Bare Optical Fiber Research Articles

Multimode fiber endoscopes for computational brain imaging.

Advances in imaging tools have always been a pivotal driver for new discoveries in neuroscience. An ability to visualize neurons and subcellular structures deep within the brain of a freely behaving animal is integral to our understanding of the relationship between neural activity and higher cognitive functions. However, fast high-resolution imaging is limited to sub-surface brain regions and generally requires head fixation of the animal under the microscope. Developing new approaches to address these challenges is critical. The last decades have seen rapid progress in minimally invasive endo-microscopy techniques based on bare optical fibers. A single multimode fiber can be used to penetrate deep into the brain without causing significant damage to the overlying structures and provide high-resolution imaging. Here, we discuss how the full potential of high-speed super-resolution fiber endoscopy can be realized by a holistic approach that combines fiber optics, light shaping, and advanced computational algorithms. The recent progress opens up new avenues for minimally invasive deep brain studies in freely behaving mice.

Influence of Adhesive Bonding on the Dynamic and Static Strain Transfers of Fibre Optic Sensors

The influence of the bonding procedure (the adhesive type, application procedure, etc.) on the static and dynamic strain transfers of bonded optical fibre sensors is studied theoretically and experimentally at room temperature. The achievable performances with four different types of adhesives (three urethane and one epoxy adhesive), and with different fibre types, are evaluated: acrylate-coated, polyimide-coated, and bare single-mode optical fibres. Static strain measurements, ranging from 20 to 200 µ strain, are performed using both fibre Bragg gratings (FBGs) and optical frequency domain reflectometry (OFDR), and are compared to reference strain-gauge measurements, and to the proposed analytical model, which is developed on the basis of stress equilibrium relations. This model is valid for bonding to all types of linear and elastic materials, as long as there is no sliding between the host material, the adhesive, and the optical fibre. The results agree between the analytical model and the experiments. Regarding the dynamic sinusoidal strain measurements, the studied dynamic range is from 10 to 100 Hz, and only the FBGs are tested. The results demonstrate that the sensitivities of strain sensors based on bonded uncoated fibres or bonded polyimide-coated fibres are comparable to those of strain gauges, and that it is possible to use bonded FBGs for precise dynamic strain measurements.

Optimization of laser dosimetry based on patient-specific anatomical models for the ablation of pancreatic ductal adenocarcinoma tumor

Laser-induced thermotherapy has shown promising potential for the treatment of unresectable primary pancreatic ductal adenocarcinoma tumors. Nevertheless, heterogeneous tumor environment and complex thermal interaction phenomena that are established under hyperthermic conditions can lead to under/over estimation of laser thermotherapy efficacy. Using numerical modeling, this paper presents an optimized laser setting for Nd:YAG laser delivered by a bare optical fiber (300 µm in diameter) at 1064 nm working in continuous mode within a power range of 2–10 W. For the thermal analysis, patient-specific 3D models were used, consisting of tumors in different portions of the pancreas. The optimized laser power and time for ablating the tumor completely and producing thermal toxic effects on the possible residual tumor cells beyond the tumor margins were found to be 5 W for 550 s, 7 W for 550 s, and 8 W for 550 s for the pancreatic tail, body, and head tumors, respectively. Based on the results, during the laser irradiation at the optimized doses, thermal injury was not evident either in the 15 mm lateral distances from the optical fiber or in the nearby healthy organs. The present computational-based predictions are also in line with the previous ex vivo and in vivo studies, hence, they can assist in the estimation of the therapeutic outcome of laser ablation for pancreatic neoplasms prior to clinical trials.

Distributed Optical Fiber Strain Sensors Packaged with Polyurethane and GFRP

To apply optical fiber sensors in an extreme environment and fulfill the rapid curing requirement of the thermosetting packaging process on glass fiber reinforced polymers (GFRP) prepreg, a novel fabricating method is proposed in this paper. It packages optical fiber by pouring and brushing the waterborne polyurethane into dry weave GFRP clothes that have bare optical fibers on one side. A special mold with a slot for laying and fixing package materials is designed. The sample with the size of 20 mm × 80 mm × 1 mm is made by adopting the developed mold. The GFRP side of the sample was pasted on the carbon fiber beam with a constant section, and a static loading test was conducted. The strain of the cantilever beam was collected by the packaged distributed optical fiber sensor and demodulated by the OFDR system. According to the Ko theory, the measured strain was converted into the displacement of the cantilever beam. The displacement of the free end of the cantilever beam was compared with the position measured by the machine vision, and the error between the two was small. The experimental results show that the ultra-thin distributed optical fiber strain sensor obtained by rapid packaging can be used for the deformation monitoring of large structures such as wind turbine blades.

Structural monitoring method for RC column with distributed self-sensing BFRP bars

Reinforced concrete (RC) columns may be damaged under earthquakes or wind loads, which will severely affect their bearing capacity. Current damage evaluation methods are mainly based on displacement measurement, which makes evaluating local structural damage difficult. Therefore, this study proposes an RC column structural monitoring and evaluation method based on a distributed self-sensing basalt fiber reinforced polymer (BFRP) bar with optical fiber sensors inside. First, the self-sensing BFRP bar is introduced, including the sensing principle, inner structure, and basic sensing performance. It is found that the strain sensing coefficient of the proposed self-sensing BFRP bar presents only 2.2 % difference from that of the bare optical fiber sensor, meaning the BFRP package take no negative effect on the strain sensitivity. Second, the analytical methods of curvature and displacement are established based on the strain distribution measurement, in which the influence of bond-slip on parameter calculations is mainly considered. Finally, the proposed method was well verified by the loading tests of one RC column with self-sensing BFRP bars. Among the results, the absolute displacement measurement error increase with the development of structure damage, which can be even up to several mm after the steel yielding. However, the relative displacement measurement error can be still controlled within 20 % until the structure failure of the RC column. Considering the additional long-term performance of the proposed self-sensing BFRP bar, the proposed method is expected to implement long-term monitoring of RC columns.

Avalanche Photodiode-Based γ-Photon Scintillation Detectors for Personalized Dosimetry in Targeted Radionuclide Therapy

<h3>Purpose/Objective(s)</h3> Targeted radionuclide therapy holds great promise as shown in neuroendocrine and prostate cancer, but highly variable biodistribution and tumor uptake requires personalized dosimetry for each patient. Optimal dosimetry requires the measurement of total integrated dose, demanding dose measurements over multiple half-lives of the radionuclide. The current state-of-art, SPECT, is often only available to a small subset of patients and is rarely done more than once. Therefore, a method for continuous radionuclide dosimetry is needed. Highly-sensitive γ-photon detectors placed on the body could solve this problem by monitoring γ decays indicative of the activity within tumors and organs at risk (OAR). We have developed a system capable of detecting γ-photons externally, in real time, with high sensitivity and dynamic range. Here we present our results for photon detection as well as a methodology to use these signals to monitor the activity in tumors and OAR in real time. <h3>Materials/Methods</h3> Y₂O₃ Eu-doped phosphor was compacted at the interface of a bare optical fiber to scintillate incoming γ-photons into 610 nm (red) light. The optical fiber was coupled to avalanche photodiodes (APDs) with a 60% peak detection efficiency at 610 nm and dead time of 50ns. The transistor-transistor logic signals from each APD are conveyed to an FPGA to compute counts per minute (CPM). The system was tested by measuring the emitted γ-photons from a titration of ¹⁷⁷Lu. Six vials were prepared by serially diluting ¹⁷⁷Lu to activities per mL ranging from 0.6∼566.2µCi/mL. The detector was placed on the outside of the vials, and the vials were incrementally changed. The CPM were recorded in real time. To determine the ability to reconstruct a SPECT image in tumors and OAR, behavioral simulations were conducted. %ID/g in these organs, paralleling the measured activity in the ¹⁷⁷Lu dilutions, was varied and the ability of the sensors to track these variations was evaluated. <h3>Results</h3> The mean CPM for the six vials ranged from 30.8 CPM (0.6µCi/mL) to 47,240 CPM (566.2µCi/mL) and was highly linear with activity. The least-squares regression fit between µCi/mL and CPM was CPM=82.28 ⨉ Concentration(µCi/mL) with an R² value of 0.9970. This range of activities is consistent with tumor and kidney uptake of an 800µCi ¹⁷⁷Lu injection in a mouse model. With this mapping from µCi/mL to CPM, the behavioral simulation demonstrated that the temporally varying trend of %ID/g of tumors and OAR could successfully be tracked in real time. It was shown that sensor placement can be optimized to predictably quantify and prevent excessive cross-talk between signals from different organs. <h3>Conclusion</h3> An APD-based γ-photon scintillation detector was developed and verified with a ¹⁷⁷Lu titration. We envision that the proposed work could be used to create a sensor network on the body capable of real time dosimetry of tumors and OAR.

Kılıfsız Optik Fiberlerde Bağıl Direnç Değişimi, Gage Faktörü ve Poisson Oranı’nın Basınç Bağımlılıkları

In this paper, a distributed sensing method relying on the principle of pressure dependencies of relative change in electrical resistance, gage factor and Poisson’s ratio of the bare optical fiber core has been proposed. Using this method, besides the pressure information, relations between pressure and relative change in electrical resistance, gage factor and Poisson’s ratio of the bare optical fiber core have been examined and then the temperature dependencies of these parameters have been mathematically analyzed and matching simulations have also been carried out in Matlab R2021b and Simulink environments. Moreover, first-order equations expressing the relations between these parameters and their temperature dependencies have been derived benefiting from the curve-fitting method. For pressure variations in the range of 2.2 × 107 Pa – 12 × 107 Pa, relative changes in electrical resistance of the fiber core have been obtained in the range of 0.41 × 10-3 – 2.13 × 10-3. In other words, the pressure dependence of relative change in electrical resistance of the fiber core can be expressed as 1.841 × 10-2 Rrc(GPa)-1, i.e. 1 GPa pressure variation occurring along the fiber core causes about 0.01841 unit of Rrc variation. Furthermore, pressure dependencies of the gage factor and Poisson’s ratio have been acquired as 2.924 × 10-2 GF(GPa)-1 and 1.462 × 10-2 σ(GPa)-1, respectively.

Fatigue Resistance of a BFRP-Encapsulated Long-Gauge FBG Strain Sensor under Cyclic Train Loads

To verify the performance of a basalt fiber reinforced polymer (BFRP) fiber-encapsulated long-gauge strain sensor in railway bridge health monitoring, this paper studies the fatigue resistance and durability of the BFRP fiber-encapsulated FBG sensor under train loads. First, the influences of the length of the anchorage section and the length ratio of the sensing section on the accuracy of the sensor were studied. Then, the BFRP sensor was applied to a sleeper for 2 million cycles of tension fatigue testing. The strain-time history of the whole fatigue test was monitored and compared. After the test, a calibration test was carried out to verify the accuracy and repeatability of the sensor. Finally, the slip and fatigue cracking of the fiber in the anchorage section of the sensor were observed by electron microscopy. The results show that the gap between the anchoring section and the bare optical fiber was filled with epoxy resin, and there was no slip behavior. No fatigue cracking occurred in the fiber, and the strain coefficient and linearity of the sensor showed no obvious changes after 2 million cycles of loading. The long-gauge strain sensor encapsulated by BFRP fibers exhibited good fatigue resistance and can meet long-term monitoring requirements under train loads.

Modeling and Optimization of Laser Cladding Fixation Process for Optical Fiber Sensors in Harsh Environments.

In order to overcome the shortcomings of the poor shear resistance of the bare optical fiber whose coating layer falls off due to harsh conditions, such as on aero-engines and the marine environment, the coaxial powder feeding laser cladding method (CPFLCM) is proposed to connect the optical fiber sensor and the substrate. The concentration field model of the powder flow is established in sections, the effective number model of particles and the corresponding laser attenuation rate are obtained. Through simulation, the influence of relevant parameters of laser cladding on the temperature field was analyzed, and the accurate parameters of laser cladding were optimized. Finally, the temperature rise trajectory of the substrate temperature field was verified by using the fiber grating temperature sensor. Through experiments, the quality of the molten pool and the optical transmission loss of the optical fiber sensor were analyzed, and the consistency of the simulation optimization parameters was verified. Through this paper, it can be concluded that the proposed CPFLCM can realize the effective connection of the optical fiber sensor to the substrate. It is of great significance in the application of optical fiber sensors in harsh environments of oceans and aerospace.

Downward viewing common-path optical coherence tomography guided hydro-dissection needle for DALK.

Deep anterior lamellar keratoplasty (DALK) is a partial-thickness cornea transplant procedure in which only the recipient's stroma is replaced, leaving the host's Descemet's membrane (DM) and endothelium intact. This highly challenging "Big Bubble" procedure requires micron accuracy to insert a hydro-dissection needle as close as possible to the DM. Here, we report the design and evaluation of a downward viewing common-path optical coherence tomography (OCT) guided hydro-dissection needle for DALK. This design offers the flexibility of using different insertion angles and needle sizes. With the fiber situated outside the needle and eye, the needle can use its' full lumen for a smoother air/fluid injection and image quality is improved. The common-path OCT probe uses a bare optical fiber with its tip cleaved at the right angle for both reference and sample arm which is encapsulated in a 25-gauge stainless still tube. The fiber was set up vertically with a half-ball epoxy lens at the end to provide an A-scan with an 11-degree downward field of view. The hydro dissection needle was set up at 70 degrees from vertical and the relative position between the fiber end and the needle tip remained constant during the insertion. The fiber and needle were aligned by a customized needle driver to allow the needle tip and tissue underneath to both be imaged within the same A-scan. Fresh porcine eyes (N = 5) were used for the studies. The needle tip position, the stroma, and DM were successfully identified from the A-scan during the whole insertion process. The results showed the downward viewing OCT distal sensor can accurately guide the needle insertion for DALK and improved the average insertion depth compared to freehand insertion.

Integrated fiber-based optoelectrode for electrochemiluminescence sensing

An optical fiber-indium tin oxide (ITO) optoelectrode was proposed and first demonstrated in electrochemical luminescence (ECL) analyze. A transparent conductive ITO film was coated onto the surface of a bare optical fiber by magnetron sputtering. The optical fiber was realized as a working electrode and a detection device at the same time. Notably, the photoelectric analysis is integrated into the optical fiber electrode, providing more information about the analyte while greatly simplifying the device. The modulation of the ECL response intensity is monitored in real-time by applying different voltages to the fiber optic electrode with a response time of 2 s. In aqueous solutions, trace amount of H2O2 can be detected in real-time by the optical fiber ECL system. A wide detection range of 1 × 10−5 M to 1 × 10−2 M is obtained. Especially, the optical fiber-ITO electrode has great potentials in environmental analysis and bioassay.

The Fiber Optic Reel System: A Compact Deployment Solution for Tethered Live-Telemetry Deep-Sea Robots and Sensors.

Tethered deep-sea robots and instrument platforms, such as Remotely Operated Vehicles (ROVs) and vertical-profiling or towed instrument arrays, commonly rely on fiber optics for real-time data transmission. Fiber optic tethers used for these applications are either heavily reinforced load-bearing cables used to support lifting and pulling, or bare optical fibers used in non-load bearing applications. Load-bearing tethers directly scale operations for deep-sea robots as the cable diameter, mass, and length typically require heavy winches and large surface support vessels to operate, and also guide the design of the deep-sea robot itself. In an effort to dramatically reduce the physical scale and operational overhead of tethered live-telemetry deep-sea robots and sensors, we have developed the Fiber Optic Reel System (FOReelS). FOReelS utilizes a customized electric fishing reel outfitted with a proprietary hollow-core braided fiber optic fishing line and mechanical termination assembly (FOFL), which offers an extremely small diameter (750 μm) load-bearing (90 lb/400 N breaking strength) tether to support live high-bandwidth data transmission as well as fiber optic sensing applications. The system incorporates a novel epoxy potted data payload system (DPS) that includes high-definition video, integrated lighting, rechargeable battery power, and gigabit ethernet fiber optic telemetry. In this paper we present the complete FOReelS design and field demonstrations to depths exceeding 780 m using small coastal support vessels of opportunity. FOReelS is likely the smallest form factor live-telemetry deep-sea exploration tool currently in existence, with a broad range of future applications envisioned for oceanographic sensing and communication.

Study of Microbending Loss Single Mode Optic Fiber in Sand Powder Against Pressure

Research has been carried out to further investigate specifically the effect of sand powder, both the size of the sand grains and the thickness of the sand powder on the photodetector output as an advanced study of the single-mode optical fiber microbending loss theory in sand grains to pressure. This was done to investigate the response of optical fibers due to microbending loss to the load and determine the size of the sand particles that are most effectively used as a compiler of load sensors. The principle works to test the response of load sensors based on single-mode fiber optic microbending loss in the form of photodetector output when given a large variety of pressure. The method used in this research is to observe the reduction in the intensity of the light transmitted through optical fibers in the form of a voltage drop that is read by MMD that is connected to the photodetector. The reduced light intensity shows that the load sensor experiences optical attenuation of the laser as a light source with a wavelength of 1550 nm and a power of 1.47 mW. Microbending loss is caused by mechanical pressure that can change the direction of optical signal transmission and the radius of the curve is equal to or less than the diameter of a bare optical fiber. Observations were made using 12 load sensors with variations in the size of the sand grains in each diameter of the hose. The results of this study obtained the size of the most effective grains of sand providing microscopic curvature in the optical fiber that is 0.05 mm in terms of the correlation between the response of sensors with various diameters to changes in pressure.

Nonlinear studies of graphene oxide and its application to moisture detection in transformer oil using D-shaped optical fibre

ABSTRACT This paper reports nonlinear optical response studies of thin layers using graphene oxide (GO) and their performance for detecting water content in transformer oil using three sets of approaches. The sensor performance is evaluated based on bare optical fibre, D-shape optical fibre integrated with GO and the effects of the covering device with polydimethylsiloxane (PDMS). The improvement of this study is to ensure the stability of the recorded signal. The nonlinear optical response of the prepared dispersion was explored using the Z-scan technique with an 800 nm continuous wave laser beam. Taking advantage of the graphene-induced evanescent field using D-shape optical fibre with PDMS covers on the device can give high sensitivity and good stability for water dissolved in the oil at 1550 nm. We measured fluctuations in transmitted light associated with varying water contents ranging from 16 to 38 ppm.

Polydopamine-Assisted Fabrication of Stable Silver Nanoparticles on Optical Fiber for Enhanced Plasmonic Sensing

We present a facile and effective method for fabrication of the localized surface plasmon resonance (LSPR) optical fiber sensor assisted by two polydopamine (PDA) layers with enhanced plasmonic sensing performance. The first PDA layer was self-polymerized onto the bare optical fiber to provide the catechol groups for the reduction from Ag+ to Ago through chelating and redox activity. As the reduction of Ag+ proceeds, Ag nanoparticles (NPs) were grown in-situ on the PDA layer with uniform distribution. The second PDA layer was applied to prevent Ag NPs from oxidating and achieve an improvement of LSPR signal. The PDA/Ag/PDA-based optical fiber sensor has an enhanced LSPR sensitivity of 961 nm/RIU and excellent oxidation resistance. The stable PDA/Ag/PDA-based LSPR sensor with high optical performance is very promising for future application in optical sensing field.

Polymer-fiber random lasers based on pumping radiation effect

In this study, we coated laser dye-doped polymethyl methacrylate on the surface of a bare polymer optical fiber (POF) to fabricate new POFs. Coherent random lasers are obtained for the gain POFs under the pump of a 532 nm laser. The mechanism for the random lasers is based on the refractive index heterogeneity scattering of the coating gain polymer. The variations of random lasers with different coating thicknesses and POF lengths under radiation pumping mode have been investigated. One important finding from our experiment is that dye-doped POF exhibits good photostability and self-healing properties. In addition, polymer-fiber random lasers as a sensor are researched in different concentrations of solutions. The intensity of the random lasers increases with the increase of the content of the surrounding ethanol solution due to swollen effect of the POF with a large gain length.

Fabrication of a Bare Optical Fiber-Based Biosensor.

A bare optical fiber-based biosensor is proposed for measuring the refractive index of different liquids and the binding kinetics of biomolecules to the sensor surface. This optical fiber sensor is based on the Kretschmann’s configuration to attain total internal reflection (TIR) for surface plasmon resonance (SPR) excitation. One end of the bare optical fiber is coated with a gold film. By guiding the light source from the other end into the optical fiber, the light is reflected from the gold-deposited end and the surface evanescent wave is excited in the gold film-transparent material interface. Methanol and ethanol solutions with different refractive indices are used for measuring the corresponding changes in the peak values of the spectra and calculating the corresponding sensitivities. These values are experimentally determined to be in the order of 10−4~10−5 refractive index unit (RIU). Binding of proteins onto the sensor surface is also monitored in real time to obtain the binding kinetics. We believe that, in the future, this optical fiber sensor can serve as a useful biosensor for in situ measurement of allergens, antibody–antigen interactions, and even circulating tumor cells in the blood.

A FBG Tilt Sensor Fabricated Using 3D Printing Technique for Monitoring Ground Movement

In this paper, 3D printing method and fiber Bragg grating (FBG) sensing technology were combined to fabricate a simple and small size tilt sensor. The raw material for creating different sensor components was Polylactic Acid (PLA) material. Bare FBG sensors were directly inscribed into PLA material using fused deposition modeling (FDM) process for complete protection. Printing test using FDM process indicate that bare optical fiber cables successfully survived the FDM process even though the melted PLA materials approached a maximum temperature of around 220°. Two FBG sensors characterized by different wavelength values were symmetrically arranged inside a sensing unit for the measurement of the tilt angle in terms of their wavelength shifts. The minimum resolution and the measurement range of this new FBG tilt sensor were 0.084° and ±75°, respectively. The measurement performance of this new tilt sensor was also verified with the conventional tilt sensor. A small slope model was built up in the laboratory to examine the measurement performance of three FBG tilt sensors which were mounted at different depths inside the slope model. The measured horizontal displacement data from FBG tilt sensors were verified with the output data of the Linear Variable Displacement Transducer (LVDT) and the obtained maximum error was less than 3% when the maximum displacements are approached.

Optical fiber bundles: Ultra-slim light field imaging probes.

Optical fiber bundle microendoscopes are widely used for visualizing hard-to-reach areas of the human body. These ultrathin devices often forgo tunable focusing optics because of size constraints and are therefore limited to two-dimensional (2D) imaging modalities. Ideally, microendoscopes would record 3D information for accurate clinical and biological interpretation, without bulky optomechanical parts. Here, we demonstrate that the optical fiber bundles commonly used in microendoscopy are inherently sensitive to depth information. We use the mode structure within fiber bundle cores to extract the spatio-angular description of captured light rays-the light field-enabling digital refocusing, stereo visualization, and surface and depth mapping of microscopic scenes at the distal fiber tip. Our work opens a route for minimally invasive clinical microendoscopy using standard bare fiber bundle probes. Unlike coherent 3D multimode fiber imaging techniques, our incoherent approach is single shot and resilient to fiber bending, making it attractive for clinical adoption.

Application of spectroelectroanalysis for the quantitative determination of mixtures of compounds with highly overlapping signals

The amount of qualitative and quantitative information provided by a UV–vis absorption spectroelectrochemistry (SEC) experiment is sometimes wasted. However, almost all electrochemical and spectroscopic data can provide valuable information. In this spirit, the main objective proposed in this work is the quantitative resolution of catechol/dopamine (CAT/DA) and dopamine/epinephrine (DA/EP) mixtures, using spectroelectrochemical sensors in long optical path length arrangement based on bare optical fibers in parallel configuration with respect to carbon nanotubes or screen-printed electrodes. These compounds show extremely similar electrochemical and spectroscopic responses at high acidic pH, being impossible to determine their concentrations in the mixtures just using univariate regression models. To our knowledge, the SEC ability to resolve complex mixtures has never been demonstrated before with signals with this degree of overlapping. The quantitative analysis of these mixtures is possible using multivariate regression analysis of a set of time-resolved spectroelectrochemical data with a powerful statistical tool such as parallel factor analysis (PARAFAC). PARAFAC enables us to extract all the information from the experiments, allowing us to quantify the different analytes in mixtures of varying concentrations with excellent results. This milestone for spectroelectroanalysis illustrates the expected capabilities of SEC and demonstrates experimentally the potential of this technique for sensing of biomolecules.