Fiber grating sensors for high-temperature measurement

Two fiber grating sensors for high-temperature measurements are proposed and experimentally demonstrated. The interrogation technologies of the sensor systems are all simple, low cost but effective. In the first sensor system, the sensor head is comprised of one fiber Bragg grating (FBG) and two metal rods. The lengths of the rods are different from each other. The coefficients of thermal expansion of the rods are also different from each other. The FBG will be strained by the sensor head when the temperature to be measured changes. The temperature is measured based on the wavelength-shifts of the FBG induced by the strain. In the second sensor system, a long-period fiber grating (LPG) is used as the high-temperature sensor head. The LPG is very-high-temperature stable CO₂-laser-induced grating and has a linear function of wavelength–temperature in the range of 0–800 °C. A dynamic range of 0–800 °C and a resolution of 1 °C have been obtained by either the first or the second sensor system. The experimental results agree with theoretical analyses.     

All-fibre optical system for exciting and detecting the vibration of silicon microresonator sensors

An all-fibre optical system for optical interrogation and detection of the vibrations of a silicon microresonator is reported. Metal-coated silicon microresonators are excited by intensity modulated laser light delivered through an optical fibre, while the vibration of the resonators is detected by an optical fibre interferometer. Measurements have shown that an average optical power of 10 μW is sufficient to maintain the flexural vibration of the resonator. When the resonator is used as a pressure sensor, its resonant frequency changes from 62 kHz to 130 kHz as the pressure varies from -0°6 bar to 1 bar (gauge). A silicon resonator with 700 nm aluminium coating functions as a temperature sensor, showing a frequency shift from 262 kHz to 251 kHz when the temperature changes from 25 °C to 80 °C.     

A novel fiber Bragg grating high-temperature sensor

A novel fiber Bragg grating (FBG) sensor for the measurement of high temperature is proposed and experimentally demonstrated. The interrogation system of the sensor system is simple, low cost but effective. The sensor head is comprised of one FBG and two metal rods. The lengths of the rods are different from each other. The coefficients of thermal expansion of the rods are also different from each other. The FBG will be strained by the sensor head when the temperature to be measured changes. The temperature is measured basis of the wavelength shifts of the FBG induced by strain. A dynamic range of 0–800 °C and a resolution of 1 °C have been obtained by the sensor system. The experiment results agree with theoretical analyses.     

Temperature measurements using fiber optic polarization interferometer

A novel measurement method of temperature based on the phenomena that the phase difference between principle polarization states in the optical retarder is function of temperature is described. The polarization state of optical beam is changed as it passes through the optical retarder, which depends on the temperature. The temperature of optical retarder is determined by comparison of the power difference between principal polarization states. We demonstrate successfully the temperature measurement by using a polarization maintaining fiber as the optical retarder. With a 100 mm length of the fiber optic retarder, the change rate of phase difference on temperature was 0.236 rad/°C and the measurement error was ±0.038°C over the temperature range of −2.6 – +3.4°C. With a 11.5 mm length of the fiber optic retarder, the change rate of phase difference on temperature was 0.021 rad/°C and the measurement error was ±0.79°C over the temperature range of −8.5 – +86.5°C.     

Thermally tuned optical fiber for true time delay generation

A new technique for generating a continuous range of true time delay values is introduced. Heating optical fiber in order to change the effective index of the guided mode produces time delays. A 45-m section of single-mode silica fiber is demonstrated to produce a continuous range of time delay values from 0 to 211 ps over a temperature tuning range of 50°C (30–80°C). A thermal time delay factor is introduced and found to be 0.096 ps/m°C for Corning LEAF fiber. A 7.66-m section of multimode Lucina polymer fiber is demonstrated to produce a range of time delay values from 0 to 32 ps over a temperature tuning range of 30°C (30–60°C). The thermal time delay factor for this fiber is −0.1427 ps/m°C.     

A simple optical method for the measurement of glass wool fiber diameter

An optical method for measuring glass wool fiber diameter has been proposed and discussed from the viewpoint of practical use. The method is based on both light scattering and light reflection on the glass wool. The method can measure the mean fiber diameter, d, in the area illuminated by laser light in realtime. The accuracy of the method, i.e., the dimensional resolution, was found to be about 0·7 μm within the diameter range of d7·0 μm.     

GaInAsSb/AlGaAsSb lasers in the wavelength range between 2.73 μm and 2.93 μm

Compressively strained multiple quantum well lasers in the GaInAsSb/AlGaAsSb material system are reported. Indium concentrations between 40% and 47.5% were chosen for the GaInAsSb quantum wells. Compressive strains varied between 1.16% and 1.43%. The lasers worked continuous wave at room temperature up to a wavelength of 2.81 μm. For a laser with 2.93 μm wavelength continuous wave operation was found up to a temperature of −23°C. This laser worked in pulsed operation at 15°C.     

Refractive index of standard oils as a function of wavelength and temperature

The refractive indices (n) of eight standard oils from Physikalisch Technische Bundesanstalt, Germany were determined with an accuracy of ±1×10⁻⁴ by using Abbe Refractometer. The measurements were performed at temperature 20°C in the spectral range 0.4–0.7 μm. The experimental data were fitted to the simple Cauchy dispersion formula and the results were found to be consistent within the limits of experimental error. In all cases, the refractive index decreased monotonically with increasing wavelength. The refractive indices (n) of these oils have been measured as a function of the temperature t (20°C up to 50°C) at λ=0.589 μm and were found to have linear temperature dependencies. The refractive indices of the studied oils and the uncertainty in their values are calculated at λ=0.589. The Lorentz–Lorenz (L–L) formula has been tested and it was found to be valid with a maximum deviation of 0.4% and was used to calculate the molecular polarizability θ.     

Intrinsic integrated optical temperature sensor based on waveguide bend loss

Curvature of a multimode integrated optical waveguide reduces the numerical aperture and induces radiation losses. In this paper, we study this phenomenon and we present a geometrical approach to calculate the local numerical aperture and the intensity attenuation. We exploit the bending effect on the local numerical aperture to make a new intrinsic temperature sensor. The simulation results are validated for the silica/silicone integrated optical case. The principal performances of the silica/silicone temperature sensor are the extended temperatures range (−50 to 200 °C) with an excellent linearity response (1%) between 20 and 200 °C.     

A multi-parameter optical fiber sensor with interrogation and discrimination capabilities

A multi-parameter and multi-function, but low-cost, optical fiber grating sensor with self-interrogation and self-discrimination capabilities is presented theoretically and experimentally. The sensor bases on three fiber Bragg gratings (FBG) and one fiber long period grating (LPG). Strain, vibration, pressure, ordinary temperature (−10 to 100 °C) and high temperature (100–800 °C) can be measured by the sensor. When high temperature (100–800 °C) is measured, the LPG is used as a high temperture sensor head and FBG₁ is used as an interrogation element. Alternatively, when one of the other four measurands is measured, FBG₁ (or FBG₂) is used as a sensor head and LPG is used as an interrogation element. When two of the other four measurands are measured simultaneously, FBG₁ and FBG₂ are used as sensor heads and LPG is used as a shared interrogation element. FBG₃ is used as a reference element to eliminate the errors resulted from light source fluctuation and the cross-sensitivity between measurand and environmental temperature. The measurands can be interrogated according to the signals of the photodiodes (PDs), which are related to the relative wavelength shift of the LPG and the FBGs. Experimental results agree well with theoretical analyses. The interrogation scheme is immune to light source fluctuation and the cross-sensitivity between measurands and enviromental temperature, and also the dynamic range is large.     

Effective spectral emissivity measurements of superalloys and YSZ thermal barrier coating at high temperatures using a 1.6μm single wavelength pyrometer

A method which employs an integrating sphere and a single-wavelength (1.6 μm) pyrometer for measuring the spectral effective emissivities of superalloys in the temperature range (=650–1050°C) is described. The spectral effective emissivities for five superalloys, namely, MARM-247, MARM-509, CMSX-4, Inconel-718, N-155 and two René-N6 samples coated with YSZ thermal barrier coating were measured. Correcting the pyrometer measurements for the variations in the object emissivity would reduce the uncertainty in the temperature measurements to <±1%.     

Effects of parylene C layer on high power light emitting diodes

20 μm parylene C layer was deposited on silicone film to enhance the oxygen and water barrier properties, deposited at typical rate of 2.0 and 5.6 μm/h by controlling different deposited pressures. Surface morphology and roughness were observed by atomic force microscopy (AFM), surface morphology images show lower deposited rate that can lead to better quality film. 20 μm parylene C layer was deposited on silicone encapsulation of high power light emitting diodes (LEDs), the samples were tested by power temperature cycling (PTC) test from −40 to 85 °C, 15 min each extreme, and no corrosion, discoloration, cracks was found on the LEDs after the 1000 h PTC reliability test, and PTC test is intended to simulate worst case conditions encountered in typical applications, and parylene C floating membrane structure can stand such high stress and strain. Optical test on white and red LED samples with and without 20 μm parylene C layer, measurement result shows the optical transmittance more than 95%. 1000 h temperature humidity bias life test (T&HB) is performed for the purpose of evaluating the reliability of LEDs in humid environments, energy-dispersive X-ray (EDX) analysis revealed much lower content of C (carbon) and O (oxygen) on the lead frame of LED with parylene C coating after T&HB test on, and no oxidation was found in the LED package.     

Bach Fabrication of Microlens at the end of Optical Fiber using Self-photolithgraphy and Etching Techniques

This paper describes a simple batch process for fabrication of microlens and microlens array at the end of an optical fiber or an optical fiber bundle using self-photolithography and etching techniques. A photoresist micro-cylinder was exactly formed at the core of the fiber end by exposing an UV light from the other end of the fiber and conventional development, rinse processes. A photoresist microlens was formed by thermal reflowing of the fiber at 170°C for 1 h. A measurement of transmissivity showed that the fabricated photoresist microlens is applicable for a wavelength that is longer than 450 nm. Alternatively, a glass microlens was fabricated at the core of the fiber by dry etching with an SF₆ gas using the photoresist microlens as a mask. The focusing of the lensed fiber was confirmed and simulation work showed that the lensed fiber could focus the light with a beam spot of 2 μm, numerical aperture (NA) of 0.285 and a depth of focus of 16 μm.     

Derivation of temperature and emittance from airborne multispectral thermal infrared scanner data

A new iterative algorithm is proposed to calculate surface temperature and emittance from data of an airborne scanner with reflective and thermal multispectral channels. For water areas the surface temperature can be determined from the thermal channels alone with an accuracy of 0.5–0.9°C, assuming a well-calibrated sensor and known atmospheric parameters. For non-water areas, information from the reflective bands of the airborne sensor is used for each image pixel to perform a pre-classification of the thermal band data by assigning a representative emittance value to one of the thermal bands. For a number of earth surface types this emittance value can be estimated with an accuracy of about±0.01, leading to a surface temperature accuracy of 0.5–1.5°C.     

Controlled fabrication of Si nanostructures by high vacuum electron beam annealing

Silicon nanostructures, called Si nanowhiskers, have been successfully synthesized on Si(1 0 0) substrate by high vacuum electron beam annealing (EBA). Detailed analysis of the Si nanowhisker morphology depending on annealing temperature, duration and the temperature gradients applied in the annealing cycle is presented. A correlation was found between the variation in annealing temperature and the nanowhisker height and density. Annealing at 935 °C for 0 s, the density of nanowhiskers is about 0.2 μm⁻² with average height of 2.4 nm grow on a surface area of 5×5 μm, whereas more than 500 nanowhiskers (density up to 28 μm⁻²) with an important average height of 4.6 nm for field emission applications grow on the same surface area for a sample annealed at 970 °C for 0 s. At a cooling rate of −50 °C s⁻¹ during the annealing cycle, 10–12 nanowhiskers grew on a surface area of 5×5 μm, whereas close to 500 nanowhiskers grew on the same surface area for samples annealed at the cooling rate of −5 °C s⁻¹. An exponential dependence between the density of Si nanowhiskers and the cooling rate has been found. At 950 °C, the average height of Si nanowhiskers increased from 4.0 to 6.3 nm with an increase of annealing duration from 10 to 180 s. A linear dependence exists between the average height of Si nanowhiskers and annealing duration. Selected results are presented showing the possibility of controlling the density and the height of Si nanowhiskers for improved field emission properties by applying different annealing temperatures, durations and cooling rates.     

Fabry-Perot optical fiber tip sensor for high temperature measurement

We propose a novel Fabry-Perot optical fiber tip sensor for high temperature measurement. The sensor consists of a short section of a special all-silica photonic crystal fiber spliced at one end to a silica single-mode fiber. Because of its all-silica structure, the sensor allows linear and stable measurements of temperature up to 1200 °C with a high sensitivity. The sensor is easy and inexpensive to fabricate and could find wide applications in mechanics, aeronautics, and metallurgy.     

New method for strength improvement of silica optical fibers

The reliability and the expected lifetime of optical fibers used in telecommunication technologies are closely related to the chemical environment action on the silica network. To ensure the long-term mechanical strength of the optical fibers, a polymer coating was applied onto the fiber surface during fiber fabrication. This external coating is vital to ensure a long optical fiber lifetime. Its protective action includes several functions, such as to protect glass fiber from any external damage, to limit chemical attack, in particular that of water, and finally to ensure fatigue protection and bending insensitivity, especially during handling and in-service installation. Since the mechanical strength of the fiber is controlled by its surface characteristics, we propose a new method for increasing fiber strength.The silica optical fibers used were 125 μm in diameter, with a 62.5 μm thick epoxy-acrylate coating. Fibers were rolled up around two similar cylinders. Using a screw, these cylinders moved away from one another and thus subjected the fibers to stretching. Submitted to this mechanical loading, the distended fibers were plunged into hot water at 65 or 85 °C and aged for several days. Then, the fibers were removed from the water and various weights were suspended on the fiber ends. Thus, the fibers were subjected to a tensile loading in static fatigue for several days. Just before fiber rupture, the fibers were unloaded and subjected to dynamic tensile tests at different velocities.Result analysis proved that the aging in hot water increased the fiber strength. The Weibull's diagram study shows a bimodal dispersion of defects on the fiber surface and the important role of polymer coating.     

高灵敏度光纤温度传感器

针对深井温度变化小,提出了一种可用于深井温度测量的高灵敏度光纤温度传感器。两根陶瓷插芯从铝管的两端插入构成外腔式光纤法珀干涉仪(EFPI)结构,用螺钉固定插芯,再用高强度的环氧树脂密封该结构,达到防水防尘效果。金属铝和陶瓷插芯具有不同的热膨胀系数,温度的变化将引起EFPI腔长变化,采用高灵敏度光纤白光干涉测量技术,就可以通过测量EFPI腔长获得被测温度。分别在固定温度和不同温度下,对腔长为146.5μm的EFPI光纤温度传感器进行了连续测量。测量结果表明,高灵敏度EFPI光纤温度传感器的腔长-温度灵敏度为260nm/℃,温度测量分辨率为0.002℃。     

Temperature-independent optical fiber Fabry–Perot refractive-index sensor based on hollow-core photonic crystal fiber

A novel optical fiber refractive index (RI) sensor, which is based on an intrinsic Fabry–Perot interferometer (IFPI) formed by a section of hollow-core photonic crystal fiber (HCPCF) and standard single mode fibers (SMF), is proposed. The external refractive index is determined according to the maximum fringe contrast of the interference fringes in the reflective spectrum of the sensor. The RI response performance is demonstrated for the measurement of the different RI solutions. The experimental data agree well with the theoretical results. Also the RI resolution and repeatability of ∼1.5 × 10⁻⁵ and ±0.5% in the linear measurement region, are achieved. In addition, the temperature response is tested from 20 °C to 120 °C, which exhibits excellent thermal stability. Therefore, such an HCPCF-based F–P sensor provides a practical way to measure RI with non-temperature compensation.     

Fiber-optic systems for temperature and vibration measurements in industrial applications

Fiber-optic systems for temperature and vibration measurements have been developed. It is shown that using the principle of wavelength conversion in the sensor, measurement systems, in which only one fiber is used for the connection of the sensor to the instrument, can be realized. The measuring accuracy is practically independent of the properties of the fiber-optic system such as fiber length, fiber diameter and type of connectors used. These properties are very important, reducing installation problems and costs. The adoption of semiconductor technology allows batch fabrication of sensor elements. For the prototype temperature instrument an accuracy of 0·5°C was obtained in the temperature range 0 to +200°C. Fiber-optic accelerometers for vibration measurements with a resolution of 0·05 g and a dynamic range of 70 dB have been realized. The systems have been tested in different industrial environments and have properties that make them suitable for many industrial applications. Advantages of fiber-optic systems are their noise immunity, galvanic isolation and inherent intrinsic safety.