Low-loss optical wave guiding along a subwavelength-diameter silica wire leaves a large amount of the guided field outside the solid core as evanescent wave and at the same time maintains the coherence of the light, making it possible to develop sensitive and miniaturized optical sensors for physical, chemical and biological applications. Here we introduce, for the first time to our knowledge, a scheme to develop optical sensors based on evanescent-wave-guiding properties of subwavelength-diameter wires. Optical wave guiding properties of these wires that are pertinent to a waveguide sensor, such as single-mode condition, evanescent field, Poynting vector and optical loss are investigated. By measuring the phase shift of the guided light, we propose a Mach-Zehnder-type sensor assembled with two silica wires. The sensitivity and size of the sensor are also estimated, which shows that, subwavelength-diameter silica wires are promising for developing optical sensors with high sensitivity and small size.
With a high melting point (over 2000°C), high transmission from ultraviolet to infrared wavelength approaching 4 μm, favorable mechanical strength and chemically inertness, the sapphire fiber is suitable for high temperature optical fiber sensor and near-infrared energy delivery. In this paper, the self-radiation and performance stability of sapphire fiber under high temperature was studied. Experiment results have shown that the self-radiation of sapphire fiber would influence the temperature measurement accuracy. The optical transmission loss increased with the time when the sapphire fiber was exposed to the high temperature environments. The most important factor that resulted in the performance deterioration was the direct contacting of the sapphire fiber with the protective tube and the dirtying of the evaporating substance. A suitable protective tube was applied to assure the performance stability. For further improvement of the optical properties of sapphire fiber, an alumina cladding is being developed to protect the sapphire fiber surface.
Based on the exact solutions of Maxwell’s equations, we have studied the basic theoretical properties of submicron and nano-diameter air-cladding silica-wire waveguides. The single-mode condition and the modal field of the fundamental modes have been obtained. Silica wires with diameters of 100-1000nm and lengths ranging from hundreds of micrometer to over 1 millimeter have been fabricated. SEM examination shows that these wires have uniform diameters and smooth surfaces, which are favorable for optical wave guiding. Light has been sent into these wires by optical coupling, and guiding light through a bent wire has also been demonstrated. These wires are promising for assembling photonic devices on a micron or submicron scale.
A multi-ions co-doped probe had been developed for radiation-based high-temperature fiber-optic sensing. The heavily (Nd3+, Er3+ and Cr3+)-co-doped probe was fabricated on one end of an Y2O3-ZrO2 waveguide by powder growth technique on an LHPG system. The wavelength-dependent absorption and emission spectra were measured, favorable results were obtained. In high temperature tests, the probe survived 2300 degree(s)C and showed a short-term stability at oxidative high temperature environment. The thermal response time of the probe is about 3.3s. Compared with metal- or oxide-coated probe, the doped probe demonstrated higher chemical and high-temperature stabilities.
High quality Y2O3-ZrO2 single crystal rectangular waveguides had been developed for high-temperature sensing applications. The waveguides were fabricated from bulky Y2O3 stabilized ZrO2 single crystal by precise cut and fine polish. Three rectangular waveguides with cross-section larger than 1mmx1mm and length of 45mm~65mm were obtained. They showed much better optical properties than Y2O3-ZrO2 single crystal fibers grown for fiber-optic sensing in previous work, optical losses of these waveguides were lower than 0.03dB/cm at wavelength of 900nm, and they were able to endure temperature as higher as 2300 degree(s)C. All of them survived a 10g vibration test with average STF(strain to failure) of about 0.25%. Experimental results show that, these waveguides are promising for fiber-optic sensing for temperature above 2000 degree(s)C.
Although Y2O3-ZrO2 fiber-optic sensor has been developed for contact measurement of temperature higher than 2000 degree(s)C, its performance is not as good as that of sapphire fiber-optic sensor below 1900 degree(s)C due to the large optical loss of the Y2O3-ZrO2 fiber. In order to improve the Y2O3-ZrO2 fiber-optic sensor for ultra-high-temperature applications, in this work, based on a newly developed rectangular Y2O3-ZrO2 single-crystal waveguide with much lower optical loss, an improved Y2O3-ZrO2 waveguide-fiber-optic sensor has been developed. The sensor has been tested up to near 2300 degree(s)C, we estimate that, the improved sensor has similar performance as the sapphire fiber-optic sensor in accuracy and resolution, except the disadvantage of relatively short waveguide. In addition, in this work, instead of the previous volatile and toxic BeO-coated probe, we use a multi-ions-doped sensor head, which is much stable and safe.
Aiming to facilitate laser surgery applications, six 0.75mm- diameter 90mm-length sapphire fiber tips were plastically bent by means of CO2 lasers with bending radii from 1.7mm to 3.6mm and bending angles of 15-120 degree(s). The average bending loss caused by the bending process of the tips is about 0.03dB with the minimum loss less than 0.02dB, the damage thresholds of these tips are higher than 160MW/cm2 for Nd:YAG laser pulses at 1.06micrometers and 2MW/cm2 for CuBr laser pulses at 510.6nm. Satisfactory optical properties of these sapphire fiber tips show that they are promising for use in laser surgery applications such as tissue ablations. 16
A novel fiber-optic thermal source, consisting of a Nd3+ doped fiber tip and a pure YAG crystal fiber that grown by LHPG(Laser heated pedestal growth) method, had been studied for laser surgery applications. Pumping by a CuBr laser, the thermal source generated temperature as high as 520 degree(s)C with stability of better than 4% (absolute temperature), the thermal response of the thermal source is about 3 seconds, and the temperature can be controlled by the input laser power. Experimental results show that, the fiber-optic thermal source is promising for laser surgery applications, and is expected to avoid overheating that likely to happen in direct laser tissue interactions.
Large-bore polystyrene-Ag-coated hollow glass waveguides with inner diameters of 3.6 mm and 8 mm had been produced for transmission of solar energy, the waveguides were fabricated by liquid phase deposition method, over 6 W focused solar radiation had been transmitted by an 8mm-thick 1m-long waveguide. Although transmission losses of the waveguides were high at the moment, they were promising for transmitting high- power solar energy after being improved.
This paper concentrates on the performance stability of sapphire fiber thermometer probe and the development of an alumina fiber cladding durable under high temperature and of high transparency. Analysis was made on the performance stability of sapphire fiber probe under high temperature when using different kinds of protective tube. It is found that direct contacting of the fiber with protective tubes is the most important factor that result in performance deterioration and the dirtying of evaporating substance is another important factor. Then it is applicable to design a suitable protective tube to assure the performance stability.
A new designed sapphire fiber-optic sensor, aiming to improve the performance of the traditional sapphire fiber- optic sensors for high temperature measurement, is provided in this paper, in this system, an additional U-shaped sapphire fiber is used together with a modulated LED reference signal. It has advantages of both high sensibility of the single-band type and the high stability of the dual- band system. It is a good method to improve the stability of the traditional radiation based sapphire fiber-optic sensor without sacrificing high sensibility.
The sensor head of the high-temperature fiber optic sensor plays a very important role in the measurement, it directly determines the frequency response characteristics, distinguishability and other features of the sensor, so the detailed and advanced research is necessary. In this paper, the frequency response characteristics of high-temperature fiber optic sensor head are studied by means of a laser heated system, relationship between sensor head's response time and its geometrical sizes is obtained, which is in accord with theoretical evaluations.
Theoretical analysis of the relation between amplitude of photothermal signals in nonuniform samples and modulating frequency used photothermal deflection spectroscopy (PDS) shows: compared with the curve of logarithmic amplitude of PDS signals of uniform substrate materials, the curve inclination of logarithmic amplitude via frequency omega increases when absorptive index of p-layer is smaller than that of n-layer in nonuniform Gap:N, otherwise, it decreases. By comparing this result with experimental results, we can obtain concentration distribution of nitrogen in p-layer and n-layer of Gap:N. Meanwhile, absorption spectra under different modulating frequencies at room temperature have been measured, and the possibility of measurement of optical and thermal characteristics at different depth of nonuniform samples under various modulating frequencies is indicated. In a word, this paper suggests a method to measure internal characteristics of nonuniform materials without damage, which has been verified in experiment.
A new kind of SCF (Single Crystal Fiber), Y-ZrO2 (Y2O3 stabilized ZrO2) SCF has been provided to substitute for Al2O3 SCF in high-temperature optic sensors, comparing for Al2O3 SCF, it has a higher melting point which can enable sensors to be operated at a higher temperature (> 2000 degree(s)C). In this paper, Y-ZrO2 SCF is grown by means of Laser Heated Pedestal Growth method, its growth and optical characteristics have been studied, fibers with diameters of 300 - 500 micrometers and lengths of 100 - 250 mm have been obtained. Experimental results show that these Y-ZrO2 SCFs can be possibly used in high-temperature optic sensors.
Accurate measurement of the optical loss of SCF is helpful to selecting high quality SCF and determining optimal growth conditions. By means of the SCF optic characteristic measurement set which made by ourselves, we have measured absorption spectra, fluorescence spectra and positional scatter spectra of Al2O3, Nd:YAG and other kinds of fibers, and some important factors which cause scattering loss have been researched. These factors include undulation of diameter, microdefects and surface wrappers of the fiber. In addition, the control of the growth conditions and improvement of the fiber qualities are mentioned.
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