Galvanometric Scanning System Research Articles

Free-space coupled, large-active-area superconducting microstrip single-photon detector for photon-counting time-of-flight imaging

Numerous applications at the photon-starved regime require a free-space coupling single-photon detector with a large active area, low dark count rate (DCR), and superior time resolutions. Here, we developed a superconducting microstrip single-photon detector (SMSPD), with a large active area of 260 µm in diameter, a DCR of ∼5kcps, and a low time jitter of ∼171ps, operated at a near-infrared of 1550 nm and a temperature of ∼2.0K. As a demonstration, we applied the detector to a single-pixel galvanometer scanning system and successfully reconstructed the object information in depth and intensity using a time-correlated photon counting technology.

A 3D reconstruction method based on one-dimensional galvanometer laser scanning system

The line-structured light galvanometer scanning system projects the laser light strip onto the object by swinging the mirror to realize the 3D profile measurement of the object. It has the advantages of compact structure, fast measurement speed and strong adaptability to complex environments. In this paper, a three-dimensional scanning system based on a line laser, a one-dimensional galvanometer and an industrial camera is constructed, and a new mathematical model of one-dimensional galvanometer laser scanner is derived. On this basis, a high-precision and simple calibration method of the laser scanning system is proposed. Compared with other methods, this method takes into account the rotation angle error caused by the installation of the galvanometer. The whole calibration process only needs a freely moving chessboard instead of a precision electric stage for operation, which is very easy and convenient for on-site use. The experimental results show that the measurement system calibrated by the proposed method has an average relative deviation of 0.14% for the distance measurement of ceramic balls, and successfully achieved 3D reconstruction of plaster objects with complex surfaces and metal parts with high or low reflectivity.

Field distortion correction in galvanometric scanning system by interpolation with symmetric polynomials

Due to its high precision and flexibility, laser process system is widely used in laser marking, drilling, cutting and projection. Laser scanning machines are usually equipped with high-speed galvanometric scanning systems, which are inevitably accompanied by field distortion. The error compensation method can help improve the marking quality that minimizes inherent and random errors. This paper introduces a facile software compensation method for this purpose. Based on symmetry of field distortion, the length of four characteristic lines is required to determine the polynomial compensation function. The experimental results verified that the technique could effectively reduce the field-distortion errors in the galvanometer laser-machining system, which verifies the feasibility and effectiveness of the compensation method. Within the processing range of the experiment (100 × 100 mm), the average relative position accuracy is 0.00068 (＜1‰). In the light of these results, high-quality laser processing is promising for achieving by the compensation method.

Enhancement of a Single-Axis Femtosecond Laser Scanning System by Using Two Galvanometers to Improve the Telecentricity and the Effective Scanning Length on Laser Process

The galvanometer scanning system plays a crucial role in modern laser material processing. With the development of this industry, the requirements for galvanometer scanners are getting higher and higher, especially to overcome the inherent disadvantages that still exist, such as image distortion, marking speed and accuracy in state-of-the-art scanning systems. In this paper, a single-axis optical scanner using two galvanometers in combination with one f-theta telecentric lens and a 343 nm femtosecond pulse laser source is proposed as a new approach for enhancing the precision of laser micromachining technology. The additional second galvanometer is used to manipulate the output laser beam of the first galvanometer to the path with less lens aberration to enhance the telecentricity correction and the effective scanning area. This is based on the international standard regulation ISO (the International Organization for Standardization) 11145:2018 requirements in optics and photonics, in which an important criterion is for the roundness of the focused beam spot to be greater than 87% to determine the effective working length of the proposed scanning system compared to the conventional scanning system. It is demonstrated by optical simulations and real optical experiments that the effective working length can be increased by 3.6 mm, corresponding to 8.1% of the effective scanning field, to achieve a laser material processing system with ISO standard. The damped least squares (DLS) algorithm in optical design software ZEMAX is used to optimize the deflected angle of the two galvanometers to obtain the optimal incident position of the f-theta lens.

Universal Calibration Method for Line Structured Light Galvanometer Scanning System

Universal Calibration Method for Line Structured Light Galvanometer Scanning System

Assessment of Laser Galvanometer Scanning System Accuracy Using Ball-Bar Standard

The laser projector based on the laser galvanometer scanning system enables the projection of CAD-based laser images onto geometrical objects. Furthermore, the system can perform a scan of components in order to control proper positioning (e.g., welded structures). We decided to conduct research aimed at determining the suitability of such systems for length measurements and assess their accuracy. These systems are commonly calibrated with the use of flat calibration boards, although their capability of projecting and scanning 3D objects. For this reason, a new method based on ISO 10360 has been proposed. Analysis of the system’s ability to perform length measurements, selection of a reference standard, and the tested device accuracy were the main objectives of the study. A ball-bar with a nominal length of 3000 mm was chosen as a reference standard. Positions of the reference element in the workspace were determined, and three series of five measured deviations were performed in each setting. The obtained values of measurement errors prove that it is possible to use ball-bar standards to assess the accuracy of the described systems, which is defined by the equation MPE(E0) = ±0.5 mm. The proposed method could be adapted to perform more complex analyzes in this area.

A scanning distortion correction method based on X – Y galvanometer Lidar system* *Project supported by the National Natural Science Foundation of China (Grant Nos. 61775048 and 62027823) and the Natural Science Foundation of Shenzhen (Grant No. JCYJ2020109150808037).

Aiming at the problem of scanning distortion in X–Y galvanometer light detecting and ranging (Lidar) scanning system, we propose a method of image scanning distortion correction with controllable driving voltage compensation. Firstly, the geometrical optics vectors model is established to explain the principle of pincushion distortion in the galvanometer scanning system, and the simulation result of scanning trajectory is consistent with experiments. The linear relationship between the driving voltage and the scanning angle of the galvanometer is verified. Secondly, the relationship between the deflection angle of the galvanometer and the scanning trajectory and the driving voltage is deduced respectively, and an image scanning correction algorithm with controllable driving voltage compensation is obtained. The simulation experiment results of the proposed method show that the root-mean-square error (RMSE) and the corresponding curve between the scan value and the actual value at different distances, have a good correction effect for the pincushion distortion. Finally, the X–Y galvanometer scanning Lidar system is established to obtain undistorted two-dimensional scanned image and it can be applied to the three-dimensional Lidar scanning system in the actual experiments, which further demonstrates the feasibility and practicability of our method.

In situ calibration of galvanometric scanning head for laser powder bed fusion machines based on a vision system

In laser powder bed fusion (LPBF) additive manufacturing machines, a laser galvanometric scanning system is used to control the movement of the laser beam, which melts material powders layer-by-layer in a powder bed. Errors between desirable positions and actual positions of the laser in the work plane of the machine directly affect the geometry accuracy of built parts. Therefore, calibrating the laser galvanometric scanning system of LPBF machines is a very important task. This system is generally calibrated by calculating correction tables for its joint components. However, most existing calibration methods require a measuring phase of actual positions of laser engraved on a calibration plate. This phase was generally performed by a measuring system in another location, thus requiring a delay time and causing an interruption of the calibration process. In the current study, an in situ calibration method for the laser galvanometric scanning head of LPBF machines was developed based on a vision system. The vision system (i.e., a high-resolution monochrome camera) was used to acquire experimental data rapidly during the calibration process, thereby avoiding the laser marking and the measuring phases of laser marks in another location. The acquired experimental data was subsequently used to calculate the correction tables/matrices for calibrating the optical chain of an LPBF machine prototype. The obtained results show that the developed measurement method is acceptable for the in situ measurement with a good accuracy. After the calibration process, errors between the positions generated by the identified LPBF machine and those measured by the proposed method are very small with an average error of 0.03 mm, and about 80% of positions present an error less than 0.04 mm in the calibration region.

Microstructures Manufactured in Diamond by Use of Laser Micromachining.

Different microstructures were created on the surface of a polycrystalline diamond plate (obtained by microwave plasma-enhanced chemical vapor deposition—MW PECVD process) by use of a nanosecond pulsed DPSS (diode pumped solid state) laser with a 355 nm wavelength and a galvanometer scanning system. Different average powers (5 to 11 W), scanning speeds (50 to 400 mm/s) and scan line spacings (“hatch spacing”) (5 to 20 µm) were applied. The microstructures were then examined using scanning electron microscopy, confocal microscopy and Raman spectroscopy techniques. Microstructures exhibiting excellent geometry were obtained. The precise geometries of the microstructures, exhibiting good perpendicularity, deep channels and smooth surfaces show that the laser microprocessing can be applied in manufacturing diamond microfluidic devices. Raman spectra show small differences depending on the process parameters used. In some cases, the diamond band (at 1332 cm−1) after laser modification of material is only slightly wider and shifted, but with no additional peaks, indicating that the diamond is almost not changed after laser interaction. Some parameters did show that the modification of material had occurred and additional peaks in Raman spectra (typical for low-quality chemical vapor deposition CVD diamond) appeared, indicating the growing disorder of material or manufacturing of the new carbon phase.

Design and implementation of the galvanometer scanning system for reflectance confocal and stimulated Raman scattering microscopy

We report on two strategies to design and implement the galvanometer-based laser-scanning mechanisms for the realization of reflectance confocal microscopy (RCM) and stimulated Raman scattering (SRS) microscopy systems. The RCM system uses a resonant galvanometer scanner driven by a home-built field-programmable gate array circuit with a novel dual-trigger mode and a home-built high-speed data acquisition card. The SRS system uses linear galvanometers with commercially available modules. We demonstrate video-rate high-resolution imaging at 11 frames per second of in vivo human skin with the RCM system and label-free biomolecular imaging of cancer cells with the SRS system. A comparison of the two strategies for controlling galvanometer scanners provides scientific and technical reference for future design and commercialization of various laser-scanning microscopes using galvanometers.

Laser-bonding of FEP/FEP interfaces for a flexible manufacturing process of ferroelectrets

This paper presents an optimized laser-bonding process for piezoelectric energy-harvesters based on thin fluorinated-ethylene-propylene (FEP) foils, using an ultra-short-pulse (USP) laser. Due to the minimized thermal stress in the material during bonding, achieved by pulse durations of few picoseconds, we created seams down to 40 µm width without generating holes in the 12.5 µm thick FEP-foils. Using a galvanometer scanning system allowed for fast bonding-speed up to several centimeters per second, making the process also suitable for large structures and areas. The achieved bond strength of the seams under influence of shearing stress was examined using tensile testing, which showed a sufficient strength of about 25 % of the maximum strength of an unbonded, single layer of FEP.

Microwelding of glass to silicon by green ultrafast laser pulses

In the present work, a direct bond between glass and silicon with optical contact was formed by 515 nm picosecond laser controlled by the galvanometer scanning system. A crack-free welding effect without edge breakage and residues was obtained with breaking strength as high as 122 MPa. New crystalline phase (SiO2) was produced and silicon and oxygen have a spatial distribution gradient in the welding area analyzed by XRD and EDS, respectively, revealing that mixing and inter-diffusion of substances have occurred during laser irradiation. The transmission of near infrared light in the welding area is significantly increased compared with the non-welding area analyzed by inverted microscope. A confocal laser scanning microscope was used to observe the morphology and width of welds, which variation trend of axial focal positions is consistent with the teardrop structure produced by nonlinear absorption effect. Scanning speeds and laser repetition frequencies also affect the width and morphology of the weld.

Rapid on-site recalibration for binocular vision galvanometric laser scanning system.

The binocular vision galvanometric laser scanning (BGLS) system, which consists of a traditional galvanometric laser scanning (GLS) system and a stereo vision system, is widely applied in various advanced manufacturing fields such as 3-dimensional (3D) laser cutting and 3D laser projection positioning. The BGLS system usually needs to be recalibrated before it is put to a certain use, since the camera parameters, as well as the relative pose between the camera and the GLS, may be changed with different applications. However, a full calibration of the BGLS system requires elaborate modeling and mass calibration data, which considerably affect the efficiency of the BGLS system. A rapid on-site recalibration method is proposed, which can substantially improve the efficiency and flexibility of the BGLS system. With the method, the BGLS system needs to be carefully off-site calibrated for only one time. The on-site precise recalibration can be quickly realized by taking only two images of the laser spots projected by the GLS on a planar board. Moreover, an ingenious linear solving method is proposed to make the whole computation process more stable and timesaving. On-site recalibration and target shooting experiments are respectively conducted to verify the proposed method.

Method of Calibration of a Laser-Based Galvanometric Scanning System with Submicron Resolution

A method used for calibration of scanning modules on the basis of galvanometric deflectors is considered. The method is based on an iterative two-step algorithm of processing of the profilometry map of the test writing of the calibrated module. It is demonstrated that high accuracy and robustness of determining the coordinates of the reference points of the test writing can be obtained by means of finding the approximate positions of such points and identifying the fragment in the vicinity of these points at the first step and searching for the exact values of the coordinates by the correlation method at the second step. The use of this method and a simple algorithm of piecewise-planar interpolation offers a possibility of laser beam positioning with a relative error of 10−5 from the writing field.

Modeling and Calibration of the Galvanometric Laser Scanning Three-Dimensional Measurement System

The traditional three-dimensional measurement system based on linear-structured light usually involves mechanical scanning platforms to perform the linear scanning, which make the structure of the system huge and complicated. The emergence of the galvanometric laser scanner solves the problem by using a galvanometer to replace the mechanical scanning platform. The employment of the galvanometer can improve the speed of scanning and simplify the structure of the system. However, there are few approaches available to calibrate this kind of system. In this paper, a high-precision calibration method is proposed to calibrate the galvanometric laser scanning three-dimensional measurement system. A precision motorized linear stage and a planar target are applied in this method. The planar target is used for the camera calibration based on Zhang’s method and the precision motorized linear stage for the laser plane calibration. The validity and accuracy of this method are evaluated by scanning the standard component. The experiments conducted suggest that the proposed method is valid and accurate for the calibration of the galvanometric laser scanning system.

Extrinsic Calibration of a Laser Galvanometric Setup and a Range Camera.

Currently, galvanometric scanning systems (like the one used in a scanning laser Doppler vibrometer) rely on a planar calibration procedure between a two-dimensional (2D) camera and the laser galvanometric scanning system to automatically aim a laser beam at a particular point on an object. In the case of nonplanar or moving objects, this calibration is not sufficiently accurate anymore. In this work, a three-dimensional (3D) calibration procedure that uses a 3D range sensor is proposed. The 3D calibration is valid for all types of objects and retains its accuracy when objects are moved between subsequent measurement campaigns. The proposed 3D calibration uses a Non-Perspective-n-Point (NPnP) problem solution. The 3D range sensor is used to calculate the position of the object under test relative to the laser galvanometric system. With this extrinsic calibration, the laser galvanometric scanning system can automatically aim a laser beam to this object. In experiments, the mean accuracy of aiming the laser beam on an object is below 10 mm for 95% of the measurements. This achieved accuracy is mainly determined by the accuracy and resolution of the 3D range sensor. The new calibration method is significantly better than the original 2D calibration method, which in our setup achieves errors below 68 mm for 95% of the measurements.

Effective Data-Driven Calibration for a Galvanometric Laser Scanning System Using Binocular Stereo Vision.

A new solution to the problem of galvanometric laser scanning (GLS) system calibration is presented. Under the machine learning framework, we build a single-hidden layer feedforward neural network (SLFN）to represent the GLS system, which takes the digital control signal at the drives of the GLS system as input and the space vector of the corresponding outgoing laser beam as output. The training data set is obtained with the aid of a moving mechanism and a binocular stereo system. The parameters of the SLFN are efficiently solved in a closed form by using extreme learning machine (ELM). By quantitatively analyzing the regression precision with respective to the number of hidden neurons in the SLFN, we demonstrate that the proper number of hidden neurons can be safely chosen from a broad interval to guarantee good generalization performance. Compared to the traditional model-driven calibration, the proposed calibration method does not need a complex modeling process and is more accurate and stable. As the output of the network is the space vectors of the outgoing laser beams, it costs much less training time and can provide a uniform solution to both laser projection and 3D-reconstruction, in contrast with the existing data-driven calibration method which only works for the laser triangulation problem. Calibration experiment, projection experiment and 3D reconstruction experiment are respectively conducted to test the proposed method, and good results are obtained.

Directional error analysis of 2D galvanometer scanning system

Directional error analysis of 2D galvanometer scanning system

Laser galvanometric scanning system for improved average power uniformity and larger scanning area

A new laser galvanometric scanning optical system incorporating a dynamic-tilt focusing lens is proposed to improve the laser spot performance in adaptive manufacturing applications. The simulations focus specifically on the laser spot size, the spot profile, the spot position, the spot energy distribution, and the size of the scanning working field. It is shown that for a designed spot size of 50 μm, the proposed system achieves an average spot size of 50.5 μm. Moreover, the maximum position deviation of the laser beam is reduced from (x=-3.02%, y=1.30%) in a traditional scanning system to (x=-0.055%, y=0.162%) in the proposed system. Finally, the maximum working field area is increased by around 240% compared to that of a traditional system. Overall, the results show that the proposed laser galvanometric scanning system achieves a small spot size, a symmetrical and round spot profile, a uniform spot energy distribution, and a large working area. As a result, it is ideally suited to rapid prototyping applications.

Reducing field distortion for galvanometer scanning system using a vision system

Laser galvanometer scanning systems are well-established devices for material processing, medical imaging and laser projection. Besides all the advantages of these devices like high resolution, repeatability and processing velocity, they are always affected by field distortions. Different pre-compensating techniques using iterative marking and measuring methods are applied in order to reduce such field distortions and increase in some extends the accuracy of the scanning systems. High-tech devices, temperature control systems and self-adjusting galvanometers are some expensive possibilities for reducing these deviations. This contribution presents a method for reducing field distortions using a coaxially coupled vision device and a self-designed calibration plate; this avoids, among others, the necessity of repetitive marking and measuring phases.