Fiber Tip Research Articles

Multiplexed optical fiber tip refractive index sensor interrogated by microwave photonics

A multiplexed and spatially-resolved optical fiber tip sensor is proposed and demonstrated for multi-point refractive index (RI) sensing based on a microwave-photonic interrogation technique. The RI sensor is simply based on an array of cleaved end facets of optical fibers with different lengths fabricated from off-the-shelf optical fiber couplers. An auxiliary optical fiber endface that serves as the reference reflector is also introduced into the system. As a result, each of the sensing reflectors from the sensor array, together with the reference reflector, forms a Michelson interferometer (MI). Interrogated by a microwave-photonic system operating at an incoherent regime, the interferogram for each of the MIs in the microwave domain can be unambiguously reconstructed. The spatial location and ambient RI of the sensing reflector can be determined based on the free spectral range and magnitude at the resonance of the microwave interferogram. An analytical model is first developed to understand the physics of the sensor, followed by a proof-of-concept demonstration. Two sensing reflectors and a reference reflector are multiplexed in the prototype system, showing high RI sensitivity and low temperature crosstalk. The unique features of the sensor, such as ease of fabrication, low cost, high sensitivity, low temperature dependence, multiplexing and spatially-resolved capabilities, make the sensor a strong candidate for biochemical and environmental sensing applications.

Model-based simulations of pulsed laser ablation using an embedded finite element method

A model of thermal ablation with application to multi-pulsed laser lithotripsy is presented. The approach is based on a one-sided Stefan–Signorini model for thermal ablation, and relies on a level-set function to represent the moving interface between the solid phase and a fictitious gas phase (representing the ablated material). The model is discretized with an embedded finite element method, wherein the interface geometry can be arbitrarily located relative to the background mesh. Nitsche’s method is adopted to impose the Signorini condition on the moving interface. A bound constraint is also imposed to deal with thermal shocks that can arise during representative simulations of pulsed ablation with high-power lasers. We report simulation results based on experiments for pulsed laser ablation of wet BegoStone samples treated in air, where Begostone has been used as a phantom material for kidney stone. The model is calibrated against experimental measurements by adjusting the percentage of incoming laser energy absorbed at the surface of the stone sample. Simulation results are then validated against experimental observations for the crater area, volume, and geometry as a function of laser pulse energy and duration. Our studies illustrate how the spreading of the laser beam from the laser fiber tip with concomitantly reduced incident laser irradiance on the damaged crater surface explains trends in both the experimental observations and the model-based simulation results.

Interferometric Fiber Optic Probe for Measurements of Cavitation Bubble Expansion Velocity and Bubble Oscillation Time.

Cavitation bubbles are used in medicine as a mechanism to generate shock waves. The study of cavitation bubble dynamics plays a crucial role in understanding and utilizing such phenomena for practical applications and purposes. Since the lifetime of cavitation bubbles is in the range of hundreds of microseconds and the radii are in the millimeter range, the observation of bubble dynamics requires complicated and expensive equipment. High-speed cameras or other optical techniques require transparent containers or at least a transparent optical window to access the region. Fiber optic probe tips are commonly used to monitor water pressure, density, and temperature, but no study has used a fiber tip sensor in an interferometric setup to measure cavitation bubble dynamics. We present how a fiber tip sensor system, originally intended as a hydrophone, can be used to track the expansion and contraction of cavitation bubbles. The measurement is based on interference between light reflected from the fiber tip surface and light reflected from the cavitation bubble itself. We used a continuous-wave laser to generate cavitation bubbles and a high-speed camera to validate our measurements. The shock wave resulting from the collapse of a bubble can also be measured with a delay in the order of 1 µs since the probe tip can be placed less than 1 mm away from the origin of the cavitation bubble. By combining the information on the bubble expansion velocity and the time of bubble collapse, the lifetime of a bubble can be estimated. The bubble expansion velocity is measured with a spatial resolution of 488 nm, half the wavelength of the measuring laser. Our results demonstrate an alternative method for monitoring bubble dynamics without the need for expensive equipment. The method is flexible and can be adapted to different environmental conditions, opening up new perspectives in many application areas.

Optical Fiber Grating-Prism Fabrication by Imprint Patterning of Ionic-Liquid-Based Resist.

We present a method of microstructure fabrication on the tip of the optical fiber using a UV soft-imprint process of polymerizable ionic liquid-based optical resist. Ionic liquid with two UV-sensitive vinylbenzyl groups in the structure was diluted in non-hazardous propylene glycol (PG) to obtain liquid material for imprinting. No additional organic solvent was required. The impact of propylene glycol amount and exposure dose on optical and mechanical properties was investigated. The final procedure of the UV imprint on the optical fiber tip was developed, including the mold preparation, setup building, UV exposure and post-laser cure. As the IL-containing vinylbenzyl groups can also be polymerized by the radical rearrangement of double bonds through thermal heating, the influence of the addition of 1-2% BHT polymerization inhibitor was verified. As a result, we present the fabricated diffraction gratings and the optical fiber spectrometer component-grism (grating-prism), which allows obtaining a dispersion spectrum at the output of an optical in line with the optical fiber long axis, as the main component in an optical fiber spectrometer. The process is very simple due to the fact that its optimization already starts in the process of molecule design, which is part of the trend of sustainable technologies. The final material can be designed by the tailoring of the anion and/or cation molecule, which in turn can lead to a more efficient fabrication procedure and additional functionalities of the final structure.

Optical fiber optrodes with silver-coated gold nanocavity ordered arrays for highly sensitive surface enhanced Raman spectrum

In this paper, we have proposed a new strategy to fabricate surface enhanced Raman scattering (SERS) optical fiber optrodes with ordered gold nanocavities modified by silver nanoparticles (AgNPs) on the fiber tip. Such nanocavity ordered array (COA) structures have strong light trapping ability and improve the utilization of light through multiple reflections of light. Besides, COAs have larger specific surface area, which can provide a large amount of SERS “hot spots”, and also enable the absorption of more probe molecules. Furthermore, secondary deposited silver can result in strong coupling between the gold nanocavities and AgNPs. Consequently, the nanocavity arrays can further enhance local surface plasmon resonance (LSPR) effect to improve the sensitivity of SERS optrodes. We have also studied such nanocavity SERS optrodes for in-situ detection of 4-aminothiophenol (4-ATP) with detection limits of 10−10 M. The SERS optrode is highly sensitive, good reproducibility and acceptable stability, and shows excellent performance in liquid-phase in-situ and remote SERS detection. Thus, it is promising for water environmental monitoring applications. And this paper provides a simple and low cost technology suitable for the large-scale fabrication of highly sensitive optical fiber SERS optrodes.

Comparative study of lab-on-fiber vector magnetic field sensor based on multimode and few-mode fiber

A kind of reflective vector magnetic field sensor with wedge-shaped fiber tip based on surface plasmon resonance (SPR) and magnetic fluid (MF) has been fabricated and investigated. Multimode fiber (MMF) and few-mode fiber (FMF) were grinded and polished to get the wedge-shaped fiber tip, which was then plated with nanoscale gold film for exciting SPR. The as-fabricated tip was immersed in MF, which was used as the magnetic field sensitive material. Experimental results show that the magnetic field intensity and direction sensitivities of the magnetic field sensors based on MMF and FMF are 2317 pm/mT, 497 pm/° and 6776 pm/mT, 2313 pm/° respectively, which proves that the proposed sensor based on wedge-shaped fiber tip has great potential for realizing lab-on-fiber devices and applications.

Кавитация на торце оптоволокна при лазерном нагреве воды в узкой щели

An experimental study of the process of growth and collapse of a cavitation bubble at the tip of an optical fiber placed between flat solid surfaces (in a slot) has been carried out. The features of the dynamics of cavitation bubbles in this configuration are explained using numerical simulation. It is shown that laser cavitation at the tip of an optical fiber can be used for selective cleaning and sanitation of surfaces in slots and channels. E. Dats, A. Kulik, M. Guzev, V. Chudnovskiy

Refractive index measurements of ethanol-water binary liquid solutions using a graded-index fiber tip sensor

The refractometric analysis of ethanol-water mixtures is hampered because this type of binary solution does not present a linear behaviour. In this work it is proposed a multimode Graded-Index Fiber (GIF) tip sensor for the measurement of ethanol in binary liquid solutions of ethanol-water. The probe is fabricated by fusion-splicing a 500 µm GIF to a Single Mode Fiber (SMF) and it operates as a refractometric sensor in reflection. Samples of ethanol-water mixtures were measured at different temperatures (20°C to 60°C) to evaluate the probe’s capability to detect variations in ethanol refractive index. The samples have different % (v/v) of ethanol, in a range between 0% and 100%.

Smart Microcavity on the Tip of Multi-Core Optical Fiber for Gas Sensing

Smart Microcavity on the Tip of Multi-Core Optical Fiber for Gas Sensing

Laser Interstitial Thermotherapy in Surgical Treatment of Vascular Anomalies.

The effects of subcooled boiling of water and blood in the experimental cuvette and rabbit hepatic vessels under the influence of optical fiber-guided continuous IR laser radiation (1.94μm) were studied using optical methods and ultrasonic diagnostics near the tip of the laser optical fiber. The liver was chosen as a model of vascular abnormality with a conglomerate of blood vessels. In the proposed approach, modification (obliteration and coagulation) of the vascular anomalies occurred as a result of intense subcooled boiling of the blood with generation of heated submerged jets.

The Self-Learning AI Controller for Adaptive Power Beaming With Fiber-Array Laser Transmitter System

In this study we consider adaptive power beaming with a fiber-array laser transmitter system in presence of atmospheric turbulence. For optimization of power transition through the atmosphere a fiber-array is traditionally controlled by stochastic parallel gradient descent (SPGD) algorithm where control feedback is provided via a radio frequency link by an optical-to-electrical power conversion sensor, attached to a cooperative target. The SPGD algorithm continuously and randomly perturbs voltages applied to fiber-array phase shifters and fiber tip positioners in order to maximize sensor signal, i.e. uses, the so-called, “blind” optimization principle. By contrast to this approach a prospective artificially intelligent (AI) control systems for synthesis of optimal control can utilize various pupil- or target-plane data available for the analysis including wavefront sensor data, photo-voltaic array (PVA) data, other optical or atmospheric parameters, and potentially can eliminate well-known drawbacks of SPGDbased controllers. In this study an optimal control is synthesized by a deep neural network (DNN) using target-plane PVA sensor data as its input. A DNN training is occurred online in sync with control system operation and is performed by applying of small perturbations to DNN’s outputs. This approach does not require initial DNN’s pre-training as well as guarantees optimization of system performance in time. All theoretical results are verified by numerical experiments.

Switchable Dual-Wavelength Tm-Doped All-Fiber Laser Integrating a Fiber Tip Modal Interferometer

A switchable dual-wavelength thulium-doped all-fiber laser incorporating a fiber tip modal interferometer (FTMI) is reported. The FTMI relies on the splice joint between a standard single-mode optical fiber and a tapered microfiber with 60- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> uniform waist diameter, providing a spectral reflection response that allows the laser to emit beyond 2000 nm, specifically at 2009.20 nm and 2013.65 nm, under a dual-wavelength operation regime. Besides, switching between both laser lines is also observed and controlled by adjustments of the intracavity polarization state, obtaining the ability to operate in a single-wavelength regime at either 2009 nm or 2013 nm. Laser emission lines exhibit linewidths of around 50 pm, side-mode suppression ratios higher than 50 dB and maximum output power of 13.4 mW.

NV nanodiamond doped fiber for magnetic field mapping

The advances in fluorescent diamond-based magnetic field sensors have led this technology into the field of fiber optics. Recently, devices employing diamond nanobeams or diamond chips embedded on an optical fiber tip enabled achieving fT-level sensitivities. Nevertheless, these demonstrations were still confined to operation over localized magnetic field sources. A new approach of volumetric incorporation of nanodiamonds into the optical fiber core enables optical fibers sensitive to magnetic field at any point along the fiber length. We show that information on the perturbed spin state of a diamond nitrogen-vacancy color center can be transmitted over a macroscopic length in an optical fiber, in presence of noise from large concentration of the color centers along the fiber. This is exploited in optical readout at the fiber output not only of the magnetic field value, but also spatially variable information on the field, which enables the localization of its source.

Ratiometric Fluorescence Optical Fiber Sensing for On-Site Ferric Ions Detection Using Single-Emission Carbon Quantum Dots

A carbon quantum dots (CQDs)-based ratiometric fluorescence fiber sensing method for Fe <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> on-site detection is developed. In the proposed ratiometric sensing system, only one kind of single-emission fluorescent material is used. The method of ratiometric sensing provides correction for environmental interferences and eliminates the fluctuations caused by the light source. In this sensing system, the LED light source is not only used to supply the light for excitation, but also provide the internal standard for ratiometric sensing. The CQDs are synthesized by a simple hydrothermal method and can be immobilized on the fiber easily in seconds without molds. The designed annular fiber tip has the advantages of high-efficiency collection of fluorescence and long service life. It is found that concentration of ferric ions can be detected in the range of 0-40 μM by the sensing system. The limit of detection (LOD) is as low as 0.3 μM, which is close to the LOD obtained by dispersing the carbon quantum dots in the solution. The response time is only 6 seconds. The sensing probe with the rapid response is only 4 mm in length with the maximum diameter of 350 μm, showing great potential on real-time detection in narrow area. The practicability and reliability of the present system is verified by actual samples.

Generation of High-Frequency Ultrasound in a Liquid upon Ex-Citation by Laser Radiation through a Light Guide with a Converter of Transparent Spheres

One of the important tasks in optoacoustics today is the development of methods and tools for generating high-frequency ultrasound (above 1 MHz) in liquids and other media. To expand the frequency range of ultrasound, it was proposed to use coatings consisting of focusing spheres on a fiber tip. The methodology of calculating the ultrasound spectra depending on the sphere size, index of refraction, and parameters of laser radiation was developed. Two cases of small and large spheres in strongly and weakly absorbing media were simulated. The experimental results were analyzed in the approximations allowing a fairly accurate estimation of the spectrum and indicatrix of the generated ultrasound upon laser excitation through a converter based on a coating of transparent spheres. A good agreement between the model and experimental result was obtained.

Fiber-sensor alignment based on surface microstructures.

Conventional methods have relied on specialized imaging equipment and advanced fabrication process to solve the problem of accurately aligning a microsensor to an optical fiber which is critical for its detection efficiency. To dramatically lower the barrier to high-precision alignment, we present a technique much easier to implement and much lower in cost. By fabricating replicable alignment and proximity structures on the surface of the sensor chip, we can achieve accurate alignment and position the fiber tip very close to the sensor without damaging it. We introduce an easy setup to examine the alignment result and demonstrate accurate alignment of dummy sensors as small as 5μm×5μm. We use our alignment method to realize efficient input coupling for a superconducting transition-edge sensor as an example of fruitful adoption in many possible applications.

A dual-mode optical fiber sensor for SERS and fluorescence detection in liquid

The in vivo detection of biomarkers in a liquid environment is very important for the early diagnosis of diseases. Spectroscopy methods are employed in ultraviolet–visible-infrared wavelengths, fluorescence or Raman spectra are detected for clinical diagnose. The dual-mode image can provide more diagnostic information and has been realized in some research work. However, there is still lacking simple and sensitive dual-mode sensors to satisfy the in vivo detecting demands. In this paper, a dual-mode fiber sensor for Surface-enhanced Raman Scattering (SERS) and fluorescence detection is proposed. The sensor is formed by a tapered optical fiber, half of the fiber tip surface is coated with Ag nanoparticles. In the detection of Rhodamine 6G (R6G) aqueous solution, the minimum detectable concentrations in SERS and fluorescence tests are of the same order of magnitude. By combining the Raman spectral features and the fluorescence intensity, the recognition and quantitation of target molecules were obtained reliably. It is the first time, to our knowledge, that the Raman-fluorescence dual-mode detection is realized in one single fiber, which was manufactured with micro-machinery techniques. It is a label-free, general-purpose fiber sensor, which can be applied for liquid biopsy, helping to diagnose and treat diseases in vivo.

Factors Affecting Residual Stump Length Following Endovenous Laser Ablation.

Current instruction for endovenous laser ablation (EVLA) recommends that the laser fibre tip should be at least 2cm caudal to the saphenofemoral or saphenopopliteal junction. This is to reduce risk of deep vein thrombosis (DVT) whilst ensuring that the Great Saphenous Vein (GSV)/Short saphenous vein (SSV) occlusion (residual stump) is close to the SFJ/SPJ to reduce recurrence rates. Our aim was to primarily assess the effects of pre-operative junction distance of the laser fibre on stump length but also look at other factors affecting this. This was a retrospective analysis of an electronic collected database performing EVLA of the GSV and SSV under local anaesthesia. Patients with previous venous thromboembolic events and incomplete data were excluded. Patients were reviewed at 6weeks with venous Duplex scan to assess for the residual stump distance and DVT. Multivariable logistic regression was used to account for confounders. Between 2012 and 2018, 2341 patients underwent EVLA and complete data were available on 825 (35.2%) patients. All patients were scanned 6weeks after procedure. Sixty-nine patients (8.4%) had flush occlusion, 3 patients (.4%) had extension into femoral or popliteal vein but not considered to be a DVT and 1 patient had a DVT. High BMI and increasing length of the pre-operative junction was associated with lower incidence of flush occlusion; higher rates of flush occlusions were noted in patients undergoing short saphenous vein (SSV) EVLT (OR 4.11, 95% CI 2.34-7.20; P < .001). Consideration should be given to increase the intra-procedure laser tip junction to more than 2cm particularly in patients undergoing SSV treatment to ensure that residual stump is sufficiently distal to SPJ and SFJ reducing risks of thrombus into the femoral or popliteal veins. Lower BMI may be associated with higher risk of this though the reasons for this are unclear.

Temperature‐ and Ambient Pressure‐Independent Sensing of Hydrogen in Fluids Using Cascaded Interferometers Incorporated in Optical Fibers

AbstractPractical gas sensors are indispensable for the healthy operation of cutting‐edge hydrogen‐based systems. An optical fiber‐based hydrogen sensor incorporating a robust Fabry–Perot interferometric structure on a fiber tip with high sensitivity, selectivity, and reliability of operation and a micrometer‐scale footprint is demonstrated. The hydrogen‐sensitive volume expansion of palladium provides bi‐metal operation with a silicon nitride mirror to tune the interferometer cavity and therefore the resonance modes by switching the mirror form factor from a flat to convex shape. It does not require any peripherals, including a power supply or data communication modules. In addition to fiber‐inherent advantages, such as remote and multiplexed monitoring without electromagnetic field interference, the sensor guarantees temperature‐ and pressure‐independent operation by adding a simple glass sub‐cavity and microwindows in the mirror layer, respectively. Critically, the sensor highlights reliable operation in a liquid fluid (electrical transformer oil) to monitor hydrogen as its “damage marker” escaping from a mechanical shield or selective gas screen. The detection limit, sensitivity, and response time of the sensor under atmospheric conditions are 15 ppm, 29.6 nm/%, and 12.5 s, respectively. In addition, the unimpaired operation of the sensor in 60 °C transformer oil is verified experimentally.

Sensitive and Efficient Fluorescent Fiber-Optic Sensor for In-Situ Hypoxia Detection in Solid Tumor

Accurate and timely assessment of malignant tumors in the early stage and therapy process is critical for making clear guidance and prognosis for the treatment, which can significantly elevate the cure rate of cancer patients. Fluorescent sensing and imaging of tumors have attracted extensive attention because of their advantages of high sensitivity and spatial resolution. A “plug-and-play” fluorescent fiber-optic sensor was reported for the diagnosis of tumor in our previous work. However, the route to obtain the optimization parameters and the spatial resolving capability of the tumor sensor were still elusive. In this work, we have systematically investigated the optimized design of the fiber from the aspects of structural morphology of the fiber tip, incident light intensity, and the concentration of the fluorescent probes that renders a limit-of-detection of 3 ng/mL and a response time less than 20 s. Furthermore, the sensor can reveal the hypoxic gradient of the solid tumor by virtue of its spatial sampling ability, enabling disease classification and margin identification potential. This proof-of-concept offers great opportunities for application in oncology, such as preoperative diagnosis, clinical operation guidance, and postoperative re-examination of tumors.