This study investigates the establishment of high-temperature storage accelerated lifetime reliability evaluation conditions for package-less micro-light-emitting diode array, so called micro-LED, based on the IES TM-21-11 LED accelerated lifetime evaluation methodology. The results confirmed that the L70 lifetime can be effectively predicted with short-duration accelerated tests alone under high-temperature conditions of 100-150 degrees C. In particular, at 120 degrees C, the exponential fit function model yielded a coefficient of determination R-2 = 0.87 and an analysis of variance (ANOVA) significance level of P < 0.0001, demonstrating that the reliability test results are statistically significant. Furthermore, the Arrhenius model provided an activation energy (E-a) of 0.517 eV and an acceleration factor of 57.2, confirming the feasibility of L70 lifetime prediction within a short test duration. This indicates that the package-less structure reflects the intrinsic high activation energy of the inorganic chip by eliminating encapsulant degradation, whereas the packaged structure has a lower overall activation energy due to the combined effect of encapsulant and chip degradation. The tests were conducted as high-temperature storage tests in which only purely thermal stress was applied without electrical operation, thereby excluding the junction temperature (T-j) generated internally during device operation and enabling independent evaluation of degradation lifetime due to external thermal energy. It is expected that subsequent operational lifetime tests incorporating driving conditions will elucidate the comprehensive reliability degradation mechanisms of micro-LEDs under both internal and external thermal energy and contribute to the establishment of an accelerated lifetime reliability evaluation framework.
Interferometry is a fundamental technique for probing relative phase changes through wave superposition and has played a central role in precision measurement. In addition to its role in precision measurement, it provides a general framework for understanding phase, coherence, and superposition in a wide range of physical systems. In this article, we discuss interferometry from the perspectives of coherence and indistinguishability, focusing on how these concepts govern interference in classical and quantum regimes. We review representative interferometer architectures, including induced-coherence schemes, nonlinear SU(1,1) interferometers, and two-photon interference, and briefly extend the discussion beyond optics to optomechanical systems. These examples illustrate the broad applicability of interferometry to interference phenomena in various physical systems.
Real-time vibration monitoring is important in precision processing, but many methods require contact sensors or complex systems. This study proposes a non-contact confocal vibration measurement system using the longitudinal chromatic aberration of a single objective lens. Two low-cost laser diodes at 405 nm and 658 nm share an optical path and form two axially separated focal planes. When the vibration target passes each plane, the reflected intensity generates peaks detected by wavelength-separated photodetectors. The vibration frequency and displacement are reconstructed by analyzing the temporal position of the detected peak signal using LabVIEW and MATLAB. In an experiment using a vibration speaker driven at 100 Hz, the vibration frequency restored under the condition of chromatic aberration length of 50 mu m and sampling frequency of 2,000 Hz was measured as 99.367 Hz and the displacement was 53 mu m. As a result of reference measurement using a commercial laser vibrometer, the displacement was confirmed to be 55.69 mu m, and an error of about 5.07% was shown. These results show that the proposed system can accurately measure micrometer-level vibrations even under low sampling conditions and demonstrate its availability as a real-time non-contact vibration monitoring technique in the processing process.
This paper proposes an optimization method for stable asymmetric light distribution in non-coaxial illumination systems. In such systems, the tilted target plane complicates the relationship between design variables and light distribution, making optimization difficult. To address this issue, a virtual target plane perpendicular to the optical axis is defined, and its illuminance distribution is used for optimization. When applied to a vacuum-cleaner illumination system, the proposed illuminance-based method satisfies both horizontal and vertical distribution requirements and achieves higher illuminance near the head region compared to conventional intensity-based optimization.
Adaptive optics (AO) has evolved from a ground-based technique for atmospheric turbulence correction into a core technology for achieving high-stability diffraction-limited imaging in spaceborne optical systems. Although space telescopes are free from atmospheric disturbances, environmental instabilities such as gravity release, launch vibration, thermal drift, and microjitter induce nanometer-to picometer-level wavefront errors, degrading image contrast and resolution. This study reviews AO principles tailored for the space environment, and analyzes representative implementations in large-aperture astronomical observatories and Earth-observation payloads. It further examines miniaturized AO technologies-MEMS deformable mirrors, fast steering mirrors, and compact wavefront sensors-for enabling real-time correction in CubeSat-and SmallSat-class missions. Based on these developments, a two-step AO demonstration mission is proposed: (1) Chromospheric Imaging Spectrograph (CHRIS) to validate low-orderAO control in orbit, and (2) Solar Coronagraph, a follow-up mission for high-contrast imaging using high-order correction. These demonstrations will establish a technological foundation for future large space telescopes, integrating intelligent control and lightweight AO modules to achieve long-term optical stability and high-contrast imaging in next-generation space missions.
The composite waveplate in a rotating-compensator spectroscopic ellipsometer consists of two or more uniaxial phase retarders. I propose an analytic method to calibrate biplate artifacts originating from the internal misalignment of two uniaxial retarders composing the composite waveplate. The suggested method enables one to suppress oscillations due to biplate artifacts, keeping their magnitude smaller than the noise level of the measured ellipsometric data. It is confirmed that the proposed calibration method secures the accuracy of measured data and user convenience for rotating-compensator spectroscopic ellipsometry.
With the miniaturization and higher integration of semiconductor devices, the size of defects to be detected has decreased to the nanometer scale, and in large-area wafer inspection there is a need for technologies that can detect such defects rapidly and in a non-destructive manner. Dark-field microscopy offers the advantage of observing fine scattering structures with high contrast; however, it has limitations in detecting subwavelength particles because their scattering is weak and it is difficult to distinguish their signals from the strong scattering originating from patterns or circuit structures. To address these issues, we implemented a laser-based dark-field microscopy system that uses a high-power laser source, speckle suppression through a rotating diffuser, reduction of pattern-scattering signals with polarization control, and a time-delay integration (TDI) camera that combines signal accumulation with high-speed detection. We confirmed that scattering signals from nanoparticles approximately 100 nm in diameter can be detected using this system.
Recent progress in ultra-intense laser technology has enabled laboratory exploration of strong-field quantum electrodynamics (QED) phenomena that may occur in extreme astrophysical environments, such as neutron stars, gamma-ray bursts, and black holes. In this study, we report an experimental demonstration of nonlinear Compton scattering, which is one of the fundamental strong-field QED processes, using a 4.2-PW Ti:sapphire laser. A multi-GeV electron beam accelerated by the ultra-intense laser interacted with a tightly focused laser pulse, resulting in a simultaneous net absorption of hundreds of photons and their conversion into a single gamma-ray photon with energies up to several hundred MeV. These results provide direct experimental evidence of nonlinear interactions between an ultra-intense laser field and a relativistic electron beam, confirming entry into the strong-field QED regime. This achievement opens the way for future studies on strong-field QED phenomena, including nonlinear Breit-Wheeler pair production and QED plasma formation.
As modern lifestyles have led to increased indoor activity, maintaining air quality in enclosed environments has become increasingly important. Vehicles represent typical confined spaces where the humid conditions inside heating, ventilation, and air conditioning (HVAC) systems facilitate microbial growth and biofilm formation, resulting in unpleasant odors, health risks, and reduced system efficiency. This study proposes a UV-C light-emitting diode (LED) based sterilization optical module designed for automotive HVAC filters. The irradiance distribution of the module was analyzed to evaluate its sterilization performance. A 3 x 3 UV-C LED array with a central wavelength of 275 nm and an optical output of 90 mW was configured to match the curved geometry of the filter. Ray-tracing simulations demonstrated uniform irradiance distribution, achieving a minimum irradiance of 110.60 mu W/cm(2), an average irradiance of 137.32 mu W/cm(2), and a uniformity of 80.6%, thereby meeting the target criterion (>= 80%).
Indoor atmospheric turbulence can significantly degrade the accuracy and repeatability of precision optical measurements by inducing wavefront distortions. Recently, the INTENSE (INdoor TurbulENce SEnsor) system, which quantifies the spatiotemporal fluctuations of the angle of arrival using multiple parallel laser beams, has gained attention as an effective method for indoor turbulence measurement. In this study, we implement simulation model based on the INTENSE configuration to evaluate how the number of measurement frames affects the estimation accuracy of turbulence parameters. Using key indicators such as the Fried parameter (r0) and outer scale (L0), we assess convergence behavior and estimation error with varying frame counts. Simulations were conducted with 512, 1024, and 2048 frames at 70 fps over a 3 m propagation path, repeated 100 times for statistical robustness. The results show that with 2048 frames, estimation errors for both the Fried parameter (r0) and outer scale (L0) converge below 3%. These findings demonstrate the accuracy and stability of the INTENSE-based simulation approach and provide practical pre-validation guidelines for selecting optimal acquisition parameters prior to physical system implementation in indoor optical turbulence environments.
Sub-THz (terahertz, 10(12) Hz) time-domain spectroscopy (TDS) system was developed to analyze the optical properties of materials within the D-band (110-170 GHz) frequency range. By directly measuring both the amplitude and phase of the electric field in the time domain, the complex refractive indices and absorption coefficients of polytetrafluoroethylene, crystalline quartz, and sapphire samples were extracted. The TDS system provides several advantages, including broad bandwidth, fast acquisition, and the capability to determine complex dielectric properties without the need for reference samples. These features highlight its potential for applications in telecommunications, biosensing, and semiconductor inspection.
This study proposes a three-dimensional (3D) gait analysis system designed to overcome the spatial limitations and distance-dependent accuracy degradation of conventional systems by employing an orthogonal arrangement of multiple depth cameras. To improve data efficiency, the system encodes depth information in RGB format and applies k-means clustering to reduce noise. Two-dimensional joint coordinates, extracted using a YOLOv8-pose model, are scale-corrected according to the subject's height and then fused with the corresponding Z-values from the registered depth images to reconstruct 3D coordinates. The proposed approach is significant in that it integrates low-cost hardware with efficient algorithms to provide accurate and convenient gait analysis, particularly in constrained clinical environments.
This paper presents an overview of the principles, implementation methods, output characteristic control, and recent research trends of dual-cavity laser structures. Representative dual-cavity configurations are compared, and their respective characteristics are introduced in terms of enabling spatial mode control, wavelength stabilization, linewidth narrowing, and pulse formation. The design principles and operating mechanisms of the dual-cavity laser are then described in detail, including demonstrations of Laguerre-Gaussian and Hermite-Gaussian mode selection, output beam shaping and real-time mode switching using an acousto-optic modulator (AOM), Q-switching-based high-energy pulse generation, and polarization-controlled dual-wavelength oscillation, along with our experimental results. This review highlights the potential of dual-cavity laser configurations to overcome the limitations of single-cavity laser systems and to offer flexible designs of wavelength, mode, temporal characteristics, and so on. Furthermore, future prospects of the dual-cavity laser configuration for next-generation laser source technology are discussed.
This paper proposes a structural optical alignment system for precisely aligning multi-wavelength lasers onto a single optical axis in active defense applications. The system not only enables precise alignment among multiple wavelengths, but also allows verification of alignment status along the operational optical path by employing a switchable path separation structure. The alignment begins with a 785 nm reference laser positioned at the center of the short-wave infrared image, followed by fine adjustment using position-sensitive detectors placed at near-field and far-field positions. Each wavelength laser is then sequentially aligned to the defined optical axis using picomotor-controlled mirrors. Experimental results demonstrate that the inter-wavelength center deviation remains within 15 mu rad. The picomotor-based fine-tuning mechanism also contributes to maintaining alignment stability during long-term operation. The proposed method meets both the high precision and operational efficiency required for optical targeting systems, making it suitable for multi-wavelength, non-lethal dazzling applications.
Free-space optical communication has recently garnered significant attention as a future-proof solution for high-speed wireless connectivity, not only serving point-to-point terrestrial links but also supporting intersatellite, satellite-ground, and deep-space communication applications. This paper reviews some technical challenges inherent to free-space optical communication systems and provides an overview of the recent state-of-the-art solutions developed to address the challenges.
Maintaining transparency of protective radar covers in adverse weather is essential for autonomous driving, yet combining high microwave transmittance with low sheet resistance in transparent heaters is constrained by the Fabry-Perot resonances set by a fixed cover thickness. To address this issue, a single metal (e.g., Cu) layer for a microwave-transparent metamaterial with a high metal filling fraction is proposed and experimentally validated. In the W band (75-110 GHz), ultrathin patterned metal imparts a precise phase delay that satisfies the Fabry-Perot resonance condition at target frequencies. By tuning the structural parameters, the transmission resonance is shifted continuously across the band while preserving high peak transmittance. Heating tests at -20 degrees C achieve complete deicing within 9 seconds, demonstrating practical suitability as a microwave transparent heater. To clarify the physical principle, the effective permittivity and permeability are retrieved from the full wave simulation, and an effectivemedium model is constructed. The model reproduces the transmission spectra within the W band, substantiating the phase-control mechanism. This approach provides a practical route to dispersion-engineered transparent heaters for radar applications, including autonomous driving.
I propose an integrated optical system and an advanced method for precise characterization of spectro-ellipsometric data to get the optical constants and thicknesses of free-standing thin pellicle films. The integrated optical system consists of a spectroscopic ellipsometer, spectroscopic reflectometer, and spectroscopic transmissometer, together with the operating software. The optical system is engineered to increase user convenience and measurement accuracy. As a consequence, the standard deviation of transmittance is reduced to below 0.02% across a wavelength range of 190-380 nm. As for the modelling analysis of spectro-ellipsometric data, a weighted error function is introduced in which weights are applied inversely proportional to the variances of measured data, and consequently both film thickness and wavelength-dependent refractive indices are determined reliably. Their standard deviations are reduced to 1/3-1/10 of previous levels, even when the ellipsometric data in blind zones are included. In addition, it is verified that the sum of the measured reflectance and the measured transmittance approaches 1.0 over the entire spectral region of 190-850 nm. From the small but noticeable discrepancy of the sum from 1.0 at shorter wavelengths, it is also confirmed that the extinction coefficients of the pellicles do not exceed 0.00022 even at 193 nm. Above all, by adopting deviation of intensity for the first time, the proposed method enables one to get the wavelength-dependent weights from a single ellipsometric measurement, and increases stability in determining thicknesses and refractive indices of pellicle films while preserving user convenience and experimental efficiency.
As numerous nuclear power plants worldwide approach decommissioning, efficient remote cutting technologies for thick metal structures have become a critical challenge. In response, domestic research has actively focused on applying high-power laser cutting technology to nuclear decommissioning. This study developed underwater laser cutting technology capable of cutting steel plates up to 100 mm thick, for dismantling a pressurized water reactor. Additionally, technology for cutting 30-mm-thick stainless steel plates of the calandria vessel for decommissioning a pressurized heavy-water reactor was established. Fundamental research on cutting zirconium alloy, a key heavy-water-reactor material, was also conducted. These results demonstrate the practical applicability of high-power laser cutting technologies to various metal structures in nuclear decommissioning sites and are expected to significantly enhance the safety and efficiency of future dismantling operations.
We investigate changes in terahertz (THz) transmission characteristics by placing micrometer-sized van der Waals materials at the gap of metal bowtie antennas resonant in the THz frequency range. Specifically, the electrical conductivity of MoS2 with a thickness of tens of nanometers is tuned by chemical doping using N-methyl-2-pyrrolidone (NMP), and DC current-voltage characteristics are measured using the bowtie antenna electrodes. Corresponding changes in THz transmission are measured to analyze the impact of conductivity modulation on the THz responses. First, MoS2 flakes with electrical conductivities of 8.8 x 10(-3) S/cm and 5.7 x 10(-3) S/cm showed no change in antenna transmission, indicating semiconducting off-state behavior. After NMP treatment, the electrical conductivities increased by up to an order of magnitude, reaching 8.7 x 10(-2) S/cm and 3.7 x 10(-2) S/cm, respectively, yet no significant change in THz transmittance was observed. This result is consistent with previous theoretical predictions that transmission changes become observable when the conductivity of the material at the bowtie gap reaches approximately 10(2) S/cm or higher. These findings enable estimation of the lower conductivity threshold required for effective THz modulation and nanomaterial detection and advance the understanding of doping effects in van der Waals materials for THz applications.
We have developed a 193 nm deep ultraviolet (DUV) Fizeau interferometer for testing semiconductor optical systems. This study presents a self-calibration method utilizing the random flat test and three flat test to minimize the shape distortion caused by systematic errors. The random flat test involves rotating and translating the test flat, performing 68 measurements, and averaging the results to cancel out the antisymmetric components of the test flat. By subtracting the average value from the obtained data, systematic errors can be removed. However, this method has limitations in removing symmetric components. To address this problem, the three flat test is applied, measuring three flats in four different ways and using mathematical calculations to eliminate both symmetric and antisymmetric components, thereby restoring the accurate shape. The self-calibration method used in this study enhances the measurement accuracy of the DUV Fizeau interferometer and will improve the performance of high-resolution optical systems under test.
