No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Real-time detection of focal position of workpiece surface during laser processing using diffractive beam samplers
... 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. ...
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. ...
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. ...
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. ...
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 ...
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. ...
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. ...
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. ...
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.
Thin film ablation with pulsed nanosecond lasers can benefit from the use of beam shaping optics to transform the Gaussian beam profile with a circular footprint into a Top-Hat beam profile with a rectangular footprint. In general, the quality of the transformed beam profile depends strongly on the beam alignment of the entire laser system. In particular, the adjustment of the beam shaping element is of upmost importance. For an appropriate alignment of the beam shaper, it is generally necessary to observe the intensity distribution near the focal position of the applied focusing optics. Systems with a low numerical aperture (NA) can commonly be qualified by means of laser beam profilers, such as a charge-coupled device (CCD) camera. However, laser systems for micromachining typically employ focus lenses with a high NA, which generate focal spot sizes of only several microns in diameter. This turns out to be a challenge for common beam profiling measurement systems and complicates the adjustment of the beam shaper strongly. In this contribution, we evaluate the quality of a Top-Hat beam profiling element and its alignment in the working area based on the ablated geometry of single pulse ablation of thin transparent conductive oxides. To determine the best achievable adjustment, we develop a quality index for rectangular laser ablation spots and investigate the influences of different alignment parameters, which can affect the intensity distribution of a Top-Hat laser beam profile.
Maintaining constant quality is challenging in laser processes where a small focal point down to few micrometers is necessary, because the beam focus must be kept within the limits of narrow depth of focus in the entire working area. We demonstrated the importance of the focus control in sapphire scribing and silicon surface material removal processes. With an online auto-focus system micromachining was made up to 1000 mm/s processing speed with a precision of ±2 μm. According to the test results, the auto-focus system significantly improved the process quality and reliability and allowed using high processing speeds and large wafer sizes with high precision.
A configuration of all-optical switching based on a Signac loop mirror that incorporates an ytterbium-doped fiber and uniform fiber Bragg grating (FBG) is proposed in this paper. It is found that the transmission spectrum of this structure is the narrow splitting of the reflection spectrum of the FBG. The shift of this ultranarrow transmission spectrum is very sensitive to the intensity of the pump power. Thus, the threshold switching power can be greatly reduced by shifting such narrow transmission spectrum. Compared with the single FBG, the threshold switching power of this configuration is reduced by 4 orders of magnitude. In addition, the results indicate that this optical switching has a high extinction ratio of 20 dB and a ultrafast response time of 3 ns. The operation regime and switching performance under the cross-phase modulation cases are also investigated.
Within the research project FEMTO, supported by the European Commission, a compact diode-pumped titanium:sapphire laser has been developed which matches the requirements of industrial systems, like compact dimensions and stable laser operation. To achieve this, the laser has been specially designed to be integrated directly into the machining system. For best process speed combined with optimal cutting quality, focus has been laid upon high repetition rates at moderate pulse energies. Typical average output powers are around 1.5W and repetition rates of up to 5 kHz. Accompanying to the laser development, a micro-machining system has been designed to meet the requirements of femtosecond laser micro-machining. In parallel to the machine development, machining processes have been investigated and optimized for different applications. The machining of delicate medical implants has been demonstrated as well as the machining system for general micro-machining of sensitive and delicate materials has been proven. Therefore, the developed machine offers the potential to boost the use of femtosecond lasers in industrial operation.
We analyze the degree of polarization of random, statistically stationary electromagnetic fields in the focal region of a high-numerical-aperture imaging system. The Richards-Wolf theory for focusing is employed to compute the full 3 x 3 electric coherence matrix, from which the degree of polarization is obtained by using a recent definition for general three-dimensional electromagnetic waves. Significant changes in the state of partial polarization, compared with that of the incident illumination, are observed. For example, a wave consisting of two orthogonal and uncorrelated incident-electric-field components produces rings of full polarization in the focal plane. These effects are explained by considering the distribution of the spectral densities of the three electric field components as well as the correlations between them.
The focusing properties of partially coherent Bessel-Gaussian beams passing through a high-numerical-aperture objective are studied based on vectorial Debye theory. Expressions for the intensity distribution, degree of coherence |mu|, and degree of polarization P are derived near the focus. Numerical calculations are performed to analyze the influences of varying corresponding parameters on the intensity distribution, |mu|, and P in the focal region. It is shown that the intensity, |mu|, and P in the focal region are all influenced by varying the effective coherent length L(c) of incident beams and maximal angle alpha determined by the numerical aperture of the objective. Also, the linearly polarized incident field is found to be depolarized after it is focused by a high-numerical-aperture objective.
We propose a new optical bistable device (OBD), which is constructed by connecting two symmetrical fiber Bragg gratings with a ytterbium-doped fiber to form a nonlinear Fabry-Perot cavity. The principle of this new OBD is described using the transfer-matrix method, and the two groups of transmitted and reflected optical bistability loops under different parameters are investigated symmetrically. Compared with single fiber Bragg grating switching, whose switching power is greater than 2 kW, this new device has evident merits in reducing the switching power to less than 45mW.
A non-intrusive optical sensor system has been developed for focus control of laser welding. This detects the light generated by the process through the laser delivery optics, and exploits the chromatic aberrations of these optics to detect any laser focal error at the workpiece. This system works for a wide range of materials and welding parameters, and example results are presented. The sensor has also been applied to laser ‘direct casting’, a process in which 3-D structures are built by flowing metal powder into a focused laser beam. In this case, melt pool temperature is also important, and so additional optics are incorporated into the sensor to provide a pyrometric temperature measurement which is used to control the laser power.
The paper describes an autofocus for reflected-light microscopy, based on oblique incidence of light that is focused on the object.The advantages and disadvantages of focusing on a small spot are pointed out, especially with regard to critical dimension metrology.In addition, the behaviour of a system with a very long focusing spot is described. It turns out that the focusing position of a system with a long spot is much more stable towards displacements of the object.
We propose a new optical bistable device (OBD), which is constructed by connecting two symmetrical fiber Bragg gratings with a ytterbium-doped fiber to form a nonlinear Fabry–Perot cavity. The principle of this new OBD is described using the transfer-matrix method, and the two groups of transmitted and reflected optical bistability loops under different parameters are investigated symmetrically. Compared with single fiber Bragg grating switching, whose switching power is greater than 2 kW, this new device has evident merits in reducing the switching power to less than 45 mW.
A new approach called dynamic-scan-detection is presented to detect the focal spot on nonplanar surfaces, which is a key technique for laser direct writing on nonplanar surfaces. From the depth response properties of a confocal system on a nonplanar surface, the approach scans the axial shift of intensity response peak by oscillating the modified pinhole in the axial direction, and demodulates the defocus by measuring the length of time between two peaks of measured intensity waveform in one period. Compared with conventional methods, this approach features easy implementation, wide depth measurement range without deterioration of the depth discrimination property, dynamically adjustable scan amplitude, and more importantly, suppression of common-mode noise or variable system factors.
A new all-optical switching device, which is constructed by connecting an erbium-doped fiber with two symmetrical long-period fiber gratings (EDF-LPFGs), is demonstrated. The performance of EDF-LPFGs switching has been investigated based on cross-phase modulation under different parameters. The theoretical analysis shows that the threshold switching power is inverse proportional to the nonlinear coefficient of the erbium-doped fiber, and is proportional to the effective area of the erbium-doped fiber and absorption coefficient. Moreover, the switching power as low as 36 mW and high extinction ratio of 18 dB are obtained in our experiment. Good agreement between the theoretical and experimental results indicate that EDF-LPFGs switching is a new design in support of switching power reduction.
In this paper, a fuzzy integral SMC (sliding mode control) strategy is developed for the intelligent focus control module
of an intelligent multi-axis laser machining CNC system and some tracking experiments with this control strategy are performed.
The results of the intelligent positioning experiment with fuzzy integral SMC strategy show that the intelligent multi-axis
laser CNC machining with this intelligent focus positioning controller has high robustness and global stability, high tracking
speed and precision.
The optical performances of the all-optical switching based on Yb3+-doped fiber Bragg grating (FBG) are investigated under the case of self-phase modulation (SPM) and cross-phase modulation (XPM). For the SPM case, the optical bistability of FBG under different parameters is investigated. It shows that the width of the hysteresis loop and threshold switching power are strongly dependent on the fiber grating length, fiber grating detuning, and coupling constant. For the XPM case, the expressions of the threshold switching power in the different detuning range are given. The influence of parameters about different detuning and coupling constant to the threshold switching power and extinction ratio are also studied. In comparison with the SPM case, the switching power of FBG under XPM case can be reduced to less than 20 mW by optimizing the parameters of FBG.
It is commonly admitted that radial polarization leads to the best light beam focalisation in terms of spot size. The work presented here shows that the choice of polarization is in fact more subtle. As an example, it is pointed out that, for numerical apertures of focusing systems below 0.8, circular polarization induces the best light confinements in air, whereas for numerical apertures beyond 0.8, radial polarization seems to be the right solution. Moreover, for numerical apertures larger than 1, we propose an original solution to achieve the ultimate focal spots in linear optical regime. They are estimated to 0.36λ/NA (λ is the wavelength and NA is the numerical aperture), the current technological limit being around 65 nm.
Any reliable automated production system must include process control and monitoring techniques. Two laser processing techniques potentially lending themselves to automation are percussion drilling and cutting. For drilling we investigate the performance of a modification of a nonintrusive optical focus control system we previously developed for laser welding, which exploits the chromatic aberrations of the processing optics to determine focal error. We further developed this focus control system for closed-loop control of laser cutting. We show that an extension of the technique can detect deterioration in cut quality, and we describe practical trials carried out on different materials using both oxygen and nitrogen assist gas. We base our techniques on monitoring the light generated by the process, captured nonintrusively by the effector optics and processed remotely from the workpiece. We describe the relationship between the temporal and the chromatic modulation of the detected light and process quality and show how the information can be used as the basis of a process control system.
We describe a focus control system for Nd:YAG laser welding based on an optical sensor incorporated into the fiber delivery system to detect light generated by the process. This broadband light is separated into two wavelength bands, and simple electronic processing gives a signal proportional to focal error as a result of chromatic aberrations in the optical delivery system. Focus control is demonstrated for bead-on-plate welds in different thicknesses of titanium alloy, aluminum alloy, mild steel, and stainless steel. The control system works for both pulsed and continuous laser radiation.