Multifocal meta-fiber based on the fractional Talbot effect.

Multi-focusing of light is a crucial capability for photonic devices that can be effectively achieved by precisely modulating the phase delay on the incident wavefront. However, integrating functional structures into optical fibers for remote light focusing remains challenging due to the complex device design and limited fabrication approaches. Here, we present the design and fabrication of metalens array on the end-face of a tailored single-mode step-index fiber for focusing light field into closely packed focal spot array. The metalenses are configured based on the fractional Talbot effect and benefit a modular design capability. Light passing through the optical fiber can be focused into different focal planes. With a synergistic 3D laser nanoprinting technique based on two-photon polymerization, high-quality meta-fibers are demonstrated for focusing light parallelly with a uniform numerical aperture (NA) as high as approximately 0.77. This may facilitate various applications such as optical trapping, generation of sophisticated beam profiles, and boosting light coupling efficiencies.

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.

Surface-wettable nonenzymatic fiber-optic sensor for selective detection of hydrogen peroxide.

A micro-nanostructure-based surface-modified fiber-optic sensor has been developed herein to selectively detect hydrogen peroxide (H2O2). In our design, phenylboronic ester-modified polymers were used as a modified cladding medium that allows chemo-optic transduction. Sensing is mechanistically based on oxidation and subsequent hydrolysis of the phenylboronic ester-modified polymer, which modulates hydrophobic properties of fiber-optic devices, which was confirmed during characterization of the chemical functional group and hydrophobicity of the active sensing material. This work illustrates a useful strategy of exploiting principles of chemical modifications to design surface-wettable fiber-optic sensing devices for detecting reactive species of broad relevance to biological and environmental analyses.

Au-NPs signal amplification ultra-sensitivity optical microfiber interferometric biosensor.

An optical microfiber interferometric biosensor for the low concentration detection of sequence-specific deoxyribonucleic acid (DNA) based on signal amplification technology via oligonucleotides linked to gold nanoparticles (Au-NPs) is proposed and experimentally analyzed. The sensor uses a "sandwich" detection strategy, in which capture probe DNA (DNA-c) is immobilized on the surface of the optical microfiber interferometer, the reporter probe DNA (DNA-r) is immobilized on the surface of Au-NPs, and the DNA-c and DNA-r are hybridized to the target probe DNA (DNA-t) in a sandwich arrangement. The dynamic detection of the DNA-t was found to range from 1.0×10-15 M to 1.0×10-8 M, and the limit of detection (LOD) concentration was 1.32 fM. This sensor exhibited not only a low LOD but also excellent selectivity against mismatched DNA-t, and it can be further developed for application in various sensing platforms.

Sensitivity enhancement of a fiber-based interferometric optofluidic sensor.

Optofluidic sensors, which tightly bridge photonics and micro/nanofluidics, are superior candidates in point-of-care testing. A fiber-based interferometric optofluidic (FIO) sensor can detect molecular biomarkers by fusing an optical microfiber and a microfluidic tube in parallel. Light from the microfiber side coupled to the microtube leads to lateral localized light-fluid evanescent interaction with analytes, facilitating sensitive detection of biomolecules with good stability and excellent portability. The determination of the sensitivity with respect to the interplay between light and fluidics, however, still needs to be understood quantitatively. Here, we theoretically and experimentally investigate the relationship between refractive index (RI) sensitivity and individual geometrical parameters to determine the lateral localized light-fluid evanescent interaction. Theoretical analysis predicted a sensitive maximum, which could be realized by synergically tuning the fiber diameter d and the tube wall thickness t at an abrupt dispersion transition region. As a result, an extremely high RI sensitivity of 1.6×104 nm/RIU (σ=4074 nm/RIU), an order of magnitude higher than our previous results, with detection limit of 3.0×10-6 RIU, is recorded by precisely governing the transverse geometry of the setup. The scientific findings will guide future exploration of both new light-fluid interaction devices and biomedical sensors.

Development of an optical microfiber immunosensor for prostate specific antigen analysis using a high-order-diffraction long period grating.

Fiber-optic biosensors are of great interest to many bio/chemical sensing applications. In this study, we demonstrate a high-order-diffraction long period grating (HOD-LPG) for the detection of prostate specific antigen (PSA). A HOD-LPG with a period number of less than ten and an elongated grating pitch could realize a temperature-insensitive and bending-independent biosensor. The bio-functionalized HOD-LPG was capable of detecting PSA in phosphate buffered saline with concentrations ranging from 5 to 500 ng/ml and exhibited excellent specificity. A limit of detection of 9.9 ng/ml was achieved, which is promising for analysis of the prostate specific antigen.

A universal strategy: Rational construction of noble metal nanoparticle-shell/conducting polymer nanofiber-core electrodes with enhanced electrochemical performances.

To address a challenge for decoration of noble metal nanoparticles (NMNPs)-shell on conducting polymer nanofiber (CPNF) electrodes (i.e. NMNP-shell/CPNF-core electrodes) for boosting electrochemical performances, a two-step strategy comprising chemical pre-deposition and electrochemical deposition is designed. The strategy shows a high universality in terms of the diversity of NMNP-shell elements (single-element: AgNP-shell, AuNP-shell, PtNP-shell, PdNP-shell; multi-element: Au/Pt/PdNP-shell) and the independence of conductive substrates of electrodes. The shells are composed of high-density NMNPs and have strong adhesion to CPNF-cores. It is demonstrated that in response to a specific applied electrical stimulus, the resulting low doping level of CPNFs facilitates the generation of high-density nucleation sites (small NMNPs) by chemical pre-deposition (as high capability of electron transfer and low resistance to electron transfer from CP chains to NM ions), which is indispensable for the formation of NMNP-shells on CPNF-cores by electrochemical deposition. The decoration of NMNP-shells can significantly enhance the electrochemical performances of CPNF electrodes. Moreover, the great practicality and reliability of NMNP-shell/CPNF-core electrodes in use as an electrocatalytic platform are confirmed. This universal strategy opens up a new avenue to construct high-dimension shell/core-nanostructured electrodes.

Calibration method of three-dimensional plane array laser radar based on space-time transformation.

As a measurement system that can realize target detection and optical imaging, the accuracy of three-dimensional laser radar is a main performance index, which makes calibration an extremely important work. Traditional calibration methods have many disadvantages, such as harsh environment requirements, complex and tedious calibration processes, and inaccurate calibration results. To solve these problems, we propose a calibration method so that the relative position of the cooperative target and the detection sensor is fixed. The principle of space-time transform is used to simulate distance, and the synchronous control of distance is realized by controlling the delay module. In addition, a simple and practical calibration device is designed. In the actual measurement, the average absolute error is 0.0019 m, and the relative error is 0.0678% in the range of 0.5-25 m. The experimental results show that this method is stable and accurate, and it can calibrate the plane array laser radar quickly and accurately.

Method of improving the measurement accuracy of plane array imaging laser radar by pixel cascade.

The plane array imaging laser radar is the product of the combination of the area array imaging optical system and the range laser radar. With extended illumination and area array detection, the target can be measured in three dimensions without mechanical scanning. In this paper, a CMOS-type built-in optical mixer is used, which is a kind of matrix depth sensor. The size of a single pixel of the sensor chip determines its maximum resolution accuracy in two-dimensional distribution measurement, which also restricts the improvement of measurement accuracy to a certain extent. In this paper, a post-processing measurement method is proposed, which expands the processed data object from a single pixel to a regional pixel, transforms the measurement of displacement into the measurement of amplitude distribution changes, and effectively utilizes the cascaded characteristics between a single pixel and pixel area. The purpose is to break through the limitation of the size of a single pixel and thus improve the measurement accuracy. The validity of the method is verified by theoretical calculation and experimental measurements, and the measurement accuracy is improved by 6 times in practical test. This method can improve the measurement accuracy and performance of existing system equipment without breaking through the limitation of pixel physical size and will be highly interesting for practical applications.

Label-Free and Reproducible Chemical Sensor Using the Vertical-Fluid-Array Induced Optical Fiber Long Period Grating (VIOLIN).

Fiber optical refractometers have gained a substantial reputation in biological and chemical sensing domain regarding their label-free and remote-operation working mode. However, the practical breakthrough of the fiber optical bio/chemosensor is impeded by a lack of reconfigurability as well as the explicitness of the determination between bulk and surface refractive indices. In this letter, we further implement the highly flexible and reproducible long period grating called "VIOLIN" in chemical sensing area for the demonstration of moving those obstacles. In this configuration, the liquid is not only leveraged as the chemical carrier but also the periodic modulation of the optical fiber to facilitate the resonant signal. The thiol compound that is adsorbed by the fluidic substrate can be transduced to the pure alteration of the bulk refractive index of the liquid, which can be sensitively perceived by the resonant drift. Taking advantage of its freely dismantled feature, the VIOLIN sensor enables flexible reproduction and high throughput detection, yielding a new vision to the fiber optic biochemical sensing field.

Optical Microfiber Reader for Enzyme-Linked Immunosorbent Assay.

In clinical diagnosis, accurate and reliable measurement technologies for the detection of disease biomarkers at ultralow concentrations can provide guidance for the initiation of treatment and potentially improve survival for patients. Here, we demonstrate an optical microfiber reader for enhanced analytical sensitivity in enzyme-linked immunosorbent assays (ELISA) that enables the detection of tiny changes of the refractive index (RI) induced by the catalyzed oxidation of substrate, owing to the strong interaction between the evanescent field and surrounding medium. By employing the microfiber reader for the C-reaction protein (CRP) and interleukin-6 (IL-6) assays after the enzymatic signal amplification in ELISA, we experimentally investigate the biosensing capacity of the device. As a result, log-linear relations of CRP and IL-6 detection in PBS and human serum between the concentration and spectral response were obtained at both nanogram and picogram levels, respectively, and anti-CRP/HRP detection as low as 9.75 pg/mL was achieved, which was undetectable by the conventional spectrophotometry. With a stable, accurate, and color-free detection capacity, this optical microfiber reader has a promising prospect in early disease diagnosis and clinical treatment.

Ultrasensitive sensing in air based on Sagnac interferometer working at group birefringence turning point.

In this paper, a gas refractometer based on microfiber Sagnac interferometer is demonstrated, which can achieve an ultrahigh sensitivity when operating at the group birefringence turning point. We undertake a theoretical analysis and a simulated calculation to study the device characteristics and obtain the specific parameters of ellipticity and long axis of the elliptic microfiber for the group birefringence turning point. In the experiment, we obtain a positive sensitivity of 0.295 nm/KPa and a negative sensitivity of -0.219 nm/KPa during gas pressure and refractive index (RI) sensing, the obtained highest RI sensitivity can reach 160,938.9 nm/RIU. To further reveal its practical potential in gas detection, we conduct CO2 gas concentration detection and the device also demonstrates ultrahigh sensitivity and good repeatability. Besides, temperature sensing is performed to explore its temperature response wherein it shows a sensitivity of 486.7 pm/ °C. These results show its potential for use in gas- and acoustic-sensing applications.

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

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

Fiber Light-Coupled Optofluidic Waveguide (FLOW) Immunosensor for Highly Sensitive Detection of p53 Protein.

Highly sensitive detection of molecular tumor markers is essential for biomarker-based cancer diagnostics. In this work, we showcase the implementation of fiber light-coupled optofluidic waveguide (FLOW) immunosensor for the detection of p53 protein, a typical tumor marker. The FLOW consists of a liquid-core capillary and an accompanying optical fiber, which allows evanescent interaction between light and microfluidic sample. Molecular binding at internal surface of the capillary induces a response in wavelength shift of the transmission spectrum in the optical fiber. To enable highly sensitive molecular detection, the evanescent-wave interaction has been strengthened by enlarging shape factor R via fine geometry control. The proposed FLOW immunosensor works with flowing microfluid, which increases the surface molecular coverage and improves the detection limit. As a result, the FLOW immunosensor presents a log-linear response to the tumor protein at concentrations ranging from 10 fg/mL up to 10 ng/mL. In addition, the nonspecifically adsorbed molecules can be effectively removed by the fluid at an optimal flow rate, which benefits the accuracy of the measurement. Tested in serum samples, the FLOW successfully maintains its sensitivity and specificity on p53 protein, making it suitable for diagnostics applications.

Sampled Bragg gratings formed in helically twisted fibers and their potential application for the simultaneous measurement of mechanical torsion and temperature.

We propose and demonstrate a novel type of sampled Bragg gratings by combining a helically twisted fiber and a Bragg grating. A comb-like spectrum with a series of harmonic narrow resonances is observed, and the influence of geometrical parameters on the resonances is studied. As a special application, the intrinsic nature of the device that contains the Bragg grating and helical fiber spectral responses permits the temperature to be detected from the former, whereas the mechanical torsion is extracted from the latter, suggesting a potential for the simultaneous measurement of these two parameters. The proposed configuration features simplification, easy fabrication, high flexibility, stability, and low cost, and therefore has good prospects for sensor applications, as well as other applications, such as multi-channel filters, distributed Bragg reflectors, etc.

Few-period helically twisted all-solid photonic bandgap fibers.

We present a type of few-period helically twisted all-solid photonic bandgap fiber (AS-PBGFs). The helical structure leads to orbital resonance of a cladding rod light, which couples with the core mode. A two-period twist structure exhibits an extremely strong resonant dip of up to 30 dB. A series of samples with twist periods of 3.31-7.92 mm (yielding twist rates of 1.90-0.79 rad⋅mm-1) in association with different resonance orders are fabricated and demonstrated. The inherent physical mechanism underlying the resonance is analyzed. Moreover, the responses of the resonance to mechanical torsion, strain, and temperature are investigated. The twisted AS-PBGFs feature high reproducibility, stability, and robustness, and have great potential in tunable in-fiber filters and sensors.

High-sensitivity DNA biosensor based on microfiber Sagnac interferometer.

Nucleic acid detection with label-free biosensors circumvents the need for costly fluorophore functionalization steps associated with conventional assays by utilizing optical fiber transducers. In spite of their technological prowess, however, these biosensors' sensitivity is limited by the design/configuration of their transducers. Therefore, it is imperative to integrate novel optical fiber transducers with existing label-free approaches to overcome those limitations. Herein, we present a high sensitivity label-free fiber optic biosensor that employs polarimetric interference of a high-birefringence (Hi-Bi) microfiber to specifically detect DNA molecules. A slight target DNA concentration change is converted into an optical wavelength shift of polarimetric interference generated by the microfiber Sagnac interferometer. The sensor provides a log-linear response to target ssDNA concentrations range from 100 pM to 1 µM and a minimum detectable concentration of 75 pM.

Higher-order diffraction of long-period microfiber gratings realized by arc discharge method.

We report novel microfiber long period gratings (MF-LPGs) characterized by higher-order diffraction, which are fabricated using an arc discharge method. It is shown that an 11-period MF-LPG can exhibit an extremely high resonant dip (>30 dB) and a low transmission loss (<1.0 dB). A series of grating samples with elongated periods, from 400 µm to 1000 µm, and different diffraction orders have been fabricated and studied in contrast to the previously reported counterparts. The proposed structures have high reproducibility, stability, flexibility, and low production costs. Moreover, the resonant wavelength has a large refractive index (RI) sensitivity (up to ~3762.31 nm/RI-unit around RI = 1.383) and a very low temperature coefficient (~3.09 pm/°C at 1401.3 nm) for a structure with a diameter of 9.6 µm. The theoretical analysis shows good agreement with the experimental results. Our study should be useful for future applications of MF-LPGs in micro-scale in-fiber devices and sensors.

Investigation on single taper-based all-solid photonic bandgap fiber modal interferometers.

We demonstrate a single taper-based all-solid photonic bandgap (AS-PBG) fiber modal interferometer that consists of a central tapered fiber region connected to the untapered via two abrupt transitions. Modal interference is given by superimposing the bandgap-guided fundamental core mode with a lower effective index and a specific index-guided cladding supermode with a higher effective index. A series of interferometers with taper diameter of 50µm ~60µm and device length of ~3mm are fabricated and studied in contrast to the conventional counterparts. The temperature coefficient of the interferometer is closely determined by the fraction of the cladding supermode energy localized within the index-raised regions of the fiber. The refractive index (RI) responsivities associated to fiber taper sizes are investigated. The measured maximal RI sensitivity is ~3512.36nm/RIU at the taper diameter of 50µm around RI = 1.423. This research gives a deep understanding to the modal-interferometric AS-PBG structure, which we believe to be valuable for the future application of the related device.

Study of lateral-drilled DBR fiber laser and its responsivity to external refractive index.

We report a lateral-drilled DBR fiber laser which contains a defective parabola-like opening inside the cavity fabricated by the CO2-laser exposure and study the laser responsivity to external refractive index (RI). Surrounding materials can readily reach the vicinity of the fiber core via the opening and interact with the laser mode. Research shows that the laser emission power mainly relies on changes of external RI while the lasing wavelength on temperature. The effects of structural parameters, pump power, and external refractive index on the RI responsivity of the device are demonstrated. The lasing threshold condition is also concerned. This work provides an opportunity for controlling emission characteristics of the DBR fiber laser through modification of external RI value, of which the results are valuable for the potential applications in optical sensing, tunable lasing, and etc.