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We propose the use of the line section method with crossed line beams for the process control of laser wire deposition. This method could be used to measure the height displacement in front of a laser spot when the processing direction changes. In laser processing, especially laser deposition of metal additive manufacturing, the laser process control technique that controls the processing parameters based on the measured height displacement in front of a laser processing spot is indispensable for high-accuracy processing. However, it was impossible to measure the height displacement in front of a processing laser spot in a processing route in which the processing direction changes as the measurement direction of the conventional light-section method comprising the use of a straight-line beam is restricted although the configuration is simple. In this paper, we present an in-process height displacement measurement system of the light-section method using two crossed line beams. This method could be used to measure the height displacement in a ±90° direction by projecting two crossed line beams from the side of a laser processing head with a simple configuration comprising the addition of one line laser to the conventional light-section method. The height displacement can be calculated from the projected position shift of the line beams irrespective of the measurement direction by changing the longitudinal position on the crossed line beams according to the measurement direction. In addition, the configuration of our proposed system is compact because the imaging system is integrated into the processing head. We could measure the height displacement at 2.8–4 mm in front of a laser processing spot according to the measurement direction by reducing the influence of intense thermal radiation. Moreover, we experimentally evaluated the height displacement measurement accuracy for various measurement directions. Finally, we evaluated continuous deposition in an “L” shape wherein the deposition direction was changed while using a laser wire direct energy deposition machine for the laser process control based on the in-process height displacement measurement result. We achieved highly accurate continuous deposition at the position wherein the processing direction changes despite the acceleration and deceleration of the stage by laser process control.

In the recent period of the miniaturization of devices, there has been a high demand for high-resolution, flexible, and fast machining technique to accommodate high production volumes. Conventional laser machining with a focused laser beam has been widely used to fabricate small devices for various applications. However, this process is limited by the optical diffraction limit of the laser beam. Therefore, the photonic nanojet (PNJ) machining technique is a promising solution to tackle this problem. This technique is based on the near-field focusing of light waves with a high-energy laser power below the surface of an irradiated dielectric microsphere. We introduce water as a medium in the proposed PNJ machining technique so that the pattern could be fabricated more efficiently, faster, and with better quality. We evaluate the characteristics of the generated PNJ in water; further, we estimate the PNJ machining results numerically using the FDTD method and confirm them experimentally. To the best of our knowledge, this is the first ever to do so. All the holes obtained from the PNJ machining experiment were consistently in the sub-micrometer order and below the optical diffraction limit value of the constructed setup.

We propose ghost imaging (GI) with deep learning to improve detection speed. GI, which uses an illumination light with random patterns and a single-pixel detector, is correlation-based and thus suitable for detecting weak light. However, its detection time is too long for practical inspection. To overcome this problem, we applied a convolutional neural network that was constructed based on a classification of the causes of ghost image degradation. A feasibility experiment showed that when using a digital mirror device projector and a photodiode, the proposed method improved the quality of ghost images.

We propose an in-process height measurement system for a weld bead and feedback control system for wire-feeding speed for high-quality laser deposition. Metal additive manufacturing, especially laser metal-wire deposition, is effective for complex shape fabrication and repair processing. However, we must control the gap between a weld bead and a feed wire in an optimal range for high-quality deposition. Conventionally, the Z-stage pitch for multi-layer deposition must be precisely adjusted by each deposition shape. In this paper, we design an in-process height measurement system that is integrated in a laser processing head, which measures the weld bead height by a line section method. We decreased the influence of the intense thermal radiation generated from a melt pool by inserting the band-pass filter of the line beam's wavelength in the imaging system and optimizing its line laser power. Consequently, our system can measure the weld bead height near the melt pool, which is 4 mm in front of it. Next we show that our proposed system can measure the weld bead height during wire-laser metal deposition with 50-μm accuracy by comparing its value to the true value. Finally, we achieved a cylinder shape deposition of 50-mm height, regardless of the Z-stage pitch and the cylinder diameter of the multi-layer deposition, by controlling the wire-feeding speed based on the measured weld bead height.

In recent years, unique functions of nano-periodic structures have attracted much attention, and there is a need for processing techniques with high processing efficiency and flexibility. Therefore, we focused on Talbot lithography, which has excellent processing efficiency for periodic structures. In this paper, we use hologram-assisted Talbot lithography to improve the processing flexibility of Talbot lithography. Hologram-assisted Talbot lithography is a method to improve the processing flexibility of structures by using CNN to estimate the incident light distribution. In order to improve the accuracy of the hologram-assisted Talbot lithography method for controlling periodic structures, we studied the learning of CNN. And we showed that the shape and period of the structure can be controlled by using CNN.

In demand for minute defect inspection, it is required to detect weak scattered light caused by small defects. Ghost imaging (GI) is known for its high sensitivity and high noise resistance method. However, it requires many measurements to obtain a high-quality image because GI is the correlation-based imaging method. Reducing the number of measurements, a method combined with deep learning has been proposed. In order to improve the estimation accuracy using CNN, we propose to parallelize the convolutional layers. Parallel convolutional layers can efficiently extract both local and global features, which contributes to the improvement of estimation accuracy. In this report, we show that parallel CNN is more accurate than conventional CNN by experiments.

We construct a full-field phase-shifting terahertz digital holography (PS-THz-DH) system by use of a THz quantum cascade laser and an uncooled, 2D micro-bolometer array. The PS-THz-DH enables us to separate the necessary diffraction-order image from unnecessary diffraction-order images without the need for spatial Fourier filtering, leading to suppress the decrease of spatial resolution. 3D shape of a visibly opaque object is visualized with a sub-millimeter lateral resolution and a sub-µm axial resolution. Also, the digital focusing of amplitude image enables the visualization of internal structure with the millimeter-order axial selectivity. Furthermore, the internal stress distribution of an externally compressed object is visualized from the phase image. The demonstrated results imply a possibility for non-destructive inspection of visibly opaque non-metal materials.

A photonic nanojet (PNJ) is a fine light beam generated from a dielectric microsphere or microcylinder illuminated by a laser and has been applied to nanometer scale measurement and machining. In this study, we attempt to control the PNJ intensity distribution flexibly by adjusting the intensity distribution of two incident laser beams: a Gaussian beam and a radially polarized beam. Then, finite-difference time-domain simulations demonstrated that a large PNJ beam diameter ( 1.08 λ ) could be obtained by Gaussian beam with a small beam radius, whereas a small PNJ beam diameter ( 0.34 λ ) could be obtained by radially polarized beam. Moreover, PNJ with isotropic intensity distribution can be obtained as well as a small PNJ beam diameter by a radially polarized beam incidence to a microsphere.

A fiber-coupled photoconductive antenna (PCA) is a powerful tool for portable terahertz (THz) systems using a compact 1550-nm mode-locked Er:fiber laser with a fiber output port. However, a low-temperature-grown GaAs (LTG-GaAs) PCA could not be used for this purpose due to the need for wavelength conversion of the 1550-nm light, regardless of the good characteristics for PCA. In this article, we achieved the fiber coupling of the 1550-nm mode-locked fiber laser light on a bowtie-shaped LTG-GaAs PCA detector without the need for wavelength conversion. While the two-step photo-absorption mediated by midgap states in the LTG-GaAs PCA makes it possible to use the 1550-nm light, the similarity of the size between the PCA gap spacing and the fiber core diameter enables the direct contact coupling between the fiber output tip and the PCA gap without any optical components. The developed lens-less fiber-coupled LTG-GaAs PCA detector was effectively applied for the absolute frequency measurement of continuous-wave THz radiation based on the photo-carrier THz frequency comb. The combination of the lens-less fiber-coupled LTG-GaAs PCA with the compact 1550-nm fiber laser will be useful for the portable apparatus for the absolute frequency measurement of practical CW-THz sources and other applications.

A photonic nanojet (PNJ) is a fine and high intensity light beam that is generated from a dielectric microsphere irradiated by a laser. A PNJ has a smaller beam diameter than the wavelength of the incident laser and can propagate for longer than 1 μm with high intensity and minimal divergence. In other words, a PNJ has a long depth of focus. Due to its outstanding optical properties, a PNJ is suitable for laser micro machining to create sub-micrometer scale structures. Depth of focus of a PNJ generated in water is longer than in air. In this paper, we experimentally investigated machining characteristics of laser micro machining using a PNJ in water medium. First, electromagnetic simulation was conducted to know the intensity distribution of PNJ in water medium. The simulation demonstrated that PNJ in water mdium has beam diameter of sub-micrometer scale and micrometer scale depth of focus. Next, machining experiments were also conducted on a silicon substrate. A femtosecond laser was used as the machining laser. By controlling the microsphere position, the PNJ position can be controlled in the propagation direction. Sub-micrometer scale hole diameters were obtained even when the PNJ position in the propagation direction was changed by 3 μm. In conclusion, the long depth of focus of a photonic nanojet in water medium enable to create sub-micrometer scale structures.

Developing a suitable production method for three-dimensional periodic nanostructures with high aspect ratios is a subject of growing interest. For mass production, Talbot lithography offers many advantages. However, one disadvantage is that the minimum period of the light intensity distribution is limited by the period of the diffraction grating used. To enhance the aspect ratio of fabricated nanostructures, in the present study we focus on multi-wave interference between diffracted waves created using the Talbot effect. We propose a unique exposure method to generate multi-wave interference between adjacent diffraction orders by controlling the angle of incidence of an ultraviolet (UV) light source. Using finite-difference time-domain simulations, we obtain fringe patterns with a sub-wavelength period using a one-dimensional periodic grating mask. Moreover, we demonstrate the practical application of this approach by using UV lithography to fabricate sub-wavelength periodic structures with an aspect ratio of 30 in millimeter-scale areas, indicating its suitability for mass production., Comment: 11 pages, 8 figures, 3 tables

A photonic nanojet (PNJ) is a fine and high intensity light beam that is generated from a dielectric microsphere irradiated by a laser. A PNJ has a smaller beam diameter than the wavelength of the incident laser and can propagate for longer than 1 μm with high intensity and minimal divergence. In other words, a PNJ has a long depth of focus. Due to its outstanding optical properties, PNJ is suitable for laser micro machining. In this paper, we theoretically and experimentally investigated machining characteristics of laser micro machining using a PNJ. First, electromagnetic simulations were conducted to estimate hole diameter when PNJ is irradiated to a sample by changing a distance from the microsphere to the sample. The simulation demonstrated that sub-micrometer scale hole diameter could be obtained even when PNJ is defocused by 1 μm due to the long depth of focus. Next, machining experiments were also conducted on a silicon substrate. A femtosecond laser was used as the machining laser. By holding the microsphere with a micropipette and controlling its position, the position of the PNJ can be controlled in the z direction. Micrometer and sub-micrometer scale hole diameters were obtained even when the position of PNJ in the z direction was changed by 1 μm. The hole diameters obtained in the experiment were consistent with the hole diameter estimated by the simulation. In conclusion, the long depth of focus of a photonic nanojet enable to create sub-micrometer scale structure.

Direct laser machining in sub-micron scale patterning at a surface of material remains a challenging task though the laser machining has been widely applied in various application. A photonic nanojet becomes a promising way to solve the problem by involving near-field focusing of light waves below the surface of a dielectric microsphere to fabricate pattern in micro- and nanometer size. By generating laser power to the microsphere and controlling the resulting photonic nanojet intensity distribution and position related to the workpiece, intended ablation size on the material could be controlled at the sub-micrometer scale. In this study, liquid is proposed as photonic nanojet machining medium due to several advantages that liquid offer during machining process. Laser trapping system is then introduced to the optical system to control the position of the microsphere during machining process. An in-liquid nanomachining by generating photonic nanojet in laser trapping configuration is a subject to study with the effect on the resulting ablation and viability of machining process from a set of parameters are investigated numerically using finite-difference time-domain (FDTD) technique. According to the findings of this study, nanometer scale, flexible, and fast novel laser nanomachining could be realized by combining photonic nanojet machining and laser trapping technique.

This paper reports for high-sensitivity imaging based on Ghost imaging (GI), which is one of the single-pixel imaging. Although the GI is correlation-based imaging between structured illumination lights and detected signals, there is an advantage in detecting weak light intensity such as fluorescence of molecules. Especially, in the case of using extream weak light intensity, a photon signal is useful for imaging. Therefore, we focused on the arrival time of the first photon and used the time as the intensity of the signal. Furthermore, to improve the detection time, we applied machine learning to reduce the measurement number. In this paper, we have proposed the principle and some experimental results.

Terahertz digital holography (THz-DH) has the potential to be used for non-destructive inspection of visibly opaque soft materials due to its good immunity to optical scattering and absorption. Although previous research on full-field off-axis THz-DH has usually been performed using Fresnel diffraction reconstruction, its minimum reconstruction distance occasionally prevents a sample from being placed near a THz imager to increase the signal-to-noise ratio in the hologram. In this article, we apply the angular spectrum method (ASM) for wavefront reconstruction in full-filed off-axis THz-DH because ASM is more accurate at short reconstruction distances. We demonstrate real-time phase imaging of a visibly opaque plastic sample with a phase resolution power of λ/49 at a frame rate of 3.5 Hz in addition to real-time amplitude imaging. We also perform digital focusing of the amplitude image for the same object with a depth selectivity of 447 μm. Furthermore, 3D imaging of visibly opaque silicon objects was achieved with a depth precision of 1.7 μm. The demonstrated results indicate the high potential of the proposed method for in-line or in-process non-destructive inspection of soft materials.

Recent advancement of fabrication technologies enables nanoscale surface patterning on microstructures, which requires multi-scale measurements for the determination of their geometric dimensions and surface texture in the same coordinate system. Previously, we have developed a micro-coordinate measuring machine with a laser-trapped microprobe that uses an optically trapped microsphere as a surface detecting sensor. In this paper, we propose a surface imaging technique using the optically trapped microsphere as a micro-lens, i.e., the surface imaging system is integrated into the coordinate measuring system. The optically trapped microsphere with a diameter of 8 μm was brought close to a measured surface to image the surface underneath it. Because there is an unknown gap between the microsphere and surface, the focal plane of the imaging system had to be adjusted to the measured surface by the developed imaging lens system. With the microsphere-based imaging system, an optical diffractive grating of sub-micrometer periodic structure with 417 nm pitch and 50 nm depth was successfully imaged and the defect on the grating was detected. Thus, we verified that development of an imaging system based on the optically trapped microsphere for multi-scale evaluation systems can be accomplished.

A photonic nanojet (PNJ) is a high intensity light beam that has beam diameter of sub-micrometers and depth of focus of several micrometers. In order to improve the accuracy and resolution of laser ablation using a PNJ, a shock wave generated in laser ablation was focused on. The shock wave pressure depends on the distance from sample surface and the shape of the surface. Thus, detection of the shock wave makes it possible to estimate the processing state. The confocal system made it possible to detect the shock wave via the vibration of the microsphere. The vibration signals in the time domain were Fourier transformed in the frequency domain. After the laser irradiation, there was a frequency peak that was not seen before the laser irradiation. Therefore, it was demonstrated that the shock wave generated by laser ablation could be detected from the vibration of the microsphere.

Recently, high-precision and in-process surface topography measurement methods are required in the field of precision machining. Laser inverse scattering has been reported as a method satisfying these characteristics. However, the dynamic range and spatial resolution of this method depend on the wavelength of the light source, which limits the surface topography that can be measured when using a single-wavelength light source. This limitation can be removed by using multi-wavelength laser. When a multi-wavelength laser is used as a light source, a method to spectrally disperse surface image of the sample and Fraunhofer diffraction pattern is necessary. In this paper, we propose a method for spectroscopy of surface image and diffraction pattern using VIPA spectroscopy. Then, the diffraction pattern using the optical frequency comb as a light source was spectrally analyzed with the proposed method and it was confirmed that the diffraction pattern of each wavelength can be measured.

Spectroscopic ellipsometry is a means of investigating optical and dielectric material responses. Conventional spectroscopic ellipsometry is subject to trade-offs between spectral accuracy, resolution, and measurement time. Polarization modulation has afforded poor performance because of its sensitivity to mechanical vibrational noise, thermal instability, and polarization-wavelength dependency. We combine spectroscopic ellipsometry with dual-comb spectroscopy, namely, dual-comb spectroscopic ellipsometry. Dual-comb spectroscopic ellipsometry (DCSE). DCSE directly and simultaneously obtains the ellipsometric parameters of the amplitude ratio and phase difference between s-polarized and p-polarized light signals with ultra-high spectral resolution and no polarization modulation, beyond the conventional limit. Ellipsometric evaluation without polarization modulation also enhances the stability and robustness of the system. In this study, we construct a polarization-modulation-free DCSE system with a spectral resolution of up to 1.2 × 10−5 nm throughout the spectral range of 1514–1595 nm and achieved an accuracy of 38.4 nm and a precision of 3.3 nm in the measurement of thin-film samples., Spectroscopic ellipsometry is an established technique to characterize the optical properties of a material. Here, Minamikawa et al. combine the method with dual-comb spectroscopy, which allows them to obtain ellipsometric parameters including the phase difference between s-polarized and p-polarized light.

This paper presents a technique that employs a stroboscopic oblique-incidence interferometer to visualize the motion of a vibrating object with a rough surface. An oblique-incidence interferometer is well suited to the analysis of a rough surface and micro displacement because light-scattering is reduced when a surface is rough. However, when continuous light is used, the fringe pattern on the vibrating surface in the ultrasonic region can not be observed for the analysis of a micrometer resolution profile. To overcome this problem, pulsed light synchronized with a vibrating sample is employed as a light source using an acousto-optic modulator (AOM). The timing between the vibrating sample and the observation light is controlled using a common oscillator, so that the time-resolved behavior of the stator can be measured. We successfully detect the periodic movement of a fringe pattern for a vibrating ultrasonic motor using the interferometer.

We propose a new prism-based surface-plasmon-resonance microscope (PB-SPRM) with a diffraction grating. Unlike the conventional PB-SPRM, the proposed one is free from distortion and blur, which were introduced because of the inclination of an object plane against one normal to the optical axis. To solve this problem, we inserted a reflection-type diffraction grating into the conventional PB-SPRM between the sample and the objective lens. Also, we placed an aperture on the back focal plane of the objective lens so that an object-side telecentric (OST) optical system was formed. By this optical configuration, a wide field of view (FOV) and a high frame rate were also obtained. We describe the theoretical background for designing the new PB-SPRM. In order to evaluate the performance of the proposed system, we show some numerical simulation results in detail. Mixing water and an ethanol–water solution along a micro-flow channel at a rate of 30 frames/s is also shown.

Conventional laser machining with a focused laser beam is difficult to use for sub-micrometre machining because of the optical diffraction limit. We therefore propose a laser machining method that uses a photonic nanojet with a fine-beam profile generated from a dielectric microsphere illuminated by laser light. The beam diameter is several hundred nanometres, which exceeds the diffraction limit. By controlling the intensity distribution and position, the processed feature size can be controlled at the sub-micrometre scale. A hole with a diameter of approximately 200 nm was machined using a photonic nanojet with a wavelength of 800 nm.

We proposed a refractive index (RI) sensing method with temperature compensation by using an optical frequency comb (OFC) sensing cavity employing a multimode-interference (MMI) fiber, namely, the MMI-OFC sensing cavity. The MMI-OFC sensing cavity enables simultaneous measurement of material-dependent RI and sample temperature by decoding from the comb spacing frequency shift and the wavelength shift of the OFC. We realized the simultaneous and continuous measurement of RI-related concentration of a liquid sample and its temperature with precisions of 1.6*10^-4 RIU and 0.08 degree. The proposed method would be a useful means for the various applications based on RI sensing., 22 pages

Digital holography (DH) is a technique to reconstruct the amplitude and phase images of a sample by calculating the wavefront propagation from the interference image. Although DH enables three-dimensional shape measurement based on the phase images, axial dynamic range of a single-optical-wavelength DH is limited to less than a full or half optical wavelength due to phase wrapping ambiguity. To extend the axial range over the optical wavelength, synthesized wavelength DH has been proposed. In this method, DH is performed at two different wavelengths, and then synthesized wavelengths between them are used. However, use of a single longer synthesized wavelength degrades the axial resolution because the axial dynamic range is limited by the phase noise. To extend the axial dynamic range, one has to increase the axial range while maintaining the axial resolution of sub-wavelength. One promising approach to do it is cascade linking between multiple synthetic wavelengths with different orders. In this paper, we present multicascadelinked synthetic wavelength DH using an optical-comb-referenced frequency synthesizer (OFS). OFS is a tunable external cavity laser diode phase-locked to an optical frequency comb, and is effectively used for multiple synthetic wavelengths within the range of 32 um to 1.20 m. A multiple cascade link of the phase images among an optical wavelength and 5 different synthetic wavelengths enables the shape measurement of a reflective millimeter-sized stepped surface with the axial resolution of 34 nm.

Parallel direct laser machining in sub-micro scale patterning at a surface of material on a large scale remains a challenging task though the laser machining has been widely applied in various applications. A photonic nanojet comes up as a promising way to solve the problem by involving near-field focusing of light waves below the surface of an introduced microsphere to fabricate structures of micro- and nanometer size. By generating laser power, the workpiece is then modified only locally at the tip of photonic nanojet that has as small as 616 nm width which provide fine machining pattern. In order to control the position of microsphere in nanomachining process, optical tweezers is introduced to the optical system. An in-liquid processing nanomachining by generating photonic nanojet in optical tweezers configuration from a laser beam is then be a subject to study by controlling the parameters investigate the viability of machining process.

Dual-comb spectroscopy (DCS) is useful for gas spectroscopy due to high potential of optical frequency comb (OFC). However, fast Fourier transform (FFT) calculation of a huge amount of temporal data spends significantly longer time than the acquisition time of an interferogram. In this article, we demonstrate frequency-domain DCS by a combination of DCS with lock-in detection, namely LID-DCS. LID-DCS directly extracts an arbitrary OFC mode from a vast number of OFC modes without the need for FFT calculation. Usefulness of LID-DCS is demonstrated in rapid monitoring of transient signal change and spectroscopy of hydrogen cyanide gas., Comment: 28 Pages, 4 figures

A terahertz (THz) frequency synthesizer based on photomixing of two near-infrared lasers with a sub-THz to THz frequency offset is a powerful tool for spectroscopy of polar gas molecules due to its broad spectral coverage; however, its frequency accuracy and resolution are relatively low. To tune the output frequency continuously and widely while maintaining its traceability to a frequency standard, we developed a photomixing THz synthesizer phase-locked to dual optical frequency combs (OFCs). While the phase-locking to dual OFCs ensured continuous tuning within a spectral range of 120 GHz, in addition to the traceability to the frequency standard, use of a broadband uni-traveling carrier photodiode for photomixing enabled the generation of CW-THz radiation within a frequency range from 0.2 to 1.5 THz. We demonstrated THz frequency-domain spectroscopy of gas-phase acetonitrile CH3CN and its isotope CH3 13CN in the frequency range of 0.600–0.720 THz using this THz synthesizer. Their rotational transitions were assigned with a frequency accuracy of 8.42 × 10−8 and a frequency resolution of 520 kHz. Furthermore, the concentration of the CH3CN gas at 20 Pa was determined to be (5.41 ± 0.05) × 1014 molecules/cm3 by curve fitting analysis of the measured absorbance spectrum, and the mixture ratio of the mixed CH3CN/CH3 13CN gas was determined to be 1:2.26 with a gas concentration of 1014–1015 molecules/cm3. The developed THz synthesizer is highly promising for high-precision THz-FDS of low-pressure molecular gases and will enable the qualitative and quantitative analyses of multiple gases.

A practical method for the absolute frequency measurement of continuous-wave terahertz (CW-THz) radiation uses a photocarrier terahertz frequency comb (PC-THz comb) because of its ability to realize real-time, precise measurement without the need for cryogenic cooling. However, the requirement for precise stabilization of the repetition frequency (f rep) and/or use of dual femtosecond lasers hinders its practical use. In this article, based on the fact that an equal interval between PC-THz comb modes is always maintained regardless of the fluctuation in f rep, the PC-THz comb induced by an unstabilized laser was used to determine the absolute frequency f THz of CW-THz radiation. Using an f rep-free-running PC-THz comb, the f THz of the frequency-fixed or frequency-fluctuated active frequency multiplier chain CW-THz source was determined at a measurement rate of 10 Hz with a relative accuracy of 8.2 × 10−13 and a relative precision of 8.8 × 10−12 to a rubidium frequency standard. Furthermore, f THz was correctly determined even when fluctuating over a range of 20 GHz. The proposed method enables the use of any commercial femtosecond laser for the absolute frequency measurement of CW-THz radiation.

We propose a novel, on-machine method of measuring the profile of the cutting edge of a tool by using the cutting fluid on the tool surface. Despite an environment of on-machine tool profile measurement, it is difficult to measure a cutting edge profile by using conventional optical methods due to interference from the cutting fluid on the tool surface. To overcome this problem, we propose a profile measurement method that uses confocal fluorescent detection from the cutting fluid on the tool surface. Moreover, for precise measurements, a method that corrects for the thickness of the cutting fluid is provided. Fluorescence from the surface of a silicon wafer coated with a fluorescent dye that is set horizontally as well as vertically to the optical axis of a developed fluorescent confocal microscope is detected. As a basic verification, the cutting edge profile of a milling tool with wear is measured using the proposed measuring and correction methods that employ a fluorescent dye. The results confirm that the proposed method can provide detailed measurements of a tool wear profile.

When using conventional optical methods in the on-machine environment, it is difficult to precisely evaluate tool geometry because the cutting fluid covering the tool surface reduces measurement accuracy owing to noisy light scattering. To overcome this problem, the principle of measurement based on the confocal detection of fluorescence emitted from the cutting fluid is proposed. The proposed method was fundamentally validated by measurement results for detailed tool wear profiles. This novel measurement technique produces results with sub-micron accuracy.

We introduce an optical frequency comb (OFC) to microscopy to coherently link the confocal microscopy and phase microscopy. One-dimensional (1D) image pixels of a sample were encoded onto OFC modes via 1D spectral encoding, in which OFC acted as an optical carrier with a vast number of discrete frequency channels. Then, a scan-less line-field confocal image with a depth resolution of 50 μm was decoded from a mode-resolved OFC amplitude spectrum obtained by dual-comb spectroscopy. Furthermore, a phase image with a depth resolution of sub-λ was decoded from a moderesolved OFC phase spectrum under the above confocality. The proposed hybrid microscopy approach will be a powerful tool for a variety of applications.

Dual comb spectroscopy (DCS) is based on the combination of Fourier transform spectroscopy with an optical frequency comb (OFC), and has a spectral resolution of MHz order over a spectral range of several tens THz. Furthermore, nonmechanical time-delay scanning enables the rapid data acquisition. While DCS imaging is required for hyperspectral imaging, an imaging sensor cannot be used for DCS imaging because of a slow response time compared to the temporal evolution of interferogram signal. Therefore, it is essential to acquire the interferogram signal by use of a single-channel detector while scanning the sample position or the focal point. If DCS imaging can be achieved without the need for such scanning, the application field of the DCS imaging will be largely expanded. One promising method to achieve the scanless imaging is a single-pixel imaging (SPI). SPI enables scan-less imaging by use of optical spatial coding on the sample with a single-channel detector. Also, the spatial averaging effect improves an image quality. In this paper, we present combination of DCS with SPI, namely a DCS-SPI. DCS-SPI provides 12,000 mode-resolved hyperspectral images in both amplitude and phase at a spatial resolution of 46 μm without the need for mechanical scanning. Furthermore, we determined thickness of a chromium layer from a phase image in the near-infrared wavelength region.

Digital holography (DH) is a promising method for non-contact surface topography because the reconstructed phase image can visualize the nanometer unevenness in a sample. However, the axial range of this method is limited to the range of the optical wavelength due to the phase wrapping ambiguity. Although the use of two different wavelengths of light and the resulting synthetic wavelength, i.e., synthetic wavelength DH, can expand the axial range up to a few tens of microns, this method is still insufficient for practical applications. In this article, a tunable external cavity laser diode phase-locked to an optical frequency comb, namely, an optical-comb-referenced frequency synthesizer, is effectively used for multiple synthetic wavelengths within the range of 32 um to 1.20 m. A multiple cascade link of the phase images among an optical wavelength (= 1.520 um) and 5 different synthetic wavelengths (= 32.39 um, 99.98 um, 400.0 um, 1003 um, and 4021 um) enables the shape measurement of a reflective millimeter-sized stepped surface with the axial resolution of 34 nm. The axial dynamic range, defined as the ratio of the maximum axial range (= 0.60 m) to the axial resolution (= 34 nm), achieves 1.7*10^8, which is much larger than that of previous synthetic wavelength DH. Such a wide axial dynamic range capability will further expand the application field of DH for large objects with meter dimensions., Comment: 19 pages, 7 figures

A method and setup are proposed for trapping and detecting nanoparticles dispersed in a nanocomposite solution using periodically localized light generated by a subwavelength transmission grating. By numerical simulations, it is shown that there is an optimum duty ratio of the grating to produce the periodically localized light. Experimental results are presented for Au/γ-Fe2O3 composite nanoparticles having a diameter of 21.0 nm trapped on a silicon subwavelength rectangular grating and detected ellipsometrically. The technique should prove useful for evaluating optical and mechanical properties of nanocomposite materials.

We compared images reconstructed by three methods: ghost imaging (GI), Hadamard transform imaging (HTI), and scan-based imaging. Although GI and HTI use a bucket (or single channel) detector, GI has attracted more attention than HTI in recent years. Nevertheless, a direct comparison between them has not yet been conducted to the best of our knowledge. In the present work, we evaluate contrast ratios of images obtained from computational GI (CGI) and HTI under various signal-to-noise ratio (SNR) conditions. Our results indicate that HTI and CGI are useful in high- and low-SNR situations, respectively, although both methods have a similar performance.

Dual comb spectroscopy (DCS) is based on the combination of Fourier transform spectroscopy with an optical frequency comb (OFC), and has a spectral resolution below MHz order over a spectral range over several tens THz. Furthermore, non-mechanical time-delay scanning enables the rapid data acquisition. However, in order to expand DCS into spectral imaging, a CCD or a CMOS camera cannot be used because a high-speed, point detector is indispensable to acquire the fast interferogram signal in DCS. Therefore, the first demonstration of DCS imaging was based on the mechanical scanning of the sample position. If DCS imaging can be achieved without the need for mechanical scanning, the application field of the DCS imaging will be largely expanded. One promising method to achieve the scan-less 2D imaging is a single-pixel imaging (SPI), enabling scan-less 2D imaging by use of pattern illumination on the sample and a point detector. Also, the accumulation effect in the random pattern illumination increases a signal-to-noise ratio. In this paper, we present combination of DCS with SPI, namely a scan-less DCS imaging. Spectral imaging of a sample indicated the effectiveness and potential of scan-less DCS imaging.

We propose a novel sensitive imaging method using a correlation based imaging by a probability of photon detection. Our proposed method was possible to observe a fluorescence sample from weaker signals than a conventional method.

We have developed a hyperspectral imaging scheme that involves a combination of dual-comb spectroscopy and Hadamard-transform-based single-pixel imaging. The scheme enables us to obtain 12,000 hyperspectral images of amplitude and phase at a spatial resolution of 46 um without mechanical scanning. The spectral resolution is 20 MHz, as determined by the linewidth of a single comb mode, and the spectral interval is 100 MHz over a spectral range of 1.2 THz centred at 191.5 THz. As an initial demonstration of our scheme, we obtained spectroscopic images of a standard test chart through an etalon plate. The thickness of an absorptive chromium-coated layer on a float-glass substrate was determined to be 70 nm from the hyperspectral phase images in the near-infrared wavelength region., Comment: 19 pages, 6figures

Combination of the dual comb spectroscopy with the single-pixel imaging is effectively applied for a scan-less hyperspectral imaging of an object having spectral-dependent absorption.

We propose a modified method for thickness measurement of a dielectric coating layer on metal based on Otto optical configuration (O-configuration). This method enables us to estimate the coating thickness that typically ranges from several tens of nanometers to more than one micrometer with precision less than a few nanometers. The common method to measure the thickness of dielectric coating layer is to utilize the frustrated total-internal reflection. In order to measure the thickness of several tens of nanometers, one can apply the surface-plasmon-resonance (SPR) phenomenon generated by the p-polarized light. For thickness larger than one hundred nanometers, a metal-clad leaky-waveguide (MCLW) mode generated by the p- or the s-polarized light can be employed without significant changes to the optical setup. The numerical and experimental verifications of the modified O-configuration reveals its effectiveness for precise measurement of moderately-thick dielectric coating layer on the metal.

To carry out precise measurements of refractive indices of nanocomposite materials, we propose a metal-diffraction-grating-based surface plasmon resonance (SPR) sensor (G-SPRS). In the G-SPRS, two SPR dips on the angular spectrum produced by two diffracted lights, the orders of which are first and negative second are utilized in a differential manner to enhance sensitivity in measurements. The principle of the G-SPRS is explained with reference to the dispersion relation of the surface plasmon induced on the metal grating. To improve precision in measurements, the fill factor of the rectangular-grooved grating is optimized. The effect of the depth of the groove on the precision is also discussed. From numerical simulations based on the rigorous coupled-wave analysis (RCWA) method, we found that the sensitivity of the proposed sensor was enhanced more than 3 times in comparison with that of the conventional sensor.

We propose a novel ultrasonic wave sensing method employing an optical-frequency-comb (OFC) cavity, namely, an OFC sensing cavity-based ultrasonic wave sensor. We confirm the fundamental characteristics of the OFC sensing cavity-based ultrasonic wave sensor by using an ultrasonic wave transducer and realize the frequency response of up to 13 MHz with a linear response. We also confirm the potential applicability in photoacoustic imaging employing an 8-ns pulsed laser. Our demonstrations confirm the capability of the concept of the OFC sensing cavity in ultrasonic wave sensing and will also expand the potential of OFC technology.