Fluorescence decay characteristic of Tm-doped YAG crystal fiber for sensor applications, investigated from room temperature to 1400/spl deg/C

An yttrium aluminum garnet (YAG) crystal fiber with a thulium-doped end tip was specially grown by means of the laser heated pedestal growth approach and designed to be incorporated in a fiber-optic temperature probe. The fluorescence decay characteristics of the crystal fiber, including the temperature dependence of both the fluorescence lifetime and intensity, were comprehensively investigated. Experimental results indicated that the crystal fiber showed a monotonic relationship between the fluorescence lifetime and temperature with an average lifetime sensitivity of 3 /spl mu/s /spl deg/C over a wide temperature range, taking measurement from room temperature to 1200/spl deg/C. Good stability (up to 1400/spl deg/C) was observed with high repeatability of the fluorescence lifetime during the annealing process carried out on the fiber over this temperature range. The fiber was found to be an excellent candidate material to be used as a fluorescence decay-based fiber thermometer probe and the results are presented on its performance.     

Gas-sensing properties of sprayed films of (CdO)/sub x/(ZnO)/sub 1-x/ mixed oxide

A novel NO/sub 2/ sensor based on (CdO)/sub x/(ZnO)/sub 1-x/ mixed-oxide thin films deposited by the spray pyrolysis technique is developed. The sensor response to 3-ppm NO/sub 2/ is studied in the range 50/spl deg/C-350/spl deg/C for three different film compositions. The device is also tested for other harmful gases, such as CO (300 ppm) and CH/sub 4/ (3000 ppm). The sensor response to these reducing gases is different at different temperatures varying from the response typical for the p-type semiconductor to that typical for the n-type semiconductor. Satisfactory response to NO/sub 2/ and dynamic behavior at 230/spl deg/C, as well as low resistivity, are observed for the mixed-oxide film with 30% Cd. The response to interfering gas is poor at working temperature (230/spl deg/C). On the basis of this study, a possible sensing mechanism is proposed.     

Process monitoring of a thermosetting resin using optical-fiber sensors in a microwave environment

Conventional means of curing thermosetting resins used in advanced fiber-reinforced composites involves heating the sample in an oven or autoclave where heat is transferred to the sample through conduction and convection. In general, the cure schedules that are used can be time consuming and expensive. Hence, significant attempts are being made to improve the processing efficiency and productivity of this class of material. Microwave-based processing has been claimed to offer the advantage of a significantly faster processing time. In this study, two remote (noncontact) sensor systems were designed and developed to facilitate spectral-based quantitative process monitoring inside a custom-modified commercial microwave oven. The two fiber-optic sensor systems were 1) a noncontact optical-fiber reflectance probe and 2) a reusable transmission probe assembly. Spectroscopic data were obtained using the fiber-optic probes at specified microwave power levels. Since conventional metal-based temperature monitoring devices cannot be used in the microwave oven, a low-cost disposable fiber-optic probe was developed. This design was based on an optical fiber-based extrinsic Fabry-Perot interferometer (FPI). The extrinsic FPI temperature sensor demonstrated an accuracy of /spl plusmn/0.5/spl deg/C over the range from ambient to 300/spl deg/C. The temperature of the resin system was also measured simultaneously, along with the spectral data, to monitor the progress of the chemical reaction (curing).     

High-sensitivity sensor of low relative humidity based on overlay on side-polished fibers

A low relative humidity (RH) sensor based on overlay on side-polished fiber is presented. The evanescent field from a single-mode optical fiber is coupled to a TiO/sub 2/ waveguide overlay. The transmission response exhibits sharp resonances whose central wavelengths are linearly shifted with RH. This behavior is due to the porous columnar nanostructure of the TiO/sub 2/ film. The water is adsorbed in the pores of the nanostructure changing the refractive index of the layer and causing a shift of the wavelength resonances. The response of the sensor is determined by the shape and size of the pores. The optical fiber evanescent field sensor developed has a linear response and high sensitivity (0.5 nm/% RH) for low RH (RH/spl sim/0%-15%) at 26.1/spl deg/C/spl plusmn/0.6/spl deg/C. The lack of hysteresis in the adsorption-desorption cycle has been checked. The development of a sensor with tailored response is envisaged using properly techniques to control the porosity of the material.     

Optical and electrical H/sub 2/- and NO/sub 2/-sensing properties of Au/In/sub x/O/sub y/N/sub z/ films

In this paper, we describe the optical and electrical gas-sensing properties of In/sub x/O/sub y/N/sub z/ films with an ultrathin gold promoter overlayer. We have fabricated In/sub x/O/sub y/N/sub z/ films with a nanocrystalline porous structure by RF-sputtering in Ar/N/sub 2/ followed by an annealing process. Gold particles with 20-30-nm diameter have been formed on top of the In/sub x/O/sub y/N/sub z/ films by dc sputtering and an annealing process. We have investigated the optical H/sub 2/and NO/sub 2/-sensing properties (change of absorbance) and also the electrical sensing effect (change of electrical resistance) for these two gases. A combined optical/electrical sensor for H/sub 2//NO/sub 2/ is proposed.     

Synthetic ferrimagnetic media: effects of thermally assisted writing

Thermally assisted writing on high-coercivity synthetic ferrimagnetic media (SFM) was demonstrated using a conventional spin stand equipped with an optical head for commercial magnetooptical drives. The laser light (/spl lambda/ = 685 nm) was focused through a glass substrate onto a recording layer. The optical spot size was 1.1 /spl mu/m and a commercial magnetic head had a writer width of /spl sim/0.25 /spl mu/m. The recording properties were measured as a function of the writing current (I/sub w/) and the laser power (P/sub w/). For the thermally stable medium with H/sub c/ = 6 kOe, a laser irradiation with an optimum power significantly improved the overwrite performance and the signal-to-noise ratio (SNR) values. The SNR values were improved by optimizing P/sub w/ over a wide writing current range. The improvements with the assist were found in both the signal and the noise. The media with a large dynamic coercivity value or with thick magnetic layers clearly showed the advantages with thermal assist.     

Study of x/spl alpha/-Fe/sub 2/O/sub 3/-(1-x)ZrO/sub 2/ solid solution for low-temperature resistive oxygen gas sensors

A noble type of oxygen-sensitive and electrical-conductive material, ZrO/sub 2/-based with /spl alpha/-Fe/sub 2/O/sub 3/ thick-film gas sensor, was investigated for low operating temperature. Amorphous-like solid solutions of x/spl alpha/-Fe/sub 2/O/sub 3/-(1-x)ZrO/sub 2/ powders were derived using the high-energy ball milling technique, and their physical and microstructural properties were characterized from DTA, XRD, TEM, and XPS. The oxygen gas-sensing properties of the screen-printed thick-film gas sensors fabricated from such mechanically-alloyed materials were characterized systematically. Very good sensing properties were obtained with a relative resistance value of 82 in 20% oxygen, and at a low operating temperature of 320/spl deg/C. AC impedance spectra and thermally stimulated current were characterized to investigate the conduction properties of the solid solution, 0.2/spl alpha/-Fe/sub 2/O/sub 3/-0.8ZrO/sub 2/, in air and nitrogen (carrier gas), respectively. It was found that the Arrhenius plots of /spl sigma/T versus 1000/T have two distinct gradients corresponding to two activation energies in the high and low temperature regions. The transition temperature occurs at about 320/spl deg/C that corresponds to an optimal operating temperature of the gas sensor. It is believed that the high oxygen vacancy concentration present in the solid solution, 0.2/spl alpha/-Fe/sub 2/O/sub 3/-0.8ZrO/sub 2/, and the dissociation of the associated oxygen vacancy defect complexes at 320/spl deg/C are the critical factors for the high relative resistance to oxygen gas at low operating temperature.     

Fabrication and characterization of nano-sized SrTiO/sub 3/-based oxygen sensor for near room-temperature operation

Nano-sized SrTiO/sub 3/-based oxygen sensors were fabricated from synthesized SrTiO/sub 3/ and commercial SrTiO/sub 3/ using the high-energy ball milling and the thick-film screen-printing techniques. The particle sizes, microstructural properties, oxygen-sensing properties, and humidity effects of the synthesized nano-sized SrTiO/sub 3/-based oxygen sensors were characterized using X-ray diffraction (XRD), transmission electron microscope, scanning electron microscope (SEM), and gas sensing measurements. Experimental results showed that the particle size of the powders was milled down to be around 27 nm. The effect of different annealing temperatures (400/spl deg/C, 500/spl deg/C, 600/spl deg/C, 700/spl deg/C, and 800/spl deg/C) on the gas sensing properties of the synthesized SrTiO/sub 3/ sensor from nitrogen to 20% oxygen was characterized. The commercial SrTiO/sub 3/ devices annealed at 400/spl deg/C, both with 0-h and 120-h milling time, were used for comparison. The optimal relative resistance (R/sub nitrogen//R/sub 20%oxygen/) value of 6.35 is obtained for the synthesized SrTiO/sub 3/ sample annealed at 400/spl deg/C and operating at 40/spl deg/C. This operating temperature is much lower than that of conventional metal oxide semiconducting oxygen gas sensors (300/spl deg/C-500/spl deg/C) and SrTiO/sub 3/ oxygen gas sensors (>700/spl deg/C). The response and recovery times are 1.6 and 5 min, respectively. The detected range is 1-20% oxygen. The impedance of the synthesized SrTiO/sub 3/ sensor with annealing at 400/spl deg/C and operating at 40/spl deg/C (from 1 mHz to 10 MHz) in 20% oxygen ambient was found to be independent of the relative humidity (dry, 20% RH, 80% RH, near 100% RH).     

Simultaneous measurement of humidity and temperature by combining a reflective intensity-based optical fiber sensor and a fiber Bragg grating

A novel sensor capable of simultaneously measuring temperature and humidity has been fabricated and demonstrated using optical fiber waveguides. The sensor head is composed of a fiber Bragg grating and a low-finesse Fabry-Perot interferometric cavity. The Fabry-Perot cavity was fabricated using the electrostatic self-assembled monolayer process for the molecular-level deposition of materials of different thicknesses that form a humidity-sensitive coating on the end of the fiber, while the in-line Bragg grating fiber element is used to monitor temperature. Experimental results for a humidity range from 11% to 97% RH and for a temperature range from 10/spl deg/C to 85/spl deg/C are shown.     

Fiber Bragg grating sensor interrogation using chirped fiber grating-based Sagnac loop

We have investigated numerically and experimentally a fiber Bragg grating (FBG) sensor interrogation scheme utilizing a linear chirped grating-based Sagnac loop as a wavelength-dependent receiver. The scheme is suitable for both static and dynamic sensor interrogation with advantages of stable and linear readout response and easily-adjustable sensing resolution and dynamic range. Static and dynamic strain resolutions as high as /spl plusmn/ 4.2 /spl mu//spl epsiv/ and 0.406 /spl mu//spl epsiv///spl radic/ Hz have been demonstrated using this scheme.     

Chemical warfare agent detection using MEMS-compatible microsensor arrays

Microsensors have been fabricated consisting of TiO/sub 2/ and SnO/sub 2/ sensing films prepared by chemical vapor deposition (CVD) on microelectromechanical systems array platforms. Response measurements from these devices to the chemical warfare (CW) agents GA (tabun), GB (sarin), and HD (sulfur mustard) at concentrations between 5 nmol/mol (ppb) and 200 ppb in dry air, as well as to CW agent simulants CEES (chloroethyl ethyl sulfide) and DFP (diisopropyl fluorophosphate) between 250 and 3000 ppb, are reported. The microsensors exhibit excellent signal-to-noise and reproducibility. The temperature of each sensor element is independently controlled by embedded microheaters that drive both the CVD process (375/spl deg/C) and sensor operation at elevated temperatures (325/spl deg/C-475/spl deg/C). The concentration-dependent analyte response magnitude is sensitive to conditions under which the sensing films are grown. Sensor stability studies confirm little signal degradation during 14 h of operation. Use of pulsed (200 ms) temperature-programmed sensing over a broad temperature range (20/spl deg/C-480/spl deg/C) enhances analyte selectivity, since the resulting signal trace patterns contain primarily kinetic information that is unique for each agent tested.     

Standardized evaluation of chemical compositions of LiTaO3 single crystals for SAW devices using the LFB ultrasonic material characterization system

The line-focus-beam ultrasonic material characterization (LIFB-UMC) system is applied to compare and evaluate tolerances provided independently for the Curie temperature T/sub C/ and lattice constant /spl alpha/ to evaluate commercial LiTaO/sub 3/ single crystals by measuring the Rayleigh-type leaky surface acoustic wave (LSAW) velocities V/sub LSAW/. The relationships between VLSAW, and T/sub C/ and /spl alpha/ measured by individual manufacturers were obtained experimentally using 42/spl deg/YX-LiTaO/sub 3/ wafers as specimens from three crystal manufacturers. In addition, the relationship between VLSAW and SH-type SAW velocities V/sub SAW/ that are actually used for the SAW device wafers was obtained through calculations, using the chemical composition dependences of the acoustical physical constants for LiTaO/sub 3/ crystals reported previously. The result of a comparison between the T/sub C/ tolerance of /spl plusmn/3/spl deg/C and the /spl alpha/ tolerance of /spl plusmn/0.00002 nm through the common scale of VLSAW or VSAW demonstrated that the /spl alpha/ tolerance is 1.6 times larger than the T/sub C/ tolerance. Furthermore, we performed a standardized comparison of statistical data of T/sub C/ and /spl alpha/ for LiTaO/sub 3/ crystals grown by two manufacturers during 1999 and 2000, using VLSAW. The results clarified the differences of the average chemical compositions and of the chemical composition distributions among the crystal ingots between the two manufacturers. A guideline for the standardized evaluation procedure has been established for the SAW-device wafer specifications by the LFB-UMC system.     

Self-referenced point temperature sensor based on a fluorescence intensity ratio in Yb(3+)-doped silica fiber

An optical fiber temperature sensor, based on the fluorescence intensity ratio from the (2)F (5/2)(a) and (2)F(5/2)(b) Stark sublevels in ytterbium-doped silica fiber, has been investigated. Results of a sensor prototype demonstrate an accuracy near 1 degrees C in a 600 degrees C temperature range. Changes in the fluorescence intensity ratio because of variation in pump power, pump wavelength, and induced fiber bending loss are demonstrated to be small, supporting development of a practical sensor based on the technique described.     

Amperometric uric acid biosensors fabricated of various types of uricase enzymes

Preparation process of an enzyme-based bipotentiostatic amperometric uric acid sensor has been investigated. The suitability of three different Uricase (EC 1.7.3.3) enzymes (from porcine liver, Candida Utilis, Bacillus Fastidiosus) is described in this paper. The sensor fabricated of Uricase from Candida Utilis showed a linear response to uric acid in the 0-0.9 mM concentration range and the response current range was 0-3.3 /spl mu/A. The sensor fabricated of Uricase from Bacillus Fastidiosus has been saturated at 0.72 mM and the response was not linear above 0.24 mM. The response current range was 0-0.9 /spl mu/A. The sensor fabricated of Uricase from porcine liver has not given detectable electrical signal due to its very low specific activity. The substrate was prepared by screen printing on sintered alumina ceramic sheets using pastes of Au or Pd-Pt as working (W) and counter (C) and Pt-Ag as a reference (R) electrode. Galvanostatic electrocopolymerization of dodecyl sulfate doped poly-N-methyl-pyrrole (pNMPy) layer was used for enzyme immobilization. The layout of the sensor consists of four electrode surfaces (W/sub 1/, W/sub 2/, R, and C). By the bipotentiostatic technique, the two working electrodes (with and without the enzyme) are identically prepared and polarized, while the currents in the two circuits are measured simultaneously; thus, the current of the W/sub 2/-C circuit (I/sub 2/) can be substracted as a nonspecific background noise. The nonspecific oxidation of uric acid on the poly-N-methyl-pyrrole layer at 0.2 V has been demonstrated in oxygen bubbled buffer solution.     

An optical fiber radiation sensor for remote detection of radiological materials

This paper demonstrates the feasibility of a portable radiation sensor system that uses the pulsed optically stimulated luminescence technique to remotely interrogate an aluminum oxide (Al/sub 2/O/sub 3/:C) radiation sensor via an optical fiber. The objective is to develop a system for applications requiring simple and inexpensive sensors for widespread monitoring of ionizing radiation levels, which can be remotely interrogated at regular periods with little or no human intervention and are easy to install, operate, and maintain. Results on the optimization and performance of the system are presented. The current minimum detectable dose is of the order of 5 /spl mu/Gy, which is already satisfactory for applications such as the monitoring of radioactive plumes from radioactive waste sites. We also discuss potential developments that could decrease the minimum detectable dose to allow radiation doses as low as the background level to be measured over short time intervals, making the system more versatile for detecting radiological materials.     

Iridium oxide as low temperature NO/sub 2/-sensitive material for work function-based gas sensors

This paper presents the results on work function-based NO/sub 2/-sensing properties of iridium-oxide thin films at 130/spl deg/C. Films of 20-nm and 100-nm thickness were deposited on silicon substrates using dc sputtering followed by annealing in oxygen ambient. Sensitivity of these films to different concentrations of NO/sub 2/, H/sub 2/, CO, Cl/sub 2/, and NH/sub 3/ in synthetic air was measured using a Kelvin probe. It was observed that work function of 20-nm-thick iridium-oxide film changed by /spl sim/100 mV on exposure to 5-ppm NO/sub 2/ (German safety limit). Cross sensitivity to other gases (except NH/sub 3/) and interference of humidity was found to be negligibly small. The film was incorporated as a gate electrode in a hybrid suspended gate field effect transistor (HSGFET) structure to examine its suitability in FET-type sensors. The films were characterized using Rutherford backscattering spectroscopy, X-ray diffraction analysis, and scanning electron microscopy to determine their composition, phase, and surface morphology. The results suggest that iridium-oxide film is a promising material for the realization of a FET-based NO/sub 2/ sensor.     

Applicability of blue/UV laser diodes for the measurement of vaporized fuel fluorescence around stoichiometric concentrations

A fluorescence-based fiber-optic probe is implemented for use with a blue/UV laser diode to form a rugged, nonintrusive fuel sensor. Measurements are obtained from a test vessel controlled to replicate the temperature and pressure environment in a typical combustion engine. A fuel containing a specific polycyclic aromatic hydrocarbon (perylene, C20H12) is used to obtain wavelength resolved information and concentration variation characteristics. Emission intensity is examined in atmospheres with and without oxygen at pressures from 1 to 10 bar. The signals from a retail gasoline fuel present a greater challenge as they result from a complex mixture of hydrocarbon components. Detection of gasoline vapor concentrations in the range of 4% to 125% of stoichiometry is demonstrated.     

Effects of calcining and sintering parameters on the magnetic properties of high-permeability MnZn ferrites

For asymmetric digital subscriber line (ADSL) technology applications, new MnZn ferrites are required with not only high initial permeability, but also with low hysteresis material constant /spl eta//sub B/. The effects of calcining and sintering temperatures and oxygen equilibrium partial pressure on the magnetic properties of MnZn ferrites are studied in this work. It was found that raising the calcining temperature required a rise in the sintering temperature to simultaneously achieve maximum permeability and the lowest /spl eta//sub B/ value. Both grain size and porosity had an effect on the initial permeability of the samples. Porosity had an even larger effect on the /spl eta//sub B/ value than the initial permeability. Oxygen equilibrium partial pressure also had an important effect on the initial permeability and /spl eta//sub B/ value. In our testing range, both the initial permeability and the /spl eta//sub B/ value decreased with increasing value of A (oxygen parameter). In order to optimize synthesis conditions for the high initial permeability and low /spl eta//sub B/ value, it was best to calcine the sample at 950/spl deg/C and sinter it at 1370/spl deg/C, with A=7.55.     

Limiting sensitivity of a differential absorption spectrometer with direct detection in the 2/spl nu//sub 3/ and /spl nu//sub 2/+2/spl nu//sub 3/ vibration bands [methane detection applications]

A differential absorption spectrometer, for methane detection, using tunable laser diodes in the 1.33-/spl mu/m and 1.66-/spl mu/m bands, has been studied. The spectral scanning is carried out by current modulation of the laser diode. We analyzed the performance with a multimode Fabry-Perot laser diode in the /spl nu//sub 2/+2/spl nu//sub 3/ band of methane and a monomode laser diode in the 2/spl nu//sub 3/ band. The theoretical results are validated by several experiments. To determine the sensitivity limit of the sensor, we have examined the influence of the noise sources. A sensitivity of 10 ppm/spl middot/m was obtained in the 2/spl nu//sub 3/ band. The main limiting factors are the relative intensity noise of the laser, optical interferences, and quantization noise. We also analyzed the influence of the temperature on the laser diode emission spectra and the methane absorption spectra.     

Single-mode rib optical waveguides on SOG/SU-8 polymer and integrated Mach-Zehnder for designing thermal sensors

This paper presents a successful design, realization,and characterization of single-mode rib optical waveguides on SOG/SU-8 polymers in order to highlight a new approach to designing heat sensors. The basic principle of this new thermal-sensing method relies on the differential thermal behavior regarding both acting arms of a micro Mach-Zehnder Interferometer(MZI). First, two families of single-mode straight rib waveguides composed of SOG/SU-8 polymers are analyzed. Hence, optical losses for TE/sub 00/ and TM/sub 00/ optical modes for structures on Si/SiO/sub 2//SU-8 have been estimated respectively as 1,36 /spl plusmn/ 0,02 and 2,01/spl plusmn/0,02 dB/spl middot/cm/sup -1/, while the second one composed of Si/SiO/sub 2//SOG/SU-8 presented losses of 2,33 /spl plusmn/ 0,02 and 2,95/spl plusmn/0,02 dB/spl middot/cm/sup -1/. Then, owing to modeling results, an experimental sensor is realized as an integrated device made up of SU-8 polymer mounted on a standard silicon wafer. When subjected to a radiant source, as a laser light (980 nm) is injected across the cleaved input face of the MZI, the significant change of output signal allows us to consider a new approach to measuring radiant heat flowrate. Experimental results are given regarding the obtained phase shift against the subjected thermal power. According to the modeling results, one can expect new highly sensitive devices to be developed in the next coming years, with advantageous prospective industrial applications.