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Defect detection requires highly sensitive and robust inspection methods. This study shows that non-overlapping illumination patterns can improve the noise robustness of deep learning ghost imaging (DLGI) without modifying the convolutional neural network (CNN). Ghost imaging (GI) can be accelerated by combining GI and deep learning. However, the robustness of DLGI decreases in exchange for higher speed. Using non-overlapping patterns can decrease the noise effects in the input data to the CNN. This study evaluates the DLGI robustness by using non-overlapping patterns generated based on binary notation. The results show that non-overlapping patterns improve the position accuracy by up to 51%, enabling the detection of defect positions with higher accuracy in noisy environments.

We explore the contribution of convolutional neural networks to correcting for the effect of the point spread function (PSF) of the optics when applying ghost imaging (GI) combined with deep learning to identify defect positions in materials. GI can be accelerated by combining GI and deep learning. However, no method has been established for determining the relevant model parameters. A simple model with different kernel sizes was built. Its accuracy was evaluated for data containing the effects of different PSFs. Numerical analysis and empirical experiments demonstrate that the accuracy of defect identification improved by matching the kernel size with the PSF of the optics.

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.

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.

Three-dimensional microstructures in the sub-micrometer scale exhibit unique properties. A flexible machining method to fabricate such structures is desired. Photonic nanojet (PNJ) is high intensity laser beam with sub-micrometer scale beam diameter and micrometer scale depth of focus. PNJs have a longer depth of focus than tightly focused laser beams with a high numerical aperture. In this study, we investigate the angular control of PNJs by controlling the propagation direction of incident light in order to realize flexible laser micro machining using PNJs. By controlling the position of the microsphere in the focused laser beam with a large defocus, the propagation direction of the laser beam incident on the microsphere is changed, and the angle of the PNJ can be controlled. Laser machining experiments on a silicon substrate showed that the PNJ angle can be controlled by incident laser angle. Furthermore, sub-micrometer scale laser machining was achieved even when using an oblique PNJ. The simulation results and experimental results are in good agreement. In conclusion, the angle control of the photonic nanojet can be applied to flexible multi-axis laser micro machining.

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.

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.

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