs) offers a highly desired platform for silicon-based active nanophotonic applications, while the prevailing tuning approaches by free carrier injections via an all-optical stimulus are yet limited to THz and infrared ranges and are less effective in visible bands. In this work, we present the realization of active manipulations on q-BICs for nanoscale optical switching in the visible by introducing a local index perturbation through a photothermal mechanism. The sharp q-BIC resonance exhibits an ultrasensitive susceptibility to the complex index perturbation, which can be flexibly fulfilled by optical heating of silicon. Consequently, a mild pump intensity of 1 MW/cm2 can yield a modification of the imaginary part of the refractive index of less than 0.05, which effectively suppresses the sharp q-BIC resonances and renders an active modulation depth of reflectance exceeding 80%. Our research might open up an enabling platform for ultrasensitive dynamic nanophotonic devices.

Marriage of a Dual-Plasmonic Interface and Optical Microfiber for NIR-II Cancer Theranostics.

The use of light as a powerful tool for disease treatment has introduced a new era in tumor treatment and provided abundant opportunities for light-based tumor theranostics. This work reports a photothermal theranostic fiber integrating cancer detection and therapeutic functions. Its self-heating effect can be tuned at ultralow powers and used for self-heating detection and tumor ablation. The fiber, consisting of a dual-plasmonic nanointerface and an optical microfiber, can be used to distinguish cancer cells from normal cells, quantify cancer cells, perform hyperthermal ablation of cancer cells, and evaluate the ablation efficacy. Its cancer cell ablation rate reaches 89% in a single treatment. In vitro and in vivo studies reveal quick, deep-tissue photonic hyperthermia in the NIR-II window, which can markedly ablate tumors. The marriage of a dual-plasmonic nanointerface and an optical microfiber presents a novel paradigm in photothermal therapy, offering the potential to surmount the challenges posed by limited light penetration depth, nonspecific accumulation in normal tissues, and inadvertent damage in current methods. This work thus provides insight for the exploration of an integrated theranostic platform with simultaneous functions in cancer diagnostics, therapeutics, and postoperative monitoring for future practical applications.

Sensing characteristics of structural microfiber long-period gratings.

We present a detailed investigation into the sensing characteristics of a structural microfiber long-period grating (mLPG) sensor. By spirally winding a thinner microfiber to another thicker microfiber, periodic refractive index modulation is formed while the optical signal transmitted in the thicker microfiber is resonantly coupled out to the thinner microfiber, and then a 5-period four-port mLPG can be obtained with a device length of only ∼570 µm demonstrated a strong resonant dip of 25 dB. We studied the sensitivity characteristics of the four-port mLPG with surrounding strain, force, temperature and refractive index, and the obtained sensitivities were -6.4 pm/µÉ›, -8418.6 nm/N, 7.62 pm/°C and 2122 nm/RIU, respectively. With the advantages of high refractive index sensitivity and wide wavelength tunable range, the four-port mLPG has great potential in applications such as tunable filters and biochemical sensor.

Fabry-Pérot Cavities with Suspended Palladium Membranes on Optical Fibers for Highly Sensitive Hydrogen Sensing.

Hydrogen (H2) sensors are critical to various applications such as the situation where H2 is used as the clean energy for industry or the indicator for human disease diagnosis. Palladium (Pd) is widely used as the hydrogen sensing material in different types of sensors. Optical fiber H2 sensors are particularly promising due to their compactness and spark-free operation. Here, we report a Fabry-Pérot (FP)-cavity-based H2 sensor that is formed with a freestanding Pd membrane and integrated on a conventional single-mode optical fiber end. The freestanding Pd membrane acts both as the active hydrogen sensing material and as one of the reflective mirrors of the cavity. When the Pd film absorbs H2 to form PdHx, it will be stretched, resulting in a change of the cavity length and thus a shift of the interference spectrum. The H2 concentration can be derived from the amplitude of the wavelength shift. Experimental results showed that H2 sensors based on suspended Pd membranes can achieve a detection sensitivity of about 3.6 pm/ppm and a detection limit of about 3.3 ppm. This highly sensitive detection scheme is expected to find applications for sensing low-concentration H2.

Compact optical fiber photoacoustic gas sensor with integrated multi-pass cell.

Optical fiber acoustic sensors with miniature size and high sensitivity are attractive to develop compact photoacoustic spectroscopy. Here, a compact photoacoustic gas sensor was demonstrated by utilizing a diaphragm-based fiber-optic Fabry-Perot cavity as both the acoustic sensor and the multipass cell. A nanoscale graphite film was used as the flexible diaphragm to increase the acoustic sensitivity of the Fabry-Perot cavity and the cavity inner surface was coated with highly-reflective Au film to form a multipass cell for amplification of the photoacoustic signal. With a laser power of 20 mW at 1532.8 nm, the sensor demonstrated a low detection limit of ∼ 50 ppb for C2H2 gas with an integration time of ∼ 100 s. The optical fiber photoacoustic gas sensor with a millimeter-scale diameter and ppb-level detection limit is promising for trace gas sensing in various areas including industrial process and environmental monitoring.

Plasmonic Coupling on an Optical Microfiber Surface: Enabling Single-Molecule and Noninvasive Dopamine Detection.

Optical fibers can be effective biosensors when employed in early-stage diagnostic point-of-care devices as they can avoid interference from molecules with similar redox potentials. Nevertheless, their sensitivity needs to be improved for real-world applications, especially for small-molecule detection. This work demonstrates an optical microfiber biosensor for dopamine (DA) detection based on the DA-binding-induced aptamer conformational transitions that occur at plasmonic coupling sites on a double-amplified nanointerface. The sensor exhibits ultrahigh sensitivity when detecting DA molecules at the single-molecule level; additionally, this work provides an approach for overcoming optical device sensitivity limits, further extending optical fiber single-molecule detection to a small molecule range (e.g., DA and metal ions). The selective energy enhancement and signal amplification at the binding sites effectively avoid nonspecific amplification of the whole fiber surface which may lead to false-positive results. The sensor can detect single-molecule DA signals in body-fluids. It can detect the released extracellular DA levels and monitor the DA oxidation process. An appropriate aptamer replacement allows the sensor to be used for the detection of other target small molecules and ions at the single-molecule level. This technology offers alternative opportunities for developing noninvasive early-stage diagnostic point-of-care devices and flexible single-molecule detection techniques in theoretical research.

ZIF-90-Derived Porous ZnO Coated Optical Microfiber Interferometer Sensor for Enhanced Humidity Sensing and Breath Monitoring.

Humidity plays an important role in many fields, and the realization of high sensitivity and fast response simultaneously for humidity detection is a great challenge in practical application. In this work, we demonstrated a high-performance relative humidity (RH) sensor made by supporting zeolitic imidazolate framework-90 (ZIF-90)-derived porous zinc oxide (ZnO) onto an optical microfiber Sagnac interferometer (OMSI). The ZIF-90-modified OMSI (ZIF-90-OMSI) sensor was in situ heated at different temperatures to obtain porous ZnO, and their humidity-sensing properties were investigated ranging from 25 to 80% RH. The experimental results showed that the porous ZnO fiber sensor prepared at 500 °C (Z500-OMSI) exhibited best humidity-sensing performance with a high sensitivity of 96.2 pm/% RH (25-45% RH) and 521 pm/% RH (50-80% RH) and ultrafast response/recovery time (62.37/206.67 ms) at 22.3% RH. These performances were attributed to the complete transformation of ZIF-90 to ZnO at 500 °C. The obtained Z500 not only retained the high porosity and specific surface area of ZIF-90 but also exhibited the exceptional hydrophilicity of ZnO. In addition, the signals of the proposed Z500-OMSI sensor changed with different breathing patterns, indicating the possibility for human respiration monitoring. This work provided a reliable candidate for an effective RH monitoring system with potential application in medical diagnoses, industrial production, environmental detection, and human health monitoring.

Near-infrared microfiber Bragg grating for sensitive measurement of tension and bending.

Fiber-optic devices working in the visible and near-infrared windows are attracting attention due to the rapid development of biomedicine that involves optics. In this work, we have successfully realized the fabrication of near-infrared microfiber Bragg grating (NIR-µFBG), which was operated at the wavelength of 785 nm, by harnessing the fourth harmonic order of Bragg resonance. The NIR-µFBG provided the maximum sensitivity of axial tension and bending to 211 nm/N and 0.18 nm/deg, respectively. By conferring the considerably lower cross-sensitivity, such as response to temperature or ambient refractive index, the NIR-µFBG can be potentially implemented as the highly sensitive tensile force and curve sensor.

Nanoscale Adsorption, Assembly, and Deionization Dynamics Recorded by Optical Fiber Sensors.

Capacitive deionization in environmental decontamination has been widely studied and now requires intensive development to support large-scale deployment. Porous nanomaterials have been demonstrated to play pivotal roles in determining decontamination efficiency and manipulating nanomaterials to form functional architecture has been one of the most exciting challenges. Such nanostructure engineering and environmental applications highlight the importance of observing, recording, and studying basically electrical-assisted charge/ion/particle adsorption and assembly behaviors localized at charged interfaces. In addition, it is generally desirable to increase the sorption capacity and reduce the energy cost, which increase the requirement for recording collective dynamic and performance properties that stem from nanoscale deionization dynamics. Herein, we show how a single optical fiber can serve as an in situ and multifunctional opto-electrochemical platform for addressing these issues. The surface plasmon resonance signals allow the in situ spectral observation of nanoscale dynamic behaviors at the electrode-electrolyte interface. The parallel and complementary optical-electrical sensing signals enable the single probe but multifunctional recording of electrokinetic phenomena and electrosorption processes. As a proof of concept, we experimentally decipher the interfacial adsorption and assembly behaviors of anisotropic metal-organic framework nanoparticles at a charged surface and decouple the interfacial capacitive deionization within an assembled metal-organic framework nanocoating by visualizing its dynamic and energy consumption properties, including the adsorptive capacity, removal efficiency, kinetic properties, charge, specific energy consumption, and charge efficiency. This simple "all-in-fiber" opto-electrochemical platform offers intriguing opportunities to provide in situ and multidimensional insights into interfacial adsorption, assembly, and deionization dynamics information, which may contribute to understanding the underlying assembly rules and the exploring structure-deionization performance correlations for the development of tailor-made nanohybrid electrode coatings for deionization applications.

In vivo endoscopic ultrasound imaging with a rotational-scanning, all-optical ultrasound probe.

All-optical ultrasound manipulates ultrasound waves based on laser and photonics technologies, providing an alternative approach for pulse-echo ultrasound imaging. However, its endoscopic imaging capability is limited ex vivo by the multifiber connection between the endoscopic probe and the console. Here, we report on all-optical ultrasound for in vivo endoscopic imaging using a rotational-scanning probe that relies on a small laser sensor to detect echo ultrasound waves. The acoustically induced lasing frequency change is measured via heterodyne detection by beating the two orthogonally polarized laser modes, enabling a stable output of ultrasonic responses and immunity to low-frequency thermal and mechanical disturbances. We miniaturize its optical driving and signal interrogation unit and synchronously rotate it with the imaging probe. This specialized design leaves a single-fiber connection to the proximal end and allows fast rotational scanning of the probe. As a result, we used a flexible, miniature all-optical ultrasound probe for in vivo rectal imaging with a B-scan rate of 1 Hz and a pullback range of ∼7 cm. This can visualize the gastrointestinal and extraluminal structures of a small animal. This imaging modality offers an imaging depth of 2 cm at a central frequency of ∼20 MHz, showing promise for high-frequency ultrasound imaging applications in gastroenterology and cardiology.

Transparent microfiber Fabry-Perot ultrasound sensor with needle-shaped focus for multiscale photoacoustic imaging.

Photoacoustic tomography emerged as a promising tool for noninvasive biomedical imaging and diseases diagnosis. However, most of the current piezoelectric ultrasound transducers suffer optical opacity and tissue-mismatched acoustic impedance, hindering the miniaturization and integration of the system for multiscale and multimodal imaging. Here, a transparent polydimethylsiloxane (PDMS) encapsulated optical microfiber ultrasound sensor was demonstrated for photoacoustic imaging with scalable spatial resolution and penetration depth. The sensor comprised a microfiber loop sandwiched by a pair of in-line Bragg gratings, which formed an ultrasound-sensitive Fabry-Perot cavity allowing free delivery of ultrasound/light beams and unique needle-shaped ultrasound focusing along the penetration depth. The sensor with a detection limit of ∼ 700 Pa and a bandwidth of ∼ 10 MHz was applied for multiscale photoacoustic imaging of mouse ear and brain vasculatures. With advantages of flexibility, optical transparence and focusing capability, the sensor offers new opportunities for developing photoacoustic/ultrasound imaging devices for biomedical and clinic applications.

Supermode Bragg grating inscribed in a strongly coupled seven-core fiber and its responses to temperature and curvature.

A Bragg grating is successfully inscribed in a piece of strongly coupled seven-core fiber (SCF). There are two separate Bragg resonance notches observed in the transmission spectrum, corresponding to backward coupling of HE11-like and HE12-like supermodes of the SCF. The mode coupling mechanism of the Bragg grating is theoretically investigated via modeling and analyzing modal properties of the SCF. The theoretical results agree well with the experimental results. Since the SCF is spliced between two standard single mode fibers with central alignments at both ends, the transmission spectrum of the device also contains a set of interference fringe due to modal interference between the supermodes. The device's responses to temperature and curvature are experimentally measured, respectively. The obtained temperature sensitivities and curvature sensitivities of the supermode Bragg grating notches are 9.55 pm/°C and 9.55 pm/°C, -1.8 pm/m-1 and -112.3 pm/m-1, respectively. The obtained temperature sensitivity and curvature sensitivity of one of the interference spectrum dips are 11.8 pm/°C and -3909.8 pm/m-1, respectively. This device is potentially useful for simultaneous measurement of temperature and curvature.