Objective Photolithography is a crucial technology in integrated circuit manufacturing. With the critical dimension of photolithography becoming smaller, integrated circuits are becoming more and more compact. For this reason, a variety of resolution enhancement techniques, such as off-axis illumination, have been proposed. The influence of pupil performance on the quality of exposure pattern and overlay accuracy is prominent with the application of this technology. Off-axis illumination modes are generated by the illumination system of the photolithography machine. The generally used illumination modes include conventional, annular, 0° quadrupole (Q0), 45° quadrupole (Q45), dipole in X direction (DX), and dipole in Y direction (DY). In the working process of the photolithography machine, the lenses are exposed to deep ultraviolet light. Thus, the transmittance of their optical materials and coating films gradually deteriorate, which may affect the pupil performance. It is necessary to adopt the pupil correction technology for illumination system to improve the pupil performance.
Recently, various pupil correction methods or devices have been proposed. However, these methods have not produced a specific correction process. Energy loss during pupil correction affects production rate of photolithography machine. To solve these problems, a high energy efficiency pupil correction method is proposed in this paper. This method can improve the performance of a corrected pupil while energy efficiency is increased maximally. As a result, production rate of photolithography machine is improved while ensuring quality of exposure pattern.
Methods The main goal of the pupil correction method is to correct an unqualified pupil to a full-field pupil correction target. In traditional correction method, the theoretical limit (zero) is set as the target. However, the reference energy value for the correction method proposed in this paper is changed. The reference energy value is selected by considering the process and assembly errors of the pupil corrector. The deviations of pupil correction results at different fields of view are also considered. In this way, the performance of the corrected pupil can meet the requirements while the energy loss during the correction is minimized. Furthermore, considering the correction requirements of different illumination modes, a pupil correction plate suitable for many illumination modes is designed by the multi-ring partition method.
To verify the feasibility of this method, a pupil correction analysis is conducted. The pupils, including three annular illumination modes and three different quadrupole illumination modes, are measured in the illumination system of a KrF step-and-scan photolithography machine with NA=0.82. These annular (quadrupole) illumination modes are corrected by a correction plate, for which the transmittances are calculated by the multi-ring partition pupil correction method. The pupils of these annular (quadrupole) illumination modes are divided into three rings ([0, 0.5), [0.5, 0.75), and [0.75, 1]) along the radial direction. According to the manufacturing and assembling errors of the pupil corrector, Δ1 is set to 0.3%. Δ2 is set to 0.2% for the difference of pupil correction results at different fields of view.
Results and Discussions For a comparison study, the traditional correction method and the method proposed in this paper (
Conclusions In this paper, the high-energy efficiency pupil correction method based on multi-ring partition is proposed. The multi-ring partition method is adopted, and the selection method of the reference energy value is changed. In this way, one pupil correction plate is suitable for several illumination modes. The energy loss caused by pupil correction can be reduced, and the performance of the corrected pupil meets the requirements. The results of pupil correction indicate that the maximum energy loss of annular and quadrupole corrected pupils can be reduced from 5.54% and 3.06% to 2.06% and 0.93% respectively, in comparison with the traditional pupil correction method. This method is of great significance to improve the production rate of the photolithography machine.
Objective With the rapid development of virtual reality technology, indoor navigation technology, and indoor positioning technology, the extraction and modeling of indoor 3D point cloud objects have become a research hotspot. Under normal circumstances, an indoor scene is quite complex, and the point cloud data obtained by scanning is usually cluttered. There are many objects and occlusions, and automatic modeling cannot be carried out. It is necessary to segment a complex indoor point cloud into simple geometric primitives to perform modeling. Because there are several plane structures in indoor scenes, such as walls and ground, plane segmentation for indoor scene point cloud is a crucial part of segmentation for indoor scene point clouds. Owing to the complexity and bulkiness of indoor scene point clouds, traditional random sample consensus (RANSAC) and 3D Hough transform methods are complex and inefficient in the process of plane segmentation for indoor scene point clouds. In this article, we propose a new method for plane segmentation of indoor scene point clouds. Compared with existing methods, this method has a great improvement in time efficiency and is more suitable for plane segmentation for indoor scene point clouds.
Methods In this article, a new plane segmentation method based on projection length point cloud layering and mean shift (MS) normal vector constraint is proposed. First, the method estimates the normal vector of the point cloud by the principal component analysis method, combines the coordinates of the point cloud to obtain the projection length, and then layers the point cloud according to the projection length by a certain step. Afterward, it takes the current maximum stratified point cloud for normal vector constraint based on the MS method to get the point cloud with the most concentrated normal vector. Next, it uses the remaining points to perform RANSAC and least squares plane fitting to obtain the plane parameters and then removes the point cloud contained in the current plane model by a certain thickness threshold. The above steps are repeated to obtain the parameters of all planes until the number of plane points extracted is less than a certain value. Finally, the model point clouds are extracted from the original point cloud based on the obtained plane parameters, and after the model optimization that includes planes merging, error point reclassification, and irrelevant point elimination, the final plane segmentation result is obtained.
Results and Discussions In this article, a new concept of projection length of point cloud is proposed that is used to segment the plane of point cloud in an indoor scene (Fig. 2). The indoor point cloud is layered on the basis of the projection length, and the resultant point cloud number histogram can initially reflect the number of planes and distance distribution in the scene (Fig. 4). The projection lengths of the planes calculated from the resulting plane parameter fall in the peak or adjacent interval in the resultant point cloud number histogram (Table 2). After the point cloud layering based on the projection length, most points in the maximum layer come from the same target plane, and there are only a small number of irrelevant points. After MS clustering, the remaining points are all from the target plane, which is convenient for plane fitting (Fig. 5). The proposed method can completely segment the plane structure of indoor scenes, including walls, ceiling, floor, and desktop. Meanwhile, other irrelevant structures, such as potted plants, chairs, and door frames, are removed in the segmentation process (Figs. 7 and 8). The distances between the obtained plane models are very close to the actual measured distances; the difference is in the millimeter level (Table 3). The deflection angles between the planes obtained in this study, and the planes obtained by single-point measurement are all within 0.2° (Table 4). Compared with the maximum likelihood sample consensus method and improved 3D Hough transform method, the proposed method is obviously better in terms of total time consumption (Table 5).
Conclusions In this article, we propose a new method of plane segmentation for indoor scene point cloud. Through the point cloud layering based on projection length and normal vector constraint based on MS, the proposed method can quickly obtain points from a single plane, thereby achieving plane fitting and segmentation rapidly and then gets the final result after model optimization. Experiments show that the proposed method can effectively segment the plane structure in the indoor scene point cloud, and the model optimization can avoid over-segmentation and remove irrelevant points. Simultaneously, the experiment proves that the segmentation result of the proposed method has higher accuracy and meets the requirements of later modeling. In addition, compared with two improved classical methods for point cloud segmentation, the proposed method is time efficient and is suitable for segmentation for a large number of point clouds.
Significance Distance measurement is a common basic technology in the field of geometric measurement and has broad applications in scientific research and industry. Currently, high-precision distance measurement is normally achieved using the interferometric method, and the distance results can be directly traced to the optical wavelength. However, the phase ambiguity hinders the application of the traditional interferometric method in long-distance absolute positioning, such as space missions, including tight formation-flying satellites, antenna measurement, spacecraft rendezvous and docking, as well as precision manufacturing and assembly, including aircraft manufacturing, satellite equipment manufacturing, and synthetic aperture optical system assembly. Fortunately, invention of the optical frequency comb (OFC) provides great opportunities for geometric measurements.
In recent years, several OFC-based methods have been proposed for the measurement of distances, e.g., the intermode beat, dispersive interferometry, pulse alignment, and dual-comb methods. Compared with conventional methods, OFC-based methods are capable of resolving the problem of phase ambiguity and measuring the absolute distance. Among them, the dual-comb ranging method makes full use of the characteristics of OFC in the time and frequency domains and exhibits reasonable dynamics, precision, and unambiguity range. The dual-comb method opens up a new direction for distance measurement and is expected to bring great benefits to optical metrology. Since 2009, many advances have been achieved in dual-comb ranging techniques. However, there are still several challenges involving principle research and industrial applications. Hence, it is necessary to summarize progress of the dual-comb ranging technique to guide future development in this field more rationally.
Progress The progress of the dual-comb ranging method is illustrated in
Conclusions and Prospects In summary, the dual-comb ranging technique provides an efficient tool for absolute distance measurement with a large unambiguity range, high precision, and high speed, and it has also become a hot spot in the field of ranging. Such an overall performance brings great benefits to various tasks in optical metrology. With the continuous in-depth and detailed explorations of dual-comb ranging, it is expected to become a portable instrument product widely used in scientific research and industry.
Significance The weak-measurement (WM) theory was proposed by Aharonov et al. in 1988. We can obtain the measurement value that is much larger than the eigenvalue using the WM theory by appropriately adjusting the pre-and post-selection states and maintaining a small interaction intensity between the system under test and the detector. The small interaction maintained here is what referred to as “weak” in the WM. In the process of weak interaction, an important parameter, named “weak value,” contained in the pointer state has always played an important role. Therefore, we call this process of significantly amplifying the actual parameters as the weak value amplification. However, because it is impossible to prove the existence of this “weak value” at the beginning, the WM theory has been questioned by the scientific community to the extent that some believed that the WM theory was an absurd idea. In 1989, Duck et al. re-explained the concept of WM, and then in 1991, Ritchie et al. verified the existence of weak values of key parameters in WM through experiments. Consequently, the WM theory is widely accepted.
Progress The WM theory provides a deeper explanation for quantum physics and shows potential for the precision measurement. However, in the next decade, WM development mainly revolved around the theoretical study, such as WM realization in some specific systems, related study on the pointer states in the WM systems, and the “weak value”. In this paper, we discussed important parameters, the WM techniques in classical and quantum physics, the prospect of applying WM techniques to the fields of high-precision measurement, and some other theoretical studies. After the first five years of the 21 st century, the WM techniques have gradually showed their own unique properties in the measurement field. In 2005 and 2007, Pryde and Jozsa, respectively, implemented the WM experiments in polarization detection and measured the complex weak value. They explained in detail the physical meaning of the real and imaginary parts of the weak value in the actual measurement. The WM has been improved to the stage of high-precision measurement. In 2008, Hosten et al. reported a related WM work that studies the spin Hall effect of observing light in Science, which refocused the spotlight on the WM, the promising techniques. Owing to the high measurement accuracy and potential, the amplification mechanism of the WM can be used to observe the physical phenomena and detect the physical parameters.
Furthermore, several related theories since 2010 have shown that the WM performance in the frequency domain has more obvious detection advantages than other fields. Particularly, in 2011, Li group from the University of Science and Technology, China, proposed that white light could be used to achieve high-sensitivity detection of optical phase in the WM system. Two years later, they verified the WM through experiments, which laid the foundation for the WM applications in frequency domain optics. In 2016, He Yonghong group used broadband high-brightness super-luminescent diode as a light source and realized an optical frequency domain WM system with a wide range of general values. Compared with the traditional optical interference detection, the WM system is 1--2 orders of magnitude higher in detection accuracy.
Currently, the WM techniques are widely implemented in four fields including the time domain, frequency domain, spatial domain, and polarization angle distribution based on the requirement of different applications. Relevant study results show that the WM technique has good applicability and high measurement accuracy in these four fields. The representative work in each field is as follows:
(1) Time-domain WM: single-photon tunneling time and observing the spin Hall effect of light.
(2) WM in frequency domain: subpulse width time delay, temperature measurement, and phase shift.
(3) WM in the spatial domain: ultrasensitive beam deflection measurement, Goos-H?nchen displacement, WM techniques to improve the SPR resolution.
(4) Weak-polarization angle measurement: polarization rotation, beam deflection angle measurement, light polarization measurement, chiral molecule measurement, and deviation of the beam angle in reflection or refraction.
Conclusions and Prospects In this paper, we introduce two types of measurement methods combining weak-value amplification based on practical applications of the weak-value amplification in high-precision measurement. These methods are used to measure changes in physical quantities by analyzing the lateral offset and frequency shift of the beam. Based on these systems, the weak-value amplification can be combined with many traditional measurement methods to improve the resolution of the system. Finally, we discuss the development trend of weak-value amplification. Combining the traditional detection methods and weak-value amplification techniques to achieve higher system resolution, indicators are a direct application of the WM in high-precision measurement. Because the weak-value amplification techniques can excellently suppress technical noise, the existing WM system has a resolution of ~1--2 orders of magnitude improvement compared with that of the traditional system. The WM techniques have potential applications in the fields of biology and chemistry.
Objective In the past 20 years, ultra-short ultra-intense laser technology has experienced rapid development. However, the maximum output power of these lasers is limited by nonlinear effects, large diameter compression grating technology, gain bandwidth limitations, and other factors. One of the most promising technologies to further enhance output ability is coherent beam combining. Effective coherent beam combining requires strict inter-beam synchronization. In recent years, many attempts have been made to improve synchronous measurement and control. The research progress of most implementations has been solely based on photoelectric detection, optical balanced cross-correlation, and temporal and spatial interferences. Nevertheless, these methods need to maintain the time interval of the two beams in coherent time, limiting the femtosecond pulse synchronous measurement range within 1 ps. The ability of an electronic oscilloscope to achieve a time resolution less than 10 ps is difficult; therefore, it is more difficult to accurately measure the pulse delay within 1--10 ps. In addition, for online synchronous measurement of a multichannel ultra-short pulse coherent beam combining system, the abovementioned methods are more complicated to implement and cannot achieve a single-shot measurement. In this paper, a single-shot measurement method for a multichannel ultra-short pulse with large dynamic range time synchronization based on all-fiber spectral interference is proposed. This method has a wider measurement range to measure synchronization than the nonlinear correlation method and a larger measurement accuracy than an oscilloscope. Our method improves efficiency in multichannel laser synchronous measurements for engineering applications and has important application potential for multichannel ultra-short pulse laser coherent beam combining systems.
Methods First, theoretical and simulation analyses based on multichannel optical fiber array spectral interferometry were carried out. Predictions of τmin and τmax for the designated measurement range were made according to Equation (6). Considering the purpose of synchronous measurements, this study created the concept of fixed time offset. The beneficial effect of this concept is that through the comparison of measured values and fixed offset time, we can determine the absolute time difference between the referenced light and the light to be measured. Moreover, with a fixed offset time, when the measured values were equal to the fixed offset times introduced by optical delay lines on the referenced light fiber paths, the two pulses reached a zero-synchronization state. In our experiment, the feasibility of the single-shot multichannel synchronous measurement method was verified. The experimental optical path was built using the path of a four-channel pulse synchronous measurement as an example (Fig.3). The three formed interference signals and one beam of reference light were input to the imaging spectrometer using a multipath fiber buncher.
Results and Discussions The spectrogram in the experiments is recorded by an imaging spectrometer, which indicates that the spectrometer has the ability to record 20 signals (Fig.4). The delay, τ, between the reference and measured beams is obtained through the data processing method described in Section 2.1. This method illustrates that τmax is equal to 14.751 ps and τmin is equal to 1.055 ps, which determine the measurable range (Fig. 5). From experimental results, the range that can be measured is slightly less than the theoretical interval, mainly due to airflow disturbances, mechanical vibration, and dark current noise from the spectrometer. For measurement precision of different offset points, the deviation of the statistical mean value of multiple measurement results is obtained from the present value. In Figure 6, it is shown that with the increase of temporal spacing (TS) between the two pulses, the β value decreases. When TS reaches 6.139 ps, the β value is at its minimum. When TS is greater than 6.139 ps, the β value increases continuously. The measured jitter, γ, is shown on the right vertical coordinate of Figure 6 and it shows the same trend as the β value (Fig.6). Measurement error is because of uncertainty of the wavelength or frequency spacing of the interference fringe in the spectrogram caused by noise. However, the degree of response of different fixed offset times to noise is different. Therefore, the measurement accuracy is varied at different fixed offset times.
Conclusions This paper demonstrates that the single-shot synchronous measurement technique for a multichannel ultra-short pulse laser based on all-fiber spectral interference is feasible through simulation and experiment. The measuring range is determined by the spectral interference fringe spacing, and the theoretical simulation results show that a fixed time offset is beneficial for the realization of a zero-synchronization state measurement. The optimal solution of the offset time is obtained using experimental statistical results. Experimental data prove that setting the fixed time offset in the center of the measurable range area can improve measurement accuracy. The minimum time synchronization accuracy is 5.3 fs and the measurement range is 1.055--14.751 ps, which are in good agreement with results of the theoretical analysis. The all-fiber spectral interference synchronization measurement method combines the characteristics of spectral interference and optical fiber array in design. The advantages of the method are easy integration of an optical fiber path, fast processing speed of spectral interference data, and low-energy demand of signals. Our method can satisfy the ultra-short ultra-intense laser facility real-time and multichannel measurement diagnosis requirements. The method also makes up for a small measurement range and poor temporal resolution when measuring the synchronization state using the nonlinear correlation method and an oscilloscope, respectively. The complexity of the configuration and difficulty of a single-shot measurement in multichannel synchronous measurements are solved. Therefore, our method has important application prospects in multichannel ultra-short pulse laser coherent beam combining systems.
Objective The safety of pavement manhole covers is crucial in urban development. The timely and accurate detection of manhole cover disease can save maintenance costs, reduce road hazards, and ensure driving safety. Traditional methods for surveying and mapping manhole covers mainly use manual measurements, which usually require considerable human and material resources. Moreover, such measurements have a low operating efficiency and poor safety, which is not conducive for the rapid update of data. Therefore, new, efficient, and automated methods and techniques are urgently required for the manhole cover measurement and disease detection. Currently, methods for manhole cover extraction and disease detection mainly include the differential polar method, ellipse feature-based fitting algorithm, and image feature detection method. These methods exhibit low robustness. Moreover, the direct image detection methods are affected by the image quality and illumination. It is difficult to obtain information about manhole cover diseases using such methods. This study develops a technical process involving the original point cloud and the manhole cover extraction and disease detection using vehicle-borne laser point cloud data. Based on the intensity image combined with the improved Hough algorithm for achieving the accurate road manhole cover position and disease information, the experimental results show a good robustness and stability of the proposed method. We hope that the proposed technical solution can help the city management department in inspecting and maintaining manhole covers to effectively improve the extraction efficiency and operation safety of manhole covers.
Methods First, based on the high-precision vehicle-borne laser point cloud data, accurate ground point cloud data were obtained using a combined filtering algorithm of the point cloud gradient and cloth simulation. They eliminate the influence of invalid features on the manhole cover extraction. Second, the intensity orthographic method was used to generate high-resolution intensity images of the ground points. Moreover, the manhole cover was binarized using the adaptive threshold method to increase the edge display effect of the road manhole cover. Then, according to the shape and position characteristics of the manhole cover circle, the edge was detected based on the image binarization segmentation result. Further, the location of the manhole cover circle was divided into potential manhole cover object detection and a real manhole cover using the Hough circle detection algorithm, which strictly limits the curvature and edge accumulation threshold. Finally, using the two processes of object detection, the precise extraction of the manhole cover position was achieved. Next, the disease information of the manhole cover was obtained by calculating the elevation value of the adjacent point cloud within a certain distance between the manhole cover position and the surrounding area. Finally, a high-precision GPS-RTK and DS3 level comparison experiment was performed to evaluate the stability and reliability of the proposed algorithm.
Results and Discussions Regarding the road surface properties of the manhole cover position, this study first proposes the combined filtering method of the point cloud gradient and cloth simulation. The latter performs secondary filtering to retain and optimize the ground point results of gradient filtering for invalid floating data elimination. Several data tests were used to obtain the accurate ground point cloud results (Fig.12). Because the intensity image contains considerable noise and requires a large number of Hough calculations, the accurate position of the manhole cover circle is obtained by detecting the edge contour (Fig.13) and setting the appropriate curvature and edge accumulation threshold in the improved Hough circle detection algorithm. Combined with the actual vehicle-borne laser point cloud experimental study, the accuracy and precision of manhole cover extraction reach 84% and 98%, respectively. Additionally, the manhole cover extraction efficiency is significantly improved and the vectorization result of the manhole cover extraction can be displayed in the real point cloud coordinates (Fig.14). Furthermore, the accuracy experimental results show the robustness and reliability of the manhole cover extraction plane position (Table 2) and settlement disease detection results (Table 3).
Conclusions In this study, in view of the difficulty and low efficiency of traditional road manhole cover measurements, the vehicle-borne laser point cloud data are directly used to achieve the precise manhole cover position and disease detection. First, a combined filtering algorithm of the point cloud gradient and cloth simulation is proposed to obtain the high-precision ground point data and generate intensity images. Then, this technique was combined with the adaptive threshold binarization method to obtain the high-discrimination manhole cover edge contour using the improved Hough circle detection algorithm. The manhole cover is approximately positioned within the circle curvature limit, and the accurate position is achieved using the edge accumulation threshold based on the previous step. Finally, the position parameters and disease information of the manhole cover are obtained. Combined with field experiment data verification, the accuracy and precision rate of manhole cover extraction of the proposed method reach 84% and 98%, respectively, greatly improving the manhole cover disease detection efficiency and operation safety compared with traditional methods. Combined with the precision analysis of the same name detection points, the manhole cover extraction results of the proposed scheme show high accuracy and the data results can meet the requirements of related projects. The technical scheme and experimental results of this research show the effectiveness and reliability of the vehicle-borne laser point cloud used in manhole cover extraction and disease detection and provide new ideas for urban intelligent management.
Objective Line structured light profile measurement is an important technique for rail profile detection. Currently, simulation analysis is instrumental in the research of rail grinding mechanism and track structure dynamics. Optical simulation design software has also been subjected to considerable research in optical system design, simulation modeling, and error analysis. However, few reports have focused on the simulation modeling of the line laser rail profile measurement system. In view of this situation, a simulation model of the rail profile measurement system based on Zemax software is proposed. The proposed simulation model is of guiding significance for designing optical systems, selecting optical elements, and improving measurement accuracy. It can provide theoretical support for the accuracy improvement and reliability evaluation of the rail profile measurement system.
Methods The rail profile measurement system is divided into image acquisition, system calibration, and profile measurement modules. The image acquisition module obtains the rail laser cross section image and mainly includes the line laser, lens, and camera. The system calibration module obtains the calibration parameters, i.e., the transformation relationship between the image plane in the pixel coordinate system and the measurement plane in the world coordinate system. The profile measurement module extracts the center pixel coordinates of the light stripe from the rail laser cross section image obtained using the image acquisition module. Then, it transforms the central pixel coordinates of the light stripe into the world coordinate system using the calibration data to determine the real rail profile. Based on the division of the system function modules, the system modeling process is divided into three steps (Fig. 3). In the first step, the image acquisition module is modeled (Fig. 8). First, the optical model of the main components is established in the Zemax non-sequential mode. Then, the system simulation model is established by combining the optical model of the components and optical structure parameters to ensure that the system simulation model has the image acquisition function. In the second step, the system calibration module is modeled based on the plane target calibration method (Fig. 10). The image acquisition module collects the calibration board images under different poses, and the system calibration parameters are calculated. In the third step, the profile measurement module simulates the rail profile measurement process (Fig. 12). The image acquisition module scans the rail at a certain sampling interval along the rail direction (extension direction) and obtains the rail laser cross-section image at equal intervals. The real rail profile is calculated using the rail laser cross section image; hence, the system simulation model has the profile measurement function (Fig. 9).
Results and Discussions To comprehensively evaluate the measurement accuracy of the system simulation model, component accuracy verification, rail simulation measurement, and actual rail measurement experiments are performed (Figs. 15--17). Experimental results show that the root mean square error (0.049 mm) obtained using the system simulation model is close to the root mean square error (0.066 mm) obtained using the actual measurement device based on the 20 repeated measurement data of rail vertical wear (Table 5). The system simulation model achieves high accuracy, and the simulation measurement results are consistent with the actual situation, thus demonstrating that the simulation model can better simulate the rail profile measurement system.
Conclusions A simulation model of the rail profile measurement system based on Zemax is proposed. The simulation model has image acquisition, system calibration, and profile measurement functions. The results show that the simulation model is consistent with the measurement results of the actual measurement system, and the simulation model can be used to simulate the rail profile measurement process using line structured light. The differences between the simulation model and the actual system are highlighted from different aspects, thus providing a reference for further improving the simulation model. The system simulation model can be used for analyzing related problems in the field of rail profile measurement, e.g., evaluating the impact of lasers on both sides of the rail that are not coplanar and generating rail surface defect samples using the system simulation model to solve the problem of a lack of negative samples in deep learning. Moreover, the system simulation model can be used for experimental verification and laboratory or field experiments can be performed simultaneously with system simulation experiments. The simulation data can not only verify the experimental results but also provide guidance for the experimental design. Finally, the system simulation model can be used to predict the results. Some tests unsuitable for field tests or parameters and cannot be well controlled can be performed using the simulation model, such as the vehicle body pose compensation test. The simulation model provides a new analysis method for studying rail profile measurements using line structured light and offers guiding significance for optical system design, optical element selection, and measurement accuracy improvement.
Significance There is no science without measurement. More accurate measurement of physical quantities is highly desired in modern science and technologies. Laser interferometric precision measurements have outstanding advantages, including traceability, nanometer or even picometer resolution, and ultralong measuring range up to several meters, kilometers, or even thousands of kilometers. It is widely used in advanced technologies and frontier research, such as IC devices, CNC machines, ultraprecision micromanufacturing, and gravitational wave detection.
However, laser interferometric precision measurements have many key problems that demand urgent solutions. The most crucial one is that the laser source requires to be independent, whereas the current domestic market cannot produce satisfactory dual-frequency lasers for heterodyne interferometry. Traditional laser sources have frequency differences lower than 3 MHz, which limits the maximum measuring speed. This severely restricts the processing efficiency of IC chips or machine tools. Furthermore, the output power of the widely used lasers is only 0.5 mW, which is low for further multidimensional measurements. More importantly, dual-frequency lasers exhibit a nonlinear error of several nanometers, thus affecting the precision of the interferometers. For example, the Agilent dual-frequency interferometer has a nonlinear error of 3 nm [
Many scientific frontier studies, such as gravitational wave detection, lithography machine positioning, and interstellar exploration, require ultrahigh precision measurement technology. Since the advent of laser interference technology, it has been crucial in precision measurements, and the demand for accurate measurements will increase from micro-nano level to picometers, or even femtometers, in the future. Therefore, independently developing novel interferometers with better performance is required for domestic laser interferometry precision measurements. Furthermore, summarizing the characteristics and limitations of existing interferometers is crucial to guide future development in this field more rationally.
Progress Owing to the above application requirements and technical problems, we have been devoted to investigating laser and laser-feedback interference in the past several decades. We have recorded great breakthroughs in dual-frequency innovative lasers with large frequency difference and high-power maintenance and in feedback interference for nanometer measurements without target mirrors.
On one hand, a new laser source based on the principle of the Zeeman-birefringence dual-frequency has been developed and produced independently with a higher dual-frequency difference and output power (
Conclusions and Prospects Herein, we presented the latest achievements and research progress in the research team on dual-frequency laser measurement and feedback interferometry technology in the past decade. We also presented the prospect of laser interferometry in precision measurements. Based on the results obtained herein, we shall focus on innovation, seek breakthroughs in new measurement principles and methods, and continuously improve the performance of the developed interferometers, bringing breakthroughs to laser interference ultraprecision measurements and their applications.
Objective Atmospheric turbulence is a key research topic in the field of atmospheric and environmental science. It greatly influences the development of aerospace, aircraft safety control, and laser communication. Random fluctuations in the refractive index of the atmosphere along the laser propagation path cause a series of transmission effects. Therefore, a laser beam projected onto a large area microcrystalline reflective film appears as irregular dynamic speckles in the far-field plane. In addition, there are constant directional moving shadows in the laser speckle images, which are caused by the path integral effect of transverse wind. Theoretically, two-dimensional (2D) laser shadow images can be used to detect the path transverse averaged wind velocity. However, due to the deformation of laser shadows and uneven transverse wind distribution, it is uncertain whether the moving velocity of laser shadows calculated using a cross-correlation algorithm can accurately reflect the moving velocity of the flow field and how the sampling frequency of images affects the calculation results. To address these problems, a new method for simulating transverse wind in atmospheric turbulence based on the dynamic phase screen theory is developed for quantitative simulation analysis. In addition, simulation results are substantially verified by experiments. We hope that our study will be helpful in the remote sensing detection of wind and other engineering applications.
Methods A complete pixel search algorithm based on normalized cross-correlation is used to calculate the displacement of laser shadows. First, the transverse wind along the laser propagation path is simulated by moving infinitely long and nonstationary phase screens. Images of lasers transmitted through the atmosphere are obtained for uniform and nonuniform transverse wind distributions. Then, the relationships between the velocity of laser shadows and wind speed are analyzed separately when the wind flow field is distributed differently. In addition, the appropriate sampling frame rates of different wind speeds are calculated. Next, a laser propagation experiment on the horizontal path is conducted, and real-time laser shadow images are taken. Finally, the calculated displacement of laser shadow images and transverse wind speed obtained using an ultrasonic anemometer are fitted to obtain their quantitative relationship. The path transverse wind velocity can be calculated directly from laser shadow images using this relation.
Results and Discussions Simulation results show that there is a linear relationship between the moving speed of laser shadows and path transverse wind speed when the distribution is uniform. Although the path transverse wind blowing from different directions introduces errors, this linear relationship still exists (Fig. 5). The shadow displacement caused by the transverse wind near the emission end is greater than the average wind speed; whereas, it is less than the average wind speed near the receiving end, that is, the influence of transverse wind on shadow displacement has a different path weight (Fig. 6). In addition, the sampling frequency of the image has a great influence on the calculation results of shadow displacement. (Fig. 7). Finally, experimental results show that the correlation coefficient between the measured and fitted wind speeds based on shadow displacement reaches 0.949, demonstrating that 2D wind vector can be obtained using laser shadow images in actual measurements. (Fig. 10).
Conclusions Simulation analysis shows that when the path transverse wind is uniformly distributed, the movement speed of laser shadows and transverse wind speed have an approximately 1∶1 linear relationship, which means that the moving velocity of laser shadows accurately reflects the moving velocity of the flow field. If the path transverse wind is in an inconsistent direction, some fitting errors are introduced, but the linear relationship between the shadow movement speed and average transverse wind speed is maintained. Moreover, the influence of path transverse wind at different positions on the shadow displacement is slightly different, causing the proportionality coefficient not to be 1. In addition, the minimum sampling frequency of CCD is estimated to ensure sufficient spatial correlation between continuous images and the accurate calculation of laser shadow displacement. Laser propagation experimental results demonstrate that 2D wind vector can be obtained using laser shadow images in actual measurements. We can quantitatively observe the motion of a 2D field and the evolution of an atmospheric vortex on a laser propagation path using laser shadow images, which is difficult to obtain using traditional methods, such as ultrasonic anemometers and wind lidar. In addition, the transverse wind is closely related to the effect of laser atmospheric transmission heat halo, and the change in wind speed in the vertical direction near the ground is related to the surface heat flow, which can be studied as a potential application direction.
Objective When the binocular vision system is used to measure underwater targets, the refraction effect of water makes the measurements differ from the actual situation. The existing methods for reducing the refraction effect include the following. 1) A high-accuracy refraction model is established for calculating the refraction path of each pixel in the camera using the model and for obtaining the actual position of the pixel. 2) The refraction effect is assumed to be an aberration of the camera model. Furthermore, a new camera-calibration method is used to estimate the camera parameters in the underwater environment through which a new measurement model is established to reduce the refraction effect. 3) By improving the traditional epipolar line-based matching method, the traditional onshore epipolar line model can be applied to underwater situations for feature point matching. However, these three methods have certain limitations. 1) High-order parameters need to be introduced when establishing a high-accuracy refraction model, making the calculation process more complex. 2) Special calibration devices are used for camera calibration to compensate for the refraction effect of water. Although an accurate underwater refraction model can be obtained using this method, the calibration devices are usually complex. 3) Although the matching method of the traditional epipolar line exhibits good practicability, the refraction effect on the epipolar line is not considered. Herein, we propose a new modeling method of underwater epipolar line using a device containing multiple line-structured light and a binocular vision system. The discrete curve model of the underwater epipolar line is established based on the ray-tracing principle. In addition, the underwater epipolar line-matching method is improved for feature point matching. The method can effectively improve the matching accuracy of underwater feature points and further improve the measurement accuracy of underwater targets.
Methods Herein, the objects of measurement included a standard ball workpiece (Fig. 9), a standard cylinder workpiece (Fig. 9), and a three-ball workpiece (Fig. 13). Our multiple line-structured light and binocular vision system comprised two Aca1300-60gm gray-scale cameras produced by the German Basler Company, 25 blue line lasers, a blue dot laser, several lighting LEDs, and a switch (Fig.2). The abovementioned devices are strictly fixed in a sealed cabin. The included angle of principal axes and the spatial distance of optical centers between the two cameras are approximately 35° and 500 mm. The laser position is fixed, and all the light planes are approximately parallel to each other. The measured object is placed within the cameras’ public field of view, and the array line laser beams are projected on the object surface. The images are captured simultaneously by the binocular vision system. The feature points of the current target objects are obtained from these images (Fig.11). According to the above system’s underwater binocular stereo vision model and ray-tracing principle, we adopted an iterative optimization method to solve the underwater epipolar line model, which corresponded to a pixel on the left image plane. Then, we could achieve a higher precision underwater feature point selection and matching according to the epipolar line model. In the experiment, the standard ball and standard cylinder workpieces are, respectively, placed in still water for measurement. The feature points of the extracted object are the waiting-for-matched points. The waiting-for-matched points are screened and matched based on the minimum distance constraint and the epipolar line model. To evaluate the matching effect, we compared the matching precision of the model with that of the traditional epipolar line model in the experiments. Furthermore, we conducted a three-dimensional (3D) reconstruction of the matched feature points to verify the effect of using the epipolar line model for underwater target object measurement.
Results and Discussions Our multiple line-structured light exhibit good penetration capability and can capture the characteristic information of underwater targets more accurately. The experimental results showed that the established underwater discrete epipolar line model is more consistent with the actual distribution of feature points than the traditional epipolar line model. The distance between the feature points and the epipolar line model is closer, and the matching accuracy is higher (Table 1). In the measurement experiments on the three-ball workpiece, the maximum measurement radius errors obtained using the traditional epipolar line model are greater than 0.5 mm, whereas those obtained using the epipolar line model are less than 0.3 mm (Table 3). In the experiments of measuring the center distance of several standard ball workpieces, the maximum center distance error obtained using the traditional epipolar line model is greater than 0.6 mm, while that of the epipolar line model less than 0.2 mm. Thus, the epipolar line model can achieve higher spatial accuracy when it is applied to 3D underwater target measurement.
Conclusions Herein, we proposed a discrete epipolar curve model-based underwater multiple line-structured light binocular measuring method. According to the underwater refraction model of the measurement system, we established the underwater discrete epipolar curve model. Then, we selected the waiting-for-matched feature points using the epipolar line model to achieve effective feature matching. The experimental results showed that the method has high measurement accuracy for the standard underwater ball and cylinder workpieces. It solves the problem of large matching error of feature points in the underwater binocular system and exhibits a good 3D reconstruction effect. Moreover, it is suitable for underwater target measurement because of the simple operation of the measurement process.