Percussion drilling in glasses and process dynamics with femtosecond laser GHz-bursts

All-fiber chirp tuning of ultrashort pulses via long chirped fiber Bragg grating pair

Chirp tuning of ultrashort pulses is crucial for nonlinear fiber amplification and nonlinear dynamics investigations. Here we demonstrate all-fiber chirp tuning via a chirped fiber Bragg grating (CFBG) pair. Two identically long CFBGs were placed reversely to cancel out most of their huge dispersion (∼40 ps/nm or ∼22.4 ps2 @1030 nm), while the controllable temperature gradient along them could be used for precise chirp tuning with a tuning range of ∼ps2, verified by dispersion measurement and ultrashort pulse broadening. This relatively large chirp tuning could be used in prechirp management in nonlinear fiber amplifiers, exemplified by the optical spectrum tailoring therein. In addition, we also show this precise chirp tuning capability could be very helpful for pulse temporal quality diagnosis, which is indispensable for seed pulse optimization. We believe this all-fiber chirp tuning technique would find wide applications in nonlinear amplification and ultrafast nonlinear dynamics investigations.

Modeling highly efficient femtosecond laser ablation of aluminum for cutting

This article presents high-efficiency ablation of aluminum using femtosecond laser processing with a pulse duration of 300 fs and a laser wavelength of 1035 nm. A series of stationary irradiation experiments were conducted at repetition rates frep = 50 kHz, 100 kHz, 200 kHz, 500 kHz, and 1 MHz. The comparison results indicated that as the repetition rate increased above a laser fluence F0 ≈ 1 J/cm2, the ablation rate ΔL decreased. This phenomenon could be attributed to particle shielding, which occurs as the density of particles increases with increasing the volume of ablated targets, corresponding to an increase in F0. The volume ablation rate ΔV was obtained in 0.53 ≤ F0 ≤ 2.59 J/cm2 at frep = 100 kHz, revealing that ΔV at F0 = 2.59 J/cm2 was ~ 3.4 times higher than that at F0 = 1 J/cm2. Multibeam laser processing, utilizing a diffractive optical element, was employed to reduce frep, thereby suppressing particle shielding while preserving the total laser fluence and scan speed. The experimental groove shapes were accurately estimated using a developed analytical model. These findings provide valuable insights for achieving high-efficiency laser cutting of aluminum in the realm of secondary battery manufacturing.

Ultrasmooth Micromilling of Stainless Steel by Ultrashort Pulsed Laser Ablation Using MHz Bursts

Ultrashort pulsed (USP) laser burst ablation has attracted numerous interests for its great potential in enhancing ablation efficiency and reducing the heat-affected zone. However, little attention has been paid to the influence of burst ablation on the processed surface quality. To fill this research gap, the present study conducts a comprehensive investigation on the surface processing of stainless steel using ultrashort pulsed laser burst ablation. Systematic experiments have been carried out to investigate influences of pulse number per burst (PpB), pulse fluence, and burst overlap ratio on surface quality and ablation efficiency. A two-dimensional model has been developed to unveil the fundamental thermodynamic process and evolution of ablation and melting in material during USP laser burst ablation. Compared to single-pulse ablation, the optimum ablation efficiency decreases with increasing PpB by less than 30% in burst ablation. Despite reduced ablation efficiency, burst-mode ablation can generate much better surface quality, achieving an ultrasmooth surface with an Sa roughness as low as 0.13 μm. Burst ablation generates distinctive surface structures compared to single-pulse ablation, and their formation mechanisms are scrutinized. The thickness of the surface melting layer is unveiled to determine surface morphology. Based on transmission electron microscopy (TEM) analysis and numerical simulation, a melting layer thickness between 100 and 320 nm is found to result in smooth surfaces. This work highlights the advantage of burst-mode ablation in achieving ultrasmooth surfaces on stainless steel and unveils the fundamental mechanisms of surface structures formation in USP laser burst ablation.

Bessel Beam Femtosecond Laser Interaction with Fused Silica Before and After Chemical Etching: Comparison of Single Pulse, MHz-Burst, and GHz-Burst

We investigate the elongated modifications resulting from a Bessel beam-shaped femtosecond laser in fused silica under three different operation modes, i.e., the single-pulse, MHz-burst, and GHz-burst regimes. The single-pulse and MHz-burst regimes show rather similar behavior in glass, featuring elongated and slightly tapered modifications. Subsequent etching with Potassium Hydroxide exhibits an etching rate and selectivity of up to 606 μm/h and 2103:1 in single-pulse operation and up to 322 μm/h and 2230:1 in the MHz-burst regime, respectively. Interestingly, in the GHz-burst mode, modification by a single burst of 50 pulses forms a taper-free hole without any etching. This constitutes a significant result paving the way for chemical-free, on-the-fly drilling of high aspect-ratio holes in glass.

Femtosecond Laser Percussion Drilling of Silicon Using Repetitive Single Pulse, MHz-, and GHz-Burst Regimes

In this contribution, we present novel results on top-down drilling in silicon, the most important semiconductor material, focusing specifically on the influence of the laser parameters. We compare the holes obtained with repetitive single pulses, as well as in different MHz- and GHz-burst regimes. The deepest holes were obtained in GHz-burst mode, where we achieved holes of almost 1 mm depth and 35 µm diameter, which corresponds to an aspect ratio of 27, which is higher than the ones reported so far in the literature, to the best of our knowledge. In addition, we study the influence of the energy repartition within the burst in GHz-burst mode.

Crackless high-aspect-ratio processing of a silica glass with a temporally shaped ultrafast laser

In this Letter, we propose a crackless high-aspect-ratio processing method based on a temporally shaped ultrafast laser. The laser pulse is temporally split into two sub pulses: one with smaller energy is used to excite electrons but without ablation so that the applied pressure to the sample is weak, and the other one is used to heat the electrons and achieve material removal after it is temporally stretched by a chirped volume Bragg grating (CVBG). Compared with the conventional ultrafast laser processing, the crack generation is almost suppressed by using this proposed method. The hole depth increases more than 3.3 times, and the aspect ratio is improved at least 2.2 times. Moreover, processing dynamics and parameter dependence are further experimentally studied. It shows that the processing highly depends on the density of electrons excited by the first pulse (P1) and the energy of the second pulse (P2). This novel, to the best of our knowledge, method provides a new route for the precise processing of wide-bandgap materials.

Supercontinuum generation in bulk solid-state material with bursts of femtosecond laser pulses

We report on experimental and numerical investigation of burst-mode supercontinuum generation in sapphire crystal. The experiments were performed using bursts consisting of two 190 fs, 1030 nm pulses with intra-burst repetition rates of 62.5 MHz and 2.5 GHz from an amplified 1 MHz Yb:KGW laser and revealed higher filamentation and supercontinuum generation threshold for the second pulse in the burst, which increases with the increase of intra-burst repetition rate. The experimental results were quantitatively reproduced numerically, using a developed model, which accounted for altered material response due to residual excitations remaining after propagation of the first pulse. The simulation results unveiled that residual free electron plasma and self-trapped excitons contribute to elevated densities of free electron plasma generated by the second pulse in the burst and so stronger plasma defocusing, significantly affecting its nonlinear propagation dynamics. The presented results identify the fundamental and practical issues for supercontinuum generation in solid-state materials using femtosecond pulse bursts with very high intra-burst repetition rates, which may also apply to the case of single pulses at very high repetition rate, where residual material excitations become relevant and should be accounted for.

Laser processing of silicon with GHz burst pumped third harmonics for precise microfabrication

Femtosecond laser processing has proved to be a valuable tool for various microfabrication applications. In order to further increase the quality and efficiency of femtosecond laser processing, processing with GHz burst mode lasers has gained attention in recent years, where packets of high-repetition rate pulses are used instead of single pulses at the fundamental repetition rate. However, the use of burst-pulses has mainly been limited to the fundamental wavelength of powerful regenerative amplifier systems, often near 1 micrometer wavelength. In this study, we explore the characteristics and potential benefits of further wavelength conversion of burst-pulses emitted at the near-infrared to the ultraviolet region via direct third-harmonic generation. We construct an in-line process evaluation setup with a chromatic confocal sensor, and evaluate the ablation characteristics of the burst-pumped and non-burst processing of silicon. We observe that burst-mode processing has significantly reduced surface roughness and debris, resulting in high-quality laser processing. To demonstrate the utility of such burst-pumped UV processing, we show the successful milling of a spherical structure enabled by in-line surface profile feedback, while similar processing with non-burst conditions did not work. We believe such results show the strong potential of burst laser sources for use in accurate microfabrication of structures with micrometer-scale resolution.

Mechanism and performance evaluation of transient and selective laser processing of glass based on optical monitoring

Femtosecond laser processing has been widely applied in glass processing owing to its ability to fabricate microscale components. To improve processing efficiency, a transient and selective laser (TSL) processing technique was previously developed, in which electron excitation was induced inside a transparent medium by a single pulse of femtosecond (fs) laser, and a single pulse of microsecond (µs) laser can be selectively absorbed in this excited region to heat and remove the material. However, because of its high speed removal process, the unclear mechanism and inefficient evaluation of its processing performance limit its further application. This study analyzes the transient spatiotemporal evolution of the induced plasma and the related material removal mechanism of the TSL processing using a side high-speed monitoring method. To achieve a rapid performance evaluation, a quantitative analysis of the optical plasma signals (on a microsecond timescale) generated in TSL processing was performed by employing a developed coaxial high-speed monitoring method using a photodetector. The variations in the shapes, intensity distribution, and dimensions of the plasma were quantitatively investigated. In addition, the relation between the plasma signal and drilling performance under different laser parameters, including hole depth, hole types, and cracks, was explored and quantitatively analyzed. The revealed mechanism is expected to contribute to the broadening of the application of TSL processing in microfabrication. Furthermore, the developed high-speed and precision monitoring technology can be utilized for high-speed evaluation and precision control of machining quality in real time during ultrahigh-speed laser machining, without time-consuming camera observations.

High-Power Femtosecond Laser Processing of SiC Ceramics with Optimized Material Removal Rate

Silicon carbide (SiC) ceramics are widely used as structural materials for various applications. However, the extraordinarily high hardness, brittleness, low material removal rate, and severe tool wear of these materials significantly impact the performance of conventional mechanical processing techniques. In this study, we investigated the influence of different parameters on the material removal rate, surface quality, and surface oxidation during the laser processing of SiC ceramic samples using a high-repetition-frequency femtosecond laser at a wavelength of 1030 nm. Additionally, an experimental investigation was conducted to analyze the effects of a burst mode on the material removal rate. Our results demonstrate that the surface oxidation, which significantly affects the material removal rate, can be effectively reduced by increasing the laser scanning speed and decreasing the laser scanning pitch. The material removal rate and surface quality are mainly affected by laser fluence. The optimal material removal rate is obtained with a laser fluence of 0.4 J/cm2 at a pulse width of 470 fs.

Comparative Study of Percussion Drilling in Glasses with a Femtosecond Laser in Single Pulse, MHz-Burst, and GHz-Burst Regimes and Optimization of the Hole Aspect Ratio

In this contribution, we present a comparative study on top-down drilling in sodalime glass, with a femtosecond laser operating in single-pulse, MHz-burst and GHz-burst modes, respectively. We investigate the hole depth, drilling rate, and hole morphology for these three regimes while keeping the same experimental conditions. We demonstrate that, for both burst regimes, the burst length has to be adapted for optimizing the hole depth. In the GHz-burst regime, the lower the ablation rate the longer the holes. The three drilling regimes lead to different hole morphologies, where the GHz-burst mode results in the best hole quality featuring glossy inner walls and an almost cylindrical morphology. Furthermore, we obtain crack-free holes, the deepest measuring 3.7 mm in length and 25 µm in entrance diameter corresponding to an aspect ratio of 150, which is the highest aspect ratio reported thus far with femtosecond GHz-burst drilling to the best of our knowledge.

Bessel Beam Dielectrics Cutting with Femtosecond Laser in GHz-Burst Mode

We report, for the first time to the best of our knowledge, Bessel beam dielectrics cutting with a femtosecond laser in GHz-burst mode. The non-diffractive beam shaping is based on the use of an axicon and allows for cutting glasses up to 1 mm thickness with an excellent cutting quality. Moreover, we present a comparison of the cutting results with the state-of-the-art method, consisting of short MHz-bursts of femtosecond pulses. We further illustrate the influence of the laser beam parameters such as the burst energy and the pitch between consecutive Bessel beams on the machining quality of the cutting plane and provide process windows for both regimes.

Advances in Femtosecond Laser GHz-Burst Drilling of Glasses: Influence of Burst Shape and Duration

The femtosecond GHz-burst mode laser processing has attracted much attention in the last few years. Very recently, the first percussion drilling results obtained in glasses using this new regime were reported. In this study, we present our latest results on top-down drilling in glasses, focusing specifically on the influence of burst duration and shape on the hole drilling rate and the quality of the drilled holes, wherein holes of very high quality with a smooth and glossy inner surface can be obtained. We show that a decreasing energy repartition of the pulses within the burst can increase the drilling rate, but the holes saturate at lower depths and present lower quality than holes drilled with an increasing or flat energy distribution. Moreover, we give an insight into the phenomena that may occur during drilling as a function of the burst shape.