Atsushi Yarai

A new technique for enhancing the measurement resolution of a fiber Bragg grating (FBG) temperature sensor is proposed. This technique uses a cavity ring-down approach to amplify optical intensity by accumulating unremarkable intensity changes. A wavelength-stabilized optical pulse with a width of 10 ns rotates several times inside an optical fiber loop that contains a FBG sensor. In other words, the loop system functions as an integrator of slight intensity transition. A temperature resolution of at least 0.02 °C was achieved at 20.0 °C. Resolution with this technique is at least five times higher than previous techniques.

A new non-destructive evaluation technique for surface mounted capacitor (SMC) based on photothermal frequency modulation is proposed. The slight change of capacitance by a thermal wave makes an oscillation frequency modulate, enabling detection by frequency demodulation. Defective samples were created artificially in our laboratory from a normal SMC by loading mechanical stress to it. It is confirmed that the capacitance change ratio decreased as the loaded stress increased. The phenomena are discussed qualitatively from the viewpoint of the reflectance of thermal wave caused by an air layer.

A new differential cavity ring-down spectroscopy (CRDS) detection technique is proposed. This technique features the wavelength differential detection by alternating two side bands generated by an optical single side band modulator. This is useful for detecting media with extremely narrow absorption spectra. The detection of carbon dioxide gas with a 1.572-mu m wavelength is attempted at the wavelength difference of 0.16 mu m. The differential CRDS signal on the oscilloscope display is distinctly observed when the dioxide gas pressure exceeded 200 Pa. (C) 2015 The Authors. Published by Elsevier Ltd.

This paper proposes a new technique for quickly measuring the viscosity of liquid with pico-liter volume based on Doppler shift by elastic wave in liquid induced by periodical photothermal expansion. Our sensing head for the sample liquids is composed of two spherical lensed fibers so that the volume of the sample liquid is no more than the order of pico-liter. The time resolved frequency of the Doppler shift is measured, and the viscosity is estimated from the shape of the curve. The experimental result was demonstrated, and the estimation of the "relative viscosity" by using a seismic system is attempted.

This article proposes an in-situ measurement method of the thermal diffusivity and the thermal conductivity of thin film during deposition inside the vacuum chamber based on a photothermal reflectance technique. Our new technique is simple and reasonable, although no adequate conventional instrument exists. A 2.5-mm-diameter FC-ferrule made from zirconium is contained in a vacuum and used to measure the thermoproperties of the thin film deposited on the end surface of the ferrule. The dependence of the thermal diffusivity and the thermal conductivity of the palladium film on the film thickness are demonstrated. These values are shown to be asymptotically adjacent to those of the bulk since the film is thicker. It is also described that the measurement error is increased, since the optical transmissivity of the thin film becomes to be high when the film is too thin.

This article proposes an optical fiber hydrogen (H) sensor based on photothermal reflectance [hereinafter modulated optical reflectance (MOR)] technique. Our H(2) sensor is based on a technique that detects the changes of MOR signals in palladium film, which is widely known to absorb H(2) gas. The sensor element is a palladium film deposited on a 2.5-mm-diameter FC-ferrule made from zirconium to realize the optical fiber sensor. Our recently developed "laptop" MOR instrument assembled with optical fiber components is applied to this technique. Thus, an extremely compact photothermal H(2) gas sensor system can be constructed. We certified that our technique has hypersensitive less than 1% with a concentration of H(2) gas and also demonstrated that the response time is approximately 5 seconds when the sensor head is filled with H(2) gas.

A multi-functional sensor based on a pulsed photoacoustic (PA) technique is proposed. This sensor can simultaneously measure the chemical concentration, temperature, and flow speed of liquid using only one fiber Bragg grating. The concentration and flow speed of a sample liquid can be estimated by analyzing both the "shape" and the "amplitude" of a PA waveform observed with an oscilloscope. The DC voltage, which is biased on the PA signal, also depends only on the static temperature of the liquid. Our metal-free sensor head, which is basically maintenance free, is substantially noise proof without electro-magnetical shielding. We demonstrated that our proposed sensor functions as a multi-functional sensor within a concentration range from 1000 to 10,000 ppm with a Rhodamine 6G solution. (C) 2009 Published by Elsevier Ltd.

An optical fiber probe for humidity monitoring based on a new reflection sensing technique for detecting thermal lens is proposed This sensing probe is based on the technique, which is originally developed in our earlier work Due to the reflection defection; individual fibers for forwarding and returning optical beams are not necessary, making the multiple sensing of several monitoring points to be easy. The measurement system for our new sensor is described, and humidity sensing characteristics are demonstrated.

Methods to achieve ultra-high-sensitivity measurement of metal thin-film thickness by photothermal mirage detection are proposed. These methods apply 1), sample surface coating and 2) a semi-cylindrical lens for pumping beam focusing. Consequently, successful measurements are taken for thicknesses of less than several hundred nm. The semi-cylindrical lens plays a critical role in the measurements, and the sample surface coating significantly improves measurement sensitivity to about 2.5 times higher than the results for samples without coating.

An optical fiber humidity sensor based on a thermal lens detection technique is proposed. This sensor allows us to measure humidity without special treatment of the clad of optical fiber in spite of a certain optical absorption path-length under 50µm. Because of this, we believe that our humidity sensor will have a long lifetime satisfying uses with severe requirements. Our sensor is also extremely small (e.g., 20mm in length and 5mm in diameter) and robust. It is also confirmed that our optical fiber humidity sensor can measure absolute humidity.

An effective and stable method to enhance the sensitivity for photothermal measurements was proposed. The effectiveness of our method was proven by measuring the thickness of metal thin-film samples. It features sample surface coatings to improve the pumping beam's absorption of the sample as well as radiation efficiencies from the sample. Two solutions, graphite and dye, were used as coating materials. As metal thin-film samples, aluminum films formed by vacuum deposition on sapphire substrate were used. The inclination of the line showing the relationship between detection signals to the aluminum film thickness (i.e. measurement sensitivity) with coatings is more than three times larger than without coatings. Resolution on the order of a few nm film thickness measurement was achieved by using this coating method for metal films whose thickness ranged from 0 to several hundred mm. The effectiveness of our method functions for other photothermal measurements.

The relationships between the various shapes in a Buddhist temple bell and the corresponding acoustic characteristics are clarified mainly by Finite Element Method (FEM) analysis. First, we show that the cross-sectional shapes of the "komazume" (lower part, which is slightly thicker than the rest of the bell) have high correlations to the vibration modes as well as to the vibration positions. As a result, the komazume has a large influence on the bell's acoustic characteristics. The beat sound is another important factor in the bell's overall sound, but its origin has not yet been fully clarified. Since the beat sound is assumed to be generated by the bell's formal or material asymmetrical factors, the influences of a few formal asymmetrical factors on the beat sound are investigated. First, the beat sound obtained by simulation using miniaturized bells is confirmed to closely match the experimental results. Then, it is shown that the "doza" (part where the bell is struck), which functions as a formal asymmetrical factor when the bell vibrates, is highly related to the beat sound. It is also clarified that the "obi" (perpendicular stripe pattern on the bell's surface) also slightly influences the beat sound.

This paper describes the performance of an optical fiber gas sensor based on thermal lens (TL) spectroscopy. This sensor features an optical absorption path whose length is extremely short compared with other optical spectroscopic techniques. A prototype of a robust sensing head is proposed whose sensitivity is demonstrated. An all-fiber TL gas sensor system built with a fiber ring laser is proposed as a method of dramatic sensitivity enhancement, and its capability is certified.

A method to enhance the sensitivity of photothermal radiometry (PTR) measurement was proposed. This method features multireflection of a pumping laser beam on the inside of a liquid vessel. This is a new, effective, and extremely simple method for measurement of a lowly thermally emissive sample. Our experimental data were compared to that of the conventional PTR method. A dye (IR-140) was used as the impurity in a solvent, and we ascertained superior detectablity of the impurity in the sample vessel-a 12 mm diameter integrating sphere-to that of the conventional method by more than a magnitude of one. A theoretical "limit of detection" value for our technique was estimated to be 0.7 ppb when the power of the pumping beam was 500 mW. (C) 2003 American Institute of Physics.

This paper describes how to achieve an adjustment-free mirage instrument with components assembled in a compact package. The deflection angle amplifier that we previously developed for mirage detection is assembled in this instrument. The features of this instrument are: 1) no optical table, 2) adjustment-free design, and 3) an extremely compact structure for mobile use. The volume of our compact mirage instrument is at least 1/25 times that of the conventional mirage instrument. The capability of the compact mirage instrument is also verified. The results show good potential for using the technique for apparatus commercialization.

This article proposes a method to utilize a commercially available high-power pulsed-laser diode (HPLD) in time sharing to induce damage by laser irradiation and to detect the threshold level of the damage. This method features: (1) damage inducement when the HPLD is under the pulsed high-power laser oscillation mode, and (2) surface reflectivity measurement when the HPLD is under the cw light-emitted-diode illumination mode. This technique allows nondestructive measurement of the damage threshold level with high sensitivity, in spite of its extremely simple approach. Accordingly, the technique is appropriate for real-time inspection in a manufacturing process. The practical data for determining the damage threshold are shown. The damage threshold for aluminum (JIS A5052) was estimated to be approximately 62 mJ/mm(2). The capability of our proposed method is shown compared with that of more conventional methods. The advantages of our technique are described. (C) 2001 American Institute of Physics.

A new Bragg reflection detection instrument for an ultrasonic measurement that is based on self-mixing effects due to an optical feedback for a laser diode is proposed. This instrument allows measurement of the velocity and/or absorption of ultrasound in an ultrahigh frequency (UHF) range, in spite of its extremely simple configuration. The velocity of an ultrasonic wave in water was measured in an UHF range. The velocity was within 5% of the velocity in the previous results. (C) 2001 American Institute of Physics.

A novel and structurally very simple ultra-high-sensitivity all-fiber thermal lens instrument has been developed. In the instrument, a fiber-coupler unifies the pumping and detecting light beams from different laser diodes, which are integrated with different optical fibers. These light beams are emitted from the hemispherical end of one fiber and efficiently captured by another hemispherically-ended fiber. The advantages of this new instrumentis configuration include not needing to adjust various optical apparatuses, very simple structure and easy mobility for commercialization, very small sample quantity, and a high signal-to-noise ratio because it does not use a pinhole. The detection limit of the pigment concentration (IR-140) was about 20 ppb, which will be improved by using solutions that have other light absorption wavelengths or a pumping laser beam with a higher output.

When using glasses as musical instruments (also referred to as the glass-harp), the pitch needs to be minutely adjusted. That is, it needs to be tuned. A wide adjustment range has been achieved by a new method that locally shaves the bottom of the cup of each vessel circumferencially. The pitch decreased in proportion to the quantity of glass shaved. This relationship between the quantity of glass shaved and the change in pitch was clarified both experimentally and analytically by Finite Element Method (FEM) analysis. The amount of pitch change accompanied with the shaving method is occasionally limited by the vessel shape. In such cases, pitch can be changed by filling wine glasses with specific quantities of water, a well-known conventional tuning method. This auxiliary method has been measured experimentally and analyzed by FEM to clarify the relationship between the water quantity in vessels and the amount of pitch change. A harmonics analysis was also performed. Using these procedures, prediction of vibration frequency could be done in advance, which means a desired pitch can be easily obtained.

A new Bragg reflection detection instrument for an ultrasonic measurement that is based OH optical feedback effects for a laser diode (LD) was proposed. This instrument allows us to measure the velocity and/or absorption of ultrasonic in an ultra high frequency (UHF) range, in spite of its extremely simple configuration. The velocity and absorption factor of an ultrasonic in water were measured in an UHF range. The velocity was in reasonable agreement with the velocity in the reference.

An optimum design method has been developed for our new beam deflection angle amplification technique for mirage detection with a cylindrical reflection mirror. This design method is based on an analysis of the quantitative influence of the probe beam expansion. Further more, we have clarified the reason for the peak tendency shown in the dependency of the mirage signal amplitude on total probe beam path length.

Artificially made narrow vertical cracks of various widths were detected and quantitatively evaluated using a photothermal beam deflection method (mirage detection method). In order to evaluate the influence of thermal characteristics, two test sample materials whose thermal conductivities largely differed from each other were chosen. Experimental investigations clarified that the vertical ''signal amplitude depth'' is proportionally related to crack widths when the crack widths are of almost the same order as the pumping beam spot size of the material.

This paper describes a new method for measuring the thermal diffusivity of liquid samples through inverse photopyroelectric detection measurement. It is proven that thermal diffusivity values dan be obtained with both amplitude-signals and phase signals in a low frequency range. At around 3 He, these values are found to correspond to their theoretical values. We experimentally prove that the piezoelectric effect, which films are known to exhibit, is negligibly small compared with the photopyroelectric effect.

In an earlier paper, we showed that a high-fidelity tomographic imaging reconstruction method for photopyroelectric thermal wave detection equipment could be achieved through a high signal-to-noise ratio detection technique and through a new calculation algorithm. However, reproducibility of the experiments was inadequate, resulting in reconstructed tomographic imagings that were often significantly deteriorated. Several subsequent investigations revealed that the adhesion method used to attach pyroelectric film to test samples influences tomographic imaging fidelity and reproducibility. This paper describes the effects of two experimental conditions: 1) adhesion methods of test samples and, 2) humidity conditions on tomographic imaging fidelity and reproducibility. PVDF (polyvinylidene fluoride) and LiTa03 were used as the pyroelectric films. Silver powdered paste as well as silicon grease were chosen for the investigations as the adhesives. Investigations, demonstrated that: 1) in the case of PVDF film, silicon grease is clearly superior for realizing high-fidelity imagings and high reproducibility and, 2) humidity conditions are very critical in the case of LiTaO3. © 1995, Taylor &amp Francis Group, LLC. All rights reserved.

This paper describes a high-fidelity tomographic imaging reconstruction method for photopyroelectric thermal wave detection equipment. The high-fidelity is achieved through a high signal-to-noise ratio detection technique and through a new calculation algorithm. A very-small-diameter detection metal probe tip can be used due to the high signal-to-noise ratio detection technique, increasing the spatial resolution. The calculation algorithm which considers thermal wave transmission paths along with defects achieves the high-fidelity tomographic imaging reconstruction.

A new technique has been developed for amplification of the photothermal beam deflection angle for mirage detection. This technique, based an a very simple operating principle, uses a cylindrical reflection mirror. The use of a new amplifier provided a signal-to-noise ratio approximately 10 times that obtained without the amplifier for equipment of the same size. By using the new amplifier, a mirage signal was obtained when a transistor array processed on a silicon wafer was measured.

A new technique for amplification of photothermal beam deflection angle was developed. This technique. based on a very simple operation principle, uses a cylindrical reflection mirror for a new deflection amplifier. This paper presents experimental data and the results of theoretical analysis. Mirage signal amplification of more than 35 times was achieved. Additionally, mirage signals obtained with this new deflection-angle amplifier did not significantly decrease even in the high modulation frequency range.

This paper describes high signal-to-noise ratio and high resolution detection techniques for photopryoelectric thermal wave imaging equipment. A high signal-to-noise ratio technique is mainly achieved by realizing a very low-noise first stage pre-amplifier and its assembling technique and by the use of very low noise second stage amplifier. High resolution detection is achieved by using a metal probe tip with a very small diameter. This equipment could be achieved mainly due to the high signal-to-noise technique and the finite element method (FEM) analysis. A high-fidelity tomographic image reconstructed by these techniques is also shown.