Compact Numerical Aperture 0.5 Fiber Optic Spectrometer Design Using Active Image Plane Tilt.

The numerical aperture of the spectrometer is crucial for weak signal detection. The transmission lens-based configuration has more optimization variations, and the grating can work approximately in the Littrow condition; thus, it is easier to acquire high numerical aperture (NA). However, designing a large aperture focusing lens remains challenging, and thus, ultra-high NA spectrometers are still difficult to acquire. In this paper, we propose a method of setting image plane tilt ahead directly when designing the large aperture focusing lens to simplify the high NA spectrometer design. By analyzing the accurate demands of the focusing lens, it can be concluded that a focusing lens with image plane tilt has much weaker demand for achromatism, and other monochromatic aberration can also be reduced, which is helpful to increase the NA. An NA0.5 fiber optic spectrometer design is given to demonstrate the proposed method. The design results show that the NA can achieve 0.5 using four lenses of two materials, and the MTF is higher than 0.5 when the spectral dispersion length is 12.5 mm and the pixel size is 25 µm, and thus, the spectral resolution can achieve 6.5 nm when the spectral sampling ratio is 2:1. The proposed method can provide reference for applications when appropriate materials are limited and high sensitivity is necessary.

Two-Level Spatio-Temporal Feature Fused Two-Stream Network for Micro-Expression Recognition.

Micro-expressions, which are spontaneous and difficult to suppress, reveal a person's true emotions. They are characterized by short duration and low intensity, making the task of micro-expression recognition challenging in the field of emotion computing. In recent years, deep learning-based feature extraction and fusion techniques have been widely used for micro-expression recognition, particularly methods based on Vision Transformer that have gained popularity. However, the Vision Transformer-based architecture used in micro-expression recognition involves a significant amount of invalid computation. Additionally, in the traditional two-stream architecture, although separate streams are combined through late fusion, only the output features from the deepest level of the network are utilized for classification, thus limiting the network's ability to capture subtle details due to the lack of fine-grained information. To address these issues, we propose a new two-level spatio-temporal feature fused with a two-stream architecture. This architecture includes a spatial encoder (modified ResNet) for learning texture features of the face, a temporal encoder (Swin Transformer) for learning facial muscle motor features, a feature fusion algorithm for integrating multi-level spatio-temporal features, a classification head, and a weighted average operator for temporal aggregation. The two-stream architecture has the advantage of extracting richer features compared to the single-stream architecture, leading to improved performance. The shifted window scheme of Swin Transformer restricts self-attention computation to non-overlapping local windows and allows cross-window connections, significantly improving the performance and reducing the computation compared to Vision Transformer. Moreover, the modified ResNet is computationally less intensive. Our proposed feature fusion algorithm leverages the similarity in output feature shapes at each stage of the two streams, enabling the effective fusion of multi-level spatio-temporal features. This algorithm results in an improvement of approximately 4% in both the F1 score and the UAR. Comprehensive evaluations conducted on three widely used spontaneous micro-expression datasets (SMIC-HS, CASME II, and SAMM) consistently demonstrate the superiority of our approach over comparative methods. Notably, our approach achieves a UAR exceeding 0.905 on CASME II, making it one of the few frameworks in the published micro-expression recognition literature to achieve such high performance.

Bibliometric analysis of the current status and trends on medical hyperspectral imaging.

Hyperspectral imaging (HSI) is a promising technology that can provide valuable support for the advancement of the medical field. Bibliometrics can analyze a vast number of publications on both macroscopic and microscopic levels, providing scholars with essential foundations to shape future directions. The purpose of this study is to comprehensively review the existing literature on medical hyperspectral imaging (MHSI). Based on the Web of Science (WOS) database, this study systematically combs through literature using bibliometric methods and visualization software such as VOSviewer and CiteSpace to draw scientific conclusions. The analysis yielded 2,274 articles from 73 countries/regions, involving 7,401 authors, 2,037 institutions, 1,038 journals/conferences, and a total of 7,522 keywords. The field of MHSI is currently in a positive stage of development and has conducted extensive research worldwide. This research encompasses not only HSI technology but also its application to diverse medical research subjects, such as skin, cancer, tumors, etc., covering a wide range of hardware constructions and software algorithms. In addition to advancements in hardware, the future should focus on the development of algorithm standards for specific medical research targets and cultivate medical professionals of managing vast amounts of technical information.

Design and Study of a Reflector-Separated Light Dispersion-Compensated 3D Microscopy System.

The secondary-phase grating-based tomographic microscopy system, which is widely used in the biological and life sciences, can observe all the sample multilayer image information simultaneously because it has multifocal points. However, chromatic aberration exists in the grating diffraction, which seriously affects the observation of the image. To correct the chromatic aberration of the tomographic microscope system, this paper proposes a system that adopts blazed gratings and angle-variable reflectors as chromatic aberration correction devices according to the principle of dispersion compensation and Fourier phase-shift theory. A reflector-separated light dispersion-compensated 3D microscopy system is presented to achieve chromatic aberration correction while solving the problem of multilayer image overlap. The theoretical verification and optical design of the system were completed using ZEMAX software. The results show that the proposed system reduced the chromatic aberration of ordinary tomographic microscopy systems by more than 90%, retaining more wavelengths of light information. In addition, the system had a relatively wide range in the color difference compensation element installation position, reducing the difficulty of dispersion compensation element installation. Overall, the results indicate that the proposed system is effective in reducing chromatic aberration in grating diffraction.

A Two-Step Simulated Annealing Algorithm for Spectral Data Feature Extraction.

To address the shortcomings in many traditional spectral feature extraction algorithms in practical application of low modeling accuracy and poor stability, this paper introduces the "Boruta algorithm-based local optimization process" based on the traditional simulated annealing algorithm and proposes the "two-step simulated annealing algorithm (TSSA)". This algorithm combines global optimization and local optimization. The Boruta algorithm ensures that the feature extraction results are all strongly correlated with the dependent variable, reducing data redundancy. The accuracy and stability of the algorithm model are significantly improved. The experimental results show that compared with the traditional feature extraction method, the accuracy indexes of the inversion model established by using the TSSA algorithm for feature extraction were significantly improved, with the determination coefficient R2 of 0.9654, the root mean square error (RMSE) of 3.6723 µg/L, and the mean absolute error (MAE) of 3.1461 µg/L.

Study of the Off-Axis Fresnel Zone Plate of a Microscopic Tomographic Aberration.

A tomographic microscopy system can achieve instantaneous three-dimensional imaging, and this type of microscopy system has been widely used in the study of biological samples; however, existing chromatographic microscopes based on off-axis Fresnel zone plates have degraded image quality due to geometric aberrations such as spherical aberration, coma aberration, and image scattering. This issue hinders the further development of chromatographic microscopy systems. In this paper, we propose a method for the design of an off-axis Fresnel zone plate with the elimination of aberrations based on double exposure point holographic surface interference. The aberration coefficient model of the optical path function was used to solve the optimal recording parameters, and the principle of the aberration elimination tomography microscopic optical path was verified. The simulation and experimental verification were carried out utilizing a Seidel coefficient, average gradient, and signal-to-noise ratio. First, the aberration coefficient model of the optical path function was used to solve the optimal recording parameters. Then, the laminar mi-coroscopy optical system was constructed for the verification of the principle. Finally, the simulation calculation results and the experimental results were verified by comparing the Seidel coefficient, average gradient, and signal-to-noise ratio of the microscopic optical system before and after the aberration elimination. The results show that for the diffractive light at the orders 0 and ±1, the spherical aberration W040 decreases by 62-70%, the coma aberration W131 decreases by 96-98%, the image dispersion W222 decreases by 71-82%, and the field curvature W220 decreases by 96-96%, the average gradient increases by 2.8%, and the signal-to-noise ratio increases by 18%.

Research and Application of Several Key Techniques in Hyperspectral Image Preprocessing.

This paper focuses on image segmentation, image correction and spatial-spectral dimensional denoising of images in hyperspectral image preprocessing to improve the classification accuracy of hyperspectral images. Firstly, the images were filtered and segmented by using spectral angle and principal component analysis, and the segmented results are intersected and then used to mask the hyperspectral images. Hyperspectral images with a excellent segmentation result was obtained. Secondly, the standard reflectance plates with reflectance of 2 and 98% were used as a priori spectral information for image correction of samples with known true spectral information. The mean square error between the corrected and calibrated spectra is less than 0.0001. Comparing with the black-and-white correction method, the classification model constructed based on this method has higher classification accuracy. Finally, the convolution kernel of the one-dimensional Savitzky-Golay (SG) filter was extended into a two-dimensional convolution kernel to perform joint spatial-spectral dimensional filtering (TSG) on the hyperspectral images. The SG filter (m = 7,n = 3) and TSG filter (m = 3,n = 4) were applied to the hyperspectral image of Pavia University and the quality of the hyperspectral image was evaluated. It was found that the TSG filter retained most of the original features while the noise information of the filtered hyperspectral image was less. The hyperspectral images of sample 1-1 and sample 1-2 were processed by the image segmentation and image correction methods proposed in this paper. Then the classification models based on SG filtering and TSG filtering hyperspectral images were constructed, respectively. The results showed that the TSG filter-based model had higher classification accuracy and the classification accuracy is more than 98%.

Broad visible spectral subwavelength polarizer with high extinction ratio using hyperbolic metamaterial.

A subwavelength polarizer with an extinction ratio greater than 50 dB in the visible region (exceeding 80 dB in the 500 to 800 nm wavelength region) is presented that uses hyperbolic metamaterial decorated on each side with a subwavelength grating. The approach is based on the spatial frequency filtering characteristics of the hyperbolic metamaterial with its alternating metal-dielectric nano-film, which enables squeezing of bulk plasmon polaritons to support transverse magnetic polarized light transmission in the high spatial wavevector modes while suppressing other polarization mode waves over the broad visible spectrum. The proposed method exhibits a more realizable fabrication process than that used for a subwavelength polarizer based on high-aspect-ratio grating for broad spectrum and high-extinction-ratio performance. The subwavelength polarizer in this work is potential for the high-sensitivity polarimetric imaging.

Design of a subwavelength all-metal grating for generating azimuthally polarized beams based on modified particle swarm optimization.

A subwavelength metal grating for generating azimuthally polarized beams was designed. The grating material was determined by calculating the total thermal conductivity coefficient. A modified particle swarm optimization (PSO), which has a two-step algorithm structure, and a particle position-determined inertia weight was performed to establish the grating's structural parameters. Results show that an Au-Ti-Cu all-metal structure provides good thermal conductivity. A high duty cycle structure gives the grating high polarization selectivity of approximately 82.63% at 10.6 µm and a minimum of 26.39% in the 9.6-11.6 µm band. The modified PSO is more efficient and requires fewer calculations while maintaining accuracy. The geometrical parameter tolerances were also analyzed. The groove depth fabrication tolerance is 200 nm, and the sidewall angle fabrication tolerance is 19°, which means that the optimized structure is insensitive to structural parameter deviations.

Convex blazed grating of high diffraction efficiency fabricated by swing ion-beam etching method.

A swing ion-beam etching method to fabricate convex blazed gratings used in shortwave infrared hyperspectral imaging spectrometers is presented. This method solves the consistency problem of blaze angles by swing etching through the meridian direction of the gratings. The mathematical relationship of the curvature, aperture, and diffraction efficiency of convex gratings is studied to demonstrate the limitation of conventional translational lithography and the necessity of swing etching. A geometric model is built to analyze the influence of swinging speed and beam slit width on groove evolution. Convex gratings with a 45.5 gr/mm groove density, 67 mm aperture, 156.88 mm radius of curvature, and 2.2° blaze angle have been fabricated and measured where the peak and average diffraction efficiency in the shortwave infrared band reach 90% and 70%, respectively. Experimental results validate that high-efficiency convex gratings of small blaze angle and high groove consistency can be produced by swing etching, which satisfy the requirements for high spectral resolution and miniaturization of imaging spectrometers.

The effect of ultrasonic vibration and surfactant additive on fabrication of 53.5gr/mm silicon echelle grating with low surface roughness in alkaline KOH solution.

In the silicon echelle grating fabrication process, the "pseudo-mask" formed by the hydrogen bubbles generated during the etching process is the reason causing high surface roughness and poor surface quality of blazed plane. Based upon the ultrasonic mechanical effect and contact angle reduced by surfactant additive, ultrasonic vibration, isopropyl alcohol (IPA) and 2,4,7,9-Tetramethyl-5-decyne-4,7-diol (TMDD) were used to improve surface quality of 53.5gr/mm echelle grating. The surface roughness Rq is smaller than 18nm, 7nm and 2nm when using ultrasonic vibration, IPA and TMDD respectively. The surface roughness Rq is smaller than 5nm and 1.5nm respectively when combining ultrasonic vibration with IPA and TMDD. The experimental results indicated that the combination of ultrasonic agitation and surfactant additive (IPA&TMDD) could obtain a lower surface roughness of blazed plane in silicon echelle grating fabrication process.

Study of influence of the removal depth of the silicon modification layer on grating structures in reaction-sintered silicon carbide substrate and improvement method.

The influence of the removal depth of a silicon modification layer on grating structures and mirrors is studied. The removal depth 6-14 µm is the optimization result for Si-modified reaction-sintered silicon carbide (RS-SiC) used as mirror substrates, but the removal depth 9-12 µm is the optimization result for Si-modified RS-SiC used as grating substrates. The diffraction efficiency and stray light of the gratings fabricated in the Si-modified RS-SiC substrates with removal depth 9-12 µm is 90.5%-94% and 5.30×10-7-5.45×10-7, respectively. Additionally, the number and scale of high-reflection points can be used as the basis for judging the removal depth of the Si-modified RS-SiC used as grating substrates. These results and the regularity have guiding significance for the application of Si-modified RS-SiC as a microstructural substrate.

Effects of ultrasonic agitation and surfactant additive on surface roughness of Si (111) crystal plane in alkaline KOH solution.

In the silicon wet etching process, the "pseudo-mask" formed by the hydrogen bubbles generated during the etching process is the reason causing high surface roughness and poor surface quality. Based upon the ultrasonic mechanical effect and wettability enhanced by isopropyl alcohol (IPA), ultrasonic agitation and IPA were used to improve surface quality of Si (111) crystal plane during silicon wet etching process. The surface roughness Rq is smaller than 15 nm when using ultrasonic agitation and Rq is smaller than 7 nm when using IPA. When the range of IPA concentration (mass fraction, wt%) is 5-20%, the ultrasonic frequency is 100 kHz and the ultrasound intensity is 30-50 W/L, the surface roughness Rq is smaller than 2 nm when combining ultrasonic agitation and IPA. The surface roughness Rq is equal to 1 nm when the mass fraction of IPA, ultrasound intensity and the ultrasonic frequency is 20%, 50 W and 100 kHz respectively. The experimental results indicated that the combination of ultrasonic agitation and IPA could obtain a lower surface roughness of Si (111) crystal plane in silicon wet etching process.

Measuring method of diffraction efficiency for plane grating based on Fourier spectral technology.

A traditional double monochromatic measurement instrument of diffraction efficiency for a plane grating involves two major problems: one is the differences of output spectrum bandwidths during measurement of a standard reflection mirror and the tested grating; the other is overlapping of diffracted spectra, which influence testing accuracy of diffraction efficiency. In this paper, a new measuring method of diffraction efficiency based on Fourier spectral technology is presented. The mathematical model of diffraction efficiency is first deduced and then verified by ray tracing and Fourier optics simulation. The influences of the moving cube corner's tilt error, lateral shift error, and maximal moving distance error on the measurement accuracy are analyzed in detail. The analyses provide theoretical references for designing diffraction efficiency instruments. Compared with the traditional diffraction efficiency measurement instrument with double monochromator structure, our method not only improves the measurement accuracy of diffraction efficiency but also has the advantage of high luminous flux, high spectral resolution, multiwavelength measurement in mean time, and high wavenumber accuracy.

Numerical simulation of ultrasonic enhancement on mass transfer in liquid-solid reaction by a new computational model.

Mass transfer coefficient is an important parameter in the process of mass transfer. It can reflect the degree of enhancement of mass transfer process in liquid-solid reaction and in non-reactive systems like dissolution and leaching, and further verify the issues by experiments in the reaction process. In the present paper, a new computational model quantitatively solving ultrasonic enhancement on mass transfer coefficient in liquid-solid reaction is established, and the mass transfer coefficient on silicon surface with a transducer at frequencies of 40 kHz, 60 kHz, 80 kHz and 100 kHz has been numerically simulated. The simulation results indicate that mass transfer coefficient increases with the increasing of ultrasound power, and the maximum value of mass transfer coefficient is 1.467 × 10(-4) m/s at 60 kHz and the minimum is 1.310 × 10(-4) m/s at 80 kHz in the condition when ultrasound power is 50 W (the mass transfer coefficient is 2.384 × 10(-5) m/s without ultrasound). The extrinsic factors such as temperature and transducer diameter and distance between reactor and ultrasound source also influence the mass transfer coefficient on silicon surface. Mass transfer coefficient increases with the increasing temperature, with the decreasing distance between silicon and central position, with the decreasing of transducer diameter, and with the decreasing of distance between reactor and ultrasound source at the same ultrasonic power and frequency. The simulation results indicate that the computational model can quantitatively solve the ultrasonic enhancement on mass transfer coefficient.