Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/TCI.2022.3183396

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Other Link： https://dblp.uni-trier.de/db/journals/tci/tci8.html#KobayashiFYONHC22

Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

In this study, a novel, to the best of our knowledge, external-cavity laser diode (ECLD) without a diffraction grating is proposed and demonstrated. In the proposed configuration, an acousto-optic deflector (AOD) acts not only as a deflector but also as a diffraction grating. Thus, the AOD functions as a wavelength-selective device, which helps improve the overall performance of the ECLD. In fact, the proposed configuration realizes a wide range of wavelength scanning with a simple configuration, highly efficient optical feedback, and a steady optical resonator with a constant cavity length. We confirm that the wavelengths scanned with the proposed ECLD agree well with theoretical calculations. The scanning range and maximum frequency response reached approximately 60 nm and 50 kHz, respectively. Moreover, reproducible measurements of the three-dimensional thickness distribution of a thin glass plate indicates that the proposed ECLD can be used for optical coherence tomography imaging systems.

DOI： 10.1364/ol.454021

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Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

A phase refractive index is measured directly from an unwrapped spectral phase distribution whose ambiguity is determined by fitting the spectral phase distribution with functions based on Cauchy’s equation. The phase refractive index of a quartz glass with 20 µm thickness is measured exactly from three spectral phase distributions detected in two different configurations of a spectrally resolved interferometer. Since there is a high possibility that the ambiguity cannot be correctly determined when there is a large difference between a function of the real refractive index and Cauchy’s equation, characteristics of the fitting are examined.

DOI： 10.1364/ao.438267

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Publishing type：Research paper (international conference proceedings)   Publisher：SPIE  

DOI： 10.1117/12.2606987

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Language：English  

DOI： 10.1038/s41540-021-00197-3

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Publishing type：Research paper (international conference proceedings)   Publisher：SPIE  

DOI： 10.1117/12.2601233

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Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

DOI： 10.1088/2631-8695/ac09d8

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Other Link： https://iopscience.iop.org/article/10.1088/2631-8695/ac09d8/pdf

Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

In order to measure exactly a large thickness of glass plate with a spectrally resolved interferometer using a spectral analyzer with a low resolution of 0.5 nm and a supercontinuum light source with a large bandwidth of about 300 nm, a new measurement method with a spectrally resolved interferometer is proposed where a variable signal position is generated by moving a reference surface with a piezoelectric stage. It is made clear how to decide the signal position by analyzing amplitude distribution of Fourier transform of the interference signal. Through four-step measurement by using four different optical configurations with four different signal positions the thickness of glass plate can be obtained from a slope of a spectral phase distribution which does not contain the refractive index of glass plate. A small measurement error of 50 nm is achieved in measuring 1 mm thickness of a glass plate.

DOI： 10.1364/osac.417141

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Authorship：Lead author,　Last author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：The Optical Society  

DOI： 10.1364/oe.425972

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier Ltd  

Utilization of a complex-valued interference signal (CVIS) in a white-light scanning interferometer is a powerful method to measure exactly surface positions of a glass plate. In this method a zero phase position nearest to a maximum amplitude position in the CVIS is a measured surface position. Proportional coefficients that relate the maximum amplitude position and the zero phase position of a rear surface to the thickness of a glass plate are required for measuring exactly the thickness. These coefficients and periodical position jump occurring on the zero phase position are investigated by simulations. The number of occurrences of the position jump is calculated from the maximum amplitude and zero phase positions. Shape measurement of a glass plate with about 20 μm-thickness is achieved by using the coefficients and the number of occurrences of the position jump.

DOI： 10.1016/j.optlaseng.2020.106505

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

<title>Abstract</title>Sound evokes sub-nanoscale vibration within the sensory epithelium. The epithelium contains not only immotile cells but also contractile outer hair cells (OHCs) that actively shrink and elongate synchronously with the sound. However, the in vivo motion of OHCs has remained undetermined. The aim of this work is to perform high-resolution and -accuracy vibrometry in live guinea pigs with an SC-introduced spectral-domain optical coherence tomography system (SD-OCT). In this study, to reveal the effective contribution of SC source in the recording of the low reflective materials with the short total acquisition time, we compare the performances of the SC-introduced SD-OCT (SCSD-OCT) to that of the conventional SD-OCT. As inanimate comparison objects, we record a mirror, a piezo actuator, and glass windows. For the measurements in biological materials, we use in/ex vivo guinea pig cochleae. Our study achieved the optimization of a SD-OCT system for high-resolution in vivo vibrometry in the cochlear sensory epithelium, termed the organ of Corti, in mammalian cochlea. By introducing a supercontinuum (SC) light source and reducing the total acquisition time, we improve the axial resolution and overcome the difficulty in recording the low reflective material in the presence of biological noise. The high power of the SC source enables the system to achieve a spatial resolution of 1.72 ± 0.00 μm on a mirror and reducing the total acquisition time contributes to the high spatial accuracy of sub-nanoscale vibrometry. Our findings reveal the vibrations at the apical/basal region of OHCs and the extracellular matrix, basilar membrane.

DOI： 10.1007/s10043-021-00654-8

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Other Link： https://link.springer.com/article/10.1007/s10043-021-00654-8/fulltext.html

Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER HEIDELBERG  

In mammals, audition is triggered by travelling waves that are evoked by acoustic stimuli in the cochlear partition, a structure containing sensory hair cells and a basilar membrane. When the cochlea is stimulated by a pure tone of low frequency, a static offset occurs in the vibration in the apical turn. In the high-frequency region at the cochlear base, multi-tone stimuli induce a quadratic distortion product in the vibrations that suggests the presence of an offset. However, vibrations below 100 Hz, including a static offset, have not been directly measured there. We therefore constructed an interferometer for detecting motion at low frequencies including 0 Hz. We applied the interferometer to record vibrations from the cochlear base of guinea pigs in response to pure tones. When the animals were exposed to sound at an intensity of 70 dB or higher, we recorded a static offset of the sinusoidally vibrating cochlear partition by more than 1 nm towards the scala vestibuli. The offset's magnitude grew monotonically as the stimuli intensified. When stimulus frequency was varied, the response peaked around the best frequency, the frequency that maximised the vibration amplitude at threshold sound pressure. These characteristics are consistent with those found in the low-frequency region and are therefore likely common across the cochlea. The offset diminished markedly when the somatic motility of mechanosensitive outer hair cells, the force-generating machinery that amplifies the sinusoidal vibrations, was pharmacologically blocked. Therefore, the partition offset appears to be linked to the electromotile contraction of outer hair cells.

DOI： 10.1007/s00424-020-02373-6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPIE  

Vibration distribution measurements using interferometers have been evaluated by many researchers. We have used a laser diode interferometer to measure two-dimensional vibration distributions using a combination of phase-shifting and downsampling techniques. Our system simultaneously measured nanometer-order sinusoidal vibration distributions of two objects vibrating at different frequencies, using a charged coupled device camera with a slower frame rate than either of the two harmonic frequencies being measured. This is a cost-efficient and compact system capable of successfully measuring high-frequency vibrations without requiring expensive high-frame-rate cameras.

DOI： 10.1117/1.OE.59.3.034112

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Language：English   Publishing type：Research paper (scientific journal)  

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE  

The cochlea of the inner ear transduces sound energy into electrical signals that are essential for audition. This transduction is processed in nano-scale vibration of the cochlear sensory epithelia. In mammals, the epithelia contain various cells and structures: inner hair cells, outer hair cells (OHCs), Deiters' cells, basilar membrane, reticular lamina, etc. The sound elicits vibration in all these constituents. Among them, only OHCs cell body actively and periodically changes in length in association with the vibration. The unique mechanical activity of OHCs modifies the sound elicited vibration in the epithelia with a feedback mechanism. Although the modification is considered to critically contribute to the high sensitivity and sharp tuning in hearing through sensory IHCs, the real motion of OHCs remains uncertain. Vibrometrical studies of cochlear mechanics has revealed important vibration of the cell bodies involving the epithelia. However, difference in vibration pattern of the apical and basal ends of the cell has remain uncertain due to low spatial resolution of the system and low reflectivity of the cells. We performed a spectral domain OCT (SD-OCT) vibrometry by using the modified commercial SD-OCT system. Because the broad spectral bandwidth and strong power of the light source improve a performance of OCT systems in both of imaging and vibrometry, we introduced a supercontinuum light source into the commercial system. Our system achieved cellular-level tomographic imaging and subnano-scale vibration measurement in the transparent epithelia with the recording time of 100 ms in in vivo animal.

DOI： 10.1117/12.2539303

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE  

The sensory epithelium in the cochlea of the inner ear transduces sound energy into electrical signals, which are essential for a neurotransmitter release. Nevertheless, the in vivo behavior and correlation of the dynamics among each layer remains unclear. To assess the wide-range motion within a living organism, we develop multifrequency-swept optical coherence microscopic vibrometry (MS-OCMV). This method employed the wide-filed heterodyne interferometric vibration measurement technique, which can capture the full-field vibration amplitude and phase distributions with optical coherence microscopy (OCM). In this system, we newly introduced the broadband multifrequency supercontinuum (SC) as a light source. Manipulating the frequency interval of the multifrequency SC can determine the depth of the region of interest where the two-dimensional biological vibration distributed as well as to conduct 3D tomographic volumetric imaging. In this manuscript, we demonstrate the accuracy of two-dimensional surface vibration measurement and the underlying performance of 3D tomographic imaging. We report the measurement results of vibrational motions induced by sound stimuli on the sensory epithelium of a living guinea pig using MS-OCMV.

DOI： 10.1117/12.2545362

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPIE  

We propose an acousto-optically tuned external-cavity laser diode (ECLD) based on the Littrow configuration to expand the wavelength-scanning range and eliminate mechanical motions. The scanning range and tuning rate of our prototype were 68.85 nm and more than 100 kHz, respectively, without any mode hop. Moreover, we implemented this improved ECLD in an optical coherence tomography imaging system and conducted a measurement of the two-dimensional thickness distribution of a thin glass plate via continuous wavelet transform analysis.

DOI： 10.1117/1.OE.58.10.104108

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/ICIP.2019.8803737

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Other Link： https://dblp.uni-trier.de/db/conf/icip/icip2019.html#FujiiYMOCONH19

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1364/BOE.10.003317

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1364/AO.58.003548

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：Japanese   Publishing type：Research paper (conference, symposium, etc.)  

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Language：Japanese   Publishing type：Research paper (conference, symposium, etc.)  

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Language：Japanese   Publishing type：Research paper (conference, symposium, etc.)  

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Language：Japanese   Publishing type：Research paper (conference, symposium, etc.)  

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Language：Japanese   Publishing type：Research paper (conference, symposium, etc.)  

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：Japanese   Publishing type：Research paper (conference, symposium, etc.)  

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE  

In this study, we propose a spectral-domain optical coherence tomography (OCT) method that employs pulse irradiation to capture an interferogram and continuous wavelet transform (CWT) for data analysis. The pulse irradiation technique enables obtaining an interferogram with a high signal-to-noise ratio, and the CWT allows accurate analysis of a nonstationary signal. Experimental results obtained using a cover glass of thickness 150 μm confirmed that the repetitive measurement error was 0.32 μm. However, this error was reduced to 0.23 μm when pulse irradiation was applied for image capture.

DOI： 10.1117/12.2534005

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE  

Full-field swept source optical coherence tomography using an acousto-optically tuned external-cavity laser diode is proposed and demonstrated. The wavelength of the light source is controlled by diffraction in an acousto-optic modulator, instead of by mechanical motion. This allows tuning rates of over 100 kHz without mode hopping. For phase analysis, we used a continuous wavelet transform. This allows for accurate signal processing that reduces calculation error caused by the nonstationary features of an interference signal. We measured the two-dimensional thickness distribution of a thin glass plate. The average thickness of the glass plate was found to be 152.2 μm.

DOI： 10.1117/12.2547786

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Language：Japanese   Publishing type：Research paper (conference, symposium, etc.)  

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Language：Japanese   Publishing type：Research paper (conference, symposium, etc.)  

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Language：Japanese   Publishing type：Research paper (conference, symposium, etc.)  

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Language：Japanese   Publishing type：Research paper (conference, symposium, etc.)  

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Language：Japanese   Publishing type：Research paper (conference, symposium, etc.)  

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Language：Japanese   Publishing type：Research paper (conference, symposium, etc.)  

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Language：Japanese   Publishing type：Research paper (conference, symposium, etc.)  

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OSA - The Optical Society  

A new signal processing is proposed in which the dispersion phase is not subtracted from the detected spectral phase distribution. The linear and bias components in the spectral phase distribution are used to calculate the complex-valued interference signal (CVIS). The simulations verify that the dispersion phase generates an inclination in the measured surface profile along one direction in which the magnitude of the dispersion phase changes linearly. The simulations also show that the position of zero phase nearest the position of amplitude maximum in the CVIS almost does not change due to the bias component, although the random phase noise contained in the interference signal changes the slope of the linear component. Measured surface profiles show that the new signal processing achieves highly accurate measurement by the CVIS.

DOI： 10.1364/AO.57.000894

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：OSA - The Optical Society  

An acousto-optically tuned external-cavity laser diode using Littrow configuration is proposed and its characteristics are investigated. The maximum wavelength scanning range of the prototype was 45 nm. Furthermore, thickness of a glass plate is measured using the developed light source and continuous wavelet transform.

DOI： 10.1364/FIO.2018.FM3D.3

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：OSA - The Optical Society  

An optical coherence tomography (OCT) has attracted considerable attention as a method of precise measurement. A swept source that possesses an external resonator using a diffraction grating and an acousto-optic deflector (AOD) was developed for swept-source (SS) OCT. We have conducted numerical simulations and reconfigured the external-cavity based on the results. We confirmed that the wavelength scanning range was expanded from 22.30[nm] to 66.56[nm].

DOI： 10.1364/OPJ.2018.31aAJ8

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE  

Advanced signal processing is required to make exact measurements with nanometer order accuracy. A complex-valued interference signal of a white-light scanning interferometer (WLSI) obtained from the detected real-valued interference signal through Fourier transform provides an accurate position of an object surface with an error less than 4 nm. Moreover, the sampling points of the interference signal of the WLSI detected with a camera are corrected with the measured scanning positions which are obtained from an interference signal detected by using an optical band-pass filter. This correction method provides more accurate surface profiles with an error less than 2 nm. In experiments a surface profile with a step shape of 3 μm-width is measured accurately.

DOI： 10.1117/12.2502790

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：FRONTIERS MEDIA SA  

Light-gated ion channels and transporters have been applied to a broad array of excitable cells including neurons, cardiac myocytes, skeletal muscle cells and pancreatic b -cells in an organism to clarify their physiological and pathological roles. Nonetheless, among nonexcitable cells, only glial cells have been studied in vivo by this approach. Here, by optogenetic stimulation of a different nonexcitable cell type in the cochlea of the inner ear, we induce and control hearing loss. To our knowledge, deafness animal models using optogenetics have not yet been established. Analysis of transgenic mice expressing channelrhodopsin-2 (ChR2) induced by an oligodendrocyte-specific promoter identified this channel in nonglial cells-melanocytes-of an epithelial-like tissue in the cochlea. The membrane potential of these cells underlies a highly positive potential in a K C -rich extracellular solution, endolymph; this electrical property is essential for hearing. Illumination of the cochlea to activate ChR2 and depolarize the melanocytes significantly impaired hearing within a few minutes, accompanied by a reduction in the endolymphatic potential. After cessation of the illumination, the hearing thresholds and potential returned to baseline during several minutes. These responses were replicable multiple times. ChR2 was also expressed in cochlear glial cells surrounding the neuronal components, but slight neural activation caused by the optical stimulation was unlikely to be involved in the hearing impairment. The acuteonset, reversible and repeatable phenotype, which is inaccessible to conventional gene-targeting and pharmacological approaches, seems to at least partially resemble the symptom in a population of patients with sensorineural hearing loss. Taken together, this mouse line may not only broaden applications of optogenetics but also contribute to the progress of translational research on deafness.

DOI： 10.3389/fnmol.2017.00300

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

This study proposes a signal analysis technique that uses continuous wavelet transform for signal processing in a spectral domain optical coherence tomography system. Our method enables us to calculate the optical path difference simply by taking advantage of the fact that the product of the phase and wavelength becomes constant. Experimental results obtained using a pair of gauge blocks with a thickness difference of 40 mu m confirm that the repetitive measurement accuracy was 65.1 nm. A demonstration of the three-dimensional surface profile measurement indicates that the rms measurement error is 0.17 mu m.

DOI： 10.1016/j.optcom.2017.03.053

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

Because conventional laser Doppler vibrometry or Doppler optical coherence tomography require mechanical scanning probes that cannot simultaneously measure the wide-range dynamics of bio-tissues, a multifrequency-swept optical coherence microscopy with wide-field heterodyne detection technique was developed. A 1024 x 1024 x 2000 voxel volume was acquired with an axial resolution of similar to 1.8 mu m and an acquisition speed of 2 s. Vibration measurements at 10 kHz were performed over a wide field of view. Wide-field tomographic vibration measurements of a mouse tympanic membrane are demonstrated to illustrate the applicability of this method to live animals.(C) 2017 Optical Society of America

DOI： 10.1364/BOE.8.000608

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Language：English   Publishing type：Research paper (scientific journal)  

The cochlear lateral wall-an epithelial-like tissue comprising inner and outer layers-maintains +80 mV in endolymph. This endocochlear potential supports hearing and represents the sum of all membrane potentials across apical and basolateral surfaces of both layers. The apical surfaces are governed by K+ equilibrium potentials. Underlying extracellular and intracellular [K+] is likely controlled by the "circulation current," which crosses the two layers and unidirectionally flows throughout the cochlea. This idea was conceptually reinforced by our computational model integrating ion channels and transporters; however, contribution of the outer layer's basolateral surface remains unclear. Recent experiments showed that this basolateral surface transports K+ using Na+, K+-ATPases and an unusual characteristic of greater permeability to Na+ than to other ions. To determine whether and how these machineries are involved in the circulation current, we used an in silico approach. In our updated model, the outer layer's basolateral surface was provided with only Na+, K+-ATPases, Na+ conductance, and leak conductance. Under normal conditions, the circulation current was assumed to consist of K+ and be driven predominantly by Na+, K+-ATPases. The model replicated the experimentally measured electrochemical properties in all compartments of the lateral wall, and endocochlear potential, under normal conditions and during blocking of Na+, K+-ATPases. Therefore, the circulation current across the outer layer's basolateral surface depends primarily on the three ion transport mechanisms. During the blockage, the reduced circulation current partially consisted of transiently evoked Na+ flow via the two conductances. This work defines the comprehensive system driving the circulation current.

DOI： 10.1038/s41540-017-0025-0

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：OSA - The Optical Society  

A technique for vibration distribution measurement using a combination of phaseshifting interferometry and the down-sampling technique is proposed and demonstrated. Our system is capable of high-speed vibration measurements and does not require expensive highspeed cameras.

DOI： 10.1364/OFT.2017.OW4B.4

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

Eukaryotic cells exhibit negative resting membrane potential (RMP) owing to the high K+ permeability of the plasma membrane and the asymmetric [K+] between the extracellular and intracellular compartments. However, cochlear fibrocytes, which comprise the basolateral surface of a multi-layer epithelial-like tissue, exhibit a RMP of +5 to +12 mV in vivo. This positive RMP is critical for the formation of an endocochlear potential (EP) of +80 mV in a K+-rich extracellular fluid, endolymph. The epithelial-like tissue bathes fibrocytes in a regular extracellular fluid, perilymph, and apically faces the endolymph. The EP, which is essential for hearing, represents the potential difference across the tissue. Using in vivo electrophysiological approaches, we describe a potential mechanism underlying the unusual RMP of guinea pig fibrocytes. The RMP was + 9.0 +/- 3.7 mV when fibrocytes were exposed to an artificial control perilymph (n = 28 cochleae). Perilymphatic perfusion of a solution containing low [Na+] (1 mM) markedly hyperpolarized the RMP to -31.1 +/- 11.2 mV (n = 10; p &lt; 0.0001 versus the control, TukeyKramer test after one-way ANOVA). Accordingly, the EP decreased. Little change in RMP was observed when the cells were treated with a high [K+] of 30 mM (+ 10.4 +/- 2.3 mV; n= 7; p = 0.942 versus the control). During the infusion of a low [Cl-] solution (2.4 mM), the RMP moderately hyperpolarized to -0.9 +/- 3.4 mV (n = 5; p &lt; 0.01 versus the control), although the membranes, if governed by Cl-permeability, should be depolarized. These observations imply that the fibrocyte membranes are more permeable to Na+ than K+ and Cl-, and this unique profile and [Na+] gradient across the membranes contribute to the positive RMP.

DOI： 10.1007/s00424-016-1853-2

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Nanovibrations in bio-tissues play pivotal roles in organ functions, but techniques for their measurement have been premature. To address this, a wide-field vibrometry is adopted in a laser microscope. 2D surface vibration distributions of paw tissues of rats could be measured without x-y mechanical beam scanning. The measurable vibration frequency and amplitude were 10 kHz and approximately 240 nm.

DOI： 10.1109/OMN.2016.7565864

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Optical wavelength is scanned linearly with time, and an optical field produced by a thin film on a detection plane is detected with a sinusoidal phase-modulating interferometer. Scanned wavelength is detected with another interferometer. The detected multiple-optical fields are backpropagated along the optical axis in a computer by use of the detected wavelengths. An optical field is reconstructed by summing the backpropagated fields over the multiple wavelengths. The intensity and phase distributions of the reconstructed optical field provide the positions of the thin film surfaces with an accuracy of a few nanometers. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.optcom.2015.08.066

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Conventional laser Doppler vibrometry and heterodyne interferometry suffer during the simultaneous measurement of the spatial distribution of vibration parameters such as the amplitude, frequency and phase in a wide field of view. Although demand is increasing for methods that can measure vibrations over a wide field of view for a wide range of applications from industrial product inspections to biological measurements, full-field (FF) techniques for high-speed vibration measurements without a spatial scan are untapped. We propose a new method for high-speed FF vibration measurement that can easily be combined with profilometry and tomographic interferometry using a conventional CCD or CMOS camera. In principle, the measurable vibration frequency is unrestricted because the heterodyne signal produced by the modulated interferogram can be controlled to accommodate the CCD frame rate. The validity of the proposed method and the measurement accuracy of the spatial vibration amplitude were evaluated through simulations and experiments. In experiments, the spatial vibration parameters of a mirror vibrated at a frequency of 1 kHz and amplitude of approximately 5-65 nm were successfully measured with a spatial fluctuation of 3%-6.5%. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.optcom.2015.08.016

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPIE-SOC PHOTO-OPTICAL INSTRUMENTATION ENGINEERS  

A phase-shifting interferometer that uses pulse modulation and downsampling is proposed and demonstrated. Pulse modulation is a direct modulating technique that realizes a rapid, large change in the wavelength of a laser diode with a constant optical power. For instance, an interference signal of 10 kHz is observed after the injection of pulse current. However, an expensive high-speed charge coupled device (CCD) camera is required to acquire a high-speed interference signal. If we consider a periodical signal, the same waveform repeats at every period. Therefore, the signal varying with high frequency can be captured by a low-frequency sampling pulse. This signal processing technique, the so-called downsampling, enables us to acquire rapid interference signals with a standard CCD camera. The experimental results using a flat mirror and a concave mirror showed measurement errors of 10 and 28 nm in root mean square (RMS), respectively. The repeatability of the experimental result between the first and the second is 13 nm in RMS. (C) 2015 Society of Photo-Optical Instrumentation Engineers (SPIE)

DOI： 10.1117/1.OE.54.8.085107

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

Microvibrations that occur in bio-tissues are considered to play pivotal roles in organ function; however techniques for their measurement have remained underdeveloped. To address this issue, in the present study we have developed a novel optical coherence tomography (OCT) method that utilizes multifrequency swept interferometry. The OCT volume data can be acquired by sweeping the multifrequency modes produced by combining a tunable Fabry-Perot filter and an 840 nm super-luminescent diode with a bandwidth of 160 nm. The system employing the wide-field heterodyne method does not require mechanical scanning probes, which are usually incorporated in conventional Doppler OCTs and heterodyne-type interferometers. These arrangements allow obtaining not only 3D tomographic images but also various vibration parameters such as spatial amplitude, phase, and frequency, with high temporal and transverse resolutions over a wide field. Indeed, our OCT achieved the axial resolution of similar to 2.5 mu m when scanning the surface of a glass plate. Moreover, when examining a mechanically resonant multilayered bio-tissue in full-field configuration, we captured 22 nm vibrations of its internal surfaces at 1 kHz by reconstructing temporal phase variations. This so-called "multifrequency swept common-path en-face OCT" can be applied for measuring microdynamics of a variety of biological samples, thus contributing to the progress in life sciences research. (C) 2015 Optical Society of America

DOI： 10.1364/OE.23.021078

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

Microvibrations that occur in bio-tissues are considered to play pivotal roles in organ function; however techniques for their measurement have remained underdeveloped. To address this issue, in the present study we have developed a novel optical coherence tomography (OCT) method that utilizes multifrequency swept interferometry. The OCT volume data can be acquired by sweeping the multifrequency modes produced by combining a tunable Fabry-Perot filter and an 840 nm super-luminescent diode with a bandwidth of 160 nm. The system employing the wide-field heterodyne method does not require mechanical scanning probes, which are usually incorporated in conventional Doppler OCTs and heterodyne-type interferometers. These arrangements allow obtaining not only 3D tomographic images but also various vibration parameters such as spatial amplitude, phase, and frequency, with high temporal and transverse resolutions over a wide field. Indeed, our OCT achieved the axial resolution of similar to 2.5 mu m when scanning the surface of a glass plate. Moreover, when examining a mechanically resonant multilayered bio-tissue in full-field configuration, we captured 22 nm vibrations of its internal surfaces at 1 kHz by reconstructing temporal phase variations. This so-called "multifrequency swept common-path en-face OCT" can be applied for measuring microdynamics of a variety of biological samples, thus contributing to the progress in life sciences research. (C) 2015 Optical Society of America

DOI： 10.1364/OE.23.021078

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE  

Multifrequency sensing technique adopting the wide field heterodyne detection technique is demonstrated for interior surface vibration measurements in thick biological tissue. These arrangements allow obtaining not only 3D tomographic images but also various vibration parameters such as spatial amplitude, phase, and frequency, with high temporal and transverse resolutions over a wide field. The axial resolution and the accuracy of vibration amplitude measurement were estimated to be 2.5 μm and 3 nm, respectively. This wide-field tomographic sensing method can be applied for measuring microdynamics of a variety of biological samples, thus contributing to the progress in life sciences research.

DOI： 10.1117/12.2204984

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

This paper proposes 3-D OCT data denoising with nonseparable oversampled lapped transform (NSOLT), and examines the effectiveness through experiments. NSOLT is a lattice-based redundant transform which simultaneously satisfies the symmetric, real-valued and compact-support property. It is possible to apply a dictionary learning technique to the design by preparing examples. NSOLT is capable of having rational redundancy by controlling the number of channels and decimation ratio. In this study, a denoising technique is proposed by combining learned NSOLT dictionary and iterative hard thresholding (IHT), and the performance of the proposed method is evaluated for 3-D OCT data. It is verified through robust median estimator of noise variance and structural similarity index measure (SSIM) that the proposed technique yields effective denoising performance with moderate redundancy.

DOI： 10.1109/APSIPA.2015.7415402

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

We propose a swept source optical coherence tomography that uses an original external-cavity laser diode and demonstrate measurements of thickness distribution in two dimensions using this equipment. We first conducted high-speed, wide-range wavelength scanning with an external-cavity laser diode that was equipped with a special antireflection-coated laser diode working at 770 nm. Using an acousto-optic deflector enabled a tuning range and rate of 22 nm and 20 kHz, respectively, with no mechanical elements. Next, we applied this source to an optical coherence tomography and measured the two-dimensional distribution of thickness of a thin glass plate.

DOI： 10.1117/12.2193027

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

Multifrequency sensing technique adopting the wide field heterodyne detection technique is demonstrated for interior surface vibration measurements in thick biological tissue. These arrangements allow obtaining not only 3D tomographic images but also various vibration parameters such as spatial amplitude, phase, and frequency, with high temporal and transverse resolutions over a wide field. The axial resolution and the accuracy of vibration amplitude measurement were estimated to be 2.5 mu m and 3 nm, respectively. This wide-field tomographic sensing method can be applied for measuring microdynamics of a variety of biological samples, thus contributing to the progress in life sciences research.

DOI： 10.1117/12.2204984

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPIE-SOC PHOTO-OPTICAL INSTRUMENTATION ENGINEERS  

A sinusoidal wavelength scanning interferometer with the four-step phase-shift method is demonstrated for a three-dimensional profile measurement. The interference phase-shift signal generated by the sinusoidal wavelength scanning contains information about optical path difference covering a nm-mu m scale structure. The interference phase-shift signal was obtained by the four-step phase-shifting method. The sinusoidal wavelength shifting with a frequency of approximately 180 Hz and tunable range of 5.7 nm was performed by the Littman-Metcalf external resonator-type tunable laser with a center of 772.1 nm. The full-field surface profile measurement was conducted by a charge coupled device image sensor with an accuracy on a nm scale. The surface profiles of gauge blocks with a step height up to about 10 mu m were successfully measured with a surface profile irregularity of 3.8 x 10(-2) mu m. The deviations of two measured surfaces of upper and lower steps were estimated to be 1.6 x 10(-2) mu m and 3.1 x 10(-2) mu m, respectively. (C) 2014 Society of Photo-Optical Instrumentation Engineers (SPIE)

DOI： 10.1117/1.OE.53.8.084110

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Recently, a variety of the optical comb-based interferometries has been developed for profilometry and tomography. However the interference amplitude and phase characteristics involving the center frequency and mode spacing of the optical comb have not been sufficiently studied. To investigate these multi-frequency interference characteristics, we proposed a broadband frequency variable quasi-comb generator utilizing 4-f optical system and a spatial spectral filter which can perform unrestricted scanning of the center frequency and mode spacing. By using a sinusoidal phase modulating interferometer with the quasi-comb generator, fundamental proof-of-principle experiments were successfully demonstrated. The interference phase fixation during the symmetrical varying of the mode spacing produced the interference amplitude peak envelope without fringes. On the other hand, it was confirmed that the interference phase was changed linearly without the amplitude change by the center frequency shift of the multi-frequency spectrum. (C) 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.optcom.2013.12.015

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The present paper proposes a low-coherence interferometry system based on continuous wavelet transform, which can simultaneously detect the peak position and frequency of a signal. Although the proposed system is analogous to conventional time-domain optical coherence tomography, it realizes accurate wide-range surface profile measurement because the wavelet transform enables accurate evaluation of the position of the reference mirror. The performance of the proposed system was evaluated through step-height measurement. (C) 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.optcom.2013.08.018

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

A sinusoidal wavelength scanning interferometer is proposed for 3-D profile measurement. The interference phase-shift signal generated by the sinusoidal wavelength scanning contains information of optical path difference (OPD) covering nm-mm scale structure. The interference phase-shift signal was obtained by the four-step phase shifting method. The sinusoidal wavelength shifting bandwidth of 5.7 nm with a frequency of approximately 180 Hz was performed by the Littman-Metcalf external resonator-type tunable laser with a center of 772.1 nm. The full-field step-height surface profile measurement and 3-D surface measurement were conducted by a CCD image sensor with an accuracy of few tens nm. The surface profile of gauge blocks with a step-height of up to 10 mu m was successfully measured.

DOI： 10.1117/12.2053539

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

A self-pulsation laser diode interferometer based on spectral-domain optical coherence tomography is proposed. A visible multimode laser diode allows easy optical alignment and an economical, configurable system. A cylindrical lens and acousto-optic deflector used in our system enable rapid and stable scanning. Experimental results confirm that full-field measurement is possible without mechanical scanning devices.

DOI： 10.1117/12.2050190

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The multi-gigahertz frequency comb-based interferometer exhibits only the interference amplitude peak without the phase fringes, which can produce a rapid axial scan for full-field profilometry and tomography. Despite huge technical advantages, there remain problems that the interference intensity undulations occurred depending on the interference phase. To avoid such problems, we propose a compensation technique of the interference signals using two frequency combs with slightly varied center wavelengths. The compensated full-field surface profile measurements of cover glass and onion skin were demonstrated experimentally to verify the advantages of the proposed method. (C) 2013 The Japan Society of Applied Physics

DOI： 10.7567/APEX.6.106601

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

Multiple-wavelength backpropagation interferometry based on a spectral interferometer is proposed for measuring thin glass sheets with nanometer accuracy. The multiwavelength backpropagation method introduced to the spectral interferometer eliminates time-encoded wavelength sweeping and mechanical scanning, which enables high-speed profile measurements. The applicability of the proposed method is experimentally demonstrated through cross-sectional profile and vibrating surface displacement measurements of a glass sheet. (C) 2013 Optical Society of America

DOI： 10.1364/AO.52.003726

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

Multiple-wavelength backpropagation interferometry based on a spectral interferometer is proposed for measuring thin glass sheets with nanometer accuracy. The multiwavelength backpropagation method introduced to the spectral interferometer eliminates time-encoded wavelength sweeping and mechanical scanning, which enables high-speed profile measurements. The applicability of the proposed method is experimentally demonstrated through cross-sectional profile and vibrating surface displacement measurements of a glass sheet. (C) 2013 Optical Society of America

DOI： 10.1364/AO.52.003726

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

The Multi-frequency sweeping Fizeau-type interferometer (MFS-FI) for optical coherence tomography (OCT) is demonstrated. The multi-frequency sweeping by a variable Fabry-Perot filter permits detection of high-order low-coherence interferometric signals in the Fizeau interferometer. The sinusoidal phase modulation technique was utilized to detect accurate interference amplitude and phase distributions of back scattered light from surfaces of a sample. OCT measurements by the MFS-FI were conducted for vibrating glass plates with a frequency of 1 kHz, and cellular tissues fixed with formalin and embedded in paraffin. The tomographic 3-dimensional volume and cross-sectional surface displacements were detected with an accuracy of nano-meters.

DOI： 10.1117/12.2023816

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Multiple traps using a combination of a piezoelectric bimorph-type actuator and a pulsed laser diode are demonstrated. Three polystyrene beads were simultaneously trapped using a laser diode driven by a pulsed current.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

A pulse-modulated phase-shifting interferometer that employs two coaxial interferometers and an undersampling technique is proposed and demonstrated. The former improves the robustness of the optical system, while the latter makes it economical. Several measurements on flat and concave mirrors confirmed a measurement accuracy of similar to 18 nm.

DOI： 10.1117/12.2023531

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

It is difficult in interferometric metrology to maintain high spatial resolution over a large measurement region. In this paper this characteristic is achieved by scanning two focused laser beams of about 6 microns diameter over an object surface and a reference mirror surface, respectively, with a rotating mirror, a lens, and a polarization beam splitter. An electric-optic phase modulator generates the phase difference of a sinusoidal waveform of 10 MHz between the two focused beams to calculate the phase of the interference signal. Because the beam focused onto a diffraction grating of 50 microns period is not diffracted strongly, a surface profile of the grating can be exactly measured by this interferometer over the measurement region of 16 mm with the spatial resolution of 2 microns.

DOI： 10.1117/12.2025328

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

A multi-frequency light source with a spatial light modulator that generated a broadband spectrum with variable interval and center frequency was demonstrated. It can carry out the arbitrary multi-frequency scan for interferometric profile measurement.

DOI： 10.1109/CLEOPR.2013.6600579

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

With the development of electronic technology in recent years, electronic components become increasingly miniaturized. At the same time a more accurate measurement method becomes indispensable. In the current measurement of nano-level, the Michelson interferometer with the laser diode is widely used, the method can measure the object accurately without touching the object. However it can't measure the step height that is larger than a half-wavelength. In this study, we improve the conventional Michelson interferometer by using a super luminecent diode and continuous wavelet transform, which can detect the time that maximizes the amplitude of the interference signal. We can accurately measure the surface-position of the object with this time. The method used in this experiment measured the step height of 20 microns.

DOI： 10.1117/12.2042640

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

A multi-gigahertz frequency comb (MGFC)-based interferometer was developed for profilometry and tomography using a frequency variable supercontinuum (SC). The comparatively flattened and broadened SC light source with variable multi-gigahertz interval frequency was developed using an optical pulse synthesizer and highly nonlinear dispersion flattened fiber. The generated SC provided a stable interference output with a full width half maximum of 19 mu m during interval frequency sweeping of over 400 MHz. We experimentally confirmed that the interference signal exhibited an envelope-only waveform without fringes, which enabled the drastic reduction of the sampling points resulting in high speed measurement. A full-field 3-D image with 320 x 256 x 300 pixels was acquired with a measurement time of only 10 seconds. It was demonstrated that the MGFC-based interferometer with the novel SC light source has the potential for application in a high speed full-field 3-D metrology. (C)2012 Optical Society of America

DOI： 10.1364/OE.20.027820

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

A phase-shifting laser diode interferometer that uses direct pulse modulation is proposed and demonstrated. We found that a laser beam with a wide range of wavelength variation at constant optical power could be generated when a pulsed current was injected into the laser diode. We constructed a highly accurate interferometer by using a pair of interferometers. Several experiments, such as observations of temporal interference signals and spatial interferograms, measurement of a concave mirror, and duplicate measurements, confirmed the characteristics of pulse modulation and demonstrated the effectiveness of our technique. (C) 2012 Optical Society of America

DOI： 10.1364/AO.51.004109

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

A phase-shifting laser diode interferometer that uses direct pulse modulation is proposed and demonstrated. We found that a laser beam with a wide range of wavelength variation at constant optical power could be generated when a pulsed current was injected into the laser diode. We constructed a highly accurate interferometer by using a pair of interferometers. Several experiments, such as observations of temporal interference signals and spatial interferograms, measurement of a concave mirror, and duplicate measurements, confirmed the characteristics of pulse modulation and demonstrated the effectiveness of our technique. (C) 2012 Optical Society of America

DOI： 10.1364/AO.51.004109

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

The positions of the front and rear surfaces of a silicon dioxide film with 4 mu m thickness is measured with a novel and simple method in which both amplitude and phase of a sinusoidal wave signal corresponding to one optical path difference of a reflecting surface is utilized in a linear wavenumber-scanning interferometer. For this utilization, the scanning width and the position of the reference mirror are adjusted exactly to distinguish the two sinusoidal waves corresponding to the two surfaces of the film. The scanning width of the wavenumber and wavelength of the light source are 0.326x10(-3) nm(-1) and 140 nm, respectively. (C) 2012 Optical Society of America

DOI： 10.1364/AO.51.002429

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

The positions of the front and rear surfaces of a silicon dioxide film with 4 mu m thickness is measured with a novel and simple method in which both amplitude and phase of a sinusoidal wave signal corresponding to one optical path difference of a reflecting surface is utilized in a linear wavenumber-scanning interferometer. For this utilization, the scanning width and the position of the reference mirror are adjusted exactly to distinguish the two sinusoidal waves corresponding to the two surfaces of the film. The scanning width of the wavenumber and wavelength of the light source are 0.326x10(-3) nm(-1) and 140 nm, respectively. (C) 2012 Optical Society of America

DOI： 10.1364/AO.51.002429

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We propose a new technique for stable interferometric measurement by calculating interferograms from obtained signals using two supercontinua whose center wavelengths are slightly different. We successfully obtained a thickness profile of a glass plate. © 2012 IEEE.

DOI： 10.1109/OECC.2012.6276664

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Language：English   Publishing type：Research paper (international conference proceedings)  

A novel Fizeau-type low-coherence interferometer for optical coherence tomography (OCT) is demonstrated. The multi-frequency sweeping by a Fabry-Perot filter (FPF) permits detection of high-order interference signals in the Fizeau interferometer. The sinusoidal phase modulation (SPM) technique was utilized to detect accurate interference amplitude and phase distributions. © 2012 IEEE.

DOI： 10.1109/OECC.2012.6276575

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

Sinusoidal wavelength-scanning (SWS) interferometry is unique in that a time-varying interference signal contains phase-modulation amplitude Zb due to the SWS besides a conventional phase a. Propagation time and wavefront of the light wave diffusely reflected from a metal surface can be detected from the amplitude Zb and the phase a, respectively. A concave shape with the depth of about a few hundreds microns is measured with an error less than a few microns by using only the amplitude Zb of the detected interference signal.

DOI： 10.1117/12.929137

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

We investigated the interference amplitude and phase characteristics of a multi-frequency sweeping interferometer with sinusoidal phase modulating technique. A novel multi frequency sweeping light source that generated an interval and center frequency variable frequency comb with a bandwidth of 14 nm was demonstrated. The interference phase and amplitude distributions were investigated by observing the zeroth and first order interference signals. In addition, the zeroth and first order interference phase variations caused by center frequency sweeping were measured.

DOI： 10.1117/12.999555

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

An optical surface profile is measured with a laser diode Fizeau interferometer using a method of absolute measurement. Wavefront aberration in the interferometer causes an undesirable phase distribution in the interference signal. To eliminate this phase distribution, the object surface is shifted in two directions orthogonal to each other and the difference wavefront of the surface profile of the object is obtained. An absolute surface profile is estimated by representing the object surface with a polynomial function and by solving the difference equations with least-squares method.

DOI： 10.1117/12.2000145

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

A displacement sensor that uses a vertical cavity surface emitting laser (VCSEL), which is superior in terms of remediation of the mode-hop issue and modulation efficiency, is proposed. The interference signal in the sensor is processed with the phase-locked technique. This device allows real-time measurement of displacement. No unstable signals due to mode-hop were observed in the experiments. Displacement measurements recorded with this device indicated that it has an rms measurement accuracy of 0.3 mu m and 50 nm for displacements of 150 mu m and 1.2 mu m, respectively.

DOI： 10.1117/12.999720

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This study proposes low-coherence interferometry based on wavelet transform. This method can simultaneously detect the peak position and frequency of a signal. The use of this method for step-height measurement is demonstrated. © 2012 OSA.

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Language：English   Publishing type：Research paper (international conference proceedings)  

This study proposes low-coherence interferometry based on wavelet transform. This method can simultaneously detect the peak position and frequency of a signal. The use of this method for step-height measurement is demonstrated. © 2012 OSA.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

High-speed, wide-range wavelength scanning is demonstrated using a laser diode with an antireflection coating and an external cavity employing an acousto-optic deflector. The narrow spectral width of 0.2 nm observed in the experiment indicates the availability of a highly coherent laser beam. The scanning range of 15 nm under rapid modulation is confirmed in the prototype system. A tuning rate of more than 100 kHz is achieved without any mode hops. To the best of our knowledge, no such high-speed wavelength tuning based on an acousto-optical configuration has been demonstrated thus far. (C) 2011 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.optcom.2011.06.012

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The multi-wavelength back-propagation (MWB) method has been developed for measuring positions of multiple reflecting surfaces longer than the optical wavelength. In this study, we demonstrate one dimensional profile measurement of a glass film by a sinusoidal phase modulating (SPM) spectral interferometer with MWB method. Displacements of front and rare surfaces of the glass film vibrating with 9.7 Hz are exactly measured. The measurement repeatability obtained from 9 repeatedly-measured results is 4.7 nm in a standard deviation. © 2011 IEEE.

DOI： 10.1109/IQEC-CLEO.2011.6194003

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

Conventional methods to measure the positions of the front and rear surfaces of thin films with multiple-wavelength interferometers are reviewed to make it clear how the method proposed here is novel and simple. Characteristics of the linear wavenumber-scanning interferometry used in the proposed method are analyzed in detail to make the measurement accuracy clearly. The positions of the front and rear surfaces of a silicon dioxide film with 4 mu m thickness is measured by utilizing the phases of the sinusoidal waves forms corresponding to each of the optical path differences contained in the interference signal. The experiments and the theoretical analysis show that the measurement error is about 15 nm.

DOI： 10.1117/12.894454

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

It is difficult in interferometric metrology to maintain high spatial resolution over a large field of view. In this paper this characteristic is achieved by scanning two focused laser beams over an object surface and a reference mirror surface, respectively, with a rotating mirror, a lens, and a polarization beam splitter. An electric-optic phase modulator generates the phase difference of a sinusoidal waveform of 10 MHz between the two focused beams. Sinusoidal phase-modulating interferometry is used to calculate the phase of the interference signal caused by the two focused beams. The scanning speed of the beams is 20 m/s, and the sampling interval of the interference signal is 1/80 microseconds. The spatial resolution is 2.0 microns over the measurement region of 18.8mm. The measurement results show that the measurement error is less than 5nm.

DOI： 10.1117/12.910102

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DOI： 10.1117/12.870985

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

A profilometry based on Fizeau interferometer using the optical comb and the sinusoidal phase modulation technique is demonstrated. The optical comb generated with the Fabry-Perot etalon and a SLD is introduced in the Fizeau interferometer. The interval wave number of comb is swept by controlling the resonance length of the Fabry-Perot etalon. The sweeping range is about 500 mu m. The displacement measurement is performed by determination of zero-phase or pi-phase positions. The accuracy estimated from 5 repeated experiments is about 0.2 rad.

DOI： 10.1117/12.870985

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

Multiple-wavelength optical fields at a detecting plane of an interferometer are generated by a computer from the detected interference signals of a thin glass sheet. The generated optical fields are backpropagated towards the glass sheet along the optical axis. An optical field along the optical axis is reconstructed by summing the backpropagated fields over the multiple wavelengths. The amplitude and phase distributions of the reconstructed optical field provide the positions of the two surfaces of the glass sheet where peak values of the amplitude and zero or p values of the phase appear. The accuracy of the position measurement is several nanometers.

DOI： 10.1117/12.861534

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

Two light beams reflected from a front and rear surfaces of a glass film of 20 micron thickness interfere with each other in a common path interferometer. Sinusoidal wavelength-scanning light with the scanning amplitude of 5 nm and frequency of 15 KHz is used to generate a sinusoidal phase-modulated interference signal with the modulation amplitude of 2.6 rad. The phase of the interference signal provides the thickness variation of the film, whose measurement accuracy is a few nanometers. Moreover, in order to achieve a high spatial resolution and a wide measurement region a focused beam is scanned on the surface of the film with a rotating mirror.

DOI： 10.1117/12.871030

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

The multi-wavelength back-propagation (MWB) method enables to determine precisely the optical path different (OPD) longer than the optical wavelength from detecting the amplitude and phase of the interference signal for the multiple wavelengths. In this study, we demonstrate a 1-dimensional thickness profile measurement by the MWB and sinusoidal phase modulation (SPM) technique with a spectral interferometer. The OPD for the front and rear reflecting surfaces of a glass film with the thickness of 100 mu m are measured. The thickness profile is successfully measured with repeatability of 2 nm estimated from a standard error between 9 repeatedly-measured profiles.

DOI： 10.1117/12.870987

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We demonstrate highly precise pulse synthesis technique using an optical pulse synthesizer for spectrally flat supercontinuum generation. The technique achieved further 4-dB suppression of background noise of synthesized pulse and 3-dB suppression of supercontinuum spectrum peak at pump pulse wavelength.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC APPLIED PHYSICS  

We propose a novel supercontinuum (SC) generation technology using an optical pulse synthesizer and highly nonlinear dispersion-shifted fiber. The optical pulse synthesizer enables us to reshape seed pulses optimized for broadband SC generation. 25.0 and 12.5GHz SC combs were successfully generated by propagating synthesized seed pulses in the highly nonlinear fiber. We investigated how seed pulse conditions including shape, peak power, repetition rate, and center wavelength affect SC spectral bandwidth. We also evaluated interferometric signal waveforms from a Michelson interferometer using the generated SC comb as the light source. Interferometric peak width improved to 35.5 mu m when peak power reached 7.9 W for the Gaussian pulse-based SC comb. Numerical simulation results almost agreed with experimental results. The difference between SC combs generated from Gaussian and sech(2) pulses was confirmed by the simulations. (C) 2009 The Japan Society of Applied Physics

DOI： 10.1143/JJAP.48.09LF01

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We present a frequency-comb-based interferometric microscope Broadband optical frequency comb (OFC) was generated using optical pulse synthesizer and non-linear fiber as the light source of interferometer Three-dimensional surface profile was successfully observed with a resolution of 44 mu m and measurement speed of 30 frame/s

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

A three-dimensional microscopic interferometer by the interval frequency sweep of the supercontinuum frequency comb generated by an optical pulse synthesizer was demonstrated. The spectral bandwidth was 80 nm. The interferometric peak width was 35 mu m. (c) 2006 Optical Society of America

DOI： 10.1109/CLEO.2008.4551160

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：INST PURE APPLIED PHYSICS  

We present a frequency-comb-based interferometric microscope using a broadband comb light source and a line-type image sensor for proflometry and optical tomography. A waveguide-type frequency comb generator and wavelength equalizer were introduced to broaden the comb spectrum. In this paper, we demonstrate the surface profilometry of a coin and the observation of a cross-sectional tomography image of transparent glass. The three-dimensional images were captured using a line-type image sensor without any mechanical moving parts with a frequency interval variation of 25 GHz. The dynamic range in the depth direction was about 1.4 mm, and the measurement resolution was 35 pm. The standard deviation of seven thickness measurements was 5 mu m.

DOI： 10.1143/JJAP.46.6842

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The polarization compensation of a Faraday rotator reflector was quantitatively measured while applying mechanical stress to an optical fiber. A novel device called a Faraday rotator transmitter was developed to illuminate a sample. It was confirmed that the installation of both a Faraday rotator reflector and a Faraday rotator transmitter in a fiber-optic interferometer greatly improves the accuracy of film thickness measurements compared with measurements obtained using an interferometer that employed polarization-maintaining optical fibers.

DOI： 10.1143/JJAP.46.6848

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We present a frequency comb based interferometry with a broadband comb light source and a line-type image sensor for profilometry and optical tomography. The cross-sectional images were captured by varying the frequency interval of 25 GHz without mechanical shift. The resolution in depth direction was 35 mu m.

DOI： 10.1109/CLEOPR.2007.4391287

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

A novel interferometry technique using a frequency-comb light source is proposed for surface profilometry and tomography of discontinuous objects. Surface profile measurement is performed by sweeping the comb interval frequency without mechanical scanning. Step heights of 0.5 and 1.0 mm are successfully measured by use of the scheme with 9 mu m accuracy. (c) 2006 Optical Society of America.

DOI： 10.1364/OL.31.001976

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A novel profilometry using a waveguide-type frequency comb generator was presented. Thickness measurement of a glass plate was successfully demonstrated by sweeping comb interval frequency without mechanical scanning. © 2006 Optical Society of America.

DOI： 10.1109/CLEO.2006.4627912

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

A novel profilometry using a frequency comb light source is proposed. This method realizes step height measurement without mechanical scanning. We have confirmed the system with high accuracy with this method.

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J-GLOBAL

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J-GLOBAL

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Language：Japanese   Publisher：The Institute of Electronics, Information and Communication Engineers  

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CiNii Article

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Language：Japanese   Publisher：The Institute of Electronics, Information and Communication Engineers  

Harmonically mode-locked erbium-doped fiber lasers, which operate in the 10〜40 gigahertz region, have attracted widespread attention in the field of optical communication as light sources for ultra high-speed optical communication, because of their high-quality output pulses. However, the amount of longitudinal linewidth in these lasers has not yet been made clear. In this study, we measure the linewidth of a regeneratively and harmonically mode-locked erbium-doped fiber ring laser (ML Fiber Laser) for the first time by using a heterodyne method, and we show that the linewidth is less than 250 Hz.

CiNii Article

CiNii Books

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Language：Japanese   Publisher：The Institute of Electronics, Information and Communication Engineers  

Harmonically mode-locked erbium-doped fiber lasers, which operate in the 10〜40 gigahertz region, have attracted widespread attention in the field of optical communication as light sources for ultra high-speed optical communication, because of their high-quality output pulses. However, the amount of longitudinal linewidth in these lasers has not yet been made clear. In this study, we measure the linewidth of a regeneratively and harmonically mode-locked erbium-doped fiber ring laser (ML Fiber Laser) for the first time by using a heterodyne method, and we show that the linewidth is less than 250 Hz.

CiNii Article

CiNii Books

researchmap

Language：Japanese   Publisher：The Institute of Electronics, Information and Communication Engineers  

Harmonically mode-locked erbium-doped fiber lasers, which operate in the 10〜40 gigahertz region, have attracted widespread attention in the field of optical communication as light sources for ultra high-speed optical communication, because of their high-quality output pulses. However, the amount of longitudinal linewidth in these lasers has not yet been made clear. In this study, we measure the linewidth of a regeneratively and harmonically mode-locked erbium-doped fiber ring laser (ML Fiber Laser) for the first time by using a heterodyne method, and we show that the linewidth is less than 250 Hz.

CiNii Article

CiNii Books

researchmap