This PDF file contains the front matter associated with SPIE Proceedings Volume 8693, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

This paper presents an algorithm for determining three-dimensional displacement of thin rod- or tether-like structures from a set of scalar strain measurements, for arbitrarily large deformations. The approach employs a material-adapted reference frame and a local linearization approach that results in an exact local basis function set for the displacement and for the material frame evolution. The basis set is shown to be robust to potential singularities from vanishing bending and twisting angle derivatives and from vanishing measured strain. Validation of the approach is performed through comparison with both finite element simulations and an experiment, with average root mean square reconstruction error of 0.01%-1% of the total length depending upon the number of sensors used. Analysis of error due to extraneous noise sources and boundary condition uncertainty shows how error propagates under those effects. (Approved for release: LA-UR-13-21066)

When fiber Bragg gratings (FBG) are tightly packed in a mesh and their peaks get close at a distance on the order of individual FBG spectrum widths, they start overlapping and there is a distance below which both peaks won’t be detectable anymore using standard peak detection method. Ability to determine locations of individual peaks even after they overlap allows more gratings in a mesh and an increase in shape sensing resolution. We use a linear interpolation method to estimate peak locations when peaks overlap and become undetectable with standard peak finding technique. We test this algorithm on experimentally obtained data and compare peak locations obtained by the algorithm to exact peak locations. We analyze the error to show that algorithm performs well when velocity of peaks stays uniform during peak crossing. However, the error rapidly increases if the velocity changes during crossing and the maximum error can occur in a situation when peaks change direction during peak crossing.

We present a filter-based spectrometer consisting of a dielectric thin film filter with a lateral-spectral transmission gradient (linear variable filter) and a position detector which allows high resolution in wavelength shift registration. The resolving capacity of two interrogation unit variants (with segmented and homogeneous position detector, respectively) is examined using a monochromatic tunable light source. On the basis of the obtained results, we discuss several advantages and drawbacks of this interrogation approach.

Driven by a demand for integrated optical sensors in structural health and environmental monitoring we present the application of plasmonic gradient structures as sensor substrate. Therefore, nanoparticle arrays of gold are fabricated by interference lithography, which exhibit localized surface plasmons (LSPs). The plasmonic properties of such nanoparticles can be tuned by altering their size. In our approach, an additional photochemical growth by exposure to HAuCl4 and light is used to manufacture gradients of nanoparticle sizes within the array. These gradients in turn induce different spectral responses depending on the illuminated region of the array gradient. To enable sensing applications, such plasmonic gradient structures are placed as a filter in front of a photodetector to allow detection of transmitted optical signals from different locations of the array. Different applications can be envisioned in this configuration: On the one hand, sensing of wavelength shifts of the illuminating light source can be enabled by comparing the photocurrents generated in adjacent sensor elements. Additionally the application of refractive index measurements is demonstrated with the same detector configuration. The change in extinction of the illuminating light at different wavelengths can be used to obtain an intensity shift at the detector elements. This shift correlates to the change of the spectral resonance conditions in the array gradient upon change of refractive index.

In this paper, we investigate the fusion of imaging data from pulsed phase thermography (PPT) with local temperature data obtained from embedded fiber Bragg grating (FBG) sensors for non-destructive evaluation of composite structures. We use the square pulse heating applied for the PPT imaging as the input thermal wave for both the imaging and sensing processes. In addition, the role of the local microstructure surrounding the FBG on the measured wavelength shift as a function of temperature is derived analytically. Fusing the FBG wavelength response with the PPT data at the corresponding pixel and depth is shown to provide a unique characterization of the local material condition.

In this research, fiber Bragg grating (FBG) temperature sensors are embedded in composites in order to detect highly localized temperature gradients in the composite structures. The primary goal is to perform structural health monitoring on a composite structure. A secondary goal is to use the sensors as a diagnostic tool to determine the optimal composite materials, architectures, or structures that are the least susceptible to thermal damage. Initial results will be discussed for two composite materials using a single sensor to measure temperature variations. The tests include measurements of the temporal and spatial thermal response of the composite resulting either from an applied heat source or to high energy radiation incident on the surface. Additional tests demonstrate the response using a 3x2 array of sensors to simultaneously measure the temperature at three varying depths in the composite, using three FBGs aligned with the heat source, and three FBGs located a short lateral distance (3cm) away from the heat source. In addition, since FBGs respond to strain as well as to temperature, any strain in the composite is coupled into the embedded fiber and is also detected by the FBG sensors. Initial measurements demonstrate the simultaneous response of FBG sensors to both temperature and strain. The various components of strain that are observed in the composite will be discussed, and possible methods to isolate these components and mitigate their response will be considered.

A regenerated grating (RG) sensor with titanium (Ti) - silver (Ag) - nickel (Ni) multilayer coatings is fabricated by magnetron sputtering and electroplating processes. Optical and thermal testing are performed to evaluate the characteristics of the Ti-Ag-Ni coated RG sensor. The profiles of the reflection spectra of the RG are slightly affected by the metallization process. The shift in the Bragg wavelength of the Ti-Ag-Ni coated RG sensor can be described as a piecewise linear function of temperature, with slope discontinuity at 250 °C. The Ti-Ag-Ni coated RG sensor exhibits more than twice higher sensitivity than that of the bare RG, with satisfactory repeatability and stability at temperatures of up to 600 °C. The surface and cross-section of the sensor are observed with scanning electron microscopy (SEM). The SEM images clear show no discontinuity at the interface between the optical fiber and titanium layer, as well as a good quality of nickel coating without visible dendritic growth. These results indicate that the Ti-Ag-Ni coated RG sensors can be successfully fabricated by combining magnetron sputtering with electroplating, and provide the great potential for high-temperature sensing because of their good sensitivity, repeatability and stability.

In this paper, intelligent statistical mode sensors are proposed and analyzed. Several statistical features are used in design of intelligent sensor systems. Force measurement experiments are conducted and experimental data is analyzed using newly proposed statistical features. After that, Artificial Neural Networks (ANNs) with sensor data fusion, which is an intelligent sensor architecture, was proposed to estimate the force values. Multilayer perceptron (MLP) with different algorithms are used in the ANN model, and all of them can predict the force values with acceptable error levels. Using sensor fusion with ANNs, statistical mode sensors can be calibrated and fault tolerance of the sensor can be decreased, hence more reliable intelligent sensors can be designed.

Fiber-optic strain sensors are increasingly used in very different technical fields. Sensors are provided with specifications defined by the manufacturer or ascertained by the interested user. In some cases, sensor specification is not sufficiently validated and must therefore additionally be validated in a laboratory, primarily to ensure reliable measurement information over the intended period of operation. Even if the performance of delivered sensor is well specified, the sensor's strain characteristics and the performance of an applied sensor can significantly differ from the virgin sensor’s performance. In this case, applied sensors do not provide full reliability and lead sometimes to uncertain measurement results. This contribution will therefore focus on the role of validation in avoiding a decrease or even deterioration of the sensor function of applied sensors. Experimental validation - not only modelling - is very important, however, before experimental investigations are planned knowledge about key issues and problems that influence the measurement results must be available. Few aspects to be considered and investigated will be discussed. Selected experimental facilities to reveal weaknesses in the sensor function will be described; an outlook to open questions is given.

SHM of composite structures is an emerging technology due to several benefits in terms of higher safety and security of human life, better material understanding, save money. Fiber-optic sensing technologies (FOS) are already proved to be reliable in the family of non-destructive testing (NDT) methods. The most frequently used are fiber-optic Bragg grating (FBG) sensors. However, several drawbacks of FBG cause a permanent improvement in the field of FOS. Within this paper the authors will confront the fiber-optic low-coherence interferometric (LCI) against FBG sensor taking into account experimental results simultaneously obtained by investigation of polyetheretherketon (PEEK) probe on stress by applying the three-point loading test.

A continuously chirped long period grating (CCLPG) was used to perform directional flow sensing of a UV-cured epoxy resin and to subsequently monitor the resin cure via the measurement of the change in refractive index. The asymmetric nature of the CCLPG led to a change in the shape of the attenuation bands of the transmission spectrum dependent on the direction of flow of the resin along the fibre. The transmission spectrum of a CCLPG was monitored during the resin cure and exhibited a wavelength shift of 1.1 ± 0.1 nm. The change in the refractive index of the resin during cure was determined in the same experiment using a fibre optic Fresnel based refractometer, and correlated well (correlation coefficient 97 %) with the response of the CCLPG.

The infusion, flow and cure of RTM6 resin in a carbon fibre reinforced composite preform have been monitored using a variety of multiplexed fibre optic sensors. Optical fibre Fresnel sensors and tilted fibre Bragg grating (TFBG) sensors were configured to monitor resin infusion/flow in-plane of the component. The results obtained from the different sensors were in good agreement with visual observations. The degree of cure was monitored by Fresnel sensors via a measurement of the refractive index of the resin which was converted to degree of cure using a calibration determined from Differential Scanning Calorimetry. Fibre Bragg grating sensors fabricated in highly linearly birefringent fibre were used to monitor the development of transverse strain during the cure process, revealing through-thickness material shrinkage of about 712 με and residual strain of 223 με. An alternative approach to infusion monitoring, based on an array of multiplexed tapered optical fibre sensors interrogated using optical frequency domain reflectometry, was also investigated in a separate carbon fibre preform that was infused with RTM6 resin.

The need for a protecting guard for the popular Ceramic Matrix Composites (CMCs) is getting a lot of attention from engine manufacturers and aerospace companies. This is because the CMC has a weight advantage over standard metallic materials and more performance benefits. They are also commonly porous material and this feature is somewhat beneficial since it allows some desirable infiltration. They further undergo degradation that typically includes coating interface oxidation as opposed to moisture induced matrix which is generally seen at a higher temperature. Variety of factors such as residual stresses, coating process related flaws, and casting conditions may influence the degradation of mechanical properties of CMC. The cause of such defects which cause cracking and other damage is that not much energy is absorbed during fracture of these materials. Therefore, an understanding of the issues that control crack deflection and propagation along interfaces is needed to maximize the energy dissipation capabilities of layered ceramics. These durability considerations are being addressed by introducing highly specialized form of environmental barrier coating (EBC) that is being developed and explored in particular for high temperature applications greater than 1100 °C ^1-3. The EBCs are typically a multilayer of coatings and are in the order of hundreds of microns thick. Thus, evaluating components and subcomponents made out of CMCs under gas turbine engine conditions are suggested to demonstrate that these materials will perform as required especially when subjected to extreme temperatures and harsh operating environment. The need exists to use advanced computational methods to assess risk associated with exposure to high temperature of EBC coated CMC specimens. In the work presented here, multiscale progressive failure analysis (PFA) approach was used to evaluate the damage growth in the coating and CMC after exposure time to cyclic and elevated temperatures. In each cycle, the specimen was heated to 1300 °C then maintained at that temperature for a period of time before cooling it down to room temperature. The PFA evaluation was carried out with the GENOA⁴ software using integrated capability inclusive of: finite element structural analysis, micro-mechanics, damage progression and tracking, fracture mechanics, and life prediction. In this paper, reverse engineered constituent properties obtained from CMC lamina properties were used as input to PFA to evaluate the degradation of specimen strength during thermal cycling. The analysis results indicated that the damage initiated in the top coat of the EBC then propagated down to the bond before reaching the CMC. Life assessment of the CMC was carried out twice, once using micromechanics properties as input and another time using macromechanics properties. It was determined that the use macromechanics properties yielded a more conservative life prediction for the CMC specimen as compared to that obtained from the use of micromechanics with fiber and matrix properties as input. Residual stresses evaluated during cooldown supported the onset of damage in the top coat. All stages of damage evolution were captured with PFA including damage initiation and damage propagation. Details on the life prediction of EBC and CMC materials are discussed next.

In this study we evaluate the measurements of a fiber Bragg grating (FBG) sensor embedded at the adhesive layer of a single composite lap joint subjected to harmonic excitation after fatigue loading. After a fully-reversed cyclic fatigue loading is applied to the composite lap joint, the full spectral response of the sensor is interrogated in reflection at 100 kHz during two states: with and without an added harmonic excitation. The dynamic response of the FBG sensor indicates strong nonlinearities as damage progresses. The short-time Fourier transform (STFT) is computed for the extracted peak wavelength information to reveal time-dependent frequencies and amplitudes of the dynamic FBG sensor response. Pulse-phase thermography indicates a progression in defect size at the adhesive layer that strongly suggests non-uniform loading of the FBG sensor.

This paper reports recent developments on high-temperature, multi-element integrated ultrasonic transducers (IUTs). The multi-element IUTs are fabricated from a sol-gel route, where piezoelectric films are deposited, poled and machined into an array of 16 elements. Electrical wiring and insulation are also integrated into a practical, simple high-temperature assembly. These multi-element IUTs show a high potential for structural health monitoring at high temperatures (in the 200-500°C range): they can withstand thermal cycling and shocks, they can be integrated to complex geometries, and they have broadband and suitable operating frequency characteristics with a minimal footprint (no backing needed). The specifics of multi-element transducers, including the phased array approach, for structural health monitoring are discussed.

Acoustic emission (AE) technique is commonly applied in different materials in order to evaluate their internal fracturing condition in real time. Apart from the number of acquired signals, which are correlated to the number of active cracking sources, qualitative features of the acoustic waveforms shed light in the dominant fracturing mode. This is due to the fact that the emitted waves depend on the relative motion of the crack sides at each incident. The fracture process of most engineering materials includes shift between modes and therefore, non-invasive and real time characterization of the dominant mode supplies information on the current condition as well as poses an early warning before final failure. Although a lot of work has been done on acoustic emission characterization of fatigue damage, the work on welded components is scarse. In the present study aluminum plates are cross-welded and loaded until fracture in tension-tension fatigue experiments at different load levels. Their full acoustic activity is recorded by four sensors along with all mechanical parameters. It is shown that study of the acoustic emission rate relatively to the applied load, and qualitative waveform parameters like the frequency content and duration can be used to study the evolution of the crack under the different modes.

Current work deals with the nondestructive evaluation (NDE) of the fracture behavior of ceramic matrix composite (CMCs) materials using combined infrared (IR) thermographic and acoustic emission (AE) characterization. IR thermography as a non-destructive, real-time and non-contact technique, allows the detection of heat waves generated by the thermo-mechanical coupling and the intrinsic energy dissipated during mechanical cyclic loading of the sample. Two different thermographic methodologies, based on the measurement of the surface temperature and on the intrinsically dissipated energy respectively, were applied in order to monitor the crack initiation and propagation and to rapidly assess the fatigue limit of cross-ply SiC/BMAS composites. Simultaneously, AE monitoring was employed to record a wide spectrum of cracking events ranging from matrix cracking to fiber fracture and pull-out. AE event rate, as well as qualitative indices like the rise time and peak frequency reveal crucial information allowing the characterization of the severity of fracture in relation to the applied load. Additionally, rapid assessment of the fatigue limit of CMCs composites was also attempted by AE. Testing a specimen at different load levels for predetermined blocks of cycles shows that the AE acquisition rate remains low for loads below the fatigue limit, while it increases abruptly for higher levels. The thermographic assesment of fatigue limit is in total agreement with the AE results enabling the reliable evaluation of the fatigue limit of the material by testing just one specimen. The application of combined NDE techniques proved very valuable for benchmarking purposes while the sensitivities of the methods act complementarily to each other providing a very detailed assessment of the damage status of the material in real time.

In the present paper, the ultrasonic strain sensing performance of large-area piezoceramic coating with Inter Digital Transducer (IDT) electrodes is studied. The piezoceramic coating is prepared using slurry coating technique and the piezoelectric phase is achieved by poling under DC field. To study the sensing performance of the piezoceramic coating with IDT electrodes for strain induced by the guided waves, the piezoceramic coating is fabricated on the surface of a beam specimen at one end and the ultrasonic guided waves are launched with a piezoelectric wafer bonded on another end. Often a wider frequency band of operation is needed for the effective implementation of the sensors in the Structural Health Monitoring (SHM) of various structures, for different types of damages. A wider frequency band of operation is achieved in the present study by considering the variation in the number of IDT electrodes in the contribution of voltage for the induced dynamic strain. In the present work, the fabricated piezoceramic coatings with IDT electrodes have been characterized for dynamic strain sensing applications using guided wave technique at various different frequencies. Strain levels of the launched guided wave are varied by varying the magnitude of the input voltage sent to the actuator. Sensitivity variation with the variation in the strain levels of guided wave is studied for the combination of different number of IDT electrodes. Piezoelectric coefficient ₁₁ e is determined at different frequencies and at different strain levels using the guided wave technique.

The development of techniques for the in-situ measurement and structural health monitoring of the rotating components in gas turbine engines is of major interest to NASA. As part of this on-going effort, several experiments have been undertaken to develop methods for detecting cracks and measuring strain on rotating turbine engine like disks. Previous methods investigated have included the use of blade tip clearance sensors to detect the presence of cracks by monitoring the change in measured blade tip clearance and analyzing the combined disk-rotor system’s vibration response. More recently, an experiment utilizing a novel optical Moiré based concept has been conducted on a subscale turbine engine disk to demonstrate a potential strain measurement and crack detection technique. Moiré patterns result from the overlap of two repetitive patterns with slightly different spacing. When this technique is applied to a rotating disk, it has the potential to allow for the detection of very small changes in spacing and radial growth in a rotating disk due to a flaw such as a crack. This investigation was a continuation of previous efforts undertaken in 2011-2012 to validate this optical concept. The initial demonstration attempted on a subscale turbine engine disk was inconclusive due to the minimal radial growth experienced by the disk during operation. For the present experiment a new subscale Aluminum disk was fabricated and improvements were made to the experimental setup to better demonstrate the technique. A circular reference pattern was laser etched onto a subscale engine disk and the disk was operated at speeds up to 12 000 rpm as a means of optically monitoring the Moiré created by the shift in patterns created by the radial growth due the presence of the simulated crack. Testing was first accomplished on a clean defect free disk as a means of acquiring baseline reference data. A notch was then machined in to the disk to simulate a crack and testing was repeated for the purposes of demonstrating the concept. Displacement data was acquired using external blade tip clearance and shaft displacement sensors as a means of confirming the optical data and for validating other sensor based crack detection techniques.

Safety and maintenance cost are among the major features that engine manufacturers strive for in their design approach to produce efficient and successful products. However, this design success is subject to manufacturing highly reliable rotating components that typically undergo high rotational loading conditions that subject them to various types of failure initiation mechanisms. To counter such design concerns; health monitoring of these components is becoming a necessity, yet, this attribute remains somewhat challenging to implement. This is mostly due to the fact that presence of scattered loading conditions, crack sizes, component geometry and material property hinders the simplicity of imposing such applications. Therefore, exploitation of suitable techniques to monitor the health of these rotating components is ongoing and investigating other means of inspections such as non-destructive approaches to pre-detect hidden flaws and mini cracks is also being considered. These approaches or techniques extend more to assess materials’ discontinuities and other defects that have matured to the level where a failure is likely. This paper is pertained to presenting data collected from a spin experiment of a turbine like rotor disk tested at a range of rotational speeds up to 12000 rpm. It further includes an analytical modeling of the rotor vibration response that is characterized by a combination of numerical and experimental data. The data include blade tip clearance, tip timing measurements and shaft displacements. The tests are conducted at the NASA Glenn Research Center’s Rotordynamics Laboratory, a high precision spin rig. The results are evaluated and scrutinized to explore their relevance towards the development of a crack detection system and a supplemental physics based fault prediction analytical model.

The Aeronautical Sciences Project under NASA’s Fundamental Aeronautics Program is extremely interested in the development of novel measurement technologies, such as optical surface measurements in the internal parts of a flow path, for in situ health monitoring of gas turbine engines. In situ health monitoring has the potential to detect flaws, i.e. cracks in key components, such as engine turbine disks, before the flaws lead to catastrophic failure. In the present study, a cross-correlation imaging technique is investigated in a proof-of-concept study as a possible optical technique to measure the radial growth and strain field on an already cracked sub-scale turbine engine disk under loaded conditions in the NASA Glenn Research Center’s High Precision Rotordynamics Laboratory. The optical strain measurement technique under investigation offers potential fault detection using an applied high-contrast random speckle pattern and imaging the pattern under unloaded and loaded conditions with a CCD camera. Spinning the cracked disk at high speeds induces an external load, resulting in a radial growth of the disk of approximately 50.0-μm in the flawed region and hence, a localized strain field. When imaging the cracked disk under static conditions, the disk will be undistorted; however, during rotation the cracked region will grow radially, thus causing the applied particle pattern to be ‘shifted’. The resulting particle displacements between the two images will then be measured using the two-dimensional cross-correlation algorithms implemented in standard Particle Image Velocimetry (PIV) software to track the disk growth, which facilitates calculation of the localized strain field. In order to develop and validate this optical strain measurement technique an initial proof-of-concept experiment is carried out in a controlled environment. Using PIV optimization principles and guidelines, three potential speckle patterns, for future use on the rotating disk, are developed and investigated in the controlled experiment. A range of known shifts are induced on the patterns; reference and data images are acquired before and after the induced shift, respectively, and the images are processed using the crosscorrelation algorithms in order to determine the particle displacements. The effectiveness of each pattern at resolving the known shift is evaluated and discussed in order to choose the most suitable pattern to be implemented onto a rotating disk in the Rotordynamics Lab. Although testing on the rotating disk has not yet been performed, the driving principles behind the development of the present optical technique are based upon critical aspects of the future experiment, such as the amount of expected radial growth, disk analysis, and experimental design and are therefore addressed in the paper.

Optical measurement technologies become more and more import in many fields of nondestructive testing. One of these technologies is the optical coherence tomography (OCT). By now mostly used in medical applications, such as ophthalmology and dermatology, relevance of OCT for testing of non-biological samples in manufacturing and research is increasing. For this reason new OCT measurement systems for fast volume inspection are needed. In this paper we present an automated high speed swept source based OCT system with a scan rate of 100.000 A-scans per second. The advantage of high scan rates of such systems until know lead into high costs for appropriate light sources. Furthermore the big size of available sources makes system integration and miniaturization difficult. With a new kind of MEMS-based light sources components with smaller dimensions and a cost reduction of more than 50% are possible. We combine the MEMS-based swept source OCT system with a table top robot for large-area inspections. The field of view of commercial systems is limited to few centimeters. Our new system is capable of measuring up to 400mm in square. With inherent software the acquisition of A-Scans at any position in the working area and of any quantity is possible. Once calibrated for a sample, the system can perform an automated measurement. Therefore our OCT system can be used as a tool for testing scenarios in research, for failure analyses and for semi-automated production processes.

We demonstrate the construction of an acoustic long period ber grating (LPG) incorporating an index tailored few-mode ber and discuss its implementation for high resolution microscopy and spectroscopy. The LPG is used to selectively excite radially or azimuthally polarized second-higher order modes. Among many others, possible applications are nano-particle characterization and optical near- eld microscopy. The goal is to increase the feasibility and reliability of nondestructive optical evaluation for micro- and nanoscale devices and nanostructured materials.

We present our recent investigations on time-resolved measurements of alterations in the temporal luminescence decay of upconversion phosphors induced by electron beam treatment. The latter is a promising alternative to low-temperature and dry sterilization of surfaces for sensitive packaging materials. Especially in the food and medical sector regulations concerning sterility are increasingly tightened. For this, a secure proof for electron-beam-assisted sterilization is required. However, no non-destructive and in situ method exists up to now. Our approach to provide a secure proof of sterilization is to place a suitable marker material based on rare-earth-doped phosphors inside or on top of the packaging material of the respective product. Upon electron irradiation the marker material changes its luminescent properties as a function of applied energy dose. We verified the energy dependence by means of time-resolved measurements of the luminescent decay of different upconversion materials. In our experimental realization short laser pulses in the near-infrared range excite the marker material. The emitted light is spectrally resolved in a monochromator, collected via a silicon photo diode, and analyzed with an oscilloscope. As the main results we observe a reduction of luminescence lifetime due to electron beam treatment dependent on the emission wavelength. Hence, the electron beam induces changes in the particles' up- and down-conversion properties from which the applied energy dose can be derived.

Monitoring intracellular ice formation (IIF) as well as understanding the cellular freezing process at temperatures of nearly -40°C is beneficial to the study of cryopreservation. This paper discusses the use of optical absorption spectroscopy to examine the thermal changes occurring in water contained in cells as they reach cryogenic temperatures. The presented arrangement employs a 0.59 m all-silica steering-wheel photonic crystal fiber (SW-PCF) filled with <20uL of de-ionized water that is fusion spliced with a 1.13 m single mode fiber (SMF). The fluid filled SW-PCF is placed on a two-stage thermoelectric cooler (TEC). One end of the SW-PCF is coupled to an optical spectrum analyzer, while the SMF couples broadband light with emission peaks at 1350nm, 1450nm, 1550nm and 1650nm to the SW-PCF. Unlike our previous cryogenic freezing arrangement, a water circulating cooling system consisting of a cold plate with an attached radiator promotes operation temperatures of nearly – 40 °C. Styrofoam insulates the fiber/TEC configuration to provide thermal stability and prevent undesired ice condensation on the thermal system. A resistance temperature detector (RTD) monitored the thermal changes occurring over a range of temperatures between 5°C to -38 °C in 5 degree increments. The measured absorption spectra of the < 20μL de-ionized water sample filled PCF show absorption characteristics consistent with standard spectra for water vapor at cryogenic temperatures.

A solid-core photonic crystal fiber (SC-PCF) with a microstructured all-silica steering wheel lattice containing a 3 μm diameter core is used to identify dye-free enzyme-linked monoclonal antibodies (mAbs) of very small volumes <0.1uL for concentrations ranging from 0.1mg/mL to 10^-12 mg/mL. The samples used in this study are mAbs, which are useful in detecting the presence of the botulism toxin. Human IgG antibodies (Southern Biotech Inc.) served as the “coating” antibody to detect an enzyme-linked mAb (Southern Biotech Inc.) - the “antigen”. The spectra for mAb samples linked with enzymes were compared to control samples without enzymes and conventional ELISA measurements. The SC-PCF at lengths 1m and 0.5m were filled with ~ 0.008 μL of each sample was coupled with a broadband LED light source with emission peaks in the 1280-1700 nm range. Absorption characteristics associated with these mAbs were measured using a standard optical spectrum analyzer. Spectra were collected for control antibody samples, enzyme-linked antibody and a third sample of a mixture of both antibodies. The measured spectra for the samples with varied concentrations were consistent with conventional ELISA measurements. Limitations due to bend loss were also investigated. A compact, real-time sensor based on this dye-free technology has the potential to benefit water quality monitoring, drug manufacturing and food quality control.

This paper presents improvements to slab-coupled optical fiber sensors (SCOS) for electric-field sensing. The improvements are based on changing the crystal cut and orientation of the slab waveguide in combination with altering the input light polarization. Traditional SCOS are fabricated using z-cut potassium titanyl phosphate crystals and are operated with TM polarized light. They have been shown to detect fields as low as 100 V/m. By using an x-cut crystal and TE polarized light, the sensitivity to electric fields is increased 8x due to, primarily, an increase in electric field penetration into the slab by exploiting a tangential boundary condition, and secondly, an increase to the effective electro-optic coefficient of the slab.