Optical Backscatter Reflectometer Research Articles

Label-free optical fiber biosensor for the detection of CD44-expressing breast cancer cells

Increased level of CD44, transmembrane glycoprotein, is observed in several cancers such as breast, colorectal, head and neck, gastric, and ovarian cancer. Current methods for analyzing CD44 expressing cells are time-consuming, semi-quantitative, use labeled reagents for analysis or are not suitable for in situ analysis. Therefore exploring novel methods which are label-free and fast with the in situ detection capability for the quantification of these cells is of importance.Using optical fiber as a sensing element in biosensors offers low cost, high sensitivity, chemical inertness, and immunity to electromagnetic interference, and possibility of being used in in situ applications. This study reports the first label-free optical fiber biosensor for detection of CD44-expressing cancer cells. A fiber-optic ball resonator (BR) sensor was fabricated and then functionalized with CD44 antibodies. Different concentrations of the cells were incubated with the biosensor to measure the reflected light using optical backscatter reflectometer. The biosensor was able to detect cancer cells with a limit of detection of 335 cells/mL. The presence of the cells on the sensor surface was demonstrated by fluorescent and scanning electron microscope analysis. The work is the first proof-of-concept biosensor based on optical fibers for CD44 cancer cells and acts as a promising tool for in situ detection of cancer cells.

Characterization of optical fibers doped with nanoparticles for distributed displacement sensing.

High-scattering optical fibers have emerged as a key component in distributed sensing systems, primarily due to their capacity to enhance signal-to-noise ratio. This paper presents an experimental characterization of optical fibers doped with oxide nanoparticles for displacement sensing. They were manufactured using the phase-separation technique and different doping compounds, including calcium, strontium, lanthanum and magnesium. The Rayleigh backscattering (RBS) signatures in time and frequency domains were acquired using an Optical Backscatter Reflectometer (OBR). The maximum representative length, backscattering gain and strain sensitivity were evaluated. The results indicate that the fiber co-doped with magnesium and erbium chlorides offered the best compromise between strain sensitivity (0.96 pm/μ ϵ) and maximum length (17 m). For conditions of single and multiple perturbations, strain saturation was reached at ≥7000 μm and <1500 μm, respectively. In addition, the results reveal that, under a condition of variable temperature (30-60 °C), the sensor response becomes significantly nonlinear over length, requiring a technique for temperature cross-sensitivity mitigation that accounts for nonlinearities in sensitivity and hysteresis.

Reflector-Less Shallow-Tapered Optical Fiber Biosensors for Rapid Detection of Cancer Biomarkers

Reflector-less biosensors based on optical fibers enable the detection of the refractive index (RI) variations, in reflection mode, without the need for any reflective element (such as a grating or an interferometer). The sensing system works by interrogating the wavelength shift occurring in the Rayleigh scattering signal at each location of a thinned fiber through an optical backscatter reflectometer (OBR); a high-scattering fiber is required in order to compensate for the propagation losses. The approaches reported so far make use of etched fibers, which are however fragile and require tens of minutes of fabrication, and U-bent fibers which are incompatible with in-situ devices. In this article, we present the working principle and main results of reflector-less biosensors fabricated through shallow tapering; this concept allows the fastest fabrication (&#x223C;20 seconds through a CO₂ automated laser splicer) reported so far, and allows maintaining a compact and robust form factor as the fiber maintains a diameter &gt;28 &#x00A0;&#x03BC;m (&gt;22&#x0025; of the initial thickness). The fabrication process allows achieving a peak sensitivity up to 1.33 nm&#x002F;RIU (1.05 nm&#x002F;RIU average over 5 tapers), using MgO-nanoparticle doped fibers. The tapered biosensors have been functionalized through the silanization method, in order to detect CD44 protein (from 100 fM to 10 nM), a breast cancer biomarker. The results show a successful detection with a low limit of detection (16.4 pM) and high specificity over two controls.

Temperature and Humidity Stability of Fibre Optic Sensor Cables for High Resolution Measurements.

Fibre optic sensors offer a means for the real-time continuous measurement of temperature or strain in concrete structures. Backscattered light along a fibre optic sensing (FOS) cable is interrogated to record a frequency shift and this shift is typically translated into a physical parameter such as strain or temperature using a calibration factor. However, when the measured frequency shift is a response to a combination of mechanical, thermal or hygral (humidity) loadings it is difficult to decouple individual influences. This presents a challenge in complex materials such as concrete where the strain, temperature and moisture levels change concurrently during the fresh and hardened states. Furthermore, depending on the application, both short- and longer-term measurements are required. As such, not only is the influence of these physical factors of interest but also the time and spatial stability of the measured frequency, which is highly dependent on the FOS cable composition. To investigate this aspect, fibre optic cables commonly used for strain (three tight-buffered cables) or temperature (two loose-buffered cables) measurement were considered. The cables were subjected to mechanical or environmental exposure and interrogated using a high-resolution optical backscatter reflectometer. The exposure regimes included three temperature cycles with sustained steps from 10 °C to 60 °C and back to 10 °C and an increasing and decreasing humidity cycle with steps between 30 to 90% relH. These ranges were selected to be indicative of typical environments for concrete. The results showed that the calibration factors back-calculated from increasing and decreasing temperature or humidity cycles differed. The third temperature cycle results were found to exhibit the smallest differences between heating and cooling suggesting that temperature pre-conditioning prior to installation could be advantageous. For all the cables, a drift in the readings was observed over the duration (2.5 h for temperature and 30 h for moisture) of the sustained steps. The magnitude of the drift depended on the cable type and exposure condition. In addition, local frequency fluctuations along the cable were observed which would need to be taken into account if only a single point along the cable length was used for analysis. The obtained results highlight the importance of the cable selection to maximise the FOS measurement fidelity for a given parameter of interest.

Investigation of the Optical Properties of Silicon-on-Insulator Microring Resonators Using Optical Backscatter Reflectometry

Introduction. Optical backscatter reflectometry is one of the most promising methods used to examine characteristic parameters relevant to the design of microring resonators. This method paves the way for experimental determination of the coupling coefficient and propagation loss. However, experimental verification of this technique by comparing the transmission characteristics obtained by reflectometry and those directly measured by an optical vector analyzer has not been carried out.Aim. To determine the parameters of microring resonators by optical reflectometry and to calculate on their basis the transmission characteristics of microring resonators. To compare the calculated transmission characteristics with those obtained experimentally using a high-resolution vector analyzer.Materials and methods. The characteristic parameters of silicon-on-insulator microring resonators were investigated using an ultra-high resolution reflectometer. An original algorithm was employed to derive the characteristic parameters of microring resonators from reflectograms. An optical vector analyzer was used to study the transmission characteristics of microring resonators. Numerical modeling of transmission characteristics considering the obtained parameters was carried out according an analytical approach based on partial wave analysis.Results. The obtained values of the power coupling coefficient κ = 0.167 and propagation losses α = 3.25 dB/cm were used for numerical simulation of the transmission characteristics of a microring resonator. These characteristics were found to agree well with those obtained experimentally. The free spectral range of 88.8 GHz and Q-factor of 45 000 were determined.Conclusion. An experimental study of the characteristic parameters of silicon-on-insulator microring resonators was conducted using an optical backscatter reflectometer. The performed comparison of the experimental and theoretical transmission characteristics showed good agreement, which indicates the high accuracy of the determined resonator parameters and, as a result, the relevance of the described method.

Strain patterns of short span reinforced concrete beams under flexural loading: A comparison between distributed sensing and concrete damaged plasticity modelling

Strains beyond the elastic limit of concrete cause cracks in concrete structures. This study analysed distributed optical fibre sensor (DOFS) network strain data to identify excessive strain readings, locations and strain patterns of four reinforced concrete (RC) beams. The strain variation inside the RC beams was investigated using a concrete damaged plasticity (CDP) based finite element analysis (FEA) model. The measured bottom surface and bottom rebar strain were considered for investigation. A three-point bending arrangement was used for the loading, and an optical backscatter reflectometer was used to measure the strain. By visual inspections, it has been found that the onset of hairline cracks at the locations of the higher strain readings. The maximum load applied was 16 kN and has not shown any yielding in strain reading acquired from bottom rebars. FEA results showed admissible agreement with the measured lower surface strain and lower rebar strain. Therefore, when developing a robust SHM system, a CDP-based FEA simulation is a valuable tool for data mining of strain data. Furthermore, the strain data extracted from CDP models can be used to train artificial intelligence based SHM models.

2D Temperature Field Reconstruction Using Optical Frequency Domain Reflectometry and Machine-Learning Algorithms.

We present experimental results on the reconstruction of the 2D temperature field on the surface of a 250 × 250 mm sensor panel based on the distributed frequency shift measured by an optical backscatter reflectometer. A linear regression and a feed-forward neural network algorithm, trained by varying the temperature field and capturing thermal images of the panel, are used for the reconstruction. In this approach, we do not use any information about the exact trajectory of the fiber, material properties of the sensor panel, and a temperature sensitivity coefficient of the fiber. Mean absolute errors of 0.118 °C and 0.086 °C are achieved in the case of linear regression and feed-forward neural network, respectively.

Biofilm Detection by a Fiber-Tip Ball Resonator Optical Fiber Sensor

Bacterial biofilms are one of the most important challenges that modern medicine faces due to the difficulties of diagnosis, antibiotic resistance, and protective mechanisms against aggressive environments. For these reasons, methods that ensure the inexpensive and rapid or real-time detection of biofilm formation on medical devices are needed. This study examines the possibilities of using optical- and fiber-based biosensors to detect and analyze early bacterial biofilms. In this study, the biofilm-forming model organism Pseudomonas aeruginosa was inoculated on the surface of the optical sensor and allowed to attach for 2 h. The biosensors were made by a fiber-tip ball resonator, fabricated through a CO2 laser splicer on a single-mode fiber, forming a weak reflective spectrum. An optical backscatter reflectometer was used to measure the refractive index detected by the sensors during different growth periods. The early biofilm concentration was determined by crystal violet (CV) binding assay; however, such a concentration was lower than the detection limit of this assay. This work presents a new approach of biofilm sensing in the early attachment stage with a low limit of detection up to 10−4 RIU (refractive index units) or 35 ± 20 × 103 CFU/mL (colony formed units).

Microscopic imaging along tapered optical fibers by right-angle Rayleigh light scattering in linear and nonlinear regime.

The evolution of the light intensity along an optical waveguide is evaluated by analysing far-field right-angle Rayleigh light scattering. The method is based on point by point spectral mapping distributed along the optical waveguide with a micrometric spatial resolution given by a confocal microscope, a high spectral resolution given by a spectrometer, and a high signal-to-noise ratio given by a highly cooled detector. This non-destructive and non-invasive experimental method allows the observation of the general Rayleigh scattering profile of the optical waveguide in a nominal operation, i.e., whatever the power or the wavelength of the light source, and can be applied to micrometer-scale waveguides of several centimeters in length, for which the longitudinal characterization is challenging. Applied to a tapered optical fiber, called nanofiber, with submicrometer final diameter and several centimeters long, the method has proved its capacity to collect different optical characteristics such as optical losses, mode beatings, transition from core-cladding to cladding-air guidance for different modes, localization of punctual defects, leaking of high order modes no longer guided by the fiber. Furthermore, the experimental results are successfully compared to measurements provided by the state-of-the-art Optical Backscatter Reflectometer system, and to numerical simulations. Moreover, coupled to the spectral resolution of the spectrometer, the method have allowed the distributed measurements of the Raman cascading process along the nanofiber, for the first time to our knowledge. The experimental technique developed in this work is complementary to other characterization methods generally focused on the optical parameters of the waveguide input or output. This technique can also be extended to others waveguides whatever its geometry which represents a strong interest for deepen optical characterization of photonics waveguides, or for other optical regimes characterized by spectral evolution of the field propagating along the waveguide.

Optical Fiber Ball Resonator Sensor Spectral Interrogation through Undersampled KLT: Application to Refractive Index Sensing and Cancer Biomarker Biosensing.

Optical fiber ball resonators based on single-mode fibers in the infrared range are an emerging technology for refractive index sensing and biosensing. These devices are easy and rapid to fabricate using a CO2 laser splicer and yield a very low finesse reflection spectrum with a quasi-random pattern. In addition, they can be functionalized for biosensing by using a thin-film sputtering method. A common problem of this type of device is that the spectral response is substantially unknown, and poorly correlated with the size and shape of the spherical device. In this work, we propose a detection method based on Karhunen−Loeve transform (KLT), applied to the undersampled spectrum measured by an optical backscatter reflectometer. We show that this method correctly detects the response of the ball resonator in any working condition, without prior knowledge of the sensor under interrogation. First, this method for refractive index sensing of a gold-coated resonator is applied, showing 1594 RIU−1 sensitivity; then, this concept is extended to a biofunctionalized ball resonator, detecting CD44 cancer biomarker concentration with a picomolar-level limit of detection (19.7 pM) and high specificity (30–41%).

Distributed Temperature Monitoring Inside Ytterbium DFB and Holmium Fiber Lasers

A distributed temperature monitoring inside a cavity of ytterbium DFB and holmium fiber lasers has been demonstrated with a spatial resolution of 1 and 5 mm, respectively, for the first time to the best of our knowledge. For this, we use an optical backscatter reflectometer, which measures intracore temperature, and compare it with the data of an IR thermographic camera, which measures temperature from the surface of a fiber. In the case of holmium fiber laser pumped at a wavelength of 1125 nm with a power of 6 W, the maximum temperature variation along the ∼3-m active fiber reaches ∼60 °C. In the case of ytterbium DFB laser, we observe a strong inhomogeneity of the temperature along the DFB cavity, which leads to a significant decrease in the lasing efficiency. When pumped by a single-mode laser diode at a wavelength of 976 nm with a power of up to 526 mW, the maximum temperature difference reaches 37 °C for the 37-mm DBF cavity.

Picomolar detection of thrombin with fiber-optic ball resonator sensor using optical backscatter reflectometry

A ball resonator, positioned on the tip of an optical fiber, has been developed as a biosensor for the prototypic detection of thrombin. The device was fabricated with a fast and repeatable CO2 laser splicing method, followed by gold-sputtering and functionalization for the measurement of various protein concentrations. The ball resonator acts as a weak interferometer with a return loss below − 50 dB, and it is interrogated with an optical backscatter reflectometer measuring the reflection spectrum. We report here a sample presenting high sensitivity (1273.74 nm/RIU, RIU = refractive index units), which allows protein detection in the range 0.4–100 pM, with a limit of detection of 1.56 pM in logarithmic response.

Polarization-sensitive optical fiber-tip ball resonators for refractive index sensing with optical backscatter reflectometer interrogator

Optical fiber-tip ball resonators are inexpensive devices characterized by a rapid, repeatable, and reliable fabrication process through a fiber splicer. The spectral features of a ball resonator appear as low-finesse interferometer, with an optical backscatter reflectometer (OBR) interrogator. The spectrum changes in amplitude when the external refractive index (RI) varies. In this work, we present a polarization-sensitive method for the detection of spectral changes in ball resonators. We achieved a sensitivity ranging from 366.6 dB/RIU to 6691.6 dB/RIU (refractive index units) on the most sensitive polarization, improving the sensitivity of a standard detection of 18.6%–47.7% without the need of any additional component.

Reflective Properties of a Polymer Micro-Transducer for an Optical Fiber Refractive Index Sensor

A polymer microtip manufactured at the end of a multi-mode optical fiber by using the photopolymerization process offers good reflective properties, therefore, it is applicable as an optical fiber sensor micro-transducer. The reflective properties of this microelement depend on the monomer mixture used, optical fiber type, and light source initiating polymerization. Experimental results have shown that a proper selection of these parameters has allowed the design of a new class of sensing structure which is sensitive to the refractive index (RI) changes of a liquid medium surrounding the microtip. An optical backscatter reflectometer was applied to test a group of micro-transducers. They were manufactured from two monomer mixtures on three different types of multi-mode optical fibers. They were polymerized by means of three optical light sources. Selected micro-transducers with optimal geometries were immersed in reference liquids with a known RI within the range of 1.3–1.7. For a few sensors, the linear dependences of return loss and RI have been found. The highest sensitivity was of around 208 dB/RIU with dynamic 32 dB within the range of 1.35–1.48. Sensing characteristics have minima close to RI of a polymer microelement, therefore, changing its RI can give the possibility to tune sensing properties of this type of sensor.

Distributed optical fibre sensor based monitoring of thermoplastic carbon composite cylinders under biaxial loading: Experimental and numerical investigations

Composite tubes are used in numerous applications including aerospace, mechanical, civil and transport sectors, where they may be susceptible to combined loading especially axial compression and torsion and suffer failure due to their intrinsic weakness under such loads. An experimental cum numerical investigation is performed to understand the behaviour of hollow composite tubes with cut-outs under combined axial compression and torsional loadings. Carbon fibre reinforced thermoplastic CF/PEEK tubes with different diameters and varied lay-up sequences, were robotically manufactured using an Automated Fibre Placement (AFP) machine. These specimens were mechanically tested using a novel biaxial test jig, designed specifically to be compatible with an INSTRON 8852 axial/torsion servo hydraulic test system. A standard acrylate coated optical fibre was affixed on the surface of the cylinder for distributed strain sensing using an optical backscatter reflectometer (OBR) interrogator to obtain spatial strain data. Surface strains measured using distributed optical fibre sensing (DOFS) and strain rosettes were compared against finite element analysis predictions to successfully validate the numerical model created using Ansys ACP Pre-Post simulation package. Directional deformations around the cut-outs of these tubes were also numerically characterised and compared with digital image correlation (DIC) results from the experimental tests.

High Speed, Localized Multi-Point Strain Measurements on a Containment Vessel at 1.7 MHz Using Swept-Wavelength Laser-Interrogated Fiber Bragg Gratings

Dynamic elastic strain in ~1.8 and 1.0 m diameter containment vessels containing a high explosive detonation was measured using an array of fiber Bragg gratings. The all-optical method, called real-time localized strain measurement, recorded the strain for 10 ms after detonation with additional measurements being sequentially made at a rate of 1.7 MHz. A swept wavelength laser source provided the repetition rate necessary for such high-speed measurements while also providing enough signal strength and bandwidth to simultaneously measure 8 or more unique points on the vessel’s surface. The data presented here arethen compared with additional diagnostics consisting of a fast spectral interferometer and an optical backscatter reflectometer to show a comparison between the local and global changes in the vessel strain, both dynamically and statically to further characterize the performance of the localized strain measurement. The results are also compared with electrical resistive strain gauges and finite element analysis simulations.

Distributed Fiber Optics Sensing Applied to Laminated Composites: Embedding Process, Strain Field Monitoring with OBR and Fracture Mechanisms

Optical fibers (OFs) are among the most promising technologies for distributed sensing in structure health monitoring of composites. Optical backscatter reflectometer (OBR) based on Rayleigh scattering enable to use the entire length of telcom OF as sensor, giving detailed information about the stain fields in the host structure. Embedded sensors can provide many advantages over surface bonded ones in terms of reliability and accuracy of the measurements. Here, OFs were embedded in between (0/0), (0/90) and (90/90) interfaces of a [02/902/02] glass fiber reinforced polymer (GFRP) and were further interrogated with OBR during tensile tests. Sensors were evaluated in terms of its sensing capabilities and fracture mechanism. OFs aligned in between (0/0) direction endured 5500 µe and deviated only 2.7% in the Young’s modulus calculation compared to mounted strain gauges. The other directions presented problems related to matrix rich area (0/90) and air bubbles imprisoning (90/90), reducing the sensing range. After reaching the cracking state, OFs fail progressively and are still capable of acting as sensor, as was showed in a simple visible laser scattering test. For the first time, embedding, interrogation with OBR and fracture mechanisms are approached in a single study, which provide valuable information for wide application of OFs in SHM.

Using OBR for pressure monitoring and BVID detection in type IV composite overwrapped pressure vessels

Type IV COPVs (Composite Overwrapped Pressure Vessels) are among the most suited structures for hydrogen storage. However, its complex modes of failure and requirement for periodic maintenance has led the industry to apply high safety factors on designs. This is one of the challenges inhibiting the widespread usage of the H2 in commercial vehicles. Structural health monitoring based on optical fibers is an emerging technology that can overcome these problems, as a neural network of sensors can be integrated to the structure during manufacturing and is readily accessible over the vessel lifetime. This gives information about the real structure condition, reducing overall maintenance costs. Here, core optical fibers were embedded in type IV COPVs during the manufacturing process and monitored with OBR (Optical Backscatter Reflectometer). Sensors were interrogated during an impact detection test and a pressurization test until burst failure. Fibers were capable of detecting the position and intensity of the damage in the first test and provided strain profiles over the entire length of the vessel for longitudinal and circumferential directions on the second. Optical microscopy of vessel sections showed matrix accumulation around the optical fiber as the main cause of sensor’s failure. During pressurization, steep peaks of strain in the dome regions from the early measurements indicated the burst failure site.

Refractive Index Sensor by Interrogation of Etched MgO Nanoparticle-Doped Optical Fiber Signature

Traditional fiber-optic refractive index (RI) sensors require a reflective or resonant device in fiber (such as a grating, an interferometer, a taper, or a plasmonic device) and a spectral domain detection. In this letter, we demonstrate and experimentally validate, the possibility to fabricate a minimalistic RI sensor, based on the wet-etching of an enhanced backscattering fiber made with a core doped with MgO-based-nanoparticles. Interrogation is performed in the length domain, by detecting and correlating the random losses of the fiber with an optical backscatter reflectometer. The sensitivity is 5.25 mm/RIU with $9.6~\mu \text{m}$ resolution, corresponding to 9.69 ps/RIU.

Distributed Fiber Optic pH Sensor for the Application in the Wellbore

pH measurement is important in many applications such as environmental science, civil engineering, biotechnology, earth science, and marine sciences. In the wellbore, the pH of the cement pore solution is around 13 at early stage, and the wellbore cement is relatively stable and less permeable above pH 10.5. However, pH drops in the CO2/H2S containing environment making the cement more porous and permeable, which leads to higher risk of wellbore integrity failure. The lower pH also makes the casing steel more susceptible to corrosion. Therefore, real time monitoring of pH in the wellbore is crucial for corrosion detection and cement failure prediction. Wellbores are often at high temperature high pressure (HTHP) conditions (> 149 °C, >1400 psi). Conventionally, pH measurement is made by a glass electrode, which is fragile, prone to drifting, and suffers from alkali interference. Furthermore, it requires a reference electrode, which becomes less stable and less reliable in HTHP conditions, hence there are only limited options available for conventional pH sensors under wellbore conditions. In the contrast, fiber optic pH sensors can offer desirable advantages such as the elimination of electrical connections and wiring and the lack of need for a separate reference electrode, making them inherently more compatible with harsh environment applications. Importantly, they can also be used for remote, distributed, and continuous pH sensing in harsh environment to enable a map of pH throughout a region of interest. Therefore, fiber optic sensors have become a promising solution for pH monitoring in the wellbore. To enable distributed pH sensing with the optical fiber platform, an optical fiber must be integrated with a pH sensitive layer along the fiber, such that the optical properties can be locally altered due to the pH change of the surrounding solution and the change is monitored by a detector in either a transmission or reflection/backscatter configuration. Therefore, the development of a stable, selective, and sensitive pH sensing layer is the critical enabling technology. Sol-gel SiO2 film is one of the most widely used substrates to immobilize pH indicators, such as organic dyes, on the optical fiber surface for pH sensing. SiO2 film has high thermal stability and excellent mechanical strength, which satisfies the HTHP wellbore conditions. The SiO2 thin film can be prepared over a large area at a comparatively low cost. Most investigations use the sol-gel SiO2 film as the substrate for immobilizing pH indicators, but very few studies have reported that the film itself can be used as a pH sensing layer. Using only SiO2 film makes the pH sensor simple to fabricate and free of leaching or photobleaching which are often encountered by the entrapped dye indicators. However, the mechanistic details of the response require elucidation to understand the potential for a selective and sensitive response as a function of pH in the presence of other chemical analytes and variations in temperature and pressure. Here we report recent results on fiber optic pH sensors utilizing porous silica based films and discuss them in the context of fundamental mechanistic considerations as well as their relevance to harsh environment sensing applications. The SiO2 film is coated onto the optical fibers through a sol-gel dip-coating method. The dip-coated fibers are calcined at temperatures of 200-600oC afterwards to obtain a porous SiO2 film of a few hundred nanometers thick. The pH sensing performance is investigated in terms of light transmission to confirm sensitivity and reversibility, and the distributed pH monitoring capability is demonstrated using an optical backscatter reflectometer. The optical fiber coated with a silica-based sol-gel and calcined at 200 °C shows good sensitivity and reversibility in the pH range from 4 to 13, and the sensor is found to be more sensitive in the alkaline region, which indicates that the developed fiber optic pH sensor has potential application in the wellbore pH range.