Femtosecond laser fabricated fiber Bragg grating in microfiber for refractive index sensing

Femtosecond-laser-inscribed Bragg grating in hollow-core fiber for highly sensitive optofluidic sensing

An optofluidic sensor based on a Bragg grating in hollow-core fiber (HCF) is experimentally demonstrated. The grating is inscribed into the HCF by femtosecond laser illumination through a phase mask. Periodic index modulation is introduced into the silica material surrounding the hollow core, causing cladding mode resonance, and multiple reflection peaks are observed in the grating spectrum. These reflection peaks later shift to longer wavelengths when high-index liquid is infiltrated into the HCF. The new reflection peak results from the backward coupling of the liquid core mode of the waveguide, the mode field of which overlaps with the grating modulation surrounding the liquid core. The resonant wavelength of the liquid-core fiber grating increases with the index value of the infiltrating liquid, and optofluidic refractive index sensing is realized with the device. The highest refractive index sensitivity, 1117 nm/RIU, is obtained experimentally in the index range of 1.476-1.54. The infiltrated hollow-core fiber Bragg grating also exhibits high temperature sensitivity due to the high thermal-optic coefficient of the liquid, and a sensitivity of -301 pm/°C is achieved in the temperature range of 25°C to 60°C.

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.

A Fully-Encircled Polymerized Microfiber Bragg Grating by 3D Femtosecond Laser Nanofabrication

Through the merits of the arbitrary three-dimensional (3D) fabrication ability and nanoscale resolution of two-photon polymerization, we demonstrated a fully encircled polymerized microfiber Bragg grating using 3D femtosecond laser nanofabrication. In order to generate strong enough polymer Bragg grating units around the microfiber surface, and to possess a possible smaller unit pitch and structure size, the composition of photoresist and grating dimensions were both experimentally optimized. A fast-curing, high-adhesion, great-heat-resistant acrylate monomer EQ4PETA was chosen as the cross-linking element, and a high-efficiency photoinitiator DETC was used. Along the tapered microfiber with a diameter of 2 microns, dozens of grating units of 300 nm thickness were successively fabricated. The resonance wavelength was approximately 1420 nm, with a unit pitch of 1 μm, slightly different with varying unit pitches. The refractive index sensitivity reached up to ~440 nm/RIU, which is much higher than other microfiber grating sensors. We also measured the temperature and strain sensitivity of this fully encircled microfiber Bragg grating, and this was estimated at 88 pm/°C and 6.3 pm/µε. It is foreseeable that with the continuous progress of fabrication technology, more highly integrated functional optical devices will emerge in the future.

Two-photon direct laser writing of micro Fabry-Perot cavity on single-mode fiber for refractive index sensing

Using the two-photon polymerization (TPP) lithography, here we propose and experimentally demonstrate a fiber-tipped Fabry-Perot interferometer (FPI) for liquid refractive index (RI) measurement. To fit the aqueous environment, the FPI is designed as an open-cell microstructure consisting of well-crafted surfaces together with supporting rods, where the major spectral interference occurs between the waveguide's facet and the printed surface. Subsequently, the sensing performances of the fiber FPI are comprehensively studied under various RI as well as temperature configurations. The RI sensitivity is obtained to be ∼1058 nm/RIU with a low detection limit of 4.5× 10^-6 RIU, which is comparable to that of previous reported FPIs. And the temperature cross-sensitivity reaches a value of 8.2 × 10^-5 RIU/°C, indicating the good reliability for RI monitoring. Compared to other fiber FPIs, our sensor exhibits substantial advantages such as ease of fabrication, highly smooth cavity surfaces, and sufficient mechanical strength, providing a practical and competitive solution for chemical and biological sensing.

Micro-Tapered Fiber Few-Mode Interferometers Incorporated by Molecule Self-Assembly Fiber Grating for Temperature Sensing Applications

We demonstrate fiber few-mode interferometers based on a self-assembly surface corrugated grating using charged nano-particles. Initially, an abrupt taper (AT) was first created using a micro flame. The AT was then further outwardly stretched to make an elongated uniformed taper until the tapered diameter achieved a micron scale. The high order core modes (HOCMs) were excited at the AT and the optical path difference (OPD) among the modes was enlarged through the uniformed taper to achieve the few-mode interference effects seen. However, to significantly enhance the interference effects with higher extinction ratios (ER) over such a short length of interferometer, an external assisted grating was made using charged nanoparticles to form surface corrugated grating with a period, Λ, of approximately 14 μm. This intermediate period of the fiber grating was helpful in scattering and attenuating some unwanted high-order modes to change the optical characteristics of the few-mode interferometer (FMI). This FMI with a self-assembly fiber grating (SAFG) was further used to make fiber temperature sensors, with a maximum resonant wavelength shift of 4.6 nm, over a temperature range from 20–60 °C. The temperature sensitivity achieved was 112.6 pm/°C and the coefficient of determination, R2, was as high as 0.99, which revealed the high linearity of the results.

Chalcogenide Glass Microfibers for Mid-Infrared Optics

With diameters close to the wavelength of the guided light, optical microfibers (MFs) can guide light with tight optical confinement, strong evanescent fields and manageable waveguide dispersion and have been widely investigated in the past decades for a variety of applications. Compared to silica MFs, which are ideal for working in visible and near-infrared regions, chalcogenide glass (ChG) MFs are promising for mid-infrared (mid-IR) optics, owing to their easy fabrication, broad-band transparency and high nonlinearity, and have been attracting increasing attention in applications ranging from near-field coupling and molecular sensing to nonlinear optics. Here, we review this emerging field, mainly based on its progress in the last decade. Starting from the high-temperature taper drawing technique for MF fabrication, we introduce basic mid-IR waveguiding properties of typical ChG MFs made of As2S3 and As2Se3. Then, we focus on ChG-MF-based passive optical devices, including optical couplers, resonators and gratings and active and nonlinear applications of ChG MFs for mid-IR Raman lasers, frequency combs and supercontinuum (SC) generation. MF-based spectroscopy and chemical/biological sensors are also introduced. Finally, we conclude the review with a brief summary and an outlook on future challenges and opportunities of ChG MFs.

Laser nano-filament explosion for enabling open-grating sensing in optical fibre

Embedding strong photonic stopbands into traditional optical fibre that can directly access and sense the outside environment is challenging, relying on tedious nano-processing steps that result in fragile thinned fibre. Ultrashort-pulsed laser filaments have recently provided a non-contact means of opening high-aspect ratio nano-holes inside of bulk transparent glasses. This method has been extended here to optical fibre, resulting in high density arrays of laser filamented holes penetrating transversely through the silica cladding and guiding core to provide high refractive index contrast Bragg gratings in the telecommunication band. The point‐by‐point fabrication was combined with post-chemical etching to engineer strong photonic stopbands directly inside of the compact and flexible fibre. Fibre Bragg gratings with sharply resolved π-shifts are presented for high resolution refractive index sensing from \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${n}_{{{{{{\rm{H}}}}}}}$$\end{document}nH = 1 to 1.67 as the nano-holes were readily wetted and filled with various solvents and oils through an intact fibre cladding.

Engineered stop bands to sense an ambient environment can enable many applications. Here, the authors demonstrate well-controlled processes to open high-aspect ratio nanoholes through optical fibre for Bragg gratings in the telecomm spectrum and to enable high-resolution refractive index sensing

Ultrasensitive interferometers based on zigzag-shaped tapered optical microfibers operating at the dispersion turning point

This work proposes and demonstrates a novel interferometric sensor based on a zigzag-shaped tapered optical microfiber (Z-OMF) working at the dispersion turning point (DTP). The Z-OMF can be fabricated in a controllable manner through a modified fiber tapering method. Our study shows that the bending taper can transfer a portion of the fundamental HE₁₁ mode to higher-order modes, and when the bending angle of the Z-OMF reaches 1.61°, high contrast interference fringes can be formed between the HE₁₁ and the HE₂₁ modes. More importantly, we find that by optimizing the diameter of the OMF, the group effective refractive index (RI) difference between HE₁₁ and HE₂₁ mode equals zero, and the refractive index sensing performance can be dramatically improved. To validate our proposed sensing mechanism, we experimentally demonstrate an ultrahigh sensitivity of 1.46×10⁵ ± 0.09×10⁵ nm/RIU. The proposed Z-OMF interferometer has the advantage of high sensitivity and low cost and shows excellent potential in chemical and biological detection.

Microfiber sensor probe integrated with a cascaded Fabry-Perot interferometer

Developing micrometer-nanometer size optical fiber sensors has promising application prospects in microenvironments, such as biological cells, micro robots, and microfluids. We propose a new strategy to fabricate a microfiber sensor probe (MSP). A femtosecond laser was applied to integrate cascaded Fabry-Perot interferometers (FPIs) into a silica microfiber. And a MSP with diameter of ∼8µm, extinction ratio of 15 dB, fitness of 24.6, and Q-factor of 2310 was demonstrated in the experiment. In addition, the MSP was applied for the refractive index and thermal measurement and the sensitivity was observed to be 10 pm/°C and 18.5 nm/RIU. The two-beam approximation model was applied to analyze the spectrum, and simulations were taken to research the refractive index sensitivity influenced by the fiber size.

A review of specialty fiber biosensors based on interferometer configuration

Optical fiber biosensors have attracted extensive research attention in fields such as public health research, environmental science, bioengineering, disease diagnosis and drug research. Accurate detection of biomolecules is essential to limit the extent of disease outbreaks and provide valuable guidance for regulatory agencies to take timely measures. Among many optical fiber sensors, optical fiber biosensors based on specialty fibers have the advantages of biocompatibility, small size, high measurement resolution, high stability and immunity to electromagnetic interference. In this paper, four types interferometer biosensors based on specialty fiber, namely Mach-Zehnder interferometer, Michelson interferometer, Fabry - Perot interferometer and Sagnac interferometer, are reviewed in terms of operating principles, sensing structure and application fields. The fiber types are further divided into micro-nano optical fiber, thin core fiber, polarization maintaining fiber, polymer fiber, microstructure optical fiber. Furthermore, this paper evaluates the advantages and disadvantages of these interferometer biosensors. Finally, main challenging problems and expectational development direction of specialty fiber interferometer biosensors are summarized. This text clearly shows the huge development potential of optical fiber biosensors in biomedical.

Femtosecond laser inscribed chirped fiber Bragg gratings

In this work, a method is proposed and demonstrated for fabrication of chirped fiber Bragg gratings (CFBGs) in single-mode fiber by femtosecond laser point-by-point inscription. CFBGs with bandwidths from 2 to 12 nm and dispersion ranges from 14.2 to 85 ps/nm are designed and achieved. The sensitivities of temperature and strain are 14.91 pm/°C and 1.21pm/µε, respectively. Compared to the present phase mask method, femtosecond laser point-by-point inscription technology has the advantage of manufacturing CFBGs with different parameter flexibilities, and is expected to be widely applied in the future.

Observation of split evanescent field distributions in tapered multicore fibers for multiline nanoparticle trapping and microsensing

The optical attractive force in tapered single-mode fibers (SMFs) is usually uniformly distributed around the tapered section and has been found to be important for trapping and manipulating targeted atoms and nanoparticles. In contrast, a peculiar phenomenon of the evanescent field splitting along the azimuth axis can be experimentally observed by tapering a weakly-coupled MCF into a strongly-coupled MCF to generate supermode interference. Moreover, the supermode interference produces a hexagonally distributed evanescent field and its six vertices give rise to the multiline optical attractive force. For such spectral resonances, the optimum extinction ratio for the transmission dips is given by 47.4 dB, this being determined using an index liquid to cover the tapered MCF. The resonant dips move to a greater extent at longer wavelengths, with the optimum tuning efficiency of 392 nm/RIU for index sensing. The split evanescent fields respectively attract the excited upconversion nanoparticles in the liquid to be linearly aligned and running down the tapered region over the fiber surface, emitting green light with 60° symmetry. The charged nanoparticles were periodically self-organized, with a period of around 1.53 µm. The parallel lines, with 60° rotational symmetry, can be useful for (1) indicating the exact locations of the side-cores or orientations of the tapered MCF; (2) as precision alignment keys for micro-optical manipulation; and (3) enhancing the upconversion light, or for use in lasers, coupling back to the MCF. The split evanescent fields can be promising for developing new evanescent field-based active and passive fiber components with nano-structures.

Mid-infrared microfiber Bragg gratings

We report mid-infrared (mid-IR) Bragg gratings fabricated on sub-wavelength-diameter chalcogenide glass (ChG) microfibers. ChG microfibers with diameters around 3 µm are tapered drawn from As₂S₃ glass fibers, and the mid-IR microfiber Bragg gratings (mFBGs) are inscribed on microfibers using interference patterns of near bandgap light at a 532 nm wavelength. At a wavelength of about 4.5 µm, the mFBG has an extinction ratio of 15 dB and a positive photo-induced refractive index change of 2×10^-2. The dependence of the grating formation on accumulated influence of exposure power density and time is investigated. The mid-IR mFBGs demonstrated here may be used as building blocks for micro-photonic circuits or devices in the mid-IR spectral range.

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.

Superstructure microfiber grating characterized by temperature, strain, and refractive index sensing

Microfiber gratings with diameters in the subwavelength scale have recently attracted much attention for developments of sensitive sensors; however, a specific structure is usually chosen for sensing one parameter according to the optical response. In this work, a superstructure microfiber grating combined with microfiber Bragg grating and long-period microfiber grating is reported for the first time. The proposed superstructure is formed by ultraviolet laser inscription and femtosecond laser scratching techniques, which simultaneously endows the unique properties of the two individual gratings. The reflection and transmission spectral characteristics differing to conventional counterparts are demonstrated. The responsivities of the two gratings to temperature, strain and refractive index are investigated, providing a possibility for simultaneous multi-parameter sensing.

Investigation of a Bragg Grating-Based Fabry–Perot Structure Inscribed Using Femtosecond Laser Micromachining in an Adiabatic Fiber Taper

This paper presents the fabrication of a fiber Bragg grating (FBG)-based Fabry–Perot (FP) structure (7 mm total length) in an adiabatic fiber taper, investigates its strain and temperature characteristics, and compares the sensing characteristics with a standard polyimide coated FBG sensor. Firstly, a simulation of the said structure is presented, followed by the fabrication of an adiabatic fiber taper having the outer diameter reduced to 70 μ m (core diameter to 4.7 μ m). Next, the sensing structure, composed of two identical uniform FBG spaced apart by a small gap, is directly inscribed point-by-point using infrared femtosecond laser (fs-laser) micromachining. Lastly, the strain and temperature behavior for a range up to 3400 μ ε and 225 ° C, respectively, are investigated for the fabricated sensor and the FBG, and compared. The fabricated sensor attains a higher strain sensitivity (2.32 pm/ μ ε ) than the FBG (0.73 pm/ μ ε ), while both the sensors experience similar sensitivity to temperature (8.85 pm/ ° C). The potential applications of such sensors include continuous health monitoring where precise strain detection is required.

Micro-/Nanofiber Optics: Merging Photonics and Material Science on Nanoscale for Advanced Sensing Technology

Micro-/nanofibers (MNFs) are optical fibers with diameters close to or below the wavelength of the guided light. These tiny fibers can offer engineerable waveguiding properties including optical confinement, fractional evanescent fields, and surface intensity, which is very attractive to optical sensing on the micro-/nano scale. In this review, we first introduce the basics of MNF optics and MNF optical sensors from physical and chemical to biological applications and review the progress and current status of this field. Then, we review and discuss hybrid MNF structures for advanced optical sensing by merging MNFs with functional structures including chemical indicators, quantum dots, dye molecules, plasmonic nanoparticles, 2-D materials, and optofluidic chips. Thirdly, we introduce the emerging trends in developing MNF-based advanced sensing technology for ultrasensitive, active, and wearable sensors and discuss the future prospects and challenges in this exciting research field. Finally, we end the review with a brief conclusion.

Highly sensitive gas refractive index sensor based on hollow-core photonic bandgap fiber

A highly sensitive gas refractive index (RI) sensor based on hollow-core photonic bandgap fiber (HC-PBF) and Fourier transform white-light interferometry was experimentally demonstrated. HC-PBFs with lower loss than hollow silica tubes render a longer air cavity for the Fabry-Perot interferometers (FPIs) without a great deal of compromise to the fringe visibility of interference. Fourier transform phase demodulation method was employed in the experiment and a directly proportional relationship between the phase sensitivity and cavity length was demonstrated. For a cavity length of ∼24.9 mm, the sensor's gas RI sensitivity reaches up to 50775.54 µm/RIU in an air RI range from 1.000 to 1.030. Considering the cavity length demodulation resolution of 0.06 µm achieved by this method, the sensor can detect gas RI change with a resolution of 10^-6 RIU, which can meet the sensing demand for almost all the gases. Moreover, the gas RI sensitivity and measurement range can be improved further by lengthening the HC-PBF. The high sensitivity, large dynamic range and good linearity of the proposed sensor make it a good candidate for biosensing, monitoring of modern chemical industry or gas laser systems.

Refractive index sensor based on etched eccentric core few-mode fiber dual-mode interferometer

A compact and high sensitivity refractive index (RI) sensor has been theoretically and experimentally demonstrated based on dual-mode interferometer (DMI) in an eccentric core few-mode fiber (ECFMF). The DMI is fabricated by fusion splicing a piece of ECFMF etched by hydrofluoric acid (HF) and two single mode fibers (SMFs) with a lateral-offset. The interference is formed by LP₀₁ and LP₁₁ modes in the eccentric core of ECFMF. The etched ECFMF-DMI based on core-core mode interference exhibits a higher RI sensitivity than the DMI based on core-cladding mode interference. The sensitivity reaches up to 2565.2 nm/RIU around the RI of 1.4. Both of the etched and unetched ECFMF-DMIs have low temperature sensitivities of 9.6 pm/°C and 33.1 pm/°C, respectively. The etched ECFMF-DMI based on the core-core mode interference possesses tremendous superiority for RI measurement due to its high RI sensitivity and low temperature cross, therefore the proposed sensor has great potentials in chemical and biological fields.

Broadband all-light-control with WS2 coated microfibers

All-optical light amplitude tuning functionality is demonstrated in a layered tungsten disulfide (WS₂) nanosheets coated microfiber (MF) structure. Due to the strong light-matter interactions between WS₂ nanosheets and the evanescent field around the MF, a large variation in the transmitted power can be observed under both external and internal pump light excitations over a broadband spectrum (~100 nm). A power variation rate of ~0.3744 dB/mW is obtained under external violet pump light excitation, whereas the power variation rate of similar devices in the state of the art are usually less than 0.3 dB/mW. In terms of the response time, a moderate rise/fall time of ∼20.5/19.6 ms is achieved, which is mainly limited by the employed structure fabrication methods. These results indicate that the optical transmitted power of the WS₂ coated MF can be modulated by different pump light with the power in the order of mW, thus the proposed device might have potential applications in all optical controllable devices and sensors, etc.

Crystallization-sapphire-derived-fiber-based Fabry-Perot interferometer for refractive index and high-temperature measurement

A crystallization-sapphire-derived-fiber (CSDF)-based Fabry-Perot interferometer (FPI) for refractive index (RI) and high-temperature measurement is proposed and demonstrated. The FPI is formed by splicing sapphire-derived fiber (SDF) to the end face of a well-cleaved single-mode fiber (SMF). CSDF is generated hundreds of micrometers away from the fusion joint resulting from arc discharge and then cuts the SDF to the edge of the CSDF. The FPI consists of two cavities, one of which is formed by CSDF, and the other is SDF, between the SMF and CSDF. The fringe contrast of the reflection spectrum varying with the RI changes of the external environment is used for RI sensing, while the wavelength shifting is for the ambient temperature sensing. In the experiment, the refractive index and temperature sensitivities are about 233.8 dB/RIU in the RI range of 1.333-1.363 and 13.571 pm/°C in the temperature range of 20°C-1000°C.

Micro/Nanofibre Optical Sensors: Challenges and Prospects

Micro/nanofibres (MNFs) are optical fibres with diameters close to or below the vacuum wavelength of visible or near-infrared light. Due to its wavelength- or sub-wavelength scale diameter and relatively large index contrast between the core and cladding, an MNF can offer engineerable waveguiding properties including optical confinement, fractional evanescent fields and surface intensity, which is very attractive to optical sensing on the micro and nanometer scale. In particular, the waveguided low-loss tightly confined large fractional evanescent fields, enabled by atomic level surface roughness and extraordinary geometric and material uniformity in a glass MNF, is one of its most prominent merits in realizing optical sensing with high sensitivity and great versatility. Meanwhile, the mesoporous matrix and small diameter of a polymer MNF, make it an excellent host fibre for functional materials for fast-response optical sensing. In this tutorial, we first introduce the basics of MNF optics and MNF optical sensors, and review the progress and current status of this field. Then, we discuss challenges and prospects of MNF sensors to some extent, with several clues for future studies. Finally, we conclude with a brief outlook for MNF optical sensors.

Bragg resonance in microfiber realized by two-photon polymerization

A new method for microfiber Bragg gratings (μ-FBGs) fabrication by means of two-photon polymerization in photosensitive resin is reported. Such polymerized μ-FBGs were cured along with the surface of microfibers without any damage or distortion to the substrate. The laser intensity was optimized to improve the spectral properties of the polymerized gratings. The refractive index measurement was performed and the maximum sensitivity obtained is ~207 nm/RIU at the refractive index value of 1.440 with the fiber diameter being 1.7 μm. This work opens a new idea for optical structure integration and further optical functionality integration.

Optofluidic gutter oil discrimination based on a hybrid-waveguide coupler in fibre

Discriminating edible oils from gutter oils has significance in food safety, as illegal gutter oils cannot meet a variety of criteria such as the acid value, peroxide value and quality. To discriminate these illegal cooking oils, we propose an ultrasensitive optofluidic detection method based on a hybrid-waveguide coupler. Prior to the straight waveguide inscription in the cladding of the silica tube using a femtosecond laser, a section of coreless fibre is firstly spliced with the ST to supply a platform for the inscription of an S-band waveguide. Then a pair of microfluidic channels are ablated on the ST using the fs laser to enable liquid analytes to flow in and out of the air channel. In the transmission spectrum, a unique resonant loss dip can be observed, which is produced by coupling the light from the laser inscribed waveguide to the liquid core when the phase-matching condition is met. This hybrid-waveguide coupler with a simplified structure realizes dynamic optofluidic refractive index sensing with an ultrahigh sensitivity of -112 743 nm RIU^-1, a detection limit of 2.08 × 10^-5 RIU and a refractive index detection range from 1.4591 to 1.4622. This novel method can be used for food safety detection, specifically, for the discrimination of gutter oils.

Fiber Bragg grating fabricated in micro-single-crystal sapphire fiber

This Letter introduces a fiber Bragg grating (FBG) in a micro-single-crystal sapphire fiber (micro-SFBG) for sensing applications in high-temperature and harsh environments. The FBG was fabricated by a point-by-point method via an IR-femtosecond laser in a large-diameter sapphire fiber that was then wet-hot acid etched to achieve microfiber size, which culminated in fabricating and characterizing a 9.6 μm diameter micro-SFBG. The refractive index measurement ranging from 1 to 1.75 and temperature measurement from room temperature to 1400°C are also reported.

Investigation of Temperature Sensitivity of a Polymer-Overlaid Microfiber Mach-Zehnder Interferometer

The temperature sensitivity of the free spectral range (FSR) for a polymer-overlaid microfiber Mach-Zehnder interferometer (MZI) is investigated both theoretically and experimentally. The waist diameter of the optical microfiber can be controlled to alter the thermal expansion and optic properties of the polymer-coated MZI. Inserting an optical microfiber with a strong evanescent field into the MZI, a low index polymer with high thermal characteristics is deposited on the surface of the microfibers to realize a polymer-overlaid microfiber MZI. It was found that the thermal expansion factor in the proposed MZI plays an important role in the temperature sensitivity of the FSR. The temperature sensitivity of the polymer-overlaid microfiber MZI is improved, which is measured to be -8.29 nm/°C at 25 °C. The optical transmission spectrum of the polymer-overlaid microfiber MZI is converted to the spatial frequency spectrum via fast Fourier transform. The temperature sensitivity of the spatial frequency in the proposed polymer-overlaid MZI is estimated to be 18.31 pm^-1 °C^-1, which is 17 times higher than that of the microfiber MZI without polymer coating (1.04 pm^-1 °C^-1).

High-sensitivity and large-dynamic-range refractive index sensors employing weak composite Fabry-Perot cavities

Most sensors face a common trade-off between high sensitivity and a large dynamic range. We demonstrate here an all-fiber refractometer based on a dual-cavity Fabry-Perot interferometer (FPI) that possesses the advantage of both high sensitivity and a large dynamic range. Since the two composite cavities have a large cavity length difference, one can observe both fine and coarse fringes, which correspond to the long cavity and the short cavity, respectively. The short-cavity FPI and the use of an intensity demodulation method mean that the individual fine fringe dips correspond to a series of quasi-continuous highly sensitive zones for refractive index measurement. By calculating the parameters of the composite FPI, we find that the range of the ultra-sensitive zones can be considerably adjusted to suit the end requirements. The experimental trends are in good agreement with the theoretical predictions. The co-existence of high sensitivity and a large dynamic range in a composite FPI is of great significance to practical RI measurements.

Roadmap on optical sensors

Sensors are devices or systems able to detect, measure and convert magnitudes from any domain to an electrical one. Using light as a probe for optical sensing is one of the most efficient approaches for this purpose. The history of optical sensing using some methods based on absorbance, emissive and florescence properties date back to the 16th century. The field of optical sensors evolved during the following centuries, but it did not achieve maturity until the demonstration of the first laser in 1960. The unique properties of laser light become particularly important in the case of laser-based sensors, whose operation is entirely based upon the direct detection of laser light itself, without relying on any additional mediating device. However, compared with freely propagating light beams, artificially engineered optical fields are in increasing demand for probing samples with very small sizes and/or weak light-matter interaction. Optical fiber sensors constitute a subarea of optical sensors in which fiber technologies are employed. Different types of specialty and photonic crystal fibers provide improved performance and novel sensing concepts. Actually, structurization with wavelength or subwavelength feature size appears as the most efficient way to enhance sensor sensitivity and its detection limit. This leads to the area of micro- and nano-engineered optical sensors. It is expected that the combination of better fabrication techniques and new physical effects may open new and fascinating opportunities in this area. This roadmap on optical sensors addresses different technologies and application areas of the field. Fourteen contributions authored by experts from both industry and academia provide insights into the current state-of-the-art and the challenges faced by researchers currently. Two sections of this paper provide an overview of laser-based and frequency comb-based sensors. Three sections address the area of optical fiber sensors, encompassing both conventional, specialty and photonic crystal fibers. Several other sections are dedicated to micro- and nano-engineered sensors, including whispering-gallery mode and plasmonic sensors. The uses of optical sensors in chemical, biological and biomedical areas are described in other sections. Different approaches required to satisfy applications at visible, infrared and THz spectral regions are also discussed. Advances in science and technology required to meet challenges faced in each of these areas are addressed, together with suggestions on how the field could evolve in the near future.

Fiber surface Bragg grating waveguide for refractive index measurements

A fiber surface Bragg grating waveguide (BGW) fabricated in the surface of single-mode fiber by direct femtosecond laser inscription is demonstrated and successfully applied for refractive index (RI) measurements. Prior to laser inscription of the fiber surface BGW, an X-coupler is first inscribed across the fiber core to couple light from the core to the fiber surface. The light transmitted in the fiber surface BGW efficiently interacts with the surrounding medium due to a strong evanescent field, and obtains an acceptable RI sensitivity approaching ∼16  nm/RIU. The novel design efficiently couples the light guided in the core with the surrounding medium using a non-destructive, single-step micromachining process, and is expected to have potential applications in fiber biochemical sensing.

Refractometer probe based on a reflective carbon nanotube-modified microfiber Bragg grating

A carbon nanotube (CNT)-modified microfiber Bragg grating (MFBG) is proposed to measure the refractive index with a strong enhancement of the sensitivity in the low refractive index region. The introduction of the CNT layer influences the evanescent field of the MFBG and causes modification of the reflection spectrum. With the increase of the surrounding refractive index (SRI), we observe significant attenuation to the peak of the Bragg resonance, while its wavelength remains almost unchanged. Our detailed experimental results disclose that the CNT-MFBG demonstrates strong sensitivity in the low refractive index range of 1.333-1.435, with peak intensity up to -53.4  dBm/refractive index unit, which is 15-folds higher than that of the uncoated MFBG. Therefore, taking advantage of the CNT-induced evanescent field enhancement, the reflective MFBG probe presents strong sensing capability in biochemical fields.

Optical detection of glucose and glycated hemoglobin using etched fiber Bragg gratings coated with functionalized reduced graphene oxide

An enhanced optical detection of D-glucose and glycated hemoglobin (HbA1c ) has been established in this study using etched fiber Bragg gratings (eFBG) coated with aminophenylboronic acid (APBA)-functionalized reduced graphene oxide (RGO). The read out, namely the shift in Bragg wavelength (ΔλB ) is highly sensitive to changes that occur due to the adsorption of glucose (or HbA1c ) molecules on the eFBG sensor coated with APBA-RGO complex through a five-membered cyclic ester bond formation between glucose and APBA molecules. A limit of detection of 1 nM is achieved with a linear range of detection from 1 nM to 10 mM in the case of D-glucose detection experiments. For HbA1c , a linear range of detection varying from 86 nM to 0.23 mM is achieved. The observation of only 4 pm (picometer) change in ΔλB even for the 10 mM lactose solution confirms the specificity of the APBA-RGO complex coated eFBG sensors to glucose molecules.

Stress-induced birefringence and fabrication of in-fiber polarization devices by controlled femtosecond laser irradiations

Optical birefringence was created in a single-mode fiber by introducing a series of symmetric cuboid stress rods on both sides of the fiber core along the fiber axis using a femtosecond laser. The stress-induced birefringence was estimated to be 2.4 × 10(-4) at the wavelength of 1550 nm. By adding the desired numbers of stressed rods, an in-fiber quarter waveplate was fabricated with a insertion loss of 0.19 dB. The stress-induced birefringence was further explored to fabricate in-fiber polarizers based on the polarization-dependent long-period fiber grating (LPFG) structure. A polarization extinction ratio of more than 20 dB was observed at the resonant wavelength of 1523.9 nm. The in-fiber polarization devices may be useful in optical communications and fiber optic sensing applications.

Silica Bottle Resonator Sensor for Refractive Index and Temperature Measurements

We propose and theoretically demonstrate a bottle resonator sensor with a nanoscale altitude and with alength several of hundreds of microns made on the top of the fiber with a radius of tens microns for refractive index and temperature sensor applications. The whispering gallery modes (WGMs) in the resonators can be excited with a taper fiber placed on the top of the resonator. These sensors can be considered as an alternative to fiber Bragg grating (FBG) sensors.The sensitivity of TM-polarized modes is higher than the sensitivity of the TE-polarized modes, but these values are comparable and both polarizations are suitable for sensor applications. The sensitivity ~150 (nm/RIU) can be reached with abottle resonator on the fiber with the radius 10 μm. It can be improved with theuse of a fiber with a smaller radius. The temperature sensitivity is found to be ~10 pm/K. The temperature sensitivity can decrease ~10% for a fiber with a radius r(co) = 10 μm instead of a fiber with a radius r(co) = 100 μm. These sensors have sensitivities comparable to FBG sensors. A bottle resonator sensor with a nanoscale altitude made on the top of the fiber can be easily integrated in any fiber scheme.

Modeling seawater salinity and temperature sensing based on directional coupler assembled by polyimide-coated micro/nanofibers

The salinity and temperature of seawater are important parameters in oceanography. Based on the directional coupler assembled by polyimide-coated micro/nanofibers, optical sensors with high sensitivity for simultaneous salinity and temperature sensing in seawater are proposed. Dependences of sensitivities on wavelength, salinity, and temperature are investigated theoretically, with which performances of such sensor under general sea conditions can be evaluated. Results show that salinity and temperature sensitivities can reach levels of nm/‰ and nm/°C, which are much higher than those of fiber Bragg gratings, knot resonators, and photonic crystal fibers. Other considerations for system design such as the length of the coupling area, the diameter difference between two fibers, and the thickness of polyimide coatings are also discussed. Sensors proposed here suggest a simple approach to realize high-sensitivity micro/nanofiber optical sensing of salinity and temperature in seawater simultaneously and may find applications in developing miniature sensors used in seawater.

Note: Dissolved hydrogen detection in power transformer oil based on chemically etched fiber Bragg grating

A fiber Bragg grating (FBG) sensor based on chemically etched cladding to detect dissolved hydrogen is proposed and studied in this paper. Low hydrogen concentration tests have been carried out in mixed gases and transformer oil to investigate the repeatability and sensitivity. Moreover, to estimate the influence of etched cladding thickness, a physical model of FBG-based hydrogen sensor is analyzed. Experimental results prove that thin cladding chemically etched by HF acid solution improves the response to hydrogen detection in oil effectively. At last, the sensitivity of FBG sensor chemically etched 16 μm could be as high as 0.060 pm/(μl/l), increased by more than 30% in comparison to un-etched FBG.

Type IIa Bragg gratings formed in microfibers

In this Letter, Type IIa Bragg gratings are inscribed into microfibers. The large germanium-doped core region of the multimode fiber provides the necessary photosensitivity to form a Type IIa grating when it is drawn down to the microscale. Reducing the diameter of the microfiber due to lower saturate modulation and the amplified tension-strain transformation effect can accelerate the formation of a Type IIa grating. This provides an efficient method for the fabrication of fiber gratings with 800°C temperature resistance.

Optical Microfibre Based Photonic Components and Their Applications in Label-Free Biosensing

Optical microfibre photonic components offer a variety of enabling properties, including large evanescent fields, flexibility, configurability, high confinement, robustness and compactness. These unique features have been exploited in a range of applications such as telecommunication, sensing, optical manipulation and high Q resonators. Optical microfibre biosensors, as a class of fibre optic biosensors which rely on small geometries to expose the evanescent field to interact with samples, have been widely investigated. Due to their unique properties, such as fast response, functionalization, strong confinement, configurability, flexibility, compact size, low cost, robustness, ease of miniaturization, large evanescent field and label-free operation, optical microfibres based biosensors seem a promising alternative to traditional immunological methods for biomolecule measurements. Unlabeled DNA and protein targets can be detected by monitoring the changes of various optical transduction mechanisms, such as refractive index, absorption and surface plasmon resonance, since a target molecule is capable of binding to an immobilized optical microfibre. In this review, we critically summarize accomplishments of past optical microfibre label-free biosensors, identify areas for future research and provide a detailed account of the studies conducted to date for biomolecules detection using optical microfibres.

Fiber-optic refractometer based on a phase-shifted fiber Bragg grating on a side-hole fiber

A fiber-optic refractive index (RI) sensor based on a π-phase-shifted fiber-Bragg-grating (πFBG) inscribed on a side-hole fiber is presented. The reflection spectrum of the πFBG features two narrow notches associated with the two polarization modes and the spectral spacing of the notches is used for high-sensitivity RI sensing with little temperature cross-sensitivity. The side-hole fiber maintains its outer diameter and mechanical strength. The side-hole fiber is also naturally integrated into a microfluidic system for convenient sample delivery and reduced sample amount. A novel demodulation method based on laser frequency modulation to enhance the sensor dynamic range is proposed and demonstrated.

Highly sensitive refractive index sensor based on cascaded microfiber knots with Vernier effect

We propose and experimentally demonstrate a refractive index (RI) sensor based on cascaded microfiber knot resonators (CMKRs) with Vernier effect. Deriving from high proportional evanescent field of microfiber and spectrum magnification function of Vernier effect, the RI sensor shows high sensitivity as well as high detection resolution. By using the method named "Drawing-Knotting-Assembling (DKA)", a compact CMKRs is fabricated for experimental demonstration. With the assistance of Lorentz fitting algorithm on the transmission spectrum, sensitivity of 6523nm/RIU and detection resolution up to 1.533 × 10(-7)RIU are obtained in the experiment which show good agreement with the numerical simulation. The proposed all-fiber RI sensor with high sensitivity, compact size and low cost can be widely used for chemical and biological detection, as well as the electronic/magnetic field measurement.

Reflectivity enhanced refractive index sensor based on a fiber-integrated Fabry-Perot microresonator

We discuss a fiber-integrated refractive index sensor with strongly improved detection performance. The resonator has been implemented by means of focused-ion beam milling of a step index fiber and shows a sensitivity of about 1.15µm/RIU. Coating the resonator walls led to a strongly improved mirror reflectivity by a factor of about 26. Design rules for device optimization and a detailed mathematical analysis are discussed, revealing that the sensor operates as an optimized Fabry-Perot resonator. We also show that the performance of such kind of Fabry-Perot sensors is, in general, limited by the detection limit function - a quantity depending on the cavitiy's finesse and on the measurement capabilities used.

Femtosecond laser fabrication of monolithically integrated microfluidic sensors in glass

Femtosecond lasers have revolutionized the processing of materials, since their ultrashort pulse width and extremely high peak intensity allows high-quality micro- and nanofabrication of three-dimensional (3D) structures. This unique capability opens up a new route for fabrication of microfluidic sensors for biochemical applications. The present paper presents a comprehensive review of recent advancements in femtosecond laser processing of glass for a variety of microfluidic sensor applications. These include 3D integration of micro-/nanofluidic, optofluidic, electrofluidic, surface-enhanced Raman-scattering devices, in addition to fabrication of devices for microfluidic bioassays and lab-on-fiber sensors. This paper describes the unique characteristics of femtosecond laser processing and the basic concepts involved in femtosecond laser direct writing. Advanced spatiotemporal beam shaping methods are also discussed. Typical examples of microfluidic sensors fabricated using femtosecond lasers are then highlighted, and their applications in chemical and biological sensing are described. Finally, a summary of the technology is given and the outlook for further developments in this field is considered.

Single mode tapered fiber-optic interferometer based refractive index sensor and its application to protein sensing

We demonstrate refractive index sensors based on single mode tapered fiber and its application as a biosensor. We utilize this tapered fiber optic biosensor, operating at 1550 nm, for the detection of protein (gelatin) concentration in water. The sensor is based on the spectroscopy of mode coupling based on core modes-fiber cladding modes excited by the fundamental core mode of an optical fiber when it transitions into tapered regions from untapered regions. The changes are determined from the wavelength shift of the transmission spectrum. The proposed fiber sensor has sensitivity of refractive index around 1500 nm/RIU and for protein concentration detection, its highest sensitivity is 2.42141 nm/%W/V.

Enhanced responsivity with skew ray excitation of reflection- and transmission-type refractometric sensors

The responsivity of optical fibers to refractive index can be enhanced using high-order skew rays compared with using meridional rays. Skew rays can have a much higher number of reflections with increased interaction length along the core-cladding interface, which gives rise to stronger interactions with the external medium. Reflection/transmission-type refractometric sensors based on twin-coupled-core and multimode fibers showed one/two orders of magnitude increase in responsivity with skew ray excitation. The responsivity and sensitivity for the two types are ~2000%/RIU, ~1400%/RIU, and 4.9×10⁻⁵  RIU, 7.0×10⁻⁵  RIU, respectively.

Mu and epsilon near zero metamaterials for perfect coherence and new antenna designs

Wave interference is a fundamental physical phenomenon. Traditionally, the coherent effect of two identical point sources only takes place when the optical path is an integer number of wavelengths. In this paper, we show that mu and epsilon near zero (MENZ) metamaterials can be used to realize a perfectly constructive and isotropic interference. No matter how many point sources are embedded in the MENZ region, the wavefronts overlap perfectly. This translates into a total relaxation of the conventional condition for coherence enabled by the apparent infinite wavelength of the fields within MENZ metamaterials. Furthermore, we investigate crucial parameters such as the shape and size of the MENZ region. We demonstrate that flat sided geometries give rise to constructive interference beams serving as a powerful design mean. We also reveal the importance of relying on deeply sub-wavelength MENZ volumes as larger sizes increase the impedance and therefore reduce the output power of the device. The proposed concepts bear significance for current trends in antenna design which are inspired by the recent developments of electromagnetic metamaterials. Moreover, the perfect coherence effect can be appealing for power combiners, especially in the terahertz where sources are dim, as the irradiation intensity scales with the square of the number of embedded sources.

Microfiber optical sensors: a review

With diameter close to or below the wavelength of guided light and high index contrast between the fiber core and the surrounding, an optical microfiber shows a variety of interesting waveguiding properties, including widely tailorable optical confinement, evanescent fields and waveguide dispersion. Among various microfiber applications, optical sensing has been attracting increasing research interest due to its possibilities of realizing miniaturized fiber optic sensors with small footprint, high sensitivity, fast response, high flexibility and low optical power consumption. Here we review recent progress in microfiber optical sensors regarding their fabrication, waveguide properties and sensing applications. Typical microfiber-based sensing structures, including biconical tapers, optical gratings, circular cavities, Mach-Zehnder interferometers and functionally coated/doped microfibers, are summarized. Categorized by sensing structures, microfiber optical sensors for refractive index, concentration, temperature, humidity, strain and current measurement in gas or liquid environments are reviewed. Finally, we conclude with an outlook for challenges and opportunities of microfiber optical sensors.

Photonic nanowires: from subwavelength waveguides to optical sensors

Nanowires are one-dimensional (1D) nanostructures with comparatively large aspect ratios, which can be useful in manipulating electrons, photons, plasmons, phonons, and atoms for numerous technologies. Among various nanostructures for low-dimensional photonics, the 1D nanowire is of great importance owing to its ability to route tightly confined light fields in single-mode with lowest space and material requirements, minimized optical path, and high mechanical flexibilities. In recent years, nanowire photonics have increasingly been attracting scientists' interests for both fundamental studies and technological applications because 1D nanowires have more favorable properties than many other structures, such as 0D quantum dots (QDs) and 2D films. As subwavelength waveguides, free-standing nanowires fabricated by either chemical growth or physical drawing techniques surpass nanowaveguides fabricated by almost all other means in terms of sidewall smoothness and diameter uniformity. This conveys their low waveguiding losses. With high index contrast (typically higher than 0.5) between the core and the surrounding or with surface plasmon resonance, a nanowire can guide light with tight optical confinement. For example, the effective mode area is less than λ(2)/10 for a dielectric nanowire or less than λ(2)/100 for a metal nanowire, where λ is the vacuum wavelength of the light. As we increase the wavelength-to-diameter ratio (WDR) of a nanowire, we can enlarge the fractional power of the evanescent fields in the guiding modes to over 80% while maintaining a small effective mode area, which may enable highly localized near-field interaction between the guided fields and the surrounding media. These favorable properties have opened great opportunities for optical sensing on the single-nanowire scale. However, several questions arise with ongoing research. With a deep-subwavelength cross-section, how can we efficiently couple light into a single nanowire? How can we fabricate a nanowire with low optical loss? How can we activate a passive nanowire for optical sensing? And lastly, how can we adapt mature optical measurement technology onto a nanowire? In this Account, we highlight our initial attempts to address the above-mentioned challenges. First, we introduce the fabrication and functionalization of low-loss photonic nanowires. We show that nanowires fabricated by either top-down physical drawing (e.g., for amorphous nanowires) or bottom-up chemical growth (e.g., for crystalline nanowires) can yield excellent geometric and structural uniformities with surface roughness down to atomic level and minimize the scattering loss for subwavelength optical or plasmonic waveguiding. Then, relying on a near-field fiber-probe micromanipulation, we demonstrate optical launching of single nanowires by evanescent coupling, with coupling efficiency up to 90% for dielectric nanowires and 80% for plasmonic nanowires. Third, we discuss the waveguiding properties of nanowires and emphasize their outstanding capability of waveguiding tightly confined optical fields with high fractional evanescent fields. In addition, we briefly show a balance between the loss, confinement, and bandwidth in a waveguiding nanowire. Fourthly, we present promising approaches to single-nanowire optical sensors. By measuring optical absorption or spectral transmission of a nanowire and activating nanowires with sensitive dopants, we demonstrate a single-nanowire optical sensor with high sensitivity, fast response, and low optical power. This may lead to a novel platform for optical sensing at nanoscale. Finally, we conclude with an outlook for future challenges in the light manipulation and sensing applications of photonic nanowires.

Highly sensitive force sensor based on optical microfiber asymmetrical Fabry-Perot interferometer

An asymmetrical Fabry-Perot interferometric (AFPI) force sensor is fabricated based on a narrowband reflection of low-reflectivity fiber Bragg grating (LR-FBG) and a broadband Fresnel reflection of the cleaved fiber end. The AFPI sensor includes a section of microfiber made by tapering and it achieves a force sensitivity of 0.221 pm/μN with a tapered microfiber of 40 mm length and 6.1 μm waist diameter. Compared with similar AFPI structure in 125 μm-diameter single mode fiber, the force sensitivity of the microfiber AFPI structure is greatly enhanced due to its smaller diameter and can be optimized for different force scales by controlling the diameter. The fabrication process of the AFPI sensor is simple and cost-effective. The AFPI sensor has better multiplexing capacity than conventional extrinsic fiber-optic Fabry-Perot sensors, while it also release the requirement on the wavelength matching of the FBG-pair-based FPI.

Polarization converters in highly birefringent microfibers

A novel type of polarization converters (PCs) based on highly birefringent (Hi-Bi) microfibers is presented. Analytical formulation based on the Jones Matrix method and a numerical code based on the Full Vectorial Finite Difference Beam Propagation Method are developed to analyze the polarization evolutions in such PCs. Two different design configurations, namely the "one-side" and "two-side" perturbation configurations, are studied by use of the two methods, and the results obtained agree well with each others. The PCs can be flexibly designed to have different operating wavelengths, spectral bandwidths, and devices lengths. A particular PC based on an elliptical microfiber demonstrates a bandwidth of ∼ 600 nm around 1550 nm with a device length of ∼ 150 μm.