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Real-time detection of focal position of workpiece surface during laser processing using diffractive beam samplers

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... Cao et al. presented an approach for real-time focal position detection using diffractive beam samplers (DBS) [9], investigations of the laser-beam profile [7] and double-hole masks [6], while using the same beam path for the fabrication laser and detection. However, the laser power had to be limited due to the same beam path [7,9]. ...
... Cao et al. presented an approach for real-time focal position detection using diffractive beam samplers (DBS) [9], investigations of the laser-beam profile [7] and double-hole masks [6], while using the same beam path for the fabrication laser and detection. However, the laser power had to be limited due to the same beam path [7,9]. Additionally, a calibration was, as in all optical systems, necessary [6,7]. ...
... The disadvantage of using optical methods during laser manufacturing is the creation of plasma, due to the high laser power, which can pollute the lens or create unwanted reflections [8,9]. Additionally, in ultra-short pulse laser ablation these need to be very sensitive and are therefore highly susceptible to errors in the beam path. ...
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Microstructures were ablated using an ultra-short pulse laser system in order to investigate the influence of focal position on the surface topography. In addition, acoustic emissions measured by a piezoelectric sensor adapted to the AISI 4140 workpiece were analyzed and correlated with the focal position and the resulting surface topography. Frequency ranges sensitive to variations of the z-axis position were determined by STFT analysis. Subsequently, significant signal components were processed to enable an inference about the focal position. The hypothesis of assessing the focal position in-process based on acoustic emissions to ensure high precision during laser ablation could be confirmed.
... Several papers and technical schemes have reported novel techniques for high-precision, high-speed laser focus inspection in different conditions, which are applied in a wide range of industrial and scientific applications [10][11][12][13][14][15][16][17][18][19][20]. A good focusing condition plays an important role in profile measurement [10], increasing the productivity of modern patterning methods such as direct laser lithography on non-planar surfaces [11][12][13], two-photon nano-fabrication [14], laser micromachining using a charge-coupled device (CCD) camera and testing of the performance of laser fabrication at the focal position as well as defocus positions with various laser powers and diffractive beam samplers [15][16][17][18], remote sensing using time delay and integration (TDI) CCD cameras [19], and photonic force microscopy [20]. ...
... Several papers and technical schemes have reported novel techniques for high-precision, high-speed laser focus inspection in different conditions, which are applied in a wide range of industrial and scientific applications [10][11][12][13][14][15][16][17][18][19][20]. A good focusing condition plays an important role in profile measurement [10], increasing the productivity of modern patterning methods such as direct laser lithography on non-planar surfaces [11][12][13], two-photon nano-fabrication [14], laser micromachining using a charge-coupled device (CCD) camera and testing of the performance of laser fabrication at the focal position as well as defocus positions with various laser powers and diffractive beam samplers [15][16][17][18], remote sensing using time delay and integration (TDI) CCD cameras [19], and photonic force microscopy [20]. In addition, some other focus positioning methods have been proposed without specific applicability in technology or the industry [21][22][23], such as focal offset inspection [21], workpiece position determination [22], and four-quadrant photodiode utilization [23], for cases with a short working range. ...
... Consider a beam waist o1 w as an object with a thin lens having a focal length f located at a distance 1 d away from it along the beam axis. The obtained image is located at a distance 2 d away from the lens with a beam waist o2 w , as shown in Fig. 3. Furthermore, according to the definitions of the beam width and the radius of curvature of the wavefronts comprising the beam [18], the beam width can be expressed as follows: Equation (2) provides the position of the beam waist or focal point of the outgoing beam with respect to the lens. In other words, if a beam with beam waist at a distance 1 d away from the lens passes through the lens, it will converge at a distance 2 d on the other side of the lens. ...
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An automatic real-time focus inspection system based on a combination of dynamic focusing and real-time focus detection is introduced for use in high-precision laser processing. The system allows for accurately and rapidly positioning the laser focus on the target surface, wherever it is located along the optical axis. The proposed method is superior to conventional methods because it not only offers accurate, versatile, and high-speed autofocusing, but also combines well with the fabrication laser to perform laser processing when the laser focus is located on the sample. In this system, the laser focus is flexibly and automatically shifted along the optical axis by a dynamic focusing device to rapidly meet the working surface, ensuring high quality and complexity of fabricated patterns. Thus, the laser focus is not restricted to the focal point of the objective lens of infinity corrected microscopes reported previously. Furthermore, the focal spot analytically maintains its size with a size deviation less than 0.1% while the focus is slightly shifted along the optical axis, guaranteeing the size and quality of fabricated patterns. The proposed method is expected to lead to a high-precision laser fabrication system that can be widely applied in both science and the industry.
... Most DOEs are transparent surface relief microstructures that can affect either the amplitude or the phase of optical radiation passing through them [2][3][4][5][6]. They are standard in use as diffusers [7], beam splitters [8], beam samplers [9], beam shapers [10], axicons [11], vortex phase plates [12], etc. Lately, they have also found application in products such as 3D displays [13], consumer electronics [14], VR/AR/MR displays [15], automotive vehicles [16], and in fields such as robotics [17] or medically-required and elective surgery [18,19]. ...
... Figure 9 shows the numerical calculations of the diffraction efficiencies of the 0th, 1st, and 2nd diffraction orders for W = 20, 10, and 5 µJ/m 2 . One can notice that curves for which θ(x) was obtained by Equation (10) give better agreement with the numerical results than the curves for which θ(x) was obtained by Equation (9). Diffraction efficiencies for s- (Figure 9) and p-polarization (Figure 8b) become similar at high field strengths. ...
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We present an experimental and theoretical investigation of the optical diffractive properties of electrically tuneable optical transmission gratings assembled as stacks of periodic slices from a conventional nematic liquid crystal (E7) and a standard photoresist polymer (SU-8). The external electric field causes a twist-type reorientation of the LC molecules toward a perpendicular direction with respect to initial orientation. The associated field-induced modification of the director field is determined numerically and analytically by minimization of the Landau–de Gennes free energy. The optical diffraction properties of the associated periodically modulated structure are calculated numerically on the basis of rigorous coupled-wave analysis (RCWA). A comparison of experimental and theoretical results suggests that polymer slices provoke planar surface anchoring of the LC molecules with the inhomogeneous surface anchoring energy varying in the range 5–20 μJ/m2. The investigated structures provide a versatile approach to fabricating LC-polymer-based electrically tuneable diffractive optical elements (DOEs).
... Optical Engineering 054106-2 May 2019 • Vol. 58 (5) a positioning accuracy of AE0.1 mm. 16 The robot was sent axial position information in real-time via ethernet communication protocol using a desktop computer. ...
... A high-speed serial communication link between the two desktop computers was established so that OCT-acquired Optical Engineering 054106-4 May 2019 • Vol. 58 (5) axial peak position could be relayed in real-time to the computer commanding the manipulator. The workflow for the system was designed as follows: ...
... Previous publications introduced the new concept of a focal position detection system using a diffractive beam splitter that allows the laser beam to be focused on a sample surface with an unpredictably rough morphology [27] . The method is based on (1) low-intensity fractional beams that indicate the distance shifted from the focal position of the target sample and (2) a high-intensity main beam that handles focus detection and fabrication. ...
... The DBS divides the laser beam into three fractional beams with different intensity distributions and a propagation angle of 2.07°. The main beam has the highest intensity of approximately 97%, and the two fractional beams have a smaller intensity of 1.5% [27] . The DHM converts the main beam into two parallel beams that also propagate through the DBS. ...
Article
In this manuscript, a system for the real-time detection of the focal position on a target sample's surface during laser micromachining is presented. This system utilizes the advantages of diffractive beam samplers, double-hole masks, and two laser sources, including a diode laser for detection and a high-powered laser for fabrication. Moreover, this system can simultaneously detect the focal position and examine the defocusing direction during fabrication. This ability gives it an advantage over conventional methods that can only conduct a single task. The off-axis detection beams generated by the diffractive beam sampler that are examined during the detection process create various configurations of the beam spots on the advanced image sensor that are enhanced to read the sizes and separation of the beam spots simultaneously. Furthermore, the analytical relationship between the beam spot spacing and the specimen-objective-lens distance is used to support the calibration process and compared with experimental results. According to the changes in the distance between beam spots, the focal point and defocusing direction can be identified with the highest precision, which is indicated by the similarity between the theory and the experimental results. In addition, images of microholes fabricated by a fabrication laser are shown as a test of the focal detection system that is consistent with theory. The resolution of the system is optimized to polish the images obtained by the image sensor. Therefore, it is demonstrated that this technique provides the most accurate focusing conditions with a high numerical aperture as well as inexpensive laser fabrication and processing.
... The measurement of focal length using diffraction grating provides the uncertainty of 0.09% [22]. Moreover, a real-time system for the detection of the focal point using a diffractive beam sampler (DBS) and/or CCD camera is able to identify the focal position of a laser beam precisely on the basis of the change in the beam-spot sizes and the distances on the CCD according to the relative position of the specimen with respect to the objective lens [23,24]. This technique can remove the error caused by the aberration on the CCD owing to the pre-adjustment of the laser power during the calibration process. ...
... The distances between the laser source and lens 1, and between lens 2 and the CCD camera (a ρ , ) were held constant during the movement of lens 2. All the distances in the analytical model from the original optical setup are illustrated in Fig. 2. The discontinuity is the position of lens 2 at which the three beams originating from the laser source converge at one point on the CCD camera. From [24], the distance between beam spots on the CCD is expressed as ...
Article
An approach for measuring the focal length of a lens using a diffractive beam sampler is presented. The high-precision measurement of the focal length is conducted while a specimen is continuously moved along the optical axis. Images of the fractional beam spots on a charge-coupled-device camera are tracked with respect to the position of the specimen to obtain the relation between the beam-spot distance and the lens–specimen distance. Accordingly, this relation can be used to infer the focal length via computation. The method is carried out by simulation, easy, and inexpensive. In addition, it is applicable to find the focal length of a thin lens as well as the effective focal length of a system of lenses because it does not require the movement or replacement of lenses. The results indicate a dramatically high precision.
... A focal length measurement based on spot pattern and spherical aberration lowers the aberration effect, but leading to lower accuracy [18]. A real-time focal length determination system using a diffractive beam sampler (DBS) and/or CCD camera can accurately determine the focal length in case of light from a laser beam accurately through the change in the beam-spot sizes and the distances on the CCD in accordance with the relative position of specimen (S) with respect to the objective lens (OL) [19,20]. This technique can overcome the error associated with the aberration on the CCD camera owing to the pre-adjustment of the laser power during the calibration process. ...
Article
A cutting-edge precision technique for computation of focal length of a positive lens with double-hole mask is described. The technique is simple and versatile due to incorporation of the updated functions of image sensor device that supports reading the distance between beam spots instantaneously while the position of the specimen is being changed, as well as the reduction in several challenging measurement steps. Furthermore, this technique does not require prior knowledge of distances in the optical setup. High accuracy in focal-length measurement is obtained by precise beam spot distance analysis using image sensor integrated software. The acquired data exhibit considerably high precision and reproducibility.
... Those methods are all brilliant, but they seem to be expensive and not reproducible when applied in the industry with an enormous number of target samples. More importantly, some other studies have already improved the optical setup to explore the focus on a non-planar sample based on diffractive beam samplers [12,13] or the macro/micro dual-drive principle [14]. Furthermore, many auto-focusing devices have been developed for laser direct writing [14-16]; laser ablation [17]; automatic microscopy and measurement [18]; laser material processing [19]; two-photon photo-polymerization (TPP)-based micro-fabrication [20]; and several other applications [21-30] in Shack-Hartmann wave-front sensor fabrication [21], parallel data processing [22], photonic force microscopy [23], controllable mirror-lens retrofocus objective [24], tunable lens focal offset measurement [25], laser micromachining [26], remote sensing [27], automated optical inspection [28], auto-focusing infinity corrected microscopes [29], and direct imaging technology [30] with detailed theoretical models. ...
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In modern high-intensity ultrafast laser processing, detecting the focal position of the working laser beam, at which the intensity is the highest and the beam diameter is the lowest, and immediately locating the target sample at that point are challenging tasks. A system that allows in-situ real-time focus determination and fabrication using a high-power laser has been in high demand among both engineers and scientists. Conventional techniques require the complicated mathematical theory of wave optics, employing interference as well as diffraction phenomena to detect the focal position; however, these methods are ineffective and expensive for industrial application. Moreover, these techniques could not perform detection and fabrication simultaneously. In this paper, we propose an optical design capable of detecting the focal point and fabricating complex patterns on a planar sample surface simultaneously. In-situ real-time focus detection is performed using a bandpass filter, which only allows for the detection of laser transmission. The technique enables rapid, non-destructive, and precise detection of the focal point. Furthermore, it is sufficiently simple for application in both science and industry for mass production, and it is expected to contribute to the next generation of laser equipment, which can be used to fabricate micro-patterns with high complexity.
... However, applications of this method are limited to simple fabrication processes that do not require high accuracy. In contrast, our previous technique used a chargecoupled device (CCD) camera to detect the focal position based on the beam profile that appears on the CCD screen [14,17,18]. This method determined the best achievable adjustment on the basis of ablated geometry; however, this approach can only be used for specimens with flat surfaces. ...
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We describe a new approach for locating the focal position in laser micromachining. This approach is based on a feedback system that uses a charge-coupled device (CCD) camera, a beam splitter, and a mirror to focus a laser beam on the surface of a work piece. We tested the proposed method for locating the focal position by using Zemax simulations, as well as physically carrying out drilling processes. Compared with conventional methods, this approach is advantageous because: the implementation is simple, the specimen can easily be positioned at the focal position, and the dynamically adjustable scan amplitude and the CCD camera can be used to monitor the laser beam's profile. The proposed technique will be particularly useful for locating the focal position on any surface in laser micromachining.
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