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An all-fiber glucose sensor is proposed and demonstrated based on a helical intermediate-period fiber grating (HIPFG) produced by using a hydrogen/oxygen flame heating method. The HIPFG, with a grating length of 1.7 cm and a period of 35 μm, presents four sets of double dips with low insertion losses and strong coupling strengths in the transmission spectrum. The HIPFG possesses an averaged refractive index (RI) sensitivity of 213.6 nm/RIU nm/RIU in the RI range of 1.33-1.36 and a highest RI sensitivity of 472 nm/RIU at RI of 1.395. In addition, the HIPFG is demonstrated with a low-temperature sensitivity of 3.67 pm/°C, which promises a self-temperature compensation in glucose detection. In the glucose-sensing test, the HIPFG sensor manifests a detection sensitivity of 0.026 nm/(mg/mL) and a limit of detection (LOD) of 1 mg/mL. Moreover, the HIPFG sensor exhibits good stability in 2 h, indicating its capacity for long-time detection. The properties of easy fabrication, high flexibility, insensitivity to temperature, and good stability of the proposed HIPFG endow it with a promising potential for long-term and compact biosensors.

A compact surface plasmon resonance sensor based on an H-shaped optical fiber is proposed and demonstrated. The H-shaped optical fiber was fabricated experimentally by using hydrofluoric acid to controllably corrode the polarization-maintaining fiber. A satisfactory distance between the outer surface of the fiber and the core can be achieved, and then the surface plasmon resonance effect can be excited by coating a metal film of appropriate thickness on the surface of the fiber. This technology can realize the preparation of multiple samples at one time, compared to the traditional side-polishing technique. The H-shaped optical fiber obtained from corrosion exhibits a high surface quality and short lengths, down to only a few hundred microns. The effects of the proposed H-shaped optical fiber on spectral properties are induced by process parameters, including fiber remaining thickness, coating thickness and fiber length, and were investigated in detail. The prepared sensor was used for the specific detection of human IgG, and the minimum human IgG concentration that the sensor can distinguish is 3.4 μg/mL. Such a compact surface plasmon resonance fiber sensor has the advantages of an easy fabrication, good consistency and low cost, and is expected to be applied in the specific detection of biomarkers.

Optical humidity sensors have evolved through decades of research and development, constantly adapting to new demands and challenges. The continuous growth is supported by the emergence of a variety of optical fibers and functional materials, in addition to the adaptation of different sensing mechanisms and optical techniques. This review attempts to cover the majority of optical humidity sensors reported to date, highlight trends in design and performance, and discuss the challenges of different applications.

A surface-plasmon-resonance-based fiber device is proposed for highly sensitive relative humidity (RH) sensing and human breath monitoring. The device is fabricated by using a polyvinyl alcohol (PVA) film and gold coating on the flat surface of a side-polished polymer optical fiber. The thickness and refractive index of the PVA coating are sensitive to environmental humidity, and thus the resonant wavelength of the proposed device exhibits a redshift as the RH increases. Experimental results demonstrate an average sensitivity of 4.98 nm/RH% across an ambient RH ranging from 40% to 90%. In particular, the sensor exhibits a linear response between 75% and 90% RH, with a sensitivity of 10.15 nm/RH%. The device is suitable for human breath tests and shows an average wavelength shift of up to 228.20 nm, which is 10 times larger than that of a silica-fiber-based humidity sensor. The corresponding response and recovery times are determined to be 0.44 s and 0.86 s, respectively. The proposed sensor has significant potential for a variety of practical applications, such as intensive care and human health analysis., Competing Interests: The authors declare no conflict of interest.

Hydrogen as an antioxidant gas has been widely used in the medical and biological fields for preventing cancer or treating inflammation. However, controlling the hydrogen concentration is crucial for practical use due to its explosive property when its volume concentration in air reaches the explosive limit (4%). In this work, a polymer-based microcantilever (μ-cantilever) hydrogen sensor located at the end of a fiber tip is proposed to detect the hydrogen concentration in medical and biological applications. The proposed sensor was developed using femtosecond laser-induced two-photon polymerization (TPP) to print the polymer μ-cantilever and magnetron sputtering to coat a palladium (Pd) film on the upper surface of the μ-cantilever. Such a device exhibits a high sensitivity, roughly -2 nm % -1 when the hydrogen concentration rises from 0% to 4.5% (v/v) and a short response time, around 13.5 s at 4% (v/v), making it suitable for medical and environmental applications. In addition to providing an ultracompact optical solution for fast and highly sensitive hydrogen measurement, the polymer μ-cantilever fiber sensor can be used for diverse medical and biological sensing applications by replacing Pd with other functional materials.

We demonstrate the microprinting of a novel suspended polymer fiber Bragg grating for high-sensitivity temperature measurements. The proposed sensor was developed using a femtosecond laser-induced multiphoton polymerization technique. The grating was cured in a single-groove silica tube spliced between two single-mode fibers. Its transmission spectrum, mode field, and temperature response were thoroughly investigated. A sensitivity of -220  pm/°C was achieved over a temperature range of 24°C to 40°C, which is meaningful in biosensing applications. This all-in-fiber polymer Bragg grating exhibits high temperature sensitivity, excellent mechanical strength, and ultrahigh integration. As such, a temperature sensing element of this type would be a beneficial tool for biological measurements.

We demonstrated a compact and highly-sensitive curvature sensor based on a Mach-Zehnder interferometer created in a photonic crystal fiber. Such a Mach-Zehnder interferometer consisted of a peanut-like section and an abrupt taper achieved by use of an optimized electrical arc discharge technique, where only one dominating cladding mode was excited and interfered with the fundamental mode. The unique structure exhibited a high curvature sensitivity of 50.5 nm/m -1 within a range from 0 to 2.8 m -1 , which made it suitable for high-sensitivity curvature sensing in harsh environments. Moreover, it also exhibited a temperature sensitivity of 11.7 pm/°C.

We report the fabrication of in-line photonic microcells (PMCs) by encapsulating tapered microfibers (MFs) inside glass tubes. The encapsulation isolates MFs from external environment and makes them more suitable for real-world applications. Based on PMCs with encapsulated highly birefringent (Hi-Bi) MFs, we demonstrated pressure, temperature and refractive index (RI) sensors as well as long period grating devices. A fiber Sagnac loop interferometer incorporating a Hi-Bi microfiber PMC demonstrated RI sensitivity of 2024 nm per RI unit (nm/RIU) in gaseous environment and 21231 nm/RIU in water.

We demonstrated a high-sensitivity strain sensor based on an in-fiber Fabry-Perot interferometer (FPI) with an air cavity, which was created by splicing together two sections of standard single-mode fibers. The sensitivity of this strain sensor was enhanced to 6.0  pm/με by improving the cavity length of the FPI by means of repeating arc discharges for reshaping the air cavity. Moreover, such a strain sensor has a very low temperature sensitivity of 1.1  pm/°C, which reduces the cross sensitivity between tensile strain and temperature.

Benefiting from the great advances of the femtosecond laser two-photon polymerization (TPP) technology, customized microcantilever probes can be accurately 3-dimensional (3D) manufactured at the nanoscale size and thus have exhibited considerable potentials in the fields of microforce, micro-vibration, and microforce sensors. In this work, a controllable microstructured cantilever probe on an optical fiber tip for microforce detection is demonstrated both theoretically and experimentally. The static performances of the probe are firstly investigated based on the finite element method (FEM), which provides the basis for the structural design. The proposed cantilever probe is then 3D printed by means of the TPP technology. The experimental results show that the elastic constant k of the proposed cantilever probe can be actively tuned from 2.46 N/m to 62.35 N/m. The force sensitivity is 2.5 nm/µN, the Q-factor is 368.93, and the detection limit is 57.43 nN. Moreover, the mechanical properties of the cantilever probe can be flexibly adjusted by the geometric configuration of the cantilever. Thus, it has an enormous potential for matching the mechanical properties of biological samples in the direct contact mode. [ABSTRACT FROM AUTHOR]

Fiber-optic biosensors have garnered significant attention and witnessed rapid development in recent years owing to their remarkable attributes such as high sensitivity, immunity to electromagnetic interference, and real-time monitoring. They have emerged as a potential tool in the realm of biomarker detection for low-concentration and small molecules. In this paper, a portable and cost-effective optical fiber biosensor based on surface plasmon resonance for the early detection of breast cancer is demonstrated. By utilizing the aptamer human epidermal growth factor receptor 2 (HER2) as a specific biomarker for breast cancer, the presence of the HER2 protein can be detected through an antigen-antibody binding technique. The detection method was accomplished by modifying a layer of HER2 aptamer on the flat surface of a gold-coated D-shaped polymer optical fiber (core/cladding diameter 120/490 μm), of which the residual thickness after side-polishing was about 245 μm, the thickness of the coated gold layer was 50 nm, and the initial wavelength in pure water was around 1200 nm. For low-concentration detection of the HER2 protein, the device exhibited a wavelength shift of ~1.37 nm with a concentration of 1 μg/mL (e.g., 5.5 nM), which corresponded to a limit of detection of ~5.28 nM. Notably, the response time of the biosensor was measured to be as fast as 5 s. The proposed biosensor exhibits the potential for early detection of HER2 protein in initial cancer serum and offers a pathway to early prevention of breast cancer. [ABSTRACT FROM AUTHOR]

A distributed optical fiber refractive index sensor based on etched Ge-doped SMF in optical frequency domain reflection (OFDR) was proposed and demonstrated. The etched Ge-doped SMF was obtained by only using wet-etching, i.e., hydrofluoric acid solution. The distributed refractive index sensing is achieved by measuring the spectral shift of the local RBS spectra using OFDR. The sensing length of 10 cm and the spatial resolution of 5.25 mm are achieved in the experiment. The refractive index sensing range is as wide as 1.33–1.44 refractive index units (RIU), where the average sensitivity was about 757 GHz/RIU. Moreover, the maximum sensitivity of 2396.9 GHZ/RIU is obtained between 1.43 and 1.44 RIU. [ABSTRACT FROM AUTHOR]

A high-spatial-resolution OFDR distributed temperature sensor based on Au-SMF was experimentally demonstrated by using step-by-step and image wavelet denoising methods (IWDM). The measured temperature between 50 and 600 °C could be successfully demodulated by using SM-IWDM at a spatial resolution of 3.2 mm. The temperature sensitivity coefficient of the Au-SMF was 3.18 GHz/°C. The accuracy of the demodulated temperature was approximately 0.24 °C. Such a method has great potential to expand the temperature measurement range, which is very useful for high-temperature applications. [ABSTRACT FROM AUTHOR]

A three-parameter sensor based on a point-by-point fiber Bragg grating (PBP-FBG) is proposed. The fabricated sensor is 40 mm long. The spectrum loss at 1336.15 nm is selected to measure the surrounding liquid level. The sensitivity for liquid level sensing is −0.046 dB/mm. The temperature is measured using the Bragg dip shifts with sensitivity of 10.06 pm/ °C. The cut-off mode shift is used to measure the surrounding refractive index with sensitivity of 535.14 nm/RIU. As a benefit of femtosecond laser direct inscription fabrication, the grating length is controllable and the liquid level sensing range is expandable. This sensor provides a simple, reliable method for accurate liquid level, temperature and refractive index measurements in hazardous environments. [ABSTRACT FROM AUTHOR]

Optical orbital angular momentum (OAM), carried by optical vortices, is a fundamental property of light that grabs broad interests. The detection of OAM modes, including fractional OAM is of great importance in diverse fields. Here, we firstly propose and develop the limited integer-component measurement method to determine the fractional topological charge (TC) of OAM states. This method indicates that a 2-integer-mode responsive device could be applied for the detection of fractional TC. An orthogonal long-period fiber grating (OLPFG) element is used as a fractional OAM TC probe to verify the Applicability. We experimentally demonstrate the fractional TC detection, ranging from −1 to 0, and recognize different fractional OAM modes with the minimum TC interval of 0.01. This scheme provides a concise, precise, continuous, and fiber-compatible fractional TC detection method that might have great potential in micro-detection, optical communication, quantum information, and chiral sensing applications. [ABSTRACT FROM AUTHOR]

In this article, an all-fiber super-variable focusing vortex beam generator is proposed and demonstrated, the focal point of which can be drastically relocated by varying the wavelength of the incident light. The all-fiber generator is fabricated by etching a spiral zone plate (SZP) on the top of the composite fiber structure composed of a single mode fiber and a quarter pitch graded index fiber using focused ion beam lithography. Numerical simulation and experimental measurements have been applied in investigation of all-fiber output light field characteristics, such as the focal spot, focal length, and vortex topological charges. Results show that these parameters can be controlled by flexibly adjusting the design parameters of SZP microstructure. In particular, the position of the donut-shaped focus spot can be dynamically tuned in a wide range by varying the incident laser wavelength with the focal spot profile remains constant. The all-fiber generator can be potentially applied in particle manipulation, high capacity communication, and fiber optic endoscopic imaging, all of which require ultra-compact, multi-dimension, and flexible focusing schemes. [ABSTRACT FROM AUTHOR]

A type of encoded FBG array for use as an optical fiber tag is inscribed using a femtosecond laser line-by-line technique. Each tag included two labels and several codes formed by grating fragments with a predetermined length. Encoded information was characterized by measuring the backscattered optical signal. Local spectra from the internal grating fragments were then used to enlarge the encoding capacity or encrypt the encoded FBG. The insertion loss of the device is as low as <0.1 dB. [ABSTRACT FROM AUTHOR]

An ultra-compact cascaded Fabry Perot (F-P) interferometer in fiber, which exhibits a high-visibility and enhanced detection limit, has been developed by chemical-assisted femtosecond laser micromachining. The reflection spectrum of the cascaded F-P interferometer shows stronger and narrower resonance than single F-P cavity. The liquid refractive index (RI) measurement demonstrates that the wavelength sensitivity and the dip intensity sensitivity are ∼888 nm/RIU and −882 dB/nm/RIU, respectively, and the obtained detection limit is 4.22 × 10−5 RIU in RI range 1.325–1.330, which is higher than that of the conventional single F-P cavity in optical fibers based on wavelength modulation (10−4 RIU). Moreover, the gas pressure measurement demonstrates that the wavelength sensitivity is 2.86 nm/MPa and the detection limit reaches 1.311 × 10−2  MPa in pressure range 1.5–2.0 MPa. The proposed in-fiber cascaded F-P interferometer will find potential applications in chemical and biological sensors, biomedical probes, and low-cost care products. [ABSTRACT FROM AUTHOR]

Conventional grating-inscription technology usually inscribes only one fiber Bragg grating (FBG) in the core of an optical fiber. A novel femtosecond laser point-by-point technology was demonstrated to parallel-inscribe multiple FBGs, i.e., so-called parallel-integrated FBGs (PI-FBGs), with either different or the same reflection wavelengths in the core of a standard single-mode fiber. Single- or multiwavelength PI-FBGs exhibited one or multiple peaks in the reflection spectrum, respectively. The length of PI-FBGs can be shortened to hundreds of micrometers to increase the spatial resolution of the FBG-based sensors. Our ultrashort PI-FBGs with a length of 500 μm could be used to realize ultrahigh temperature sensing with a high spatial resolution of less than 1 mm, a linear sensitivity of 15.0 pm/°C, and a large measurement range of up to 1100 °C, which overcomes the shortcomings of conventional FBG-based sensors with a low spatial resolution of more than 1 cm and a small measurement range of up to 300 °C. Moreover, the grating parameters, such as reflectivity, reflection wavelength, polarization-dependent loss, and full width at half maximum, can be improved to achieve desired PI-FBGs by changing the spatial distribution and grating number of the PI-FBGs. The PI-FBGs are insensitive to bending, which reduces bend-induced measurement errors. Therefore, our PI-FBGs will find potential applications in the fields of optical fiber sensors, communications, and lasers. [ABSTRACT FROM AUTHOR]

We report a reproducible optofluidic laser (OFL) for multichannel biochemical sensing. A hollow optical fiber (HOF) serves as both microring resonator for lasing and microfluidic channel. The lasing mechanism is analyzed in spectral domain. Thanks to the precise control of the fiber geometry and the rotational symmetry, the HOF-OFL emission is uniformly distributed in the angular direction (σ = 0.6%) and can be employed conveniently for disposable or arrayed use. An array of ten OFLs is demonstrated with good reproducibility (σ = 3.9%) in laser threshold. The arrayed sensing capability of the OFL is also investigated. The HOF-OFLs are highly reproducible, easy to integrate in chip-scale, and have great potential for biochemical detection. [ABSTRACT FROM AUTHOR]

A Fabry–Perot interferometer (FPI) composed of a miniature fiber collimator is proposed and demonstrated for high-sensitivity bend measurements. The device consists of a quarter-pitch graded index fiber (GIF) spliced with a section of silica tube, which is interposed between single-mode fibers. The divergence angle of the included miniature fiber collimator was 0.058 rad, which was found to enhance the sensitivity of the proposed FPI to –3.353 dB/degree over the applied angle range. In comparison, the divergence angle without the collimator was 0.130 rad and FPI sensitivity was –0.618 dB/degree over the applied angle range. The fiber bend sensors demonstrated in this paper are compact, robust, and highly sensitive, and hence, are promising in various biomedical applications. [ABSTRACT FROM AUTHOR]

We demonstrated femtosecond laser inscription of fiber Bragg gratings (FBGs) in a twin-core few-mode fiber (TC-FMF) for directional bend sensing. An FBG was selectively inscribed in one core of the TC-FMF by using an 800 nm femtosecond laser through a phase mask. Three resonance peaks at the wavelengths of 1549.05, 1547.65, and 1546.08 nm were observed in the reflection spectrum of the TC-FM FBG, and were generated by the LP01 mode resonance, LP01–LP11 mode cross-coupling resonance, and LP11 mode resonance, respectively. Moreover, the TC-FM FBG exhibited the capability of directional bend sensing and achieved a maximum bend sensitivity of −37.41 pm/m−1. Hence, the proposed TC-FM FBG directional bend sensors could further be developed as promising solutions for detecting vectorial seismic or acoustic waves and 3-D shape sensing. [ABSTRACT FROM PUBLISHER]

We experimentally demonstrate an automatic arc discharge technology for inscribing high-quality helically twisted long period fiber gratings (H-LPFGs) with greatly improved inscription efficiency for single mode fibers. The proposed technology has been developed by implementing an embedded program in a commercial fusion splicer, which employs an ultraprecision motorized translation stage while the tensioning mass required by conventional inscribing technology is eliminated. More significantly, the arc-induced H-LPFGs have been reported to have potential usage as sensors in temperature, refractive index, twist stress, and strain. [ABSTRACT FROM PUBLISHER]

We report a new fiber optic sensor for temperature measurement at a temperature range of up to 900  °C with excellent stability and repeatability. The sensing head is comprised of a short hollow-core fiber segment spliced between two single-mode fibers using a commercial splicer. Optical power from the lead-in fiber is partly coupled to silica cladding of the hollow-core fiber with the remainder propagating in air core by short distance. The two beams are then re-coupled to the core of the lead-out fiber, owing to the offset splicing joint. Due to the effective index difference between the two beams, an interference pattern in the transmission spectrum is obtained. The distinct thermo-optic coefficients between silica and air result in a high-temperature sensitivity of 41 pm/ °C, and the enclosed structure ensures its immunity to the external refractive index. [ABSTRACT FROM PUBLISHER]

We demonstrate an ultrasensitive temperature sensor based on a unique fiber Fabry–Pérot interferometer (FPI) that was created by means of splicing a mercury-filled silica tube with a single-mode fiber. The air cavity of the fiber FPI was formed by the surface of the mercury column and the endface of the single-mode fiber. Moreover, the mercury-based fiber FPI exhibits an ultrahigh temperature-sensitivity of up to −41.9 nm/°C, which is about one order of magnitude higher than that of the ordinary FPI-based fiber-tip sensors reported so far. Hence, such a mercury-based fiber FPI temperature sensor could be used for highly sensitive ambient temperature sensing applications. [ABSTRACT FROM PUBLISHER]

Single-mode-no-core-single-mode (SNS) optical fiber structures have valuable potential to encapsulate as high-precision temperature sensors, due to their great sensitivity. This paper is to improve the temperature sensitivity of SNS fibers by studying the parameters of fiber length, diameter, and refractive index of the seal liquid. Transmission spectra of the fibers are given in both aspects of experiment and numerical simulation. We find that using a slim no-core fiber section sealed with liquid of high refractive index can effectively improve the sensitivity, while the choice of fiber length mainly affects the width of transmission peak. The results may contribute to the development of high-sensitivity optic temperature sensor. [ABSTRACT FROM AUTHOR]

A novel optical fiber sensing technology based on intensity distribution change in orbital angular momentum (OAM) mode is proposed and implemented herein. The technology utilizes a chiral long-period fiber grating (CLPFG) to directly excite the 1st-order OAM (OAM1) mode. The intensity changes in the coherent superposition state between the fundamental mode and the OAM1 mode at the non-resonant wavelength of the CLPFG is tracked in order to sense the external parameters applied to the grating area. Applying this technology to temperature measurement, the intensity distribution change has a good linear relationship with respect to temperature in the range of 30 °C to 100 °C. When the intensity was denoted by the number of pixels with a gray value of one after binarization of collected images, the sensitivity was 103 px/°C and the corresponding resolution was 0.0097 °C. Meanwhile, theoretical and experimental results show that the sensitivity and resolution can be further improved via changing the area of the collected image. Compared with sensing methods based on spiral interference pattern rotation in previous work, this sensing technology has the advantage of exquisite structure, easy realization, and good stability, thus making it a potential application in practices. [ABSTRACT FROM AUTHOR]

A nondestructive measurement method based on an Optical frequency domain reflectometry (OFDR) was demonstrated to achieve Young's modulus of an optical fiber. Such a method can be used to measure, not only the averaged Young's modulus within the measured fiber length, but also Young's modulus distribution along the optical fiber axis. Moreover, the standard deviation of the measured Young's modulus is calculated to analyze the measurement error. Young's modulus distribution of the coated and uncoated single mode fiber (SMF) samples was successfully measured along the optical fiber axis. The average Young's modulus of the coated and uncoated SMF samples was 13.75 ± 0.14, and 71.63 ± 0.43 Gpa, respectively, within the measured fiber length of 500 mm. The measured Young's modulus distribution along the optical fiber axis could be used to analyze the damage degree of the fiber, which is very useful to nondestructively estimate the service life of optical fiber sensors immersed into smart engineer structures. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, a new technique for discrimination of temperature and strain exploiting the polarization properties of a single normal fiber Bragg grating (FBG) is firstly proposed and experimentally demonstrated. The spectral characterization of the differential group delay (DGD) of the FBG is studied. The evolution of the DGD response of the FBG as a function of temperature and strain is analyzed. The numerical simulations and experimental results suggest that implementing two independent measurements of the resonance wavelength shift and the changes of the DGD can discriminate the temperature and strain effects. [Copyright &y& Elsevier]

A versatile technique is demonstrated to fill selectively a fluid into desired air holes in a microstructured optical fibers (MOFs). A fan-shaped groove is carved on the fiber surface to expose selected air holes to atmosphere by a micromachining system consisting of a femtosecond IR laser and a microscope. Then a fluid is filled into the exposed air holes through the carved groove with the well-known capillarity action. Such a technique can be used to fill selectively several different types of fluid samples into desired air holes in an MOF by means of carving fan-shaped grooves on different orientations of the fiber surface. [ABSTRACT FROM PUBLISHER]

Abstract: We report about fiber Bragg gratings (FBGs) inscribed in two different types of small-core Ge-doped photonic crystal fibers with a UV laser. Sensing properties of the FBGs were systematically investigated by means of demonstrating the responses of Bragg wavelengths to temperature, strain, bending, and transverse-loading. The Bragg wavelength of the FBGs shifts toward longer wavelengths with increasing temperature, tensile strain, and transverse-loading. Moreover, the bending and transverse-loading properties of the FBGs are sensitive to the fiber orientations. The reasonable analyses for these sensing properties also are presented. [Copyright &y& Elsevier]

Abstract: Many theoretical and experimental studies have been developed to characterize the spectral response of an optical fiber Bragg grating (FBG) in axial strain fields in recent years. However, comparatively few works were devoted to the evolution of the spectrum when a FBG is subjected to non-uniform transverse load. In this paper, the effects of distributed birefringence on FBG under non-uniform transverse load are analyzed and a numerical simulation based on the piecewise-uniform approach is also discussed to simulate the responses of FBG under some typical non-uniform transverse strain fields. Experiment was carried out using different loads applied at different locations of the FBG. Good agreements between experimental results and numerical simulations have been obtained. [Copyright &y& Elsevier]

In the process of fiber optic tweezer manipulating cells, an adjustable and long working distance is very important for cell capture. To address this issue, a graded-index fiber (GIF) optical tweezer based on a UV glue cavity is proposed in this paper, and its ability to capture yeast cells was characterized. The initial air cavity between the single-mode fiber (SMF) and the GIF cone is filled with ultraviolet glue. By adjusting the length of the ultraviolet glue cavity and the laser power, the manipulation distance of the cells can be controlled. The light field distribution of the UV glue cavity is numerically investigated, which is compared with that of the GIF optical tweezers based on conventional air cavity. The contrastive result proved that the UV glue filled between the SMF and the GIF has a longer working distance than the original air cavity. The experimental results indicate that the fiber optical tweezer has a good capture distance controllability. The fiber optical tweezer also has the advantages of simple structure and convenient operation. [ABSTRACT FROM AUTHOR]

Laser direct writing (LDW) is versatile in structuring fibers with micro‐sized functional elements such as fiber Bragg grating (FBG) and backscattering centers by finely manipulating back and side scattering from laser‐induced refractive index modified (RIM) points. However, the side‐scattering is a lesser‐explored property in laser‐structured fibers. In this work, a concise physical model is established to understand the side‐scattering as a combined effect of microstructure and geometry of RIM points. Based on a single‐pulsed LDW method, the parametric decoupling between scattering loss (α) and coupling strength (κ) coefficients of FBGs is reported, whose cross‐section is customized to have a flattened ellipse with thoroughly positive RIM, enabling controllable reflectivity from −21.33 dB to −0.0018 dB while maintaining narrow bandwidth and low loss. Exemplarily, a designed FBG realizes ultra‐low loss of 0.008 dB with a resonance attenuation of 10.81 dB, exhibiting a record‐breaking κ/α of 2083. Using this FBG as the high‐reflective mirror of a home‐made bismuth‐doped fiber laser, narrow‐band lasing with a high optical signal‐to‐noise ratio of ≈43 dB is achieved, demonstrating flexibilities of the proposed approach in customizing both back‐ and side‐scattering in fibers and opening up wide opportunities for combining multifunctional components into optical fibers and realizing all‐fiber networks. [ABSTRACT FROM AUTHOR]

This paper presents a review of our work concerning the recent progress in fabrications and applications of heating-induced long period fiber gratings (LPFGs). Firstly, three kinds of heating fabrication techniques based on CO2 laser, hydrogen–oxygen flame and arc discharge are demonstrated to fabricate LPFGs, i.e., standard LPFGs (SLPFGs) and helical LPFGs (HLPFGs), in different types of optical fibers such as conventional fibers, photonic crystal fibers, and photonic bandgap fibers. Secondly, the all-fiber orbital angular momentum (OAM) mode converters based on heating-induced SLPFGs and HLPFGs in different types of fibers are studied to increase the transmission capacity. Finally, the heating-induced SLPFGs and HLPFGs are investigated to develop various LPFG-based strain, pressure, torsion and biochemical sensors. [ABSTRACT FROM AUTHOR]

Optical manipulation is a non-contact and non-destructive method to capture and manipulate micro/nano particles, biological macromolecules and cells with application of optical forces. With the rapid development of optical manipulation, various kinds of optical fiber tips have been presented and demonstrated in recent years. In this letter, a novel structure of optical fiber tip with an imbedded hollow cavity is presented. The electric field distribution of the fiber tip and the influence of cavity geometry are simulated with finite-difference time-domain method. The unique advantage is that the electric field intensity will be significantly enhanced when the fiber tip is filled with high refractive index medium in the cavity, which brings the potential to generate a three-dimensional optical trap for particles. Through experiments, we also demonstrate that the velocity of the particles driven by the optical force has a quadratic relationship with the laser power. The relationship can be used in microfluidic technology to measure the velocity of micro fluid. And the balance between the scattering force and the fluid force can be used to study non-invasive migration of particles. [ABSTRACT FROM AUTHOR]

Abstract: Characteristics of cladding index modulated fiber gratings for ambient refractive index sensing is studied in this paper. The modes propagation properties and coupling properties in cladding index modulated FBG are studied based on the classical coupling mode theory while considering interactions among multiple modes and developed on a three-layer step-index fiber geometry. Evolution of cladding mode resonance spectra of cladding index modulated FBG with the decreasing of fiber diameter is simulated and compared with the core grating. Sensing characteristics of cladding modes resonance of cladding index modulated FBG to the variation of ambient refractive index is detailed and emphasized for cladding modes resonances has a much narrower bandwidth. Results show that cladding index modulated fiber grating has better spectrum for ambient refractive index sensing applications under the same sensitivity with core index modulated fiber grating and may be applied to many field including biochemical sensing and environmental monitoring. [Copyright &y& Elsevier]

• A new method for doughnut shaping based on 3D nanoprinted microlens on fiber tip is proposed. • The microlens was designed and optimized based on illumination optics, and it was fabricated by femtosecond laser induced two-photon polymerization. • Compared with diffractive optical elements, our proposed method has a larger manufacturing tolerance. • Both simulation and experiment show that the microlens can successfully transfer the Gaussian beam output into doughnut beam. Spatial beam intensity shaping is of great significance in many applications, such as laser fabrication and medical treatment. In this study we present a new method for doughnut beam shaping based on 3D nanoprinted microlens on fiber tip. The optical structure of the microlens was first designed and optimized based on illumination optics in software, and then the microlens was fabricated on a single-mode-fiber tip by femtosecond laser induced two-photon polymerization. Both simulation and experiment results have demonstrated that the Gaussian beam output from a single-mode fiber can be successfully transferred to a high-quality doughnut beam by using the fiber-tip microlens. The proposed microlens shows larger tolerance in fabrication resolution compared with optical diffraction beam shaping elements. Our research shows that the femtosecond laser induced two-photon polymerization provides a new and flexible method for the integration of micro-scaled beam shaping elements on optical fibers. [ABSTRACT FROM AUTHOR]