Laser Micromachining Applications Research Articles

Observation of Boyer-Wolf Gaussian modes

Stable laser resonators support three fundamental families of transverse modes: the Hermite, Laguerre, and Ince Gaussian modes. These modes are crucial for understanding complex resonators, beam propagation, and structured light. We experimentally observe a new family of fundamental laser modes in stable resonators: Boyer-Wolf Gaussian modes. By studying the isomorphism between laser cavities and quadratic Hamiltonians, we design a laser resonator equivalent to a quantum two-dimensional anisotropic harmonic oscillator with a 2:1 frequency ratio. The generated Boyer-Wolf Gaussian modes exhibit a parabolic structure and show remarkable agreement with our theoretical predictions. These modes are also eigenmodes of a 2:1 anisotropic gradient refractive index medium, suggesting their presence in any physical system with a 2:1 anisotropic quadratic potential. We identify a transition connecting Boyer-Wolf Gaussian modes to Weber nondiffractive parabolic beams. These new modes are foundational for structured light, and open exciting possibilities for applications in laser micromachining, particle micromanipulation, and optical communications.

Multipulse operation in Mamyshev oscillator: influence of external seed source

We study the influence of an external seed source on the startup of a fiber Mamyshev oscillator (MO). Depending on the seed source parameters, while keeping MO parameters fixed, we observe diverse mode-locked operations from the MO, generating stable single, double, triple and even up to a group of seven pulses. The probability of the occurrence of such mode-locked operations is studied. There can be different operating regimes in a particular multipulse operation. We find that there is an optimum spectral width of the seed source for which the probability of generating mode-locked stable train of single pulses from a fiber MO is maximum. Further, we find that the power of the seed source has less influence on the mode-locking state of the fiber MO; however, the probability of single or double pulse operation is relatively higher than that of multipulse operation at higher power of the seed source. These experimental observations will enrich the database of multipulse patterns and help in understanding pulse dynamics in the fiber MO. Suitably chosen seed source parameters can generate the desired pulse pattern. Such tailored outputs could have prospective applications in laser micromachining and high bit rate data transfer in optical networks.

Generation of a linear array of focal spots with prescribed characteristic using the radiation pattern from a tapered line source antenna

We report an approach for generating a linear array of focal spots with prescribed characteristics through the use of the antenna radiation theory and time reversal technology. A virtual line source antenna with a cosine-squared tapered distribution, placed at the vicinity of the common foci of a 4π-system, is employed to obtain the required illumination in the pupil plane. The polarization direction of the generated focal spots and the alignment direction of the linear array are determined by the spatial orientation of the line source antenna. The amount of focal spots depends on the period of the amplitude distribution. The position and spacing of the focal spots are related to the length and the period of the amplitude distribution. The generated linear arrays of focal spots have potential applications in laser micromachining, optical imaging and optical micromanipulation that require multiple focal points or synchronous optical operations.

Investigation of precession laser machining of microholes in aerospace material

Sidewall tapering is one of the main limitations in ultrashort pulse (USP) laser machining and is associated with the beam shape and self-limiting effect. Laser processing with a precession beam is a potential solution to overcome this limitation. A study into the effects of precession parameters on the taper angle in microhole drilling of a nickel alloy is reported in this paper. The effects of three key precession parameters, i.e., incident angle, relative distance between the focuses of the precession and individual beams, and scanning speed, have been investigated in detail. Experiments were performed to drill through holes with aspect ratios up to 20:1 and diameters ranging from 100 to 500 μm over 0.6–2 mm thick nickel alloy substrates. Experiment results showed that all the considered parameters/factors were significant and affected the hole tapering in different ways. In addition, there were important interaction effects between two of the factors, i.e., incident angle and focus position, in some cases. The optimal parameters to minimize the tapering effect are suggested, and the mechanism is discussed in detail. The precession laser machining showed clear advantages in overcoming the limitations to associated with conventional USP laser machining. Fabricating microholes with high geometrical accuracy, i.e., with straight side walls and zero taper angles, is feasible with the use of a precession beam. The results clearly show the potential of precession laser processing and the capabilities that the technology can offer for a range of laser micromachining applications in different industries, such as microelectronics, automotive, and aerospace.

Self‐Referencing 3D Characterization of Ultrafast Optical‐Vortex Beams Using Tilted Interference TERMITES Technique

AbstractFemtosecond light pulses carrying optical angular momentums (OAMs), possessing intriguing properties of helical phase fronts and ultrafast temporal profiles, enable many applications in nonlinear optics, strong‐field physics, and laser micromachining. While generation of OAM‐carrying ultrafast pulses and their interactions with matters are intensively studied in experiments, 3D characterization of ultrafast OAM‐carrying light beams in spatiotemporal domain has, however, proved difficult to achieve. Conventional measurement schemes rely on the use of a reference pulsed light beam, which needs to be well characterized in its phase front and to have sufficient overlap and coherence with the beam under test, largely limiting practical applications of these schemes. Here a self‐referencing set‐up is demonstrated based on a tilted interferometer that can be used to measure complete spatiotemporal information of OAM‐carrying femtosecond pulses with different topological charges. Through scanning one interferometer arm, the spectral phase over the pulse spatial profile can be obtained using the tilted interference signal, and the temporal envelope of the light field at one particular position around its phase singularity can be retrieved simultaneously, enabling 3D beam reconstruction. This self‐referencing technique, capable of measuring spatiotemporal ultrafast optical‐vortex beams, may find many applications in fields of nonlinear optics and light–matter interactions.

All-PM Fiber Mamyshev Oscillator Delivers Hundred-Nanojoule and Multi-Watt Sub-100 fs Pulses

An all-fiber Mamyshev oscillator with a single amplification arm is experimentally demonstrated to achieve high-energy and high-average-power ultrafast pulse output, with the initiating of an external seed pulse. In the high-energy operation, a maximum single-pulse energy of 153 nJ is achieved at a repetition rate of 9.77 MHz. After compression with a pair of diffraction gratings, a measured pulse width of 73 fs with a record energy of 122.1 nJ and a peak power of 1.7 MW is obtained. In the high-average-power operation, up to 5th harmonic mode locking of the oscillator is realized via slightly adjusting the output coupling ratio and the cavity length. The achieved maximum output power is 3.4 W at a repetition rate of 44.08 MHz, while the corresponding pulse width is compressed to around ~100 fs. Meanwhile, the system is verified to be operated reliability in both high-energy and -average-power operation regimes through assessing its short- and long-term stabilities. To the best of our knowledge, these are the highest records in pulse energy and average power delivered from a single all-fiber ultrafast laser oscillator with picosecond/femtosecond pulse duration. It is believed that even higher-energy and -average-power ultrafast laser can be realized with the proposed laser scheme through further increasing the core diameter of the all-fiber cavity, providing promising sources for advanced fabrication, biomedical imaging, laser micromachining, and other practical applications, as well as an unprecedented platform for exploring undiscovered nonlinear dynamics.

Laser-induced emission spectra of stainless steels and aluminum irradiated with nanopulse lasers without setting delay: potential applications to remote sensing and laser micromachining.

Spectral lines from the impurities held in stainless steel and in aluminum can be clearly identified in the UV and the visible spectra when emission is laser induced. These spectroscopic lines can be initiated in metal irradiated at moderate laser optical densities of about 2.5×108W/cm2. In addition to the lines arising from impurities found in some metals, it was found that some spectroscopic lines from iron oxide formed during irradiation were also detected at the above-mentioned power density. It was found that lines observed from iron oxide are consistent with what is reported in the literature. The investigations reported were produced on samples at optical densities that are sufficient to create an electric field that is about 10 times the air electrical breakdown near the focal point. The results reported were obtained without setting any delay between the laser Q-switch and the data acquisition. The spectroscopic data are comparable to those shown in the literature by laser-induced breakdown spectroscopy in term of signal-to-noise ratio and are promising in detecting impurities such as heavy metals in remote sensing applications, where pulse delay is not always practical due to atmospheric conditions and power requirements. As a marking procedure is used during the investigations, the method demonstrates how spectroscopic monitoring in real time can be applied during a procedure in laser micromachining applications.

Focus shaping by tightly focusing locally linear polarized vortex beams with Pancharatnam–Berry tailored

Based on vector diffraction theory, we demonstrated that the three-dimensional spatial separation of the two foci can be achieved by modulating the polarization distribution of the input vector light field. The Pancharatnam–Berry tailored vector light field combined with a vortex phase can realize both foci possess vortices of arbitrary topological charges. The simulation results show that the tightly focused vector vortex beam can generate two vortices with independent topological charges, and the spatial distance between the two vortex foci can be adjusted. Moreover, the focal field distribution with optical cage or flat-topped profile can be easily obtained by appropriately choosing the polarized structure and vortex phase of the input beams. We generate a perfect optical cage that is almost surrounded by uniform light intensity. The flat-topped focus can be formed by focusing of a single vortex, as well as the superposition focusing of the two vortices. These engineered focus profiles may find potential applications in optical imaging, laser micromachining, particle trapping and manipulations.

High-speed temporal and spatial beam-shaping combining active and passive elements.

Temporal and spatial shaping of laser beams is common in laser micromachining applications to improve quality and throughput. However, dynamic beam shaping (DBS) of ultrashort, high-power pulses at rates of hundreds of kHz has been challenging. Achieving this allows for full synchronization of the beam shape with high repetition rates, high-power lasers with zero delay time. Such speeds must manipulate the beam shape at a rate that matches the nanosecond to microsecond process dynamics present in laser ablation. In this work, we present a novel design capable of alternating spatial and temporal beam shapes at repetition rates up to 330 kHz for conventional spatial profiles and temporal shaping at nanosecond timescales. Our method utilizes a unique multi-aperture diffractive optical element combined with two acousto-optical deflectors. These high damage threshold elements allow the proposed method to be easily adapted for high power ultrashort lasers at various wavelengths. Moreover, due to the combination of the elements mentioned, no realignment or mechanical movements are required, allowing for high consistency of quality for high throughput applications.

A parametric study of sub-picosecond laser ablation of thin metal foils

With the properties of ultrashort pulse width and ultrahigh peak power, femtosecond lasers excel at processing materials whose thickness is less than 500 μm. Numerous experiments and theoretical analyses have testified to the fact that there are solid grounds for the future applications of femtosecond laser micromachining [1, 2]. However, with the high costs and complexity of these devices, it is the sub-picosecond laser that might be an alternative when it comes to micromachining of thin metal foils. Furthermore, investigating the sub-picosecond laser interactions with matter could provide a better understanding of the ablation mechanisms and experimental verification of existing models concerning the ultrashort pulse regime. We present research on sub-picosecond laser interaction with metal foils with a thickness of less than 250 μm under various laser pulse parameters. The research was conducted by two types of ultrafast lasers: a lab-designed sub-picosecond Yb:KYW laser (650 fs) and a commercial femtosecond Ti:S laser (35 fs). The results show how variables, such as pulse duration, energy, repetition rate, wavelength and irradiation time, affect the micromachining process.

Experimental Investigations on Laser Ablation of Aluminum in Sub-Picosecond Regimes

Due to high power and ultrashort pulses, femtosecond lasers excel at (but are not limited to) processing materials whose thicknesses are less than 500 microns. Numerous experiments and theoretical analyses testify to the fact that there are solid grounds for the applications of ultrafast laser micromachining. However, with high costs and complexity of these devices, a sub-picosecond laser that might be an alternative when it comes to various micromachining applications, such as patterns and masks in thin metal foils, micro-nozzles, thermo-detectors, MEMS (micro electro-mechanical systems), sensors, etc. Furthermore, the investigation of sub-picosecond laser interactions with matter could provide more knowledge on the ablation mechanisms and experimental verification of existing models for ultrashort pulse regimes. In this article, we present the research on sub-picosecond laser interactions with thin aluminum foil under various laser pulse parameters. Research was conducted with two types of ultrafast lasers: a prototype sub-picosecond Yb:KYW laser (650 fs) and a commercially available femtosecond Ti:S laser (35 fs). The results show how the variables such as pulse width, energy, frequency, wavelength and irradiation time affect the micromachining process.

Investigation of an interlaced laser beam scanning method for ultrashort pulse laser micromachining applications

Abstract This article investigates picosecond and sub-picosecond laser micromachining of Borofloat®33 glass and provides clear evidence that a simple modification of the laser beam scanning strategy can lead to significant improvement of machining efficiency and hence process throughput. Besides studying the impact of the fundamental laser machining parameters, such as laser fluence, pulse overlap, pulse repetition frequency (PRF), pulse duration and laser spot diameter, on the machined depth, surface roughness and material removal rate (MRR), it also compares the machining results for two different laser beam scanning strategies, called here sequential” method (SM) and “interlaced” method (IM). By changing the scanning strategy from SM to IM, the MRR can be significantly increased because IM allows high-quality machining of the glass at higher PRF values. The experimental results show that this simple, cost-free modification allows the MRR value to be increased by more than 4 times, i.e. from 0.12 mm3/s to 0.53 mm3/s. Moreover, by using a Phantom V2512 high-speed camera, the picosecond laser micromachining process using both SM and IM was filmed. The videos show that SM leads to the accumulation of glass particles within the laser-machined area, whereas in IM the glass material is removed layer by layer which leads to the generation of “cleaner” and deeper areas. The mechanisms associated with these machining improvements are discussed.

Femtosecond Laser-Micromachining of Glass Micro-Chip for High Order Harmonic Generation in Gases.

We report on the application of femtosecond laser micromachining to the fabrication of complex glass microdevices, for high-order harmonic generation in gas. The three-dimensional capabilities and extreme flexibility of femtosecond laser micromachining allow us to achieve accurate control of gas density inside the micrometer interaction channel. This device gives a considerable increase in harmonics’ generation efficiency if compared with traditional harmonic generation in gas jets. We propose different chip geometries that allow the control of the gas density and driving field intensity inside the interaction channel to achieve quasi phase-matching conditions in the harmonic generation process. We believe that these glass micro-devices will pave the way to future downscaling of high-order harmonic generation beamlines.

Effect of Process Parameters and Material Properties on Laser Micromachining of Microchannels.

Laser micromachining has emerged as a promising technique for mass production of microfluidic devices. However, control and optimization of process parameters, and design of substrate materials are still ongoing challenges for the widespread application of laser micromachining. This article reports a systematic study on the effect of laser system parameters and thermo-physical properties of substrate materials on laser micromachining. Three dimensional transient heat conduction equation with a Gaussian laser heat source was solved using finite element based Multiphysics software COMSOL 5.2a. Large heat convection coefficients were used to consider the rapid phase transition of the material during the laser treatment. The depth of the laser cut was measured by removing material at a pre-set temperature. The grid independent analysis was performed for ensuring the accuracy of the model. The results show that laser power and scanning speed have a strong effect on the channel depth, while the level of focus of the laser beam contributes in determining both the depth and width of the channel. Higher thermal conductivity results deeper in cuts, in contrast the higher specific heat produces shallower channels for a given condition. These findings can help in designing and optimizing process parameters for laser micromachining of microfluidic devices.

Probing multipulse laser ablation by means of self-mixing interferometry.

In this work, self-mixing interferometry (SMI) is implemented inline to a laser microdrilling system to monitor the machining process by probing the ablation-induced plume. An analytical model based on the Sedov-Taylor blast wave equation is developed for the expansion of the process plume under multiple-pulse laser percussion drilling conditions. Signals were acquired during laser microdrilling of blind holes on stainless steel, copper alloy, pure titanium, and titanium nitride ceramic coating. The maximum optical path difference was measured from the signals to estimate the refractive index changes. An amplitude coefficient was derived by fitting the analytical model to the measured optical path differences. The morphology of the drilled holes was investigated in terms of maximum hole depth and dross height. The results indicate that the SMI signal rises when the ablation process is dominated by vaporization, changing the refractive index of the processing zone significantly. Such ablation conditions correspond to limited formation of dross. The results imply that SMI can be used as a nonintrusive tool in laser micromachining applications for monitoring the process quality in an indirect way.

Focus shaping and optical manipulation using highly focused second-order full Poincaré beam.

Generation of vectorial optical fields with arbitrary polarization distribution is of great interest in plenty of applications. In this work, we propose and experimentally demonstrate the generation of a second-order full Poincaré (FP) beam and its application in two-dimensional (2D) flattop beam shaping with spatially variant polarization under a high numerical aperture focusing condition. In addition, the force mechanism of the focal field with 2D flattop beam profile is numerically studied, demonstrating the feasibility to trap a dielectric Rayleigh particle in three-dimensional space. The results show that the additional degree of freedom provided by the FP beam allows one to control the spatial structure of polarization, to engineer the focusing field, and to tailor the optical force exerted on a dielectric Rayleigh particle. The findings reported in the work may find useful applications in laser micromachining, optical trapping, and optical assembly.

Enhancement of laser ablation via interacting spatial double-pulse effect

A novel spatial double-pulse laser ablation scheme is investigated to enhance the processing quality and efficiency for nanosecond laser ablation of silicon substrate. During the double-pulse laser ablation, two splitted laser beams simultaneously irradiate on silicon surface at a tunable gap. The ablation quality and efficiency are evaluated by both scanning electron microscope and laser scanning confocal microscope. As tuning the gap distance, the ablation can be significantly enhanced if the spatial interaction between the two splitted laser pulses is optimized. The underlying physical mechanism for the interacting spatial double-pulse enhancement effect is attributed to the redistribution of the integrated energy field, corresponding to the temperature field. This new method has great potential applications in laser micromachining of functional devices at higher processing quality and faster speed.

Hybrid high energy femtosecond laser system based on Yb:YAG single crystal fiber amplifier

A hybrid high energy femtosecond laser system based on Yb:YAG single crystal fiber amplifier is experimentally studied. Due to the low nonlinearity and high gain of the Yb:YAG single crystal fiber, laser pulses with high energy of 146μJ at the repetition of 100KHz are obtained. The beam quality of 1.233 in the horizontal direction and 1.239 in the vertical direction is detected. After compression, pulses with the duration of 859fs and energy as high as 63.5μJ are achieved at 100kHz repetition rate in a compact size, this compact and simple laser system will find various applications in ultrafast laser micro-machining.

New borate crystal boosts UV optical applications

The ability of inorganic crystals known as nonlinear optical (NLO) materials to alter the properties of a beam of laser light—for example, doubling its frequency—makes them indispensable for applications in fiber optics, photolithography, and laser micromachining. Few NLO materials can generate coherent light deep in the ultraviolet range (< 200 nm). KBe2BO3F2 (KBBF) is an exception. But the toxicity of beryllium and the low intensity of KBBF’s NLO properties limit its application. A team of researchers led by Shilie Pan of Xinjiang Technical Institute of Physics & Chemistry and Kenneth R. Poeppelmeier of Northwestern University may have come up with a solution—NH4B4O6F (ABF), a beryllium-free deep-ultraviolet NLO material (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.7b05943). The researchers report that ABF’s nonlinear coefficients are roughly 2.5 times as large as KBBF’s values. They also note that their synthesis method, based on the high-temperature reaction of B2O3 with NH4F, leads to high-quality

Low-loss single-mode negatively curved square-core hollow fibers.

We introduce a novel design of anti-resonant fibers with negative-curvature square cores to be employed in 1.55 and 2.94 μm transmission bands. The fibers have low losses and single-mode operation via optimizing the negative curvature of the guiding walls. The first proposed fiber shows a broadband transmission window spanning 0.9-1.7 μm, with losses of 0.025 and 0.056 dB/m at 1.064 and 1.55 μm, respectively. The second proposed fiber has approximately a 0.023 dB/m guiding loss at 2.94 μm with a small cross-sectional area, useful for laser micromachining applications.