Fiber Tip Research Articles

Nanodiamond-Based Optical-Fiber Quantum Probe for Magnetic Field and Biological Sensing.

Owing to the unique electronic spin properties, nitrogen-vacancy (NV) centers hosted in diamond have emerged as a powerful quantum tool for detecting various physical parameters and biological species. In this work, an optical-fiber quantum probe, configured by chemically modifying nanodiamonds on the surface of a cone fiber tip, is developed. Based on the continuous-wave optically detected magnetic resonance method and lock-in amplification technique, it is found that the sensing performance of probes can be engineered by varying the nanodiamond dispersion concentration and modification duration during the chemical modification process. Combined with a pair of magnetic flux concentrators, the magnetic field detection sensitivity has reached 0.57 nT/Hz1/2@1 Hz, a new record among the fiber magnetometers based on nanodiamonds. Taking Gd3+ as the demo, the capability of probes in paramagnetic species detection is also demonstrated experimentally. Our work provides a new approach to develop NV centers as quantum probes featuring high integration, multifunction, high sensitivity, etc.

Determination of the intraocular irradiance and potential retinal hazards at various positions in the eye during transscleral equatorial illumination for different applied pressures

PurposeWith diaphanoscopic illumination of the eye, the intensity of light entering its interior depends on the transmission properties of the eyewall. Light that passes through the eyewall can cause damage to the retina. Therefore, in this study, the intraocular irradiances are determined at different positions on the retina, directly behind the illuminated eyewall, the opposite eyewall and near the macula of ex-vivo porcine eyes. These irradiances are examined for their dependence on the pressure applied on the eyewall with the illuminating fiber and for the influence of the pigmentation of the eye. MethodsIn total 221 ex-vivo porcine eyes were investigated. For transscleral illumination an illumination fiber with a diffusing adapter cap is pressed against the equatorial eyewall. The illumination fiber is pressed onto the eye and the pressure is measured in the anterior chamber. Three different pressures are applied, 23, 78 and 132 mmHg. A detection fiber with diffusing fiber tip is inserted into the eye at the desired position. The eyes were divided in groups with high and less pigmentation to investigate the influence of the pigmentation on the intraocular irradiance. ResultsThe intraocular irradiances Eintra increases for various increasing applied pressures with the illumination fiber on the eyewall and for various positions inside the eye. With this the irradiances weighted with the photochemical and thermal hazard weighting function, EA-R and EVIR-R, also increases. Differences in Eintra, EA-R and EVIR-R could be found for different pigmented eyes as these values are higher for less pigmented eyes than for strong pigmented ones. ConclusionThe hazard to the retina during diaphanoscopic illumination of the eye depends on how strong the surgeon presses the illumination fiber on the eyewall. Depending on the applied pressure and the measuring position in the eye, the specified limit for the photochemical hazard to the retina is partly exceeded. The pigmentation of the eye also plays a role. The irradiance in less pigmented eyes appears to be higher than in strongly pigmented eyes. Because of this, the surgeon should be able to adjust the intensity of the light source to the color of the patient’s eye.

Tapered tip optical fibers for measuring ultra-small refractive index changes with record high sensitivity.

Here we demonstrate an inexpensive, simple, and ultra-sensitive refractive index sensor based on a tapered tip optical fiber combined with a straightforward image analysis method. The output profile of this fiber exhibits circular fringe patterns whose intensity distribution dramatically changes even with ultra-small refractive index variations in the surrounding medium. The sensitivity of the fiber sensor is measured using different concentrations of saline solutions with a transmission setup consisting of a single wavelength light source, a cuvette, an objective lens, and a camera. By analyzing the areal changes in the center of the fringe patterns for each saline solution, we obtain an unprecedented sensitivity value of 24,160 dB/RIU (refractive index unit), which is the highest value reported so far among intensity-modulated fiber refractometers. The resolution of the sensor is calculated to be 6.9 ×10-9. Moreover, we measure the sensitivity of the fiber tip in the backreflection mode using salt-water solutions and obtained a sensitivity value of 620 dB/RIU. This sensor is ultra-sensitive, simple, easy to fabricate, and low-cost, which makes it a promising tool for on-site measurements and point-of-care applications.

Optical fiber sensors for monitoring cement paste carbonation

The use of concrete has been widespread in our society in housing and infrastructure, despite the environmental cost associated with its production. Its decay poses a social, economic, and environmental problem. Currently, the carbonation of cement paste is monitored through the measurement of its pH, with several optical fiber sensors (OFS) have been produced for this purpose. In the current work the focus is, also, on the carbonation monitoring of cement paste through an OFS, but not through pH measurements. Single fiber reflectance spectroscopy, previously employed to measure cement paste durability, is used to monitor the discoloration of cement paste caused by carbonation. As the carbonation front reaches the fiber tip embedded in the cement paste, the signal reflected onto the fiber increases. The accelerated carbonation of two limestone cement paste samples in an atmosphere of 100% CO2 was successfully monitored. The applicability of the sensor for operational use with ambient CO2 was confirmed through the measurement of carbonation at 3% CO2. The cross interference from water ingress and egress was also evaluated, and it didn’t hinder the measurements of carbonation. Therefore, a novel OFS capable of measuring cement paste carbonation and durability, was achieved.

Contribution of intrusive and symplastic growths in wood fibre tip development

Key messageAt initial stages of fibre development, radial enlargement of growing fibre tip is achieved, on average, in 37.8% intrusively and in 62.2% symplastically, whereas tip tangential enlargement is purely intrusive.In this study, we have investigated the mode of growth of black locust wood fibre tips at initial stages of their development using detailed measurements. Growth of fibre tips may be considered in three directions: axial, tangential, and radial. An axial elongation of a fibre tip was described as intrusive and related to the separation of walls of neighbouring cells. However, determination of the contribution of intrusive vs. symplastic component of growth in tangential and radial directions was missing. Semi-thin transverse sections of the vascular cambium and adjacent tissues were obtained by ultramicrotome and stained with PAS and toluidine blue. An anatomical analysis of contribution of intrusive and symplastic growths in fibre tip radial and tangential enlargement was performed. Our study showed that during its development an average wood fibre tip grows only intrusively in tangential direction and shows intrusive-symplastic growth in radial direction. On average, at initial stages of its development, a fibre tip radial enlargement is achieved in 37.8% by intrusive growth and in 62.2% by coordinated (symplastic) growth.

Wedged Fiber Optic Surface Plasmon Resonance Sensor for High-Sensitivity Refractive Index and Temperature Measurements.

Here, we experimentally demonstrate a wedged fiber optic surface plasmon resonance (SPR) sensor enabling high-sensitivity temperature detection. The sensing probe has a geometry with two asymmetrical bevels, with one inclined surface coated with an optically thin film supporting propagating plasmons and the other coated with a reflecting metal film. The angle of incident light can be readily tuned through modifying the beveled angles of the fiber tip, which has a remarkable impact on the refractive index sensitivity of SPR sensors. As a result, we measure a high refractive index sensitivity as large as 8161 nm/RIU in a wide refractive index range of 1.333-1.404 for the optimized sensor. Furthermore, we carry out a temperature-sensitivity measurement by packaging the SPR probe into a capillary filled with n-butanol. This showed a temperature sensitivity reaching up to -3.35 nm/°C in a wide temperature range of 20 °C-100 °C. These experimental results are well in agreement with those obtained from simulations, thus suggesting that our work may be of significance in designing reflective fiber optic SPR sensing probes with modified geometries.

Suppression of amyloid fibril formation by UV irradiation

Ultraviolet (UV) irradiation originating in the space environment has been increasing because of the destruction of the ozone layer. Although UV irradiation damages DNA, peptides, and cell structures, it also has the beneficial effects of suppressing excessive immune responses and cell proliferation. However, the effects of UV irradiation on amyloid fibril formation are unknown. Here, we analyzed its effects on amyloid fibril formation of two types of amyloidogenic proteins, human amyloid β42 (Aβ42) and human transthyretin (TTR). To study these effects, we incubated Aβ42 samples in neutral pH buffer and wild-type TTR (TTRwt) samples in acidic buffer, with and without UV irradiation. We analyzed amyloidogenicity via a thioflavin T (ThT) method and morphological changes by using electron microscopy. UV irradiation reduced the ThT fluorescence intensity of the human Aβ42 samples. For the electron microscopic analysis of Aβ42 samples, the UV irradiation induced thinning of the amyloid fiber tips. Although UV irradiation reduced the ThT fluorescence intensity of human TTRwt samples, the amounts of the aggregates did not differ with or without UV irradiation. Our experiments suggest that UV irradiation may become a useful tool for reducing the amyloid fibrils and amyloid like substances.

Photothermomechanical Nanopump: A Flow-Through Plasmonic Sensor at the Fiber Tip.

Optical fibers equipped with plasmonic flow sensors at their tips are fabricated and investigated as photothermomechanical nanopumps for the active transport of target analytes to the sensor surface. The nanopumps are prepared using a bottom-up strategy: i.e., by sequentially stacking a monolayer of a thermoresponsive polymer and a plasmonic nanohole array on an optical fiber tip. The temperature-dependent collapse and swelling of the polymer is used to create a flow-through pumping mechanism. The heat required for pumping is generated by exploiting the photothermal effect in the plasmonic nanohole array upon irradiation with laser light (405 nm). Simultaneous detection of analytes by the plasmonic sensor is achieved by monitoring changes in its optical response at longer wavelengths (∼500-800 nm). Active mass transport by pumping through the holes of the plasmonic nanohole array is visualized by particle imaging velocimetry. Finally, the performance of the photothermomechanical nanopumps is investigated for two types of analytes, namely nanoscale objects (gold nanoparticles) and molecules (11-mercaptoundecanoic acid). In the presence of the pumping mechanism, a 4-fold increase in sensitivity was observed compared to the purely photothermal effect, demonstrating the potential of the presented photothermomechanical nanopumps for sensing applications.

Gate‐Tunable Graphene Optical Modulator on Fiber Tip: Design and Demonstration (Advanced Optical Materials 22/2022)

Gate‐Tunable Graphene Optical Modulator on Fiber Tip: Design and Demonstration (Advanced Optical Materials 22/2022)

Long Hanging Structure of Collagen VII Connects the Elastic Fibers and the Basement Membrane in Young Skin Tissue.

Aging leads to substantial structural changes in the skin. Elastic fibers maintain skin structure, but their degeneration and loss of function with age result in wrinkle formation and loss of skin elasticity. Oxytalan fiber, a type of elastic fiber, extends close to the dermal-epidermal junction (DEJ) from the back of the dermis. Oxytalan fibers are abundant in the papillary layer and contribute to skin elasticity and texture. However, to accurately understand the mechanisms of skin elasticity, the interaction between elastic fibers and DEJ should be elucidated. Here, we investigated elastic fibers and DEJ and their structural alterations with aging. Several basement membrane proteins [collagen (COL) IV, COLVII, and laminin 332], fibrous tropoelastin, and fibrillin-1 in excised human skin tissue were observed using three-dimensional imaging. Age-related alterations in COLVII, elastic fibers, and fibrillin-1 were evaluated. We found that COLVII forms long hanging structures and is co-localized with fibrous tropoelastin in young skin but not aged skin. Fibrillin-1-rich regions were observed at the tips of elastin fibers in young skin tissue, but rarely in aged skin. This co-localization of elastic fiber and COLVII may maintain skin structure, thereby preventing wrinkling and sagging. COLVII is a potential therapeutic target for skin wrinkling.

Analysis of electron emission characteristics of triggered vacuum switch based on cold cathode materials

A trigger vacuum switch that works for a long time is subject to the stable emission of initial electrons. Cold cathode materials such as velvet and carbon fiber have the characteristics of large emission electron area, uniformity, and stability. In this paper, two cold cathode materials, namely T4 rayon velvet and T300 carbon fiber board, are attached. They were used on the surface of the vacuum switch cathode, the trigger electrode was located in the middle of the cathode, and the trigger vacuum switch was triggered by a positive polarity high-voltage pulse along the surface flashover. The breakdown voltage and dispersion were studied in the form of capacitive discharge, and the electron emission characteristics of the two materials were explored from a microscopic perspective, such as through scanning electron microscope and energy dispersive spectrometer element analysis. The results show that the triggering vacuum switch using this cold cathode material has stable triggering, wide operating voltage, and low breakdown voltage dispersion. The electron emission of the velvet has both fiber tip emission and lateral flashover mechanism, and carbon fiber is prone to surface damage; the anode metal surface is partially carbonized due to electron sputtering.

Fully reconfigurable optomechanical add-drop filters

Fully reconfigurable add-drop filters (ADFs) have important applications in optical communication and information processing. Here, we demonstrate a broadly tunable add-drop filter based on a double-disk cavity optomechanical system, side-coupled with a pair of tapered fiber waveguides. We investigate the dependence of the through (drop) efficiency on coupling rates between the cavity and two waveguides by varying cavity-waveguide distances. By optimizing the cavity-waveguide coupling rates, a drop efficiency of 89% and a transmission of 1.9% have been achieved. Furthermore, tuning of the ADF is realized by changing the air gap of the double disk using a fiber tip, which is controlled by a piezoelectric nanostage. Benefiting from the large optomechanical coupling coefficient and the mechanical compliance of the double-disk microcavity, a tuning range of 8 nm has been realized using a voltage of 7 V on the piezoelectric nanostage, which is more than one free spectral range of the cavity. As a result, both the through and drop signals can be resonant with any wavelength within the transparent window of the cavity material, which indicates that the ADF is fully reconfigurable.

A Micron-Range Displacement Sensor Based on Thermo-Optically Tuned Whispering Gallery Modes in a Microcapillary Resonator.

A novel micron-range displacement sensor based on a whispering-gallery mode (WGM) microcapillary resonator filled with a nematic liquid crystal (LC) and a magnetic nanoparticle- coated fiber half-taper is proposed and experimentally demonstrated. In the proposed device, the tip of a fiber half-taper coated with a thin layer of magnetic nanoparticles (MNPs) moves inside the LC-filled microcapillary resonator along its axis. The input end of the fiber half-taper is connected to a pump laser source and due to the thermo-optic effect within the MNPs, the fiber tip acts as point heat source increasing the temperature of the LC material in its vicinity. An increase in the LC temperature leads to a decrease in its effective refractive index, which in turn causes spectral shift of the WGM resonances monitored in the transmission spectrum of the coupling fiber. The spectral shift of the WGMs is proportional to the displacement of the MNP-coated tip with respect to the microcapillary's light coupling point. The sensor's operation is simulated considering heat transfer in the microcapillary filled with a LC material having a negative thermo-optic coefficient. The simulations are in a good agreement with the WGMs spectral shift observed experimentally. A sensitivity to displacement of 15.44 pm/µm and a response time of 260 ms were demonstrated for the proposed sensor. The device also shows good reversibility and repeatability of response. The proposed micro-displacement sensor has potential applications in micro-manufacturing, precision measurement and medical instruments.

Advanced Lab-on-Tip ultrasound detectors: A numerical analysis

The Lab-on-Fiber technology is giving an important contribution to the development of compact and minimal invasive devices for biomedical applications. By exploiting the high degree of miniaturization combined with the biocompatibility of the materials used, these devices can be integrated inside medical needles and taken in the human body for in vivo diagnosis and treatments, with important advantages in terms of performances, effectiveness and invasiveness. In this framework, with a look toward high-resolution ultrasound based imaging, here we analyze a novel Lab-on-Fiber 3D micro-structure for ultrasound detection, consisting in a polymeric membrane sustained by six pillars, directly integrated above the tip of a standard single mode fiber. Such a structure essentially works as a Fabry-Perot cavity, providing interference fringes in the fiber reflection spectrum, which shift accordingly with the structure vibrations caused by the incident acoustic waves. Our numerical studies, based on finite element method, demonstrated that by opportunely dimensioning the geometrical parameters involved it is possible to tune the working frequency range up to tens of MHz, reaching sensitivities higher than the standard configurations proposed so far, given by a polymeric slab directly attached above the fiber tip. Interestingly, the investigated structure can be effectively realized by exploiting the Two-Photon polymerization technique directly applied to the optical fiber tip.

Cyclic Epipedography─Monitoring Contact Angle Changes in Dipping a Microsphere into a Liquid and Lifting from It.

We propose cyclic epipedography to monitor the wetting characteristics of a microsphere on a stem with a liquid by tracking the level of a liquid line on the sphere when the sphere is dipped into and lifted from the liquid, and therefore the level of the liquid plane at infinity changes with respect to the sphere. Analysis of still images of a video taken with a horizontally held microscope determines the two levels. The sphere allows viewing of the liquid line without being obscured by the meniscus the liquid forms with its container's wall. A nearly perfect sphere is fabricated by melting the tip of a silica optical fiber while being rotated to form a ball 0.5-0.6 mm across. A dip-lift cycle is represented by a cyclic epipedogram─a closed loop in a plot of the level of the liquid line as a function of the level of the liquid plane. The position and shape of the loop allow us to estimate the contact angle at different stages in the dip-lift cycle. We applied the method to a hydroxylated silica microsphere in water, and the same sphere reacted with octylchlorosilane. Then, the method was applied to monitor the progress of the reaction of aminopropyldimethylethoxysilane with surface silanol. We find that the amphiphilic aminopropylsilane makes the surface conform to the environment. In air, the hydrophobic portion comes on top, while in water, hydrophilic part faces the surroundings. This conformity-caused Janus characteristics of the surface were almost absent with the hydroxylated silica and weak with the octylsilane-treated silica.

Microbubble end-capped fiber-optic Fabry-Perot sensors.

We report on a simple fabrication technique for Fabry-Perot (FP) sensors formed by a microbubble within a polymer drop deposited on the tip of an optical fiber. Polydimethylsiloxane (PDMS) drops are deposited on the tips of standard single-mode fibers incorporating a layer of carbon nanoparticles (CNPs). A microbubble inside this polymer end-cap, aligned along the fiber core, can be readily generated on launching light from a laser diode through the fiber, owing to the photothermal effect produced in the CNP layer. This approach allows for the fabrication of microbubble end-capped FP sensors with reproducible performance, showing temperature sensitivities as large as 790 pm/°C, larger than those reported for regular polymer end-capped devices. We further show that these microbubble FP sensors may also prove useful for displacement measurements, with a sensitivity of ∼5.4 nm/µm.

Trapezoidal Angled Fiber Coupler for Microsphere Resonator With Enhancement of Fano Resonance

We propose and demonstrate a trapezoidal angled fiber coupler for excitation of whispering-gallery mode microsphere resonator, with enhanced Fano resonance. The reflection from the angled fiber is enhanced when the oblique angle is greater than critical angle, which is obtained as 30° theoretically for angled fiber. To enhance the endface reflection, the front tip of the angled fiber is polished flat to achieve the trapezoidal angled fiber. Compared with angled fiber, the proportion of Fano lineshape in spectra for trapezoidal angled fiber is nearly doubled and the slope is greatly improved, which reaches 304.55 dB/nm. Moreover, through the coupling of dual microspheres, the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> factor is improved severalfold and the thermal tuning efficiency is increased by 50%. With the advantage of low cost, easy fabrication procedure and reflection mode operation, the proposed device shows a great potential for sensing, optical tuning and optical switching applications.

Experimental investigations of underwater laser propulsion microspheres based on a tapered fiber propulsion system

An underwater propulsion microsystem is proposed in this work, which employs a nanosecond laser pulse out from the tapered fiber tip. Noteworthily, the system can generate a directional shock wave (or plasma) to propel the polystyrene (PS) microsphere. Through simulation, the shock wave propagation characteristics and the bubble dynamic are investigated. Experimentally, high-speed photography method is employed to obtain the motion image of microsphere. The results show that the propulsion efficiency is dependent on the laser energy. Meanwhile, we explain the role of the bubble dynamic process in propelling microsphere, and find that the bubble diameter increases with the laser energy. In addition, an experiment is performed to separate and remove the PS microsphere clusters in water at fixed point. Compared with conventional technology, this new method has advantages of high controllability, directional and non-contact, and can be used for directional manipulation of underwater microstructures and removal of contaminated microspheres in water environments.

Microfluidic Pumps with Laser Streaming from Tips of Optical Fibers and Sewing Needles

AbstractThe discovery of photoacoustic laser streaming has opened up a new avenue to manipulate and drive fluids with light, but the necessity of an in situ “launch pad” has limited its utility in real‐world microfluidic applications due to both the size constraint and the complexity of fabrication. Here, it is demonstrated that 1) a versatile microfluidic pump can be materialized by using laser streaming from an optical fiber, and 2) laser streaming can be generated from a flat metal surface without any fabrication process. In the latter case, by focusing laser on the tip of a sewing needle tip, the needle can be turned into a micropump with controllable flow direction. Additionally, high‐speed imaging of the fluid motion and computational fluid dynamics simulations to confirm the photoacoustic principle of laser streaming are employed, and it is revealed that the streaming direction is determined by the direction of strongest intensity in the divergent ultrasound wavefront. Finally, the potential of laser streaming for microfluidic and optofluidic applications is demonstrated by successfully driving fluid inside a capillary tube.

Vortex-Bessel beam generation by 3D direct printing of an integrated multi-optical element on a fiber tip.

Shaping light beams as they propagate out of the tips of optical fibers is a desired ability, as the light could be tailored for various applications in a miniaturized, integrated, and cost-effective manner. However, fabricating sophisticated refractive elements directly onto fibers is challenging. By using 3D-direct laser writing (3D-DLW), high-quality optical devices could be fabricated directly on top of the fiber's facet by the two-photon absorption process. Here, we demonstrate how a high-order Bessel beam carrying orbital angular momentum (OAM) could be generated by using this lithography process. The beam is shaped using an integrated micro-optical system that consists of a twisted axicon and parabolic lens in an adapted fiber configuration. This work provides the analysis and measurements of the generated beam, along with simulated predictions. The far-field pattern, at a distance of 2 mm from the fiber, was examined, and we have found that the size of the central ring remained nearly unchanged, as expected for this type of beam. The beam's OAM value was measured using either an interference pattern or a mode convertor. Furthermore, the near-field and far-field Bessel beam profiles were investigated simultaneously at various laser power values, reaching intensities of up to 3.8 MW/cm2. This work may pave the way for future integrated beam manipulation on fibers, enabling the use of higher laser outputs.