CO2 Laser Polishing Research Articles

Morphological evolution mechanism of microstructures involved in shaping micro-pillar arrays into microlens arrays on fused silica surfaces by CO2 laser polishing

The periodic micro-pillars on the hard-brittle fused silica surface can be shaped into the smooth and defect-free microlens arrays (MLA) by CO2 laser polishing, which can be applied in imaging and beam-shaping. However, the morphology evolution mechanism of the micro-pillars is not clear during polishing, which seriously affects the rapid preparation of high-quality MLAs. Herein, a multi-physics coupling model is established to investigate the interaction between CO2 lasers and micro-pillars on the fused silica surface, and is verified from the perspectives of temporal and spatial temperature. The steady surface temperature evolution law with laser power and spot radius is explored, based on which, the parameter ‘Window’ for polishing is determined to make the material melt and flow without ablation removal. The morphology evolution of micro-pillars is investigated as well as laser-material interaction. It is found that under laser irradiation, the micro-pillar height first increases and then decreases, and its top curvature radius increases. The surface tension makes the dominant contributions to the melting and flow of the micro-pillar materials, while the Marangoni effect and the gravity have little effect. Furthermore, to fabricate the target MLAs with the specific dimensions, the influence of micro-pillar aspect ratio (φ) on microlens surface morphology is investigated. It is found that there are two critical φ for the target MLAs. For the required MLA with the target period of 200 μm and the target height of 40 μm, when the φ is larger than 5.1, the micro-pillar array fails to be shaped into the target MLA by CO2 laser polishing. As the φ decreases, the junction of micro-pillar and the substrate sags downwards significantly, and the deviation between the shaped- and the ideal spherical contours increases. To successfully prepared the target MLA and guarantee the geometry accuracy, the minimum φ is selected to be 0.71. This work has important theoretical significance for obtaining the smooth and defect-free target fused silica MLAs with the specific dimensions by CO2 laser non-evaporative polishing.

Role of fictive temperature distribution involved in CO2 laser polishing of fused silica and its optimization for achieving even heat-affected zones

CO2 laser polishing process is a highly effective way for enhancing the surface quality and laser-induced damage threshold of fused silica optics. Nevertheless, due to the thermal history and spatial gradient of the thermodynamic temperature, the heat-affected zones can be formed unevenly on the surface with an increased fictive temperature, which causes the material densification and residual stress. In this work, a 3D model of structural relaxation coupled with temperature and flow fields was innovatively established to explore the role of fictive temperature distribution involved in CO2 laser polishing of fused silica. The Raman spectrum was applied to verify this model. Subsequently, the impacts of laser beam power, scanning speed, track overlapping, substrate temperature on the fictive temperature were discussed. The results indicate that decreasing the scanning speed and increasing the substrate temperature can decrease the fictive temperature. Additionally, a higher track overlapping can reduce the height of the fictive temperature “tool mark”. Moreover, an optimization strategy based on laser beam scanning speed was proposed based on the uniform distribution of fictive temperature. The depth difference of the heat-affected zones was reduced from 164.26 μm to 31.74 μm. This work could be used to quantitatively predict the 3D fictive temperature distribution and proposed an optimization strategy based on scanning speed to suppress the non-uniformity of fictive temperature distribution, which can provide significant theoretical guidance for CO2 laser polishing technology of fused silica optics.

Parametric optimization of 3D printing process hybridized with laser-polished PETG polymer

The potential of Fused Filament Fabrication (FFF) technology to produce very complex geometrics has a profound impact on the physical world. Practitioners in the industry are developing predictive methods for assessing key parameters and responses of engineering materials. Enterprises are trying to solve problems such as Flexural Strength (FS), Average Surface Roughness (Ra), Tensile Strength (TS), sustainability including energy efficacy, and time management problems. This study's objective is divided into two main parts. The First part includes the parametric investigations of Polyethylene Terephthalate Glycol (PETG) polymer to optimize the mechanical properties, Ra, and sustainability, including Print Time (T) and Printer Energy consumption (E) of samples using the FFF technique. In second part, samples were printed at optimized parametric values and further post-processed to enhance the responses such as TS, FS, Ra, and Laser Scan Time (Tl) using CO2 laser polishing treatment. FFF with PETG, a biodegradable material, is commonly used in various fields, despite the traditionally significant surface roughness of FFF products. In pre-processing, by optimizing 3D printed parameters, FS of 69.9 MPa, lowest Ra of 6.8 μm, TS of 45.1 MPa, lowest T of 53 min and E of 0.20 kWh were attained. In post-processing, the Ra decreased by more than 58.38% due to laser polishing (6.8 μm–2.81 μm). The TS increased by 8.89%, from 45.1 MPa to 49.11 MPa. SEM investigation revealed that the gaps in the material lowered the FS by 5.09%, from 69.9 MPa to 66.34 MPa and Tl for 0.22 min. The fracture morphologies were studied as a means of learning more about the reinforcing process in the future. These outcomes prove that laser scanning can improve and alter the surface of an FFF product.

Measurement of Fine/Ultrafine Dust Using Lenticular Fiber-Based Particulate Measurement Devices

Measurement of particulate matter (PM), i.e., fine/ultrafine dust, is important for monitoring our surrounding environment. We demonstrate the fabrication of lenticular fiber-based fine/ultrafine dust measurement devices for monitoring the density and size of dust. The plano-convex lenticular fibers were fabricated by the CO2 laser polishing technique. The optical lens, used in the conventional light scattering-based fine/ultrafine dust measurement devices, was replaced by a plano-convex lenticular optical fiber to reduce the size and complexity of the dust measurement device. We utilized a light-emitting diode as the light source of the fine/ultrafine dust density measurement device. The fabricated device could indicate the dust density level (good, normal, bad, and very bad) according to the Korean standard. To estimate the size of the dust particulates, we fabricated a dust size measurement device that consists of a compact laser diode, a lenticular fiber, and a photodetector. We are successful in estimating the size of both fine and ultrafine PMs by detecting the power of the scattered light coming from the dust particulates. The dust size down to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.9 \mu \text{m}$ </tex-math></inline-formula> was detected using our dust measurement device. Our proposed dust measurement devices are simple, small, and cost-effective and can be considered as a candidate for portable applications.

Core-modulated long-period fiber gratings formed by heating and stretching

In this paper, we developed and experimentally evaluated a new type of long-period fiber grating. This structure is fabricated by CO2 laser polishing and subsequent heating with an oxyhydrogen flame. Firstly, a single-mode fiber is produced using a high-frequency CO2 laser to form periodic trapezoidal structures. It is then heated with an oxyhydrogen-flame-based fiber optic tapering system to bend the core slightly toward the polished area. The micro-bent cores exhibit strong refractive index modulation, which make the core-modulated LPFG more sensitive to tensile strain. The experimental results show that the proposed sensor exhibits a strain sensitivity as high as 30.43 pm/με over the 0–100 με range and a temperature sensitivity of 9.29 pm/∘C. Based on the results of a general theoretical analysis and simulations, we attribute the threshold point of its strain sensitivity to the degree of micro-bending of the core. Furthermore, the structure is low cost and compact and exhibits high mechanical strength and thus can be used in a variety of applications in several fields.

Investigation on hybrid laser ablation and its application in fused silica damage mitigation.

We present and investigate a hybrid laser-based method of surface shaping for damage mitigation on fused silica surfaces. Damage sites were removed and precisely shaped into an optically-benign cone by a procedure of femtosecond laser ablation with a subsequent CO2 laser polishing process. The morphology of the cone rim was quantitatively predicted by a numerical model. Since the heat-affected zone (HAZ) of the laser polishing process was effectively confined by the optimization of ablation parameters, the dimensions of the raised rim were reduced by an order of magnitude. The intensity of the on-axis hotspot was positively related to the dimensions of the raised rim, and thus an inapparent downstream intensification was achieved by the rim reduction. Laser-induced damage threshold (LIDT) of the cone was tested to be ∼14 J/cm2 on the input surface. Therefore, the presented method is appropriate to mitigate damage and also provides a promising approach to manufacturing functional microstructures for high-power applications.

Helical sensor for simultaneous measurement of torsion and temperature

Based on our current research on long-period fiber gratings (LPFGs), we propose a novel fiber-based sensor for simultaneous measurement of torsion and temperature. The proposed sensor cleverly combines CO2 laser polishing with pre-twist that can introduce elliptical birefringence in the fiber to improve sensing performance. Through comparative measurement experiments, we investigate the torsion characteristic of the sensor and the effect of different degrees of pre-twist on it. Experimental results show that the highest torsion sensitivity of two resonant dips can reach 0.635 nm/(rad/m) and −0.654 nm/(rad/m) in the range of 0 rad/m − 12.6 rad/m, while in the range of −12.6 rad/m − 0 rad/m the sensitivities are 0.14 nm/(rad/m) and −0.083 nm/(rad/m) respectively due to inhomogeneous elliptical birefringence. Meanwhile, the temperature sensitivity of the two resonant dips are calculated to be 62.5 pm/°C and 66.8 pm/°C, respectively. Due to the different sensitivities of temperature and torsion, the matrix method can be used for simultaneous measurement of two parameters to ignore crosstalk. As a result, the proposed sensor shows competitiveness in practical engineering applications.

Stability of samples in coating research: From edge effect to ageing

Mechanical and optical thermal noises play an important role in many precise opto-mechanical experiments, in which positions of test bodies are monitored by laser beams. Much of the research in this area was driven by the physics of gravitational wave interferometers, to counteract the mirror’s multi-layered dielectric coating thermal noise. Coating thermal noise is directly related to structural dissipation inside the material through the loss angle. In view of future upgrades of gravitational wave detectors, increasing the coatings mechanical performances, by lowering the loss angle and retaining their outstanding optical and morphological properties, is fundamental. The measurement of the coating loss angle requires substrates to be stable with respect to their dissipative behavior. It has been seen that fused silica disc losses are resonant-mode shape dependent and are subject to ageing effects, compromising the accuracy of mechanical characterizations of the substrates and, consequently, of the coatings. In commercial samples, the source of this deteriorations can be related to the ground, unpolished lateral surface. In this work we show that the polishing of the sample’s edge reduces the amount of spurious losses and ageing effects. A new procedure through CO2 laser polishing of the edge surface is proposed, explained and put in place. The results of these procedures, in terms of roughness and loss behavior is shown. The loss angle measurements are compared with an edge loss model and other existing models.

3D in-plane integrated micro reflectors enhancing signal capture in lab on a chip applications.

The integration of micro-optics in lab on a chip (LOCs) devices is crucial both for increasing the solid angle of acquisition and reducing the optical losses, aiming at improving the signal-to-noise ratio (SNR). In this work, we present the thriving combination of femtosecond laser irradiation followed by chemical etching (FLICE) technique with CO2 laser polishing and inkjet printing to fabricate in-plane, 3D off-axis reflectors, featuring ultra-high optical quality (RMS ∼3 nm), fully integrated on fused silica substrates. Such micro-optic elements can be used both in the excitation path, focusing an incoming beam in 3D, and in the acquisition branch, harvesting the optical signal coming from a specific point in space. The flexibility of the manufacturing process allows the realization of micro-optics with several sizes, shapes and their integration with photonic circuits and microfluidic networks.

Laser surface polishing of Ti-6Al-4V parts manufactured by laser powder bed fusion

Poor surface quality of Additively Manufactured (AM) components, can greatly increase the overall cost and lead time of high-performance components. Examples are medical devices where surfaces may contact the patient's skin and hence need to be smooth and aerospace components with high fatigue strength requirements where surface roughness could reduce fatigue life. The average surface roughness (Ra) of AM parts can reach high levels greater than 50 μm and maximum distance between the high peaks and the low valleys of more than 300 μm. As such, there is a need for fast, cost effective and selective finishing methods of AM produced components targeted at high-performance industries. In this paper Ti-6Al-4V Grade 23 ELI, popular for medical devices and aerospace parts production, was L-PBF processed to manufacture parts which were subsequently treated via laser polishing. Here in this work, CO2 laser polishing was used for the surface modification of the Ti-6Al-4V produced samples. The most significant processing parameters were optimised to achieve approximately an 80% reduction in the average surface roughness and a 90% reduction in the peak-to-valley distance with a processing time of 0.1 s/mm2 and cost of 0.2 €/cm2.

Simulation and experimental study on CO2 laser polishing quartz glass

High quality surface can be quickly obtained by laser polishing quartz glass. With the improvement of application requirements of quartz glass, the combination of process parameters for optimizing CO2 laser polishing large area quartz glass sheet is proposed. The untreated quartz ground glass was scanned by CO2 laser. Taking the roughness as the research index value, the parameters affecting CO2 laser polishing were studied and analyzed. The transient model was established by COMSOL software to simulate the polishing process, predict the combination of polishing parameters, and obtain the temperature field change in the polishing process and the flow of quartz glass during melting. 300W continuous CO2 laser was used in the experiment. The effect of surface improvement was the best when the laser power was 300W, the scanning rate was 100~200mm/s. The micro area roughness can reach Ra=10.18nm. Then, the chemical corrosion was combined with CO2 laser polishing, and the quartz glass was corroded by hydrofluoric acid and sodium hydroxide solution, and then polished to obtain the average minimum roughness Ra=5.63nm. The research results can be put into practical industrial production, which is conducive to improving production efficiency and promote the development of laser polishing quartz glass technology.

Refractometer based on fiber Mach-Zehnder interferometer composed of two micro bending cores

In this paper, a novel refractometer based on Mach-Zehnder Interferometer (MZI) is proposed and experimentally investigated. The MZI is composed of 2 micro bending cores (MBCs), one of which excites the cladding modes and the other couples the modes back. This structure is formed by high-frequency CO2 laser polishing and oxyhydrogen flame heating. With the unique deformation method, the interaction between the fiber core and the external status gets enhanced, moreover, higher modes in the cladding are excited, which leads to a high refractive index (RI) sensitivity. Due to the high temperature of the oxyhydrogen flame, the core of CO2 polished fiber is modulated, furthermore, the cladding shape of MBC tends to be circular. Hence, relatively small modulating regions of 500 μm can form for interference. In the experiment, 2 transmission dips are chosen for RI measuring, which possesses the wavelength of 1530.4 nm and 1600.8 nm, respectively. The RI sensitivities of the 2 transmission dips are -271.7 nm/RIU and -333.8 nm/RIU with the RI range of 1.33-1.42. The temperature characteristic is also experimentally analyzed and the temperature sensitivities of which are 0.121 nm/℃ and 0.171 nm/℃ in the range of 34℃-154℃. By solving the matrix equation, the proposed sensor can be applied for simultaneous measurement of RI and temperature.

CO2 laser polishing of laser-powder bed fusion produced AlSi10Mg parts

Poor surface quality represents an important issue that needs improvement for metal Additively Manufactured parts. Both short or long wavelength lasers have been applied for surface polishing in order to improve the surface finish. In this work, the possible use of a CO 2 laser for the surface polishing of AlSi10Mg parts made by Laser-based Powder Bed Fusion (L-PBF) was explored. The high surface roughness in the as-built condition can lead to increased laser energy absorption. In order to assess the effects of the main laser-related process parameters, the experiments were carried out on L-PBF samples built vertically with respect to the build platform. Their effect on the surface were evaluated by means of the surface arithmetical mean height (S a ), surface skewness (S sk ) and primary profile root mean square slope (P dq ), obtained via confocal microscopy . Microstructure evolution was also investigated by means of SEM and EDS analysis. The results showed a large reduction in surface roughness, ranging from the 67% to the 85% of the starting value. Microstructure of the re-molten layer revealed an increased grain size and an increased Si content that led to an overall hardness increase from 85 to 121 HV. • A CO 2 laser was used to perform surface re-melting of AlSi10Mg L-PBF samples. • The surface roughness (S a ) was reduced up to the 85%. • Surface symmetry, analyzed by means of areal skewness (S sk ), was improved. • Profile root mean square slope (P dq ) was also evaluated and reduced up to the 90%. • Vickers microhardness increased up to the 30% in the polished area.

A strain sensor with low temperature crosstalk based on re-modulation of D-shaped LPFG

In this paper, a sensor with high strain sensitivity and low temperature crosstalk is proposed. The proposed sensor is configured by re-modulating the D-shaped long-period fiber grating (D-LPFG). D-LPFG is fabricated by CO2 laser polishing method. Re-modulation refers to etching on the D-LPFG with CO2 laser. A new resonant dip appears after re-modulation. The re-modulation based on CO2 laser etching method changes the distribution of refractive index, which leads to the variation of coupling coefficients. Resonant dip A and dip B with wavelengths at 1301.5 nm and 1565.7 nm are selected to measure strain and temperature. Experimental results show that the strain and temperature sensitivities of the resonant dip A are 9.37 pm/με and 57.00 pm/°C, respectively. The strain and temperature sensitivities of the resonant dip B are −1.38 pm/με and 57.14 pm/°C, respectively. Through calculation, the strain sensitivity of the proposed LPFG reaches 10.75 pm/με with only 0.14 pm/°C temperature sensitivity. The temperature crosstalk is 0.013 με/°C. Therefore, the proposed LPFG can be used as a sensor for strain measurement with extremely low temperature crosstalk.

Compact Beam Homogenizer Module with Laser-Fabricated Lens-Arrays

We report on manufacturing of a compact beam homogenizer module including two lens arrays and an aperture. Lens arrays are fabricated by an all laser-based technology employing a precise femtosecond pulsed laser ablation and a CO2 laser polishing step. Each lens array is processed revealing a high contour accuracy and a roughness of 25 nm. The 8x8 lens arrays are designed to have a square footprint to generate a quadratic Top-Hat beam profile and focal length of 10 mm to realize compact packaging. Firstly, the lens arrays are tested in an experimental setup using commercial lens holders with their functionality being demonstrated by shaping a uniform 4.5 mm squared Top-Hat beam profile, as being calculated. Afterwards, a 3D printer is used to additively manufacture the housing for the beam homogenizer module having a length of only 16 mm. After assembling the laser-fabricated lens arrays and a laser-cutted aperture into the housing, the functionality of the miniaturized module is proven.

High Torsion Sensitivity Sensor Based on LPFG With Unique Geometric Structure

In this paper, we propose a sensor with high torsion sensitivity. This sensor has the ability to distinguish the torsion direction. A long-period fiber grating (LPFG) with a unique geometric structure is used to configure this sensor. The proposed LPFG is prepared by CO2 laser polishing method and etching method. First, we periodically polish the two opposite planes which are parallel to the axis direction of the fiber to get sheets structure. Then, the V-shaped grooves are etched on the surface orthogonal to the sheets to fabricate the proposed LPFG. Because the V-shaped grooves are arranged along the axis direction of the optical fiber, we name it by V-LPFG. This unique geometric structure results in the sensor with high torsion sensitivity. Experimental results prove that the torsion sensitivity of the proposed sensor reaches 0.399 nm/(rad/m) in the range of–10.5 rad/m to 10.5 rad/m. More importantly, the torsion sensitivity is significantly increased from 0.041 nm/(rad/m) to 0.399 nm/(rad/m) after the V-shaped grooves are etched. The temperature sensitivity of the proposed sensor is 50 pm/°C at the temperature range of 30–150 °C. Due to the high torsion sensitivity, relatively easy fabrication process and high price to quality ratio, the proposed sensor has the potential of carrying out torsion measurement with high sensitivity.

Fabrication and evaluation of negative axicons for ultrashort pulsed laser applications

We report on the fabrication and evaluation of a sharp tip negative axicon paving the way for applications in high-power ultrashort pulsed laser systems. The negative axicon is manufactured by applying a two-step all laser-based process chain consisting of ultrashort pulsed laser ablation and CO2 laser polishing finishing the component in less than 5 minutes. The finalized negative axicon reveals a surface roughness of 18 nm, fulfilling optical quality. Two measurement setups, including the ultrashort pulsed laser itself, are used to evaluate the formation of Bessel beams in detail. By applying a focusing lens behind the negative axicon, well-developed Bessel beams are generated while their lengths depend on the distance between the negative axicon and the lens. Furthermore, the diameter of the Bessel beams increase strongly with the propagation distance. By adding a second focusing lens, Bessel beams are generated at its focal position, being almost invariant of its position. Hence, the typical Bessel beam intensity distribution is observed over an entire moving range of this second lens of 300 mm. While these Bessel beams show superior quality in terms of sharp peaks with homogeneous concentric rings, only minor deviations in intensity and diameter are observed over the moving range.

Laser-fabricated axicons challenging the conventional optics in glass processing applications.

Laser-based fabrication can be an alternative technology to mechanical grinding and polishing processes. However, the performance of these elements in real applications still needs to be validated. In this paper, we demonstrate that the subtractive fabrication technology is able to produce high-quality axicons from fused silica, which can be efficiently used for glass processing. We comprehensively investigate axicons, fabricated by ultrashort pulsed laser ablation with subsequent CO2 laser polishing, and compare their performance with commercially available axicons. We show that laser-fabricated axicons are comparable in quality with a precision commercial axicon. Furthermore, we demonstrate the intra-volume glass modification and dicing, utilising mJ-level laser pulses. We show that the tilting operation of the laser-fabricated axicons results in the formation of directional transverse cracks, which significantly enhance the 1 mm-thick glass dicing process.

Formation mechanism of a smooth, defect-free surface of fused silica optics using rapid CO2 laser polishing

Surface defects introduced by conventional mechanical processing methods can induce irreversible damage and reduce the service life of optics applied in high-power lasers. Compared to mechanical processing, laser polishing with moving beam spot is a noncontact processing method, which is able to form a defect-free surface. This work aims to explore the mechanism of forming a smooth, defect-free fused silica surface by high-power density laser polishing with coupled multiple beams. The underlying mechanisms of laser polishing was revealed by numerical simulations and the theoretical results were verified by experiments. The simulated polishing depth and machined surface morphology were in close agreement with the experimental results. To obtain the optimized polishing quality, the effects of laser polishing parameters (e.g. overlap rate, pulse width and polishing times) on the polishing quality were experimentally investigated. It was found that the processing efficiency of fused silica materials by carbon dioxide (CO2) laser polishing could reach 8.68 mm2 s−1, and the surface roughness (Ra) was better than 25 nm. Besides, the cracks on pristine fused silica surfaces introduced by initial grinding process were completely removed by laser polishing to achieve a defect-free surface. The maximum laser polishing rate can reach 3.88 μm s−1, much higher than that of the traditional mechanical polishing methods. The rapid CO2 laser polishing can effectively achieve smooth, defect-free surface, which is of great significance to improve the surface quality of fused silica optics applied in high-power laser facilities.

Thermal noise reduction for future gravitational wave detectors

The coating thermal noise limits the sensitivity in gravitational wave detectors from few tens to hundreds Hz, where first gravitational signals were detected and others are expected. In view of future upgrades, the increase in the mechanical performances of reflective coatings, retaining their outstanding optical and morphological properties, is fundamental. In the Virgo collaboration a coating R&D group working with this aim is born. One of the research lines regards the mechanical characterization of both substrates and coatings, looking for materials with low mechanical loss angle. To perform a precise coating mechanical characterization, the substrate on which it is deposited must be characterized as well and must be stable with respect to its dissipative behaviour. Commercial SiO2 substrates are subject to effects that change their mechanical condition during time, compromising the characterization. The source of these spurious losses is related to the rough lateral surface: after its polishing, this behaviour is largely reduced. We designed and assemble a facility for the CO2 laser polishing of the substrates barrel, to provide a reliable heat treatment, reducing spurious losses to a negligible level. Other further treatments have been tested, with the aim of taking under control ϕ deterioration. We develop a procedure to prepare SiO2 samples for coating deposition. Procedure steps, mechanical characterizations and first results are shown.