Triple-sensor multiplexed frequency-modulated continuous-wave interferometric fiber-optic displacement sensor

A triple-sensor multiplexed fiber-optic displacement sensor, which can measure the displacements of three different objects or the three-dimensional displacement of a single object, is introduced. The sensor is based on the principles of optical frequency-modulated continuous-wave interference and frequency-division multiplexing. The beat signals from the individual sensors are assigned in the frequency domain and separated with different electrical bandpass filters. The displacements of objects can be determined simultaneously by detecting the phase shifts of the corresponding signals. The cross talk between the individual sensors is evaluated, and an accuracy of 0.08 microm in a dynamic range of 1000 microm is achieved.     

Smart value-added fiber-optic modules using spatially multiplexed processing

Intelligent fiber-optic value-added modules (VAMs) are proposed using what we believe to be a novel spatially multiplexed processing technique implemented with both reconfigurable and nonreconfigurable predesigned pixels per impinging beam that enables desired optical power split states needed for realizing a two state reconfigurable VAM. The preferred design uses broadband micromirrors such as ones fabricated via optical microelectromechanical systems technology. The basic VAM design uses two broadband micromirror pixels, where each pixel has its specific location and area and only one of these pixels is electrically driven to adjust its small tilt angle. The areas of the pixels are chosen to obtain the desired tap power. A proof-of-concept VAM with 100% digital repeatability is demonstrated using a Texas Instruments Digital Micromirror Device (DMD) where several micromirrors per beam are used to produce the dual-pixel effect. Example tap ratios experimentally implemented at 1550 nm include 10:90, 20:80, 66.66:33.33, 50:50, 30:70, and 25:75. DMD multipixel diffraction limits output port optical losses to 3.2 and 3.6 dB. The proposed VAM can have an impact in both digital electronic and analog RF optically implemented systems.     

Intrinsic fiber-optic ultrasonic sensor array using multiplexed two-wave mixing interferometry

An intrinsic multiplexed laser interferometer is presented that allows for the simultaneous detection of acoustic waves by an array of fiber-optic sensors. The phase-modulated signals from each sensor are demodulated by use of an adaptive two-wave mixing setup. The light from each sensing fiber in the array is mixed with a reference beam in a single photorefractive crystal (PRC), and the output beams from the PRC are imaged onto separate photodetectors to create a multiplexed two-wave mixing (MTWM) system. The sensing fibers are embedded in graphite-epoxy composite panels, and detection of both acoustic emission and ultrasonic signals in these materials is demonstrated. The intrinsic MTWM system is an effective tool for the simultaneous demodulation of signals from a large fiber sensor array. Also, the adaptive nature of the MTWM setup obviates the need for active stabilization against ambient noise.     

Low-coherence Michelson interferometric fiber-optic multiplexed strain sensor array: a minimum configuration

A minimum configuration Michelson fiber-optic low-coherence interferometric quasi-distributed sensing system is proposed that permits absolute length measurement in remote reflective sensor arrays. The sensor's reflective signal characteristics have been analyzed, and the relationship between intensities of light and number of sensors is given for evaluation of multiplexing potential. The proposed sensing scheme will be useful for the measurement of strain distribution. An important application may be strain monitoring in smart structures. Experimentally, four-sensor array has been demonstrated.     

Theory of fiber-optic, evanescent-wave spectroscopy and sensors

A general theory for fiber-optic, evanescent-wave spectroscopy and sensors is presented for straight, uncladded, step-index, multimode fibers. A three-dimensional model is formulated within the framework of geometric optics. The model includes various launching conditions, input and output end-face Fresnel transmission losses, multiple Fresnel reflections, bulk absorption, and evanescent-wave absorption. An evanescent-wave sensor response is analyzed as a function of externally controlled parameters such as coupling angle, f number, fiber length, and diameter. Conclusions are drawn for several experimental apparatuses.     

Array of opto-chemical sensors based on fiber-optic spectroscopy

A compact, flexible platform for reading out the variation of the optical absorption spectra in the visible range in a number of sensing materials is illustrated in this paper. This apparatus is based on an integrated spectrophotometer, an array of suitably controlled LEDs, optical fibers to carry and collect light, and a mechanical arrangement that makes possible the measurement, in sequence, of up to 15 different sensing layers. The unit was tested with a number of metalloporphyrins, known for their outstanding sensorial and optical properties. Data were analyzed using a multiway chemometrics approach. In this regard, a methodology to investigate the properties of an array of chemical sensors is introduced. This approach allowed an evaluation of the role played in the array by each sensing material in each spectral region to be performed. The analysis revealed interesting insight into the classification properties of the sensor array and the interaction mechanisms of porphyrins. The set of metalloporphyrins showed a variety of interaction mechanisms, and the relation of these mechanisms to the structure of the metalloporphyrins was evidenced by an accurate interpretation of the loadings of the multiway analysis.     

Fluorocarbon fiber-optic Raman probe for non-invasive Raman spectroscopy

We report the development of a novel fiber-optic Raman probe using a graded index fluorocarbon optical fiber. The fluorocarbon fiber has a simple Raman spectrum, a low fluorescence background, and generates a Raman signal that in turbid media serves as an intense reference Raman signal that corrects for albedo. The intensity of the reference signal can easily be varied as needed by scaling the length of the excitation fiber. Additionally, the fluorocarbon probe eliminates the broad silica Raman bands generated in conventional silica-core fiber without the need for filters.     

Optically multiplexed multi-gas detection using quantum cascade laser photoacoustic spectroscopy

We report high-throughput, nondispersive optical multiplexing of laser beams using a scanning galvanometer. We have utilized this technique for multispecies trace-gas detection using multiple quantum cascade laser photoacoustic spectroscopy. We demonstrate switching from one laser to another in less than 1 s, a performance level needed for a comprehensive multispecies sensor, and a high signal-to-noise ratio detection of five gaseous components, NH(3), NO(2), dimethyl methyl phosphonate (DMMP, a simulant for nerve agents), acetone, and ethylene glycol, in a room air gas mixture containing approximately 3 ppb of NH(3), approximately 8 ppb of NO(2), approximately 20 ppb of DMMP, approximately 30 ppb of acetone, and approximately 40 ppb of ethylene glycol.     

Rational design of fiber-optic probes for visible and pulsed-ultraviolet resonance Raman spectroscopy

We investigated the performance of fiber-optic resonance Raman probes with a series of experiments to determine the working curves of such probes using model analytes and to investigate the effects of absorbing media. A computer model designed to simulate these experiments is presented, and numerical results are found to be in agreement with the experimental data. Design considerations resulting from these studies are discussed, and novel designs for overcoming problems of coupling efficiency, damage threshold, and sensitivity in absorbing samples are presented. These findings are applied to the design of fiber-optic probes for ultraviolet resonance Raman spectroscopy involving nanosecond pulsed-ultraviolet excitation (225 and 266 nm). These probes have been used to collect what is, to our knowledge, the first reported fiber-optic-linked ultraviolet resonance Raman spectra of tryptophan and DNA.     

Calibration and X-ray spectroscopy with silicon CCDs

It is shown that the novel mode of operation of charge coupled devices (CCDs) allows a new method of measuring the X-ray energy-to-charge conversion factor, without requiring externally calibrated circuits etc., thereby reducing possible contributions to systematic uncertainties in this measurement. In addition, the low noise operation of CCDs is shown to lead to possibilities for measuring the Fano factor in silicon with improved precision. Advances in CCD X-ray detection performance are described, including energy resolutions of 80 eV FWHM at a temperature of 180 K, and detection efficiencies of greater than 90%. Such improvements are shown to have potential benefit for various X-ray spectroscopic applications.     

Calibration of probe volume in fluorescence correlation spectroscopy

In fluorescence correlation spectroscopy (FCS), an accurate evaluation of the probe volume is the basis of correct interpretation of experimental data and solution of an appropriate diffusion model. Poor fitting convergence has been a problem in the determination of the dimensional parameters, the beam radius, omega, and the distance along the optical axis of the probe volume, l. In this work, the instability of fitting during the calibration process is investigated by examining the chi(2) surfaces. We demonstrate that the minimum of chi(2) in the omega dimension is well defined for both converging and diverging data. The difficulty of fitting comes from the l dimension. The uncertainty in l could be significantly larger than that in omega, as determined by F-statistics. A modified calibration process is recommended based on examining two data treatment methods, combining several short data sets into a single long run and averaging the correlation functions of several short data sets. It is found that by using the mean of several converging correlation functions from short data sets instead of a long time correlation, more stable and consistent dimensional parameters are extracted to define the probe volume.     

Mid-infrared fiber-optic attenuated total reflection spectroscopy of the solid-liquid phase transition of water

Measurements of mid-infrared (MIR) absorption spectra of water and heavy water were carried out by fiber-optic evanescent wave spectroscopy, using silver halide (AgClBr) infrared fibers. Such measurements were performed for the first time on one sample, during the solid-liquid phase transition. From the variation of the spectra with temperature we found a new isosbestic point (at 3280 cm(-1) for H(2)O or at 2475 cm(-1) for D(2)O) and we identified five components of the O-H (O-D) stretch band. These phenomena have provided new information about the molecular structure of water.     

Noninvasive method for monitoring ethanol in fermentation processes using fiber-optic near-infrared spectroscopy

Short-wavelength near-infrared (SW-near-IR) spectroscopy (700-1100 nm) is used for the determination of ethanol during the time course of a fermentation. Measurements are performed noninvasively by means of a photodiode array spectrometer equipped with a fiber-optic probe placed on the outside of the glass-wall fermentation vessel. Pure ethanol/water and ethanol/yeast/water mixtures are studied to establish the spectral features that characterize ethanol and to show that determination of ethanol is independent of the yeast concentration. Analysis of the second-derivative data is accomplished with multilinear regression (MLR). The standard error of prediction (SEP) of ethanol in ethanol/water solutions is approximately 0.2% over a range of 0-15%; the SEP of ethanol in ethanol/yeast/water solutions is 0.27% (w/w). Results from the mixture experiments are then applied to actual yeast fermentations of glucose to ethanol. By use of a gas chromatographic method for validation, a good correlation is found between the intensity of backscattered light at 905 nm and the actual ethanol. Additional experiments show that a calibration model created for one fermentation can be used to predict ethanol production during the time course of others with a prediction error of 0.4%.     

Investigation of interferometric noise in fiber-optic gas sensors with use of wavelength modulation spectroscopy

We report on interferometric noise limitation of fiber-optic gas sensors with highly coherent lasers and wavelength modulation spectroscopy. Interference between signal wave and reflected waves causes signal fluctuation in the output, which limits the performance of the sensing system. Sensor resolution limited by interferometric noise is calculated for a fiber-optic gas sensor with the Q(6) absorption line of methane gas at approximately 1650 nm. The results are useful for system designers of this particular type of gas sensor.     

A chemometric analysis for evaluation of early-stage cartilage degradation by infrared fiber-optic probe spectroscopy

In vivo identification of early-stage cartilage degradation could positively impact disease progression in osteoarthritis, but to date remains a challenge. The primary goal of this study was to develop an infrared fiber-optic probe (IFOP) chemometric method using partial least squares (PLS1) to objectively determine the degree of cartilage degradation. Arthritic human tibial plateaus (N = 61) were obtained during knee replacement surgery and analyzed by IFOP. IFOP data were collected from multiple regions of each specimen and the cartilage graded according to the Collins Visual Grading Scale of 0, 1, 2, or 3. These grades correspond to cartilage morphology that displayed normal, swelling or softening, superficially slight fibrillation, and deeper fibrillation or serious fibrillation, respectively. The model focused on detecting early cartilage degradation and therefore utilized data from grades 0, 1, and 2. The best PLS1 calibration utilized the spectral range 1733-984 cm(-1), and independent validation of the model utilizing 206 spectra to create a model and 105 independent test spectra resulted in a correlation between the predicted and actual Collins grade of R2 = 0.8228 with a standard error of prediction of 0.258 with a PLS1 rank of 15 PLS factors. A preliminary PLS1 calibration that utilized a cross-validation technique to investigate the possibility of correlation with histological tissue grade (33 spectra from 18 tissues) resulted in R2 = 0.8408 using only eight PLS factors, a very encouraging outcome. Thus, the groundwork for use of IFOP-based chemometric determination of early cartilage degradation has been established.     

Autofluorescence and diffuse reflectance spectroscopy of oral epithelial tissue using a depth-sensitive fiber-optic probe

Optical spectroscopy can provide useful diagnostic information about the morphological and biochemical changes related to the progression of precancer in epithelial tissue. As precancerous lesions develop, the optical properties of both the superficial epithelium and underlying stroma are altered; measuring spectral data as a function of depth has the potential to improve diagnostic performance. We describe a clinical spectroscopy system with a depth-sensitive, ball lens coupled fiber-optic probe for noninvasive in vivo measurement of oral autofluorescence and diffuse reflectance spectra. We report results of spectroscopic measurements from oral sites in normal volunteers and in patients with neoplastic lesions of the oral mucosa; results indicate that the addition of depth selectivity can enhance the detection of optical changes associated with precancer.     

Calibration of Raman spectroscopy at 1064 nm for beeswax quantification

In the early sixties, coating with molten beeswax was considered a valuable method for preventing the erosive action of weather and/or salinity on the surface of granite sculptures and monuments. This technique had been traditionally employed by the Galician stoneworkers for partial repair of historical monuments. For this purpose, beeswax was applied to the Renaissance Frieze in the Cloister of the Cathedral of Santiago de Compostela in Galicia (Northwest Spain). The beeswax treatment was counterproductive. An intense grain disaggregation of the granite can be observed in the Frieze, owing to the crystallization of salts. As a consequence, the restoration of the Cloister presents many problems. This fact imposes the need for an exhaustive study of the wax-stone system and the demand for a nondestructive method to measure the beeswax thickness at the stone surface. The aim of this contribution is the evaluation of a laser-based method, namely Fourier transform Raman spectroscopy, for analyzing the wax presence in specific rocky material of the Frieze to be restored. To obtain a reliable quantitative calibration, we prepared beeswax films of five different thicknesses on aluminum plates (26.6-97.2 microm). Nylon was selected as external reference to obtain the Raman emission independently from the laser beam power. The ratios of the relative intensities of the Raman bands corresponding to beeswax and nylon were used for the construction of a calibration curve used for the quantitative analysis. The intensities at 2879 cm(-1), I(c2879), and 2880 cm(-1), I(n2880), for beeswax and nylon, respectively, in the Raman spectra of each material were used. A linear dependence was found for the ratio I(c2879)/I(n2880) with the beeswax thickness. The validation of this calibration curve was tested with a second validation set of samples that spans beeswax film thicknesses both inside and outside the calibration range (12.1 to 180 mum), in order to evaluate in addition the accuracy of the model at extrapolation. Without complex sample preparation, near-infrared Raman spectroscopy resulted in an effective technique for localizing the wax with lateral resolution of tens of micrometers, and for determining wax layer thickness in the stone with an uncertainty of a few micrometers.     

Phase modulation spectroscopy using an all-fiber piezoelectric transducer modulator for a resonator fiber-optic gyroscope

The theoretical analysis and experimental demonstration of phase modulation spectroscopy employing an all-fiber piezoelectric transducer modulator for a fiber ring resonator fiber-optic gyroscope is presented for the first time as far as we know. The results support the feasibility of such a technique as a rotation detection scheme for a resonator fiber-optic gyroscope.     

Mid-infrared fiber-optic reflection spectroscopy (FORS) analysis of artists' alkyd paints on different supports

Mid-infrared Fourier transform fiber-optic reflection spectroscopy (mid-IR FORS) is a noninvasive and flexible spectroscopic technique. It is ideal in the art conservation field because of its portability for on-site and in situ analysis of art objects, analyses that require delicate handling, or analyses of objects that cannot be sampled. This paper studies the applicability of mid-IR FORS for the characterization of commercial artists' alkyd paints cast on different supports. As predicted, the quality of the spectra and intensity of characteristic peaks varied according to reflectivity, roughness, and materials used in the supports. The presence of organic binder was best identified by its carbonyl peak (the most intense) and CH(2) stretching peaks; however, this was not sufficient to distinguish between oil and alkyd binders. The differentiation and identification of alkyds and oils must rely on the unique fingerprint peaks. However, in some cases, the fingerprint peaks were difficult to interpret because of strong absorptions caused by inorganic paint fillers, often present in modern paint formulations, resulting in anomalous dispersion and reststrahlen distortions.