High-power Picosecond Laser Research Articles

Realization of Joints of Aluminosilicate Glass and 6061 Aluminum Alloy via Picosecond Laser Welding without Optical Contact.

To achieve laser direct welding of glass and metal without optical contact is hard, owing to the large difference in thermal expansion and thermal conductivity between glass and metal and an insignificant melting area. In this study, the high-power picosecond pulsed laser was selected to successfully weld the aluminosilicate glass/6061 aluminum alloy with a gap of 35 ± 5 μm between glass and metal. The results show that the molten glass and metal diffuse and mix at the interface. No defects such as microcracks or holes are observed in the diffusion mixing zone. Due to the relatively large gap, the glass collapsed after melting and caulking, resulting in an approximately arc-shaped microcrack between modified glass and unmodified glass or weakly modified glass. The shape of the glass modification zone and thermal accumulation are influenced by the single-pulse energy and linear energy density of the picosecond laser during welding, resulting in variations in the number and size of defects and the shape of the glass modification zone. By reasonably tuning the two factors, the shear strength of the joint reaches 15.98 MPa. The diffusion and mixing at the interface and the mechanical interlocking effect of the glass modification zone are the main reasons for achieving a high shear strength of the joint. This study will provide reference and new ideas for the laser transmission welding of glass and metal in the non-optical contact conditions.

Extending the operational limit of a cooled spatial light modulator exposed to 200 W average power for holographic picosecond laser materials processing

The phase range of a Spatial Light Modulator (SLM) requires full 2π response for accurate phase modulation of the incident laser wavefront to create the desired structured intensity distribution. While cooled SLM’s allow increased power exposures, degradation in performance occurs with average powers P>100 W in a Gaussian beam due to residual absorption and consequent heating of the liquid crystal (LC) layer which seriously limits the phase stroke and diffraction efficiency. By introducing a refractive flat top intensity generator ahead of a cooled SLM, we have extended the phase range to Δφ = 2π at incident power P=210 W with only a small light field depolarisation. The operational limit has therefore been extended significantly, allowing efficient, parallel laser materials processing with a high-power picosecond laser. The calculation and implementation of binary Damman gratings for multi-beam laser processing is also shown to be useful at high power exposure.

Characterization of sub-nanosecond pulse compression based on frequency-detuning SBS.

High-frequency, high-power picosecond lasers have important and wide-ranging applications in laser ranging, optoelectronic countermeasures, and ultrafine industrial processing. Pulse compression based on stimulated Brillouin scattering (SBS) can achieve a highly efficient picosecond laser output, while improving the peak power and beam quality of the laser. In this paper, a generator-amplifier two-cell structure with frequency-detuning was proposed to achieve a pulse output that combines high compression ratio and high energy reflectivity. The experiment proved that under a pump pulse width of 15 ns and repetition frequency of 10 Hz, when the generator cell and amplifier cell media were selected as HT-230, the highest energy reflectivity of 46% and narrowest compression pulse width of 1.1 ns were achieved, and the pulse compression ratio was 13.6. When the amplifier cell was selected as FC-770 and the generator cell was selected as HT-230, an energy reflectivity of 52% and a compression pulse width of 840 ps could be achieved simultaneously, and the pulse compression ratio was 18.

Targeting new ways for large-scale, high-speed surface functionalization using direct laser interference patterning in a roll-to-roll process

Direct Laser Interference Patterning (DLIP) is used to texture current collector foils in a roll-to-roll process using a high-power picosecond pulsed laser system operating at either fundamental wavelength of 1064 nm or 2nd harmonic of 532 nm. The raw beam having a diameter of 3 mm @ 1/e 2 is shaped into an elongated top-hat intensity profile using a diffractive so-called FBS®-L element and cylindrical telescopes. The shaped beam is split into its diffraction orders, where the two first orders are parallelized and guided into a galvanometer scanner. The deflected beams inside the scan head are recombined with an F-theta objective on the working position generating the interference pattern. The DLIP spot has a line-like interference pattern with about 15 μm spatial period. Laser fluences of up to 8 J cm−2 were achieved using a maximum pulse energy of 0.6 mJ. Furthermore, an in-house built roll-to-roll machine was developed. Using this setup, aluminum and copper foil of 20 μm and 9 μm thickness, respectively, could be processed. Subsequently to current collector structuring coating of composite electrode material took place. In case of lithium nickel manganese cobalt oxide (NMC 622) cathode deposited onto textured aluminum current collector, an increased specific discharge capacity could be achieved at a C-rate of 1 °C. For the silicon/graphite anode material deposited onto textured copper current collector, an improved rate capability at all C-rates between C/10 and 5 °C was achieved. The rate capability was increased up to 100% compared to reference material. At C-rates between C/2 and 2 °C, the specific discharge capacity was increased to 200 mAh g−1, while the reference electrodes with untextured current collector foils provided a specific discharge capacity of 100 mAh g−1, showing the potential of the DLIP technology for cost-effective production of battery cells with increased cycle lifetime.

A Study of the Laser Removal Process of Al-Si Coating from 22MnB5 Steel.

22MnB5 hot forming steel is widely used in the automotive industry due to the increasing demand for lightweight vehicles. As surface oxidation and decarburization occur during hot stamping processes, an Al-Si coating is often precoated on surfaces. The coating tends to melt into the melt pool during the laser welding of the matrix and reduce the strength of the welded joint; therefore, it should be removed. The decoating process by sub-nanosecond and picosecond lasers and process parameter optimization were conducted in this paper. The corresponding analysis of the different decoating processes, the mechanical properties and the elemental distribution was carried out after laser welding and heat treatment. It was found that the Al element has an influence on the strength and elongation of the welded joint. The high-power picosecond laser has a better removal effect than the lower power sub-nanosecond laser. The best mechanical properties of the welded joint were obtained under the process conditions of 1064 nm center wavelength, 15 kW power, 100 kHz frequency, and 0.1 m/s speed. In addition, the content of the coating metal elements (mainly Al) melted into the welded joint is reduced with increasing coating removal width, which significantly improves the mechanical properties of the welded joints. Al in the coating rarely melts into the welding pool when the coating removal width is not less than 0.4 mm, and its mechanical properties can meet the automotive stamping requirements for the welded plate.

Efficiency Enhancing Technique for Rod Fiber Picosecond Amplifiers with Optimal Mode Field Matching

A high power and high quality picosecond laser is crucial in MEMS fabrication regarding micromachines. Optimal seed beam coupling is an important precondition to enhance laser efficiency. However, empirical coupling limits its development. In this paper, the physical parameters related to coupling are determined. The relationships among them are established under optical mode matching constraints to satisfy optimal seed beam coupling. According to a theoretical analysis, the focal length cut-off and the optimal coupling position of the coupling lens are acquired. A maximum transmittance of 87.2% is acquired with a 6 W input seed power in the validation experiment. In further power amplification experiments, a diffraction-limited beam quality is achieved, with M2X = 1.111, M2Y = 1.017, an optical efficiency of 60.5% and a slope efficiency of 66%, benefiting from the previous theoretical guidance.

Analysis of the Heat-Affected Zone and Ablation Efficiency in Terms of Burst Mode Parameters During High Power Picosecond Laser Micromachining of Metals

Abstract This paper presents a systematic study on using the burst mode ablation to limit the heat-affected zone (HAZ) while maintaining a high ablation efficiency using a high-power industrial picosecond laser with burst fluence larger than 10 J/cm2. An extended three-dimensional two-temperature model (3D-TTM) was employed to study the mechanism of the HAZ development and to predict the ablation efficiency with experimental validation. The essentiality of including the lattice heat conduction to predict accurate HAZ was discussed. The effect of the number of pulses per burst and pulse-to-pulse separation time was investigated. The optimal number of pulses per burst was obtained by using the 3D-TTM for copper and stainless steel. The 3D-TTM suggested that by using the optimal number of pulses per burst, a maximum reduction of 77% and 61% in HAZ could be achieved for copper and stainless steel respectively. And the corresponding ablation efficiency will be increased by 24% and 163% for copper and stainless steel at the same time. This study showed that burst mode laser machining at high fluence is an effective way of increasing efficiency while limiting the HAZ.

178-W picosecond green laser with active beam-pointing stabilization

Picosecond lasers with high average power and high beam quality have been widely used for precision processing and space exploration. In this study, we report a high-power picosecond green laser using a multistage Yb-doped rod-shaped photonic crystal fiber as an amplifier combined with a beam combination. The single amplification module achieves a 1,030 nm laser output of 146.8 W, and the maximum second harmonic generation (SHG) power is 92 W with a frequency conversion efficiency of 63.5%. The combined beam of the two SHGs resulted in a final output of 178 W with a repetition frequency of 24.07 MHz, pulse width of 50.1 ps, and beam quality factor of M2 = 1.16. Furthermore, an adaptive filter control method of a two-axis fast-steering mirror was applied to suppress the beam jitter to up to 45 Hz.

Modified Frantz-Nodvik equation and numerical simulation of a high-power Innoslab picosecond laser amplifier.

A modified Frantz-Nodvik (F-N) equation and a simple one-dimensional unfolded slicing model for numerically simulating high-power Innoslab picosecond amplifier are developed for the first time. The anisotropic stimulated emission cross-section of laser crystal, the influence of the tilted optical path, the spatial overlap of the seed and pump laser, as well as the pump absorption saturation effect are considered. Based on the as-developed model, 4-, 6- and 8-pass schemes high-power Nd:YVO4 Innoslab picosecond amplifiers are designed with output powers of 76.2 W, 81.4 W, and 85.5 W, respectively. The experimental results agree well with that of numerical simulation, indicating that our model is a powerful tool and paves a new way for designing and optimizing high-power Innoslab picosecond laser amplifier.

High sensitivity Thomson spectrometry: analysis of measurements in high power picosecond laser experiments

Thomson spectrometers (TS) are designed to detect and distinguish protons from heavier ions in experiments of intense laser-matter interaction. The combination of electric and magnetic field allows for deflecting ion species with different mass-to-charge ratio on different trajectories. However, even small distortions of the internal fields of the device can lead to a degradation of the measurement quality. Hence, TS are sensitive to both high electromagnetic pulses (EMPs) and fields due to static charge accumulation caused by the interaction. Here we report on the analysis of data obtained with a TS designed to have high sensitivity and robustness with, optimized shielding against EMPs, even when the device is placed at short distances from the interaction point, where the electromagnetic radiation is more intense. To test this, the spectrometer was thus placed ∼50 cm far from the target during an experiment at the PHELIX laser at GSI (∼180 J energy, >1020 W/cm2 intensity, sub-picosecond laser pulses on solid targets). Despite the presence of strong EMPs (beyond 100 kV/m at 1 m distance from the target), the tests were successful and the TS was able to retrieve a good-quality signal. Indeed, the close proximity to the interaction point caused a significant number of electrons, produced by the intense laser-target interaction, entering the TS and causing internal electrostatic fields up to tens of kV/m. These induced fields altered the trajectories of the detected ions, making the interpretation and characterization of the particle species not straightforward. This effect was analyzed with ad-hoc particle tracking simulations. This study is of high importance for the effective implementation of this type of high-sensitivity TSs in experiments with PW-power lasers.

310 W picosecond laser based on Nd:YVO4 and Nd:YAG rod amplifiers

Herein, a high-power picosecond laser system based on master oscillator power amplifier structure is demonstrated. The seed source is a SESAM mode-locked fiber oscillator; the pulse repetition frequency is selected by an acoustic optical modulator with a tunable range from 100 kHz to 4.27 MHz. The amplification system includes one fiber amplifier stage, four stages of end-pumped Nd:YVO4 amplifiers, and two stages of side-pumped Nd:YAG amplifiers. Finally, the average output power of >300 W is achieved. The maximum peak power is 86.6 MW at 300 kHz with a pulse energy of ∼700 μJ and a pulse duration of ∼8.08 ps.

High-Throughput Picosecond Laser Machining of Aerospace Nickel Superalloy

This manuscript discusses the experimental results on 300 W picosecond laser machining of aerospace-grade nickel superalloy. The effect of the laser’s energetic and beam scanning parameters on the machining performance has been studied in detail. The machining performance has been investigated in terms of surface roughness, sub-surface thermal damage, and material removal rate. At optimal process conditions, a picosecond laser with an average power output of 300 W can be used to achieve a material removal rate (MRR) of ∼140 mm3/min, with thermal damage less than 20 µm. Shorter laser pulse widths increase the material removal rate and reduce the resultant surface roughness. High scanning speeds improve the picosecond laser machining performance. Edge wall taper of ∼10° was observed over all the picosecond laser machined slots. The investigation demonstrates that high-power picosecond lasers can be used for the macro-machining of industrial components at an acceptable speed and quality.

Energy consumption and process characteristics of picosecond laser de-coating of cutting tools

Abstract Laser de-coating is a promising technology for selective coating removal to facilitate reuse of coated tools in a circular economy. It is timely to understand and optimise the quality, energy requirements and environmental footprint of the laser de-coating process. This paper reports on an investigation into process characteristics, energy demands and carbon footprints in coating removal from tungsten carbide tool inserts using high power picosecond laser sources with different pulse widths and wavelengths. The results demonstrate that although the 355 nm wavelength, 10 ps pulse width laser, has higher processing efficiency in terms of production rates, and lower specific energy for laser beam and material interaction, and results in better surface quality, the overall laser system energy consumption is much higher than that of using the 1064 nm wavelength, 150 ps pulse width laser for the removal of coatings on tungsten carbide cutting tools. This is largely due to the high energy demand of the water cooling system needed for the 355 nm wavelength picosecond laser whilst the 1064 nm ps is air cooled. There is therefore a need to optimise the total system energy consumption in the 355 nm ps laser, to support its superior laser de-coating performance. The paper shows the importance and challenges of optimising the energy consumption at different system boundaries.

Compact 50 W All-Solid-State Picosecond Laser System at 1 kHz

Compact, stable, high-power and high repetition rate picosecond laser systems are excellent sources for optical parametric chirped pulse amplification systems and laser satellite ranging systems. Compared with the traditional complex high-power amplifier, this article reports a compact high-power picosecond laser system at a repetition rate of 1 kHz based on Nd:YAG bulk crystal. The thermal lens effect limits the regenerative amplifier to directly output higher energy. For this reason, multi-stage traveling-wave amplifiers are usually used to gradually increase the laser pulse energy. So as to achieve a compact structure, a regenerative amplifier that can output higher power at 1 kHz is designed in the laser system. The regenerative amplifier can output the power of 6.5 W at the pump power of 41.5 W; the beam quality of M2 factor was about 1.3. A more flexible thermal depolarization compensation structure is applied in the side-pumped amplifier, which can effectively compensate for thermal lens effect and thermal depolarization at different pump powers. Finally, the laser pulse can achieve an output power higher than 50 W at 1 kHz after passing through an end-pumped traveling-wave amplifier and a side-pumped traveling wave amplifier.

Raman-Kerr comb generation based on parametric wave mixing in strongly driven Raman molecular gas medium

We report on experimental and theoretical demonstration of an optical comb spectrum based on a combination of cascaded stimulated Raman scattering and four-wave mixing mediated by Raman-induced nonresonant Kerr-type nonlinearity. This combination enabled to transform a conventional quasi-periodic Raman comb into a comb with a single and smaller frequency spacing. This new phenomenon is realized using a hollow-core photonic crystal fiber filled with 40 bars of deuterium, and pumped with a high-power picosecond laser. The resultant comb shows more than 100 spectral lines spanning over 220 THz from 800 nm to 1710 nm, with a total output power of 7.1 W. In contrast to a pure Raman comb, a 120 THz wide portion of the spectrum exhibits denser and equally-spaced spectral lines with a frequency spacing of around 1.75 THz, which is much smaller than the lowest frequency of the three excited deuterium Raman resonances. A numerical solution of the generalized nonlinear Schr\"odinger equation in the slowly varying envelope approximation provides very good agreement with the experimental data. The additional sidebands are explained by cascaded four-wave mixing between pre-existing spectral lines, mediated by the large Raman induced optical nonlinearity. The results open a new route to the generation of optical frequency combs that combine large bandwidth and high power controllable frequency spacing.

The preliminary study based on milling dental glass ceramics with visible and infrared picosecond laser pulse

In this study, a high-power picosecond pulsed laser in non-contact glass ceramics fabricating model was investigated based on the CAD/CAM system. Aiming to reduce the wear cost of diamond milling tools and improve the cutting efficiency and precision in the traditional manufacture of all-ceramic dental restorations, the picosecond pulsed laser (1064-nm or 532-nm wavelength) was tested for cutting three dental ceramic blocks. The relationship of laser pulse repetition or laser parameters to the ceramic remove rates was demonstrated. The remove rates of 20-W picosecond 1064-nm laser reached 0.115 mm3/s, 0.126 mm3/s, and 0.760 mm3/s for cutting VITA Mark II, IPS e.max CAD, and LAVA Ultimate respectively. The roughness and edge breakage were 5.85 μm and 15.59 μm (VITA Mark II), 3.54 μm and 14.27 μm (IPS e.max CAD), and 6.79 μm and 16.58 μm (LAVA Ultimate) respectively when high-power pulsed laser milled dental ceramics. This study verified the feasibility and practicability of the picosecond pulsed laser used for dental ceramic block milling. However, further studies on femtosecond laser’s milling efficiency and precision and the comparison of the laser milling with conventional diamond-based CAD/CAM are necessary in order to find out the manufacturing advantages of high-power pulsed laser milling system used for dental ceramic restorations.

Search for De-excitation of the 186mRe and 110mAg Nuclear Isomers in Plasma of High-Power Picosecond Laser Pulse

The possibility of observing the stimulated de-excitation of nuclear isomers (SDENI) in plasma (plasma lifetime ∼1.5 ps, temperature of electrons ∼ 10 keV) formed by the impact of a high-power laser pulse (∼1018 W cm−2) on a target is studied experimentally. Preliminary experiments are carried out with the 110mAg (T1/2 = 250 days) and 186mRe (T1/2 = 2 × 105 years) isomers. A weak SDENI effect is observed only for the 110mAg isomer on a platinum backing.

Comparison of the productivity and ablation efficiency of different laser classes for laser ablation of gold in water and air

In this study, we compare different laser systems used for the synthesis of nanoparticles. The productivity and ablation efficiency of laser ablation of gold in water and in air are determined for three pulsed laser systems with comparable pulse energy but different pulse duration and repetition rate. All experiments are performed in a fluence range of up to $$20\,\hbox {J}/\hbox {cm}^{2}$$ . The highest productivity among the considered lasers is found for a high-power picosecond laser, which shows 12 times higher ablation rate for the ablation in air compared to ablation in liquid. Further, we find that the threshold fluence for ablation in air is up to 1.9 times higher than for ablation in water. The highest ablation efficiency, which is defined as an energy specific ablation volume by the ablation rate divided by the laser power, is found for the low power, compact nanosecond laser system.

Transverse distribution of electron beam produced by relativistic picosecond laser in underdense plasma

Energetic electron beam can be generated through the directlaser acceleration (DLA) mechanism when high power picosecond laser propagates in underdense plasma, and the electron yield can reach several hundred nC, which has a great application in driving secondary radiations, such as bremsstrahlung radiation and betatron radiation. When a linearly polarized laser is used, the beam divergence is always larger in the laser polarization direction. What is more, the forked spectral-spatial distribution is observed in the experiments driven by femtosecond laser where DLA is combined with the laser wakefield acceleration (LWFA). The forked distribution is regarded as an important feature of DLA. However, an analytical explanation for both the bigger divergence and the forked spectral-spatial distribution is still lacking. Two-dimensional (2D) particle-in-cell simulations of picosecond laser propagating in underdense plasma are conducted in this paper to show how the fork is formed in DLA. The fork structure is a reflection of the distribution of electron transverse velocity. We find that when electrons are accelerated longitudinally, the transverse oscillation energy in the laser polarization direction increases correspondingly. If the electron energy is high enough, the transverse oscillation energy will increase linearly with the electron energy. As a result, the most energetic electrons will have an equal amplitude of <i>v<sub>y</sub></i>, where <i>v<sub>y</sub></i> denotes the velocity in the laser polarization direction. For a single electron, the distribution of its transverse velocity over a long period <inline-formula><tex-math id="M1">\begin{document}$\dfrac{{{\rm d}P}}{{{\rm d}{v_y}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="19-20191106_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="19-20191106_M1.png"/></alternatives></inline-formula>, will peak at ±<i>v<sub>m</sub></i> (<i>v<sub>m</sub></i> denotes the amplitude of <i>v<sub>y</sub></i>). If all the electrons have the same <i>v<sub>m</sub></i>, the distribution of <i>v<sub>y</sub></i> at a given time will be the same as <inline-formula><tex-math id="M2">\begin{document}$\dfrac{{{\rm d}P}}{{{\rm d}{v_y}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="19-20191106_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="19-20191106_M2.png"/></alternatives></inline-formula>. That means they will split transversely, leading to a forked spectral-spatial distribution. By using a simplified model, the analytical expression of <i>v<sub>m</sub></i> is derived, showing good agreement with <i>v<sub>m</sub></i> in the PIC simulation. However, the oscillation energy in the direction perpendicular to polarization will decrease when electrons are accelerated longitudinally (acceleration damping). As a consequence, the divergence perpendicular to the polarization direction will be smaller. Our research gives a quantitative explanation for the transverse distribution of electrons generated by DLA. With some modification, it can also be used in DLA combined LWFA to better control the dephasing length.

Stretching of picosecond laser pulses with uniform reflecting volume Bragg gratings

This study shows that a uniform reflecting volume Bragg grating (VBG) can be used as a compact monolithic stretcher of high-power picosecond laser pulses, which is important for cases in which chirped Bragg gratings with the required chirp rate are difficult to fabricate. When an incident short pulse propagates along a grating and experiences local Bragg diffraction, a chirp-free reflected stretched pulse with an almost rectangular shape is generated. The increase in the duration of the reflected pulse is approximately equal to twice the propagation time along the grating. We derive an analytic expression for the diffraction efficiency, which incorporates the incident pulse duration, grating thickness, and amplitude of the refractive index modulation, enabling selection of the optimum grating for pulse stretching. Theoretical models of the extended pulse profiles are found to be in good agreement with experimental autocorrelation measurements. We also propose a simple and reliable method to control the temporal parameters of high-power picosecond pulses using the same laser source and a VBG of variable thickness, which can simplify experiments requiring different pulse durations significantly.