Improving the reliability of deep learning computational ghost imaging with prediction uncertainty based on neighborhood feature maps.

Defect inspection is required in various fields, and many researchers have attempted deep-learning algorithms for inspections. Deep-learning algorithms have advantages in terms of accuracy and measurement time; however, the reliability of deep-learning outputs is problematic in precision measurements. This study demonstrates that iterative estimation using neighboring feature maps can evaluate the uncertainty of the outputs and shows that unconfident error predictions have higher uncertainties.

Lensless single-fiber ghost imaging.

We demonstrate lensless single-fiber ghost imaging, which allows illumination and collection using a single optical fiber without a transmission-type system. Speckle patterns with relative coincidence degrees of 0.14 were formed by image reconstruction using improved differential ghost imaging.

Noise-robust deep learning ghost imaging using a non-overlapping pattern for defect position mapping.

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.

Relationship between the kernel size of a convolutional layer and the optical point spread function in ghost imaging using deep learning for identifying defect locations.

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.

Fabrication of three-dimensional high-aspect-ratio structures by oblique-incidence Talbot lithography.

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.

Visualization of internal structure and internal stress in visibly opaque objects using full-field phase-shifting terahertz digital holography.

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.

Refractive index sensing with temperature compensation by a multimode-interference fiber-based optical frequency comb sensing cavity.

We proposed a refractive index (RI) sensing method with temperature compensation by using an optical frequency comb (OFC) sensing cavity including 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.

Multicascade-linked synthetic wavelength digital holography using an optical-comb-referenced frequency synthesizer.

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.

Refractive-index-sensing optical comb based on photonic radio-frequency conversion with intracavity multi-mode interference fiber sensor.

Optical frequency combs (OFCs) have attracted attention as optical frequency rulers due to their tooth-like discrete spectra together with their inherent mode-locking nature and phase-locking control to a frequency standard. Based on this concept, their applications until now have been demonstrated in the fields of optical frequency metrology. However, if the utility of OFCs can be further expanded beyond their application by exploiting new aspects of OFCs, this will lead to new developments in optical metrology and instrumentation.

Strain sensing based on strain to radio-frequency conversion of optical frequency comb.

We propose an optical frequency comb (OFC)-based strain sensing method, namely OFC sensing cavity, which is capable of radio-frequency (RF)-based strain measurement. We developed a null-method-based strain sensing system with a comb-spacing-stabilized OFC generator. We realized strain measurement from 1.

Scan-less hyperspectral dual-comb single-pixel-imaging in both amplitude and phase.

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 µm without mechanical scanning. The spectral resolution given by the data point interval in the frequency domain is 20 MHz and the comb mode interval is 100 MHz over a spectral range of 1.

Dual-comb spectroscopic ellipsometry.

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.

Dynamic terahertz spectroscopy of gas molecules mixed with unwanted aerosol under atmospheric pressure using fibre-based asynchronous-optical-sampling terahertz time-domain spectroscopy.

Terahertz (THz) spectroscopy is a promising method for analysing polar gas molecules mixed with unwanted aerosols due to its ability to obtain spectral fingerprints of rotational transition and immunity to aerosol scattering. In this article, dynamic THz spectroscopy of acetonitrile (CH3CN) gas was performed in the presence of smoke under the atmospheric pressure using a fibre-based, asynchronous-optical-sampling THz time-domain spectrometer. To match THz spectral signatures of gas molecules at atmospheric pressure, the spectral resolution was optimized to 1 GHz with a measurement rate of 1 Hz.

Adaptive sampling dual terahertz comb spectroscopy using dual free-running femtosecond lasers.

Terahertz (THz) dual comb spectroscopy (DCS) is a promising method for high-accuracy, high-resolution, broadband THz spectroscopy because the mode-resolved THz comb spectrum includes both broadband THz radiation and narrow-line CW-THz radiation characteristics. In addition, all frequency modes of a THz comb can be phase-locked to a microwave frequency standard, providing excellent traceability. However, the need for stabilization of dual femtosecond lasers has often hindered its wide use.

Real-time absolute frequency measurement of continuous-wave terahertz radiation based on dual terahertz combs of photocarriers with different frequency spacings.

Real-time measurement of the absolute frequency of continuous-wave terahertz (CW-THz) radiation is required for characterization and frequency calibration of practical CW-THz sources. We proposed a method for real-time monitoring of the absolute frequency of CW-THz radiation involving temporally parallel, i.e.

Double-modulation reflection-type terahertz ellipsometer for measuring the thickness of a thin paint coating.

We constructed a double-modulation, reflection-type terahertz (THz) ellipsometer for precise measurement of the thickness of a paint film which is coated on a metal surface and which is not transparent to visible or mid-infrared light. The double-modulation technique enabled us to directly obtain two ellipsometric parameters, Δ(ω) and Ψ(ω), as a function of angular frequency, ω, with a single measurement while reducing flicker noise due to a pump laser. The bias voltage of a photoconductive antenna (PCA) used as a THz pulse emitter was modulated at 100 kHz, and a first lock-in amplifier (LA1) was connected to the output of an electro-optic (EO) signal-sampling unit.

Ellipsometric measurement technique for a modified Otto configuration used for observing surface-plasmon resonance.

In order to carry out precise measurements of the thickness of a dielectric layer deposited on a metal surface, we have introduced an ellipsometric measurement technique (EMT) to the modified Otto's configuration (MOC) that is used for observing surface plasmon resonance (SPR). For that purpose, we have measured the thickness of the Au layer by the EMT at four different locations on an elliptic fringe pattern obtained from the MOC basing on a four-layer structure model: prism (BK7)-air-Ausubstrate (BK7). Then, we have measured that of a TiO(2) layer deposited on the Au layer by the EMT basing on a five-layer structure model: prism (BK7)-air-TiO(2)-Au-substrate (BK7).

Autoregressive-model-based fluorescence-lifetime measurements by phase-modulation fluorometry using a pulsed-excitation light source and a high-gain photomultiplier tube.

We propose a novel method for measuring fluorescence lifetimes by use of a pulsed-excitation light source and an ordinary or a high-gain photomultiplier tube (PMT) with a high-load resistor. In order to obtain the values of fluorescence lifetimes, we adopt a normal data-processing procedure used in phase-modulation fluorometry. We apply an autoregressive (AR)-model-based data-analysis technique to fluorescence- and reference-response time-series data obtained from the PMT in order to derive plural values of phase differences at a repetition frequency of the pulsed-excitation light source and its harmonic ones.