Bare Optical Fiber Research Articles

Linearly polarised three-colour lasing emission from an evanescent wave pumped and gain coupled fibre laser

A novel Whispering–Gallery–Mode (WGM) fibre laser, emitting linearly polarised three-colour light, is demonstrated by pumping and gain coupling with evanescent waves. The pump light is longitudinally coupled into a bare optical fibre immersed in a dye solution of lower refractive index. The dye molecules around the bare fibre are excited by the evanescent waves of pump light when they propagate along the fibre in a total internal reflection. When the pump beam within the fibre is a meridian beam, the WGM lasing emission from the fibre laser is a linearly polarised transverse electric wave, while it is a mixed wave of the linearly polarised transverse electric and magnetic waves if the pump beam is a skew beam within the fibre. Because the excited molecules are located within the evanescent field of WGM, a good spatial overlap between the dye gain and the evanescent field leads to a high pumping efficiency and a longer gain distance along the fibre. Once the bare fibre is inserted into three glass capillaries filled with Rhodamine 6G, 610 and 640 dye solutions, respectively, WGM laser oscillations at the wavelengths of 567–575, 605–614 and 656–666 nm occur simultaneously, and a linearly polarised three-colour lasing emission is achieved in a single optical fibre.

Thermal Effects from Modified Endodontic Laser Tips Used in the Apical Third of Root Canals with Erbium-Doped Yttrium Aluminium Garnet and Erbium, Chromium–Doped Yttrium Scandium Gallium Garnet Lasers

To evaluate the temperature changes occurring on the apical third of root surfaces when erbium-doped yttrium aluminium garnet (Er:YAG) and erbium, chromium-doped yttrium scandium gallium garnet (Er,Cr:YSGG) laser energy was delivered with a tube etched, laterally emitting conical tip and a conventional bare design optical fiber tip. Thermal effects of root canal laser treatments on periodontal ligament cells and alveolar bone are of concern in terms of safety. A total of 64 single-rooted extracted teeth were prepared 1 mm short of the working length using rotary nickel-titanium Pro-Taper files to an apical size corresponding to a F5 Pro-Taper instrument. A thermocouple located 2 mm from the apex was used to record temperature changes arising from delivery of laser energy through laterally emitting conical tips or plain tips, using an Er:YAG or Er,Cr:YSGG laser. For the Er:YAG and Er,Cr:YSGG systems, conical fibers showed greater lateral emissions (452 + 69% and 443 + 64%) and corresponding lower forward emissions (48 + 5% and 49 + 5%) than conventional plain-fiber tips. All four combinations of laser system and fiber design elicited temperature increases less than 2.5 degrees C during lasing. The use of water irrigation attenuated completely the thermal effects of individual lasing cycles. Laterally emitting conical fiber tips can be used safely under defined conditions for intracanal irradiation without harmful thermal effects on the periodontal apparatus.

Nitromethane ignition observed with embedded PDV optical fibers

For a long time, the nitromethane (NM) ignition has been observed with different means such as high-speed cameras, VISAR or optical pyrometry diagnostics. By 2000, David Goosmann (LLNL) studied solid high-explosive detonation and shock loaded metal plates by measuring velocity (Fabry-Pérot interferometry) in embedded optical fibers. For six years Photonic Doppler Velocimetry (PDV) has become a major tool to better understand the phenomena occurring in shock physics experiments. In 2006, we began to use in turn this technique and studied shock-to-detonation transition in NM. Different kinds of bare optical fibers were set in the liquid; they provided two types of velocity information; those coming from phenomena located in front of the fibers (interface velocity, shock waves, overdriven detonation wave) and those due to phenomena environing the fibers (shock or detonation waves). We achieved several shots; devices were composed of a high explosive plane wave generator ended by a metal barrier followed by a cylindrical vessel containing NM. We present results.

Study on Stress Transfer Property for Embedded FBG Strain Sensors in Concrete Monitoring

Fiber Bragg grating (FBG) sensors already have been the focus for structural health monitoring (SHM) due to their distinguishing advantages. However, as bare optical fiber is very fragile, bare FBG strain sensor without encapsulation can not properly be applied in practical infrastructures. Therefore encapsulation techniques for making encapsulated FBG strain sensor show very important in pushing forward the application of FBG strain sensors in SHM. In this paper, a simplified approximate method to analyze the stress transferring rules for embedded FBG strain sensors in concrete monitoring is put forward according to mechanics of composite materials. Shear lag theory is applied to analyze the stress transferring rule of embedded FBG strain sensor in measured host material at the first time. The measured host objects (concrete) and the encapsulated FBG strain sensor are regarded as a composite, and then the stress transfer formula and stress transfer coefficient of encapsulated FBG strain sensor are obtained.

Single fiber-in-capillary annular column for gas chromatographic separation

A method for preparation of a stationary phase-adjustable column with in-column stationary phase-coated fused-silica fiber annular column was successfully developed. The surface of a 0.12 mm o.d. bare optical fiber was first coated with a stationary phase and then inserted into a fused-silica capillary (non-coated or coated) as an annular column for gas chromatographic study. The optical fiber and capillary were coated with polydimethylsiloxane (SE-30) and polyethylene glycol 20M (PEG-20M) as nonpolar and polar stationary phases, respectively. Among the investigated annular and open tubular columns, the PEG-20M-coated fiber-in-PEG-20M-coated capillary annular column showed the highest column efficiency with a minimum plate height of 0.35 mm and an optimum gas velocity of 25 cm/s. When a SE-30/PEG-20M-coated fiber-in-uncoated capillary annular column was applied to separate a 9-component complex mixture, the total analysis time was 5.3 min and the column length was 12 m. By contrast, when a SE-30-coated fiber-in-PEG-20M-coated capillary annular column was used to separate the same 9-component mixture, the analysis time was reduced to 3.5 min and the column length was shortened by half to 6 m. Our results show that the stationary phase-coated fiber-in-stationary phase-coated capillary annular column is a better choice for gas chromatographic separation as it is more efficient and flexible. In addition, the proposed annular column design provides flexibility in using two or even more types of stationary phases to achieve optimal analytical separation.

Fabrication and Photoelectrical Behavior of Macroscopic-Long Silver Nanowire Ribbon/Bulk Metal Contact

Macroscopic-long silver nanowire ribbon including thousands of parallel aligned silver nanowires was prepared on a bare optical fiber surface in the solid phase using a solid state ionics method. Their structural characters were described at different spatial scales. The photoelectrical behavior between two contact interfaces of the ribbon and bulk metal (Ag or Ni) electrodes was explored using a local illumination by a 532 nm laser beam. Photoinduced currents were found dramatically depend on the laser spot position relative to the contact interface. In addition, the magnitudes of photoinduced current were different from been contact with Ag electrodes to Ni electrodes.

Distributed Strain Measurement in Steel Bridge with Fiber Optic Sensors: Validation through Diagnostic Load Test

Fiber optic sensing technologies are emerging as valid alternatives for the health monitoring of civil structures. Distributed sensors based on Brillouin scattering add the unique capability of measuring strain and temperature profiles along optical fibers. Measurement is performed by establishing the correlation between fiber strain and temperature, and the frequency shift of the Brillouin backscattered light induced by a monochromatic light pulse. The technology holds potential for use on large structures and integrated transportation infrastructure. Its effectiveness has been assessed through scaled laboratory experiments, whereas field validation is limited to very few demonstration projects conducted to date. This paper presents a pilot application of Brillouin optical time domain reflectometry to measure strain profiles along the steel girders of a continuous slab-on-girder bridge subjected to diagnostic load testing. One of the exterior continuous girders required heat-straightening after falling during construction due to wind. The significance of applying a distributed measurement technique lies in the potential to assess the global girder response, which would be impractical and uneconomical using discrete measurement techniques. A 1.16km long sensing circuit was installed onto the web of four girders. The circuit comprises bare optical fiber sensors, and a novel adhesively bonded fiberglass tape with embedded sensing fibers for strain measurement and thermal compensation. The strain profiles were first converted into deflection profiles and validated against discrete deflection measurements performed with a high-precision total station system. Structural assessment based on comparison of the strain profiles with the results of three-dimensional finite-element analysis of the bridge superstructure, and with specification mandated criteria, indicated that the response of the girder under investigation was within the design limits, and did not pose serviceability concerns. Factors that may affect measurement accuracy are finally discussed on the basis of the experimental and numerical results.

Wavefront sensing using speckles with fringe compensation

Wavefront sensing with numerical phase-error correction system is carried out using a random phase plate and phase retrieval using multiple intensity measurements of axially-displaced speckle patterns and the wave propagation equation. Various wavefronts with smooth curvatures incident on the developed phase plate (DPP) are examined: planar, spherical, cylindrical, and a wavefront passing through the side of a bare optical fiber. Spurious fringe pattern in the wavefront reconstructions due to a small tilt (Delta theta=0.212 degrees) in the plane illumination wave is detected and numerically corrected for. Fringe pattern of the illumination wave obtained for the setup without the phase object being investigated is used as reference fringe pattern. Fringe compensation yields wavefronts with the correct shape and numerical value based on the specifications of the setup. The numerical phase-error correction system described in this study can be extended to other types of phase errors such as those due to aberrations if optical elements are present in the setup or due to perturbations in the environment.

Fiber-reinforced polymer-packaged optical fiber sensors based on Brillouin optical time-domain analysis

Brillouin optical time-domain reflectometry (BOTDR) and Brillouin optical time-domain analysis (BOTDA) are considered to be promising and practical sensing techniques for large structures. However, there is still a major obstacle to applying BOTDR or BOTDA on large-scales; the high cost and unreliability associated with sensor installation and failure. We report a novel, low-cost, and highly reliable BOTD sensor using a rebar consisting of a bare optical fiber (OF) packaged in fiber-reinforced polymer (FRP) and named BOTD-FRP-OF. We investigate the surface bonding and its mechanical strength scanning-electron-microscope and intensity experiments. Considering the strain difference between OF and host matrix, which may result in measurement error, the strain transfer from host to OF has been studied theoretically. Furthermore, the sensing properties of glass FRP-OFs for strain and temperature at different gauge lengths were tested under different spatial and readout resolutions using commercial BOTDA. Finally, an absolute dual-BOTD-FRP-OF temperature compensation method is proposed and has been tested. This novel FRP-OF rebar shows both high strength and good sensing properties, which can be used in long-term structural health monitoring for civil infrastructure.

Interface transferring mechanism and error modification of embedded FBG strain sensors

As the strain sensing element of a structural health monitoring, the study and the application of the fibre-optic bragg grating (FBG) have been widely accepted. The accuracy of the FBG sensor is highly dependent on the physical and the mechanical properties of the strain interface transferring characteristics among the layers of bare optical fibre, protective coating, adhesive layer and host material. In this paper, firstly, the general expression of the multilayer interface strain transferring mechanism is derived. Secondly, based on the defined average strain, the error-modified equation of the FBG sensor is obtained. Finally, in the light of the embedded tube-packaged FBG and the fibre reinforced polymer-optical fibre bragg grating (FRP-OFBG) strain sensors, developed in the Harbin Institute of Technology (HIT), the corresponding strain transferring laws have been studied, and the corresponding error modification coefficients have also been given, which are validated by experiments. The research results provide theories for the development and application of the embedded FBG sensors.

Evaluation of Rugged 'Smart Patch' Fibre-Optic Strain Sensors

Fibre-optic sensors have advantages over existing electrical sensors in many strain and stress monitoring applications. However, bare optical fibres are fragile and packaging techniques must be developed before these sensors can be used more widely. One such method is the Smart Patch, in which the fibre Bragg grating is encapsulated between plies of glass-reinforced epoxy where rugged cables are anchored. This forms a flat flexible patch in which the fibre is protected from mechanical and environmental damage. However, it is important that the mechanical strength of the patch is not achieved at the expense of good strain transfer characteristics. To confirm this, fibre Bragg gratings with acrylate and polyimide coatings were embedded in a glass-epoxy patch that was bonded to an aluminium tensile specimen. An electrical strain gauge was also bonded to the specimen to provide a strain reference. Tests were carried out at different loading rates and at temperatures from -30°C to +80°C. There was good agreement between the fibre-optic sensors and the electrical strain gauge demonstrating that the patch performed in a practically identical manner to the conventional gauges. A second experiment on a representative part of ship structure demonstrated the versatility of the patch.

Focusing characteristics of optical fiber axicon microlens for near-field spectroscopy: Dependence of tip apex angle

We fabricated axicon microlenses on a single-mode bare optical fiber by using selective chemical etching technique. By varying the concentration and mixture ratios of etching solutions, we could make axicon microlenses with different apex angles (118°, 107° and 90°). We measured the illumination and collection efficiency of these microlenses by both objective-lens imaging and near-field photoluminescence (PL) spectroscopy of semiconductor quantum dots with respect to the distance between the sample and the axicon lens probe. We found that axicons with 118° and 107° [90°] apex angle tend to exhibit the laser focusing spot sizes of about 406 and 478nm [382nm] and maintain the high [slightly low] collection PL intensity with the distance between the sample and the axicon probe.

Fractal Binding and Dissociation Kinetics of Heart-Related Compounds on Biosensor Surfaces

A fractal analysis is presented for the binding and dissociation of different heart-related compounds in solution to receptors immobilized on biosensor surfaces. The data analyzed include LCAT (lecithin cholesterol acyl transferase) concentrations in solution to egg white apoA-I rHDL immobilized on a biosensor chip surface (), native, mildly oxidized, and strongly oxidized LDL in solution to a heparin-modified Au-surface of a surface plasmon resonance (SPR) biosensor (), and TRITC-labeled HDL in solution to a bare optical fiber surface (). Single-and dual-fractal models were used to fit the data. Values of the binding and the dissociation rate coefficient(s), affinity values, and the fractal dimensions were obtained from the regression analysis provided by Corel Quattro Pro 8.0 (). The binding rate coefficients are quite sensitive to the degree of heterogeneity on the sensor chip surface. Predictive equations are developed for the binding rate coefficient as a function of the degree of heterogeneity present on the sensor chip surface and on the LCAT concentration in solution and for the affinity as a function of the ratio of fractal dimensions present in the binding and the dissociation phases. The analysis presented provided physical insights into these analyte-receptor reactions occurring on different biosensor surfaces.

Alignment and fixing of fiber optics based on electrothermal MEMS actuators

An aligning and fixing method for fiber optics is described in this letter. The alignment is performed using a chevron-type thermal microactuator while the fixing is achieved by ultraviolet curing of an epoxy adhesive once the optical fibers are aligned. The microactuator has been extended so that the heaters and the optical adhesive are separated to avoid curing of the adhesive by temperature. A prototype has been fabricated using 80-/spl mu/m-thick metallized silicon-on-insulator and mechanical and optical results from the alignment of two bare single-mode optical fibers before, during, and after the fixing are presented.

Effect of interstitial laser hyperthermia in a murine model of colorectal liver metastases: scanning electron microscopic study.

Interstitial laser hyperthermia (ILH) is an in situ ablative technique used to treat colorectal liver metastases. The relatively high recurrence of tumor after treatment by ILH may be related to incomplete destruction. Little is known about the effectiveness of ILH for destroying tumor microvasculature. The aim of this study was to define the changes to the microvascular architecture of tumors after treatment with ILH, specifically focusing on the completeness of tumor vasculature destruction. An intrasplenic induction model of liver metastases in 4- to 6-week-old male inbred CBA mice was used. Laser hyperthermia was applied to liver and tumor tissue using a bare optical quartz fiber from a Medilas Fibertom 4100 Nd:YAG surgical laser generator. The animals underwent microvascular corrosion casting of the livers immediately after application of ILH. Microvascular casts were then prepared and studied by scanning electron microscopy. ILH produced complete, uniform destruction of the tumor microvasculature with compete hemostasis. Blood flow in vessels larger than 100 microm diameter had a relatively protective effect, although ILH was able to overcome this barrier effectively by increasing the energy applied. ILH produces complete destruction of tumor microvasculature with hemostasis. The protective effect of blood flow in larger vessels can be overcome by the appropriate use of higher energy levels.

Dual-light source excitation for mode-filtered light detection

The sensitivity of a multi-channel mode-filtered light detection system has been enhanced by using a dual-light source irradiation technique. The detection system was constructed from an annular column consisting of a bare optical fibre inserted into a capillary. Sample was introduced through the gap between the fibre and the capillary. A multi-channel charge-coupled device was set on the side of the capillary at which four detection windows could be simultaneously monitored. The changes in the intensities of the mode-filtered light on exposure to various concentrations of ethanol samples from each detection window were monitored. The theoretical studies on the sensitivity of detection of the detection system using dual-light source irradiation have been described. The sensitivity of detection was enhanced when a dual-light source instead of a single-light source was employed. The working concentration range for ethanol was 0–80% (v/v) ethanol. The limit of detection was determined to be 1% (v/v) ethanol. The proposed method has been successfully applied to the determination of ethanol contents of some wine samples. The results were satisfactory compared with values obtained from a standard reference method.

Interstitial laser hyperthermia for colorectal liver metastases: the effect of thermal sensitization and the use of a cylindrical diffuser tip on tumor necrosis.

Our aim in this study was to investigate the characteristics of a diffuser-tipped optical fiber in producing tumor necrosis, compared to a standard bare-tipped fiber. The potential synergistic effect between thermal sensitization by metronidazole and interstitial laser hyperthermia (ILH)-induced tumor necrosis is also evaluated. ILH is a minimally invasive technique for the treatment of colorectal liver metastases. One of the major limitations is the size of tissue necrosis achieved by a single optical fiber. Use of cylindrical diffuser-tipped fibers and thermal sensitization of tumor cells by metronidazole may increase the size of tumor necrosis achieved by a single optical fiber. A model of colorectal cancer liver metastases in male inbred CBA mice was used. Laser hyperthermia was applied to tumor tissue using either a bare optical quartz fiber or a cylindrical diffuser-tipped fiber from a Medilas fibertom 4100 Nd:YAG surgical laser generator. Six hundred joules of energy was applied at two power settings, 5 and 10 watts, using bare- and diffuser-tipped fibers, respectively. The extent of necrosis was assessed by histological techniques. A similar study with three experimental groups was treated with 300 J of applied energy. Extent of immediate tumor necrosis was compared to that seen 24 h after ILH treatment. The third group, which had been treated with intraperitoneal metronidazole prior to ILH, was also assessed for tumor necrosis after 24 h and results compared with both the previous groups. ILH delivered using a cylindrical diffuser-tipped fiber resulted in a significantly larger diameter of tumor necrosis when compared to a bare-tipped fiber, for a given amount of applied energy. The differences were more significant at higher power settings. Six hundred joules of energy applied by ILH using a bare-tipped fiber at 5 and 10 watts produced 6.7 +/- 1.1 mm and 5.9 +/- 0.6 mm diameter of tumor necrosis, respectively. At equivalent settings, the diffuser-tipped fiber produced 7.7 +/- 1.0 mm and 8.1 +/- 0.6 mm diameter of tumor necrosis (p = 0.02 and p < 0.001). Using a diffuser-tipped fiber and an applied energy of 300 J delivered at 5 watts power, mean diameter of tumor necrosis immediately after treatment was 6.7 +/- 1.1 mm and after 24 h 7.9 +/- 1.3 mm (p = 0.006). Mean diameter of tumor necrosis 24 h after ILH in animals treated with metronidazole was 8.3 +/- 1.9 mm (p = 0.11). Diffuser-tipped optical fiber appears to significantly increase the diameter of ILH-induced tumor necrosis compared to the bare fiber. In contrast to the bare fiber, it enables the application of laser energy using higher power settings without compromising the diameter of tumor necrosis achieved. In animals treated with metronidazole, a trend towards increased tumor destruction at the tumor-host interface was seen on histolopathology. In addition, a trend towards increased diameter of tumor necrosis was also seen; however, statistical significance was not achieved.

Effect of Blood Flow Occlusion on Laser Hyperthermia for Liver Metastases

Background. Interstitial laser hyperthermia (ILH) is an in situ ablative technique for the treatment of colorectal liver metastases. A significant factor limiting tumor destruction is hepatic blood flow. Modulation of hepatic blood flow may increase the size of tumor necrosis achieved. Our aim was to investigate the effect of blood flow occlusion on ILH-induced necrosis in both tumor and normal liver tissue.Materials and methods. A model of colorectal liver metastases in male inbred CBA mice was used. ILH was applied to normal liver and tumor tissue using a bare optical quartz fiber from an SYL500 Nd:YAG surgical laser generator, with and without hepatic blood flow occlusion, and the extent of necrosis was studied. Tumor blood flow was assessed by laser Doppler flowmetry and scanning electron microscopy.Results. Hepatic blood flow occlusion resulted in a significant reduction in blood flow in normal liver tissue (37.9% ± 5.8, P < 0.001) and in the periphery of the tumor (17.5% ± 7.8, P < 0.001). It did not affect the blood flow in the center of the tumor (13.4% ± 4.3, P = 0.07). ILH of normal liver tissue, at low power (2 W), with hepatic blood flow occlusion, resulted in a significant increase in the diameter of necrosis. This effect was not seen when higher power (5 W) was used in normal liver. No significant effect was noted within tumor tissue at either power setting.Conclusion. The overall effect of hepatic blood flow occlusion in ILH-induced tissue necrosis appears to be negligible in tumor tissue. Its main applicability appears to be at the tumor–host interface, where a decrease in blood flow may lead to higher temperatures and therefore to a greater degree of tumor cell destruction.

Effect of interstitial laser hyperthermia in a murine model of colorectal liver metastases

Interstitial laser hyperthermia (ILH) is an in situ ablative technique used for the treatment of colorectal liver metastases. At present, few data exist concerning the optimum power settings required to maximize tissue necrosis.The aim of this study was to establish the dose-response relationship between the laser power setting and the extent of tissue necrosis produced in liver and tumor tissue, as well as the pattern of necrosis in a murine model of liver metastases. An intrasplenic induction model of liver metastases in 4- to 6-week-old male inbred CBA mice was used. Laser hyperthermia was applied to liver and tumor tissue using a bare optical quartz fiber from a Laserex SLY500 Nd:YAG surgical laser generator. Two-watt and 5-watt power settings were used at specific time intervals. The livers were then excised, fixed in formalin, and the extent and degree of necrosis were measured. Results were expressed as mean ± standard deviation and were normally distributed. Analysis of variance was performed, and the least significant difference was used for post hoc tests. A P value of less than 0.05 was considered significant. Interstitial laser hyperthermia at 5 watts of power produced larger diameters of necrosis than did 2 watts for specific exposure times in normal liver tissue. However, when the total energy applied was compared, there was no significant difference in the diameters of tissue necrosis produced by the two power settings. The diameter of tissue necrosis in the normal liver increased from 2 mm at 10 joules to 8 mm at 600 joules of energy. Within tumor tissue, ILH at 2 and 5 watts produced similar diameters of necrosis for specific exposure times. When amounts of total energy applied were compared, ILH at the lower power setting (2 watts) produced a significantly larger diameter of necrosis than the higher power setting (5 watts). The diameter of necrosis achieved in tumor tissue was significantly larger than that in normal liver tissue at both power settings, for an equivalent amount of applied energy. The difference was more pronounced when ILH was performed at the lower power setting. The maximum diameter of necrosis achieved was 6.8 ± 0.7 mm in normal liver tissue and 7.7 ± 0.8 mm in tumor tissue. Charring of the fiber tip was delayed when the lower power setting was used, occurring after 20 seconds of exposure, compared to 5 seconds at the higher power setting. Similarly, cavitation occurred initially at 50 seconds at 5 watts of power and was delayed until 90 seconds of exposure at 2 watts of power. Histopathologic findings revealed an elliptical area of homogeneous necrosis, with a central acellular coagulum surrounded by intact but non-viable tissue. ILH is capable of producing highly reproducible, uniform, and complete tissue necrosis. The diameter of necrosis is related to the total energy applied. At low-power settings at any given amount of applied energy, a significantly larger diameter of tissue necrosis was achieved in tumor tissue compared to normal liver tissue.

Chemical sensing with microbent optical fiber

We propose and demonstrate the possibility of using a permanently microbent bare optical fiber for detecting chemical species. Two detection schemes, viz., a bright-field detection scheme (for the core modes), and a dark-field detection scheme (for the cladding modes) have been employed to produce a fiber-optic sensor. The sensor described here is sensitive enough to detect concentrations as low as nanomoles per liter of a chemical species, with a dynamic range of more than 6 orders of magnitude.