Chinese Version Light学术出版中心
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Issue 5 ， 2022
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Ultra-precision spatial-separated heterodyne Littrow grid encoder displacement measurement system
Abstract：An ultra-precision spatial separation heterodyne littrow grating encoder displacement measurement system was proposed to meet the requirements of an ultra-precision position measurement of dual workpiece table of immersion lithography machine. This study mainly includes the principle and scheme design of the measurement system， the design and manufacture of each component of the system， the measurement principle derivation of the encoder， and the test verification. The subdivision rate of the phase card of the displacement measurement system of the designed planar grating encoder was 4 096， and the measurement resolution was x50 pm/z25 pm. The experimental results showed that the planar grating displacement measurement system can measure the x and z direction displacement simultaneously， and the z direction motion stroke was ±1 mm， which met the vertical focusing requirements of the dual workpiece of the lithography machine. When the limit rotation angle of Rx/Ry/Rz uniaxial rotation or triaxial combined rotation was ± 1.5 mrad， the AC signal quality still met the measurement requirements. Moreover， it met the requirements of Rx/Ry/Rz leveling rotation of the dual workpiece table of lithography machine. To summarize， the displacement measurement system of the designed planar grating encoder met the corresponding measurement requirements of the photoengraving machine dual workpiece table and achieved higher performance indexes.Keywords：immersion lithography scanner;displacement measurement;grid encoder;spatial-separated;heterodyne;Littrow angle- 369
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Published：2022-03-25 -
Three-dimensional light field endoscope calibration based on light field disparity amplifier and super-resolution network
Abstract：Three-dimensional （3D） light field imaging in laparoscopic surgery is an emerging technology， which has the potential to enable 3D imaging. Calibration is fundamental for the 3D light field endoscope （LFE） to accomplish 3D imaging and is essential but challenging as the light field bandwidth product is limited； moreover， the light field disparity of a 3D LFE is smaller than that of the conventional light field camera， which makes it difficult to achieve acceptable light field calibration results. In this paper， the small light field disparity was amplified by computing the distance between two feature points in a 3D scene. Compared with the conventional light field disparity between different feature points in different sub-aperture images， the distance between the two feature points enlarges the distance， i.e.， the disparity between point-to-point and point-to-line， in the light field image， which leads to better calibration accuracy. Furthermore， an improved super-resolution network based on SRDenseNet was proposed， where cascaded channel attention dense blocks were applied to acquire the features of low-resolution light field images. The super-resolution network improved the two-dimensional （2D） spatial resolution and 2D angular resolution in the 4D light field data simultaneously and improved 3D LFE calibration accuracy indirectly. The experimental results show that the amplified light field disparity and higher resolution facilitated a higher calibration performance， and the re-projection error of the 3D LFE calibration decreased by 16%， while the R-square increased by 6%.Keywords：medical imaging;light field endoscope;light field disparity;Super-resolution;calibration- 199
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Published：2022-03-25 -
Fringe projection profilometry by ternary-gray encoded phase unwrapping method
Abstract：Multiple fringe images are required in fringe projection profilometry for time phase expansion. To obtain faster three-dimensional （3D） measurement by reducing the number of fringe images projected and collected by fringe projection profilometry， in this paper， a ternary-gray encoded phase unwrapping method was proposed for fringe projection profilometry. First， five fringe images were projected by the projector onto the surface of the measured object， which included three sinusoidal phase-shift fringe images and two ternary-gray encoded images. The deformed fringe images modulated by the surface of the object were collected by a camera. Second， the wrapped phase was calculated from the three deformed sinusoidal phase-shift fringe images collected by the camera. After spatial gray averaging， gray ternarization， gray pseudocode removal， the ternary-gray encoding value was obtained using the ternary-gray encoded images collected by the camera. After obtaining the encoding value， the spatial neighborhood information of the encoding value was decoded to obtain the unwrap phase level， and the wrapped phase was unfolded. The ultimate unwrap phase result was obtained by removing the incorrect phase points caused by the alignment error. Third， the 3D topography of the measured object surface was obtained through the calibration coefficients obtained by the system calibration and the ultimate unwrap phase. The experimental results showed that， compared with the optimum three-frequency with four-step phase shift method， the method proposed in this paper not only had the same measurement accuracy but improved the measurement speed by 2.4 times. The proposed method improved the efficiency of 3D topography measurement without reducing the measurement accuracy. It has practical application value for the rapid measurement of complex surfaces.Keywords：fringe projection profilometry;ternary-gray encoded;phase unwrapping;wrong phase points- 194
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Published：2022-03-25 -
Flat field correction technique for time delay integration fluorescence microscopy imaging
Abstract：Owing to their high frame rate and sensitivity， time delay integration （TDI） image sensors are widely used in fluorescence microscopy imaging systems with high throughput and large field of view. Uniformity of response in the field of view of the microscope objective is the basis for accurate acquisition of fluorescence energy distribution. To improve the imaging quality and measurement accuracy of the system， this study investigated the response non-uniformity correction method applicable to the TDI fluorescence microscopy imaging system. First， a laser-induced fluorescence imaging model was devised based on the working principle of the TDI fluorescence imaging system， and the mechanism of image uniformity degradation was analyzed. A two-step correction method based on microarray biochips was then proposed， which divided the nonuniformity of system response into high frequency and low-frequency parts to be corrected separately. The former can be corrected by traditional two point correction， whereas the latter is corrected using the proposed microarray biochip-based method. Finally， experiments based on a high-throughput TDI fluorescence microscopy imaging system were conducted to verify the calibration method employed in this study. The experimental results show that the proposed approach reduces the response nonuniformity of the TDI fluorescence imaging system from 25.21% to 2.87%， implying a significant improvement in the system performance. Moreover， it shows that the correction method proposed in this paper can significantly correct the response nonuniformity of TDI fluorescent imaging systems， and is therefore effective and practical.Keywords：fluorescence microscopy imaging;time delay integration;nonuniformity correction;microarray biochip- 155
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Published：2022-03-25 -
Hybrid millimeter wave RF/MIMO-FSO relay system
Abstract：To accurately evaluate the performance of the hybrid millimeter wave radio frequency （RF） and multiple input multiple output （MIMO） free space optical （FSO） relay system， the Meijer G function is used to derive the end-to-end interruption probability， bit error rate and traversal of the hybrid mmW RF/MIMO FSO relay system. Further， the proposed approach employs the closed analytical expression of the capacity. The relay of the hybrid system adopts a fixed gain scheme， and the receiving end adopts an equal gain aggregation scheme to combine multiple equivalent signals. The mmW RF channel obeys Nakagami-m fading， whereas the FSO channel obeys the Gamma-Gamma distribution. The results of numerical simulation show that the MIMO technology can effectively alleviate the performance degradation of the hybrid relay system caused by the intensification of atmospheric turbulence. The performance of the hybrid relay system is gradually enhanced by increasing the number of transceiver apertures. However， this also increases the complexity of the system. Comprehensive analysis shows that the MIMO-FSO link performs best when the number of apertures receiving the laser signal from the transmitter is equal to the number of receiving apertures on the photo detector （i.e. two apertures）.Keywords：space optical communication;hybrid radio frequency /free space optical relay system;millimeter wave;multiple input multiple output;fixed gain- 126
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Published：2022-03-25 -
Scintillation suppression along 12.7 km space laser communication horizontal path
Abstract：The scintillation effect of light intensity caused by atmospheric turbulence leads to the decline in signal-to-noise ratios， which seriously affects the normal operation of free-space laser communication. Spatial diversity technology can overcome this impact effectively. To suppress the scintillation of atmospheric turbulence over a 12.7 km urban horizontal channel， a spatial diversity receiving and transmitting system was designed. The suppression effects of maximum ratio combining scheme， equal gain combining scheme and selective combining scheme for diversity reception were compared and verified. The results show that the equal gain combination scheme has simple engineering implementation and an obvious suppression effect. Compared with single-channel signal， the scintillation index can reduce to 40.1%. Finally， according to the change of scintillation index throughout a day， the variations of atmospheric refractive index structure constant are inversed. The trend is consistent with the simulation result of PAMELA model. The results have great significance to improve the quality of free-space optical communication.Keywords：free space optical communication;light intensity scintillation;spatial diversity technology;turbulence suppression;diversity merging- 146
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Published：2022-03-25
Modern Applied Optics
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Large-area nanoimprint lithography： processes and device applications
Abstract：Large-area nanoimprint lithography is a patterning technology for fabricating micro-/nano-structures on a large scale with good repeatability， low cost， and high resolution. These advantages of rapidly constructing various polymers make large-area nanoimprint lithography a unique technology for the fabrication of micro/nano optical and optoelectronic devices for several applications， such as light emitting diodes， displays， augmented reality waveguides， and microfluidic devices. Therefore， large-area lithography is crucial for the commercialization of nanotechnologies. This review summarizes recent developments in the field of large-area nanoimprint lithography， including basic processes， fabrication methods for imprint molds， and typical device applications. Lastly， the prospects and challenges for large-area nanoimprint lithography are discussed.Keywords：nanoimprint lithography;large-area imprint template;large-area patterning;devices applications- 227
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Published：2022-03-25 -
Rapid-precision velocity measurement of linear motor based on weighted peak fitting of transformed sequence
Abstract：To achieve real-time robust precision measurement of linear motor velocity， a measurement method based on weighted phase correlation peak fitting was proposed in this study. First， a sawtooth stripe target image was constructed， and a continuous sawtooth fringe sequence was obtained by using a line scan camera installed on the side of the mover. Then， a phase difference algorithm was used to determine the cross power spectrum of the adjacent signals， and an inverse proportion function was introduced by applying the Fourier inverse transform to the Dirac function. To obtain the sub-pixel displacement of the fringe signal more quickly and stably， simple weighting was applied to the peak point and two neighboring points. Finally， the real-time velocity of the linear motor was calculated based on the system calibration coefficient and a fixed shooting interval. The experimental results show that the average time required by this method is approximately 0.02 ms， with a measurement accuracy within 0.05 pixel and an accuracy of velocity measurement of 0.03 mm/s.Keywords：linear motor;image velocimetry;sawtooth stripe image;weighted phase correlation peak fitting;sub-pixel measurement- 138
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Published：2022-03-25 -
Improving the forming quality of laser dynamic flexible micro-bulging by laser pre-shocking
Abstract：In this study， a novel micro forming process called laser pre-shocking assisted laser dynamic flexible micro-bulging is proposed to improve the quality of micro-formed parts. The laser shocking was used to pretreat copper foil to improve its formability， and the forming quality of micro-bulging parts was examined from the perspective of bulging depth， surface quality， thickness reduction rate， forming symmetry， and cross-section morphology. The experimental results indicated that the bulging depth and surface roughness increased with the increase in laser energy. When the laser energy increased from 325 mJ to 575 mJ， the bulging depth increased from 216.3 μm to 388.7 μm. The forming quality of micro-bulging parts deteriorated with the increase in laser energy. The maximum thickness reduction of the micro-bulging parts and local necking occurred in the central area of the bulging and mold entrance area. However， fracture occurred in the mold entrance area. With the increase in laser energy， the fracture mode changed from a mixed mode of shear and ductile fractures to shear fracture. Laser pre-shocking is beneficial to improve the forming quality of micro-bulging parts and restrain the occurrence of local necking.Keywords：laser pre-shocking;laser dynamic flexible micro-bulging;copper foil;forming quality;fracture mode- 127
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Published：2022-03-25 -
Difference iteration method of tool arc
Z -direction compensation for stabilizingX -axisAbstract：This paper proposes a difference iteration method based on slow tool servo technology to accurately determine the Z-direction of the tool arc as well as the compensation in planning the free-form surface tool path. Further， the proposed approach aims to reduce the tracking error caused by the inertia of the X-axis and hence improve the cutting efficiency. First， the polar diameter of the ideal tool point is compared with that of the actual tool point， which is calculated from the given tool contact， to determine whether the difference is less than the resolution of the lathe guide rail. If the difference is smaller， the tool point is adopted. However， if the resolution is exceeded， the difference value is compensated into the initial tool contact， the compensated tool position is recalculated， and the resulting polar diameter is then compared with the ideal polar diameter. This is an iteration， and the final tool position can be established through multiple iterations. The experimental analysis proves that the polar diameter and position of the tool point can meet the gradient of the Archimedean spiral after three iterations， after which the X-axis of the lathe is fed stably， the Z-axis compensation is smaller， and the final machining accuracy is up to PV0.226λ@632.8 nm. The difference iterative method does not require complex equations and surface fitting， making it simple， accurate， and efficient.Keywords：optical fabrication;free-form surface;stabilize X-axis;Z-direction compensation;difference iteration- 124
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Published：2022-03-25
Micro/Nano Technology and Fine Mechanics
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Visible and infrared image matching based on CycleGAN-SIFT
Abstract：Traditional infrared and visible image matching algorithms generally have the problems of low matching accuracy and poor robustness due to the different imaging mechanisms of source images. To solve this problem， a visible-infrared image matching algorithm based on the CycleGAN-SIFT was proposed. To reduce the influence of feature differences between visible and infrared images， a pseudo-infrared image was generated by CycleGAN by applying transfer learning and sharing weight. The feature extraction algorithm， known as scale-invariant feature transform（SIFT）， was used to extract and match the feature points of the pseudo-infrared image with those of the infrared image. Then， to reduce the false matching rate， random sample consensus（RANSAC） was used to eliminate the false matching point pairs. Finally， the feature points of the pseudo-infrared image were mapped to the visible image， thus finalizing the match between the visible and infrared images. To verify the effectiveness of the proposed algorithm， four groups of heterogeneous images were selected from the OTCBVS and TNO image fusion dataset and tested under the three conditions of no noise， noise， and angle distortion. Experimental results show that the matching accuracy of the proposed algorithm can reach over 95% when the angle distortion and noise interference are not considered. In the presence of angle distortion and noise interference， the matching accuracy still remains above 95%， thereby confirming the high matching accuracy and strong robustness of the proposed algorithm.Keywords：image matching;heterogeneous image;CycleGAN;scale-invariant feature transform（SIFT）;random sample consensus（RANSAC）- 157
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Published：2022-03-25 -
Deep hyperspectral image denoising by fusing space spectrum-gradient features
Abstract：To remove the noise generated during the process of hyperspectral image acquisition and to improve the performance of subsequent image processing， a deep hyperspectral image denoising method was proposed based on the fusion of spacial spectral and gradient features. It included spacial spectral and gradient characteristic networks， and each network was optimized with a dense connection and separable convolution strategy. The spacial spectral network model extracted the noise features， and the gradient network model supplemented the texture features of the noise. Finally， the feature extraction results of the two networks were fused to achieve characterization of the noise features and to restore clean images. In this study， the effectiveness of the proposed method was verified on synthetic-noise and real-noise images. Experimental results showed that the method had a significant effect on the restoration of the internal structure of images. Under the condition of noise standard deviation of 50， the mean PSNR reached 29.426 dB， while the mean SSIM reached 0.967 8 dB. The denoising results and the original image were classified by SVM algorithm， and the classification accuracy reached 90.89%.Keywords：hyperspectral image;denoise;spacial spectral network;gradient network;dense connection;separable convolution- 193
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Published：2022-03-25 -
Restoration of blurred infrared image by adaptive double edge method
Abstract：Owing to the low contrast and high noise present in infrared images， image edges are frequently blurred， severely affecting the extraction and recognition of information from the images. Based on the principle of phase congruence， an adaptive method of double-edge extraction for blurred infrared image restoration was proposed. First， the phase congruence points in the frequency domain were used as features to extract image edges. Then， two edges were adaptively selected according to the optimal angle principle to fit the Point Spread Function （PSF） of the degraded image. Finally， the fuzzy image was restored by using the Fast Total Variation （FTV） regularization model， with the PSF as prior information. Experimental results show that the method based on phase congruence is efficient and robust and can extract edges having varying shapes. Furthermore， under various conditions of image degradation， such as defocusing， optical diffraction， and relative motion， the average gradient and information entropy of the restoration improve by 11.5% and 1.4%， respectively， compared with the results of the adaptive single-edge algorithm. The restoration effect of the elliptical PSF generated by the double-edge method is considerably better than that of the circular PSF generated by the traditional single-edge method.Keywords：image restoration;phase congruence;self-adaption;double edges;the optimal angle principle;point spread function- 192
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Published：2022-03-25
Information Sciences
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