Average Laser Power Research Articles

Safe limits for the application of nonlinear optical microscopies to cultural heritage: A new method for in-situ assessment

Nonlinear optical microscopies (NLOM) are emerging techniques for the monitoring and preservation of cultural heritage, providing morphological, structural and compositional information on a large variety of artistic materials and objects. Being non-invasiveness the main requirement in artwork diagnostics, it becomes increasingly necessary to verify the absence of physicochemical alterations possibly induced by the use of high intensity radiation sources, and thus to find the exposure limits for safe applications. In this work, we propose a new methodology to set the criteria of damage evaluation for near infrared femtosecond pulsed lasers used for examining painted samples by means multi-photon excited fluorescence (MPEF), modality of NLOM. The method is based on the detection of changes in the MPEF signal upon repetitive irradiation on a series of modern acrylic painted samples, complemented by Raman spectroscopy measurements performed before and after irradiation. The obtained results allowed determining a safe average laser power range for the non-invasive determination of thickness in paint layers.

High-throughput micromachining with ultrashort pulsed lasers and multiple spots

A one step functionalization of large surfaces and layers could be enabled by high power ultrashort pulsed lasers. Despite today's availability of high power ultrashort pulsed lasers (up to several hundred watts), it is still a challenge to structure large surface areas, which is required on embossing rollers, within an acceptable processing time for industrial roll-to-roll production. Furthermore, fluencies near the ablation threshold are necessary to provide the highest depth resolution with minor side effects. So, it is a challenge to convert the increasing high average laser power into a high processing speed by maintaining the well-known quality of ultrashort pulsed lasers constant. One approach is the combination of a high-speed application and several parallel ablating laser spots. In this contribution, a newly developed high compact picosecond laser with high pulse repetition rates and an average power of 500 W was distributed into 8 and 16 parallel beamlets by a diffractive optical element. The power was controlled by single acousto-optical modulators per beamlet. Different functional surface geometries have been realized on an embossing roller as a master, which is used for the replication of the structures in a roll-to-roll or a roll-to-plate process. Feature sizes from 2 μm to 10 μm in areas of 2 m2 could be processed. With this approach, functional structures such as reduction in friction, improved soft touch, or light guiding elements can be generated on large surfaces within short processing times.

Laser heating of cantilevered tips: Implications for photoinduced force microscopy

Photoinduced force microscopy (PiFM) measures changes in the force between an atomically sharp tip and the sample under the influence of applied radiation. Several mechanisms contribute to the overall force, including forces introduced due to thermal heating of the sample and the tip. In this paper, we study the effect of laser heating of the retracted tip under illumination conditions relevant to PiFM and how it affects the mechanical resonance properties of the cantilever. Using a gold-coated silicon cantilever with its tip irradiated by a tightly focused 532-nm laser beam, we find that the tip temperature increases linearly at 7.5 K/mW of average laser power, irrespective of whether the laser is pulsed or continuous wave. The temperature rise gives rise to a decrease of the cantilever's spring constant and an increase of the damping coefficient. We demonstrate that for retracted tips, these thermally induced changes to the mechanical resonance only moderately impact the measured photoinduced force, as the measured scattering force is at least an order of magnitude stronger than the effective force introduced by laser heating under experimentally relevant conditions.

Investigation the effect of pulsed laser parameters on the temperature distribution and joint interface properties in dissimilar laser joining of austenitic stainless steel 304 and Acrylonitrile Butadiene Styrene

Direct laser joining of metal to plastic materials is one of the cost effective methods of joining. The demand for laser welding of stainless steels and thermoplastics is going on increase because of having many applications such as automotive, aerospace and aviation industries. This paper presents the experimental investigation of direct laser joining of stainless steel 304 and Acrylonitrile Butadiene Styrene (ABS). The effects of pulsed laser parameters including laser welding speed, focal length, frequency and power on the themperature field and tensile shear load was investigated. The results showed that excessive increase of the joint interface temperature mainly induced by high laser power density results in exiting of the more volume of the molten ABS from the stainless steel melt pool. Also, increasing the laser power density through decreasing the focal length or increasing the laser power led to an increase in the surface temperature, higher beam penetration and high volume of molten ABS. Decreasing the focal length from 5 to 2 mm significantly rose the temperature from 150 to 300 °C. By increasing the laser pulse frequency, the number of bobbles at the ABS interface surface remarkably increased where the temperature increased from 120 to 180 °C. The X-ray spectroscopy results showed the existence of the polymer elements on the metal surface at the joint interface zone. The tensile shear load clearly increased from 280 to 460 N with augmentation of laser average power from 180 W to 215 W. Applying higher levels of laser power has clearly decreased the tensile shear load due to creating bigger bobbles and more cavities at the adhesive zone.

An investigation of recast behavior in laser ablation of 4H-silicon carbide wafer

4H-silicon carbide (SiC) is a suitable semiconductor material for high-speed power devices but also a typical difficult-to-process and hard-brittle material. Laser ablation is an efficient approach to process SiC. However, recast layer is an inevitable thermal damage during laser ablation. The present study experimentally and numerically investigated the recast behavior in laser ablation of 4H–SiC. The recast humps are with height of 1.4–3.3 μm and width of 12–22 μm, consist of SiC, Si, and SiO2, where 15.08 wt% of oxygen is detected. Microcracks and pore clusters are observed on the recast humps under SEM. Recoil pressure and surface tension in the molten pool dominate the process of recast hump formation. The recoil pressure of Si vapor is 2.27 MPa at 5000 K, as high as 22 times of atmospheric pressure. The simulation results show that the fluid in the middle flows around while the surrounding fluid flows upwards. The upward flow is almost laminar while the flow at the bottom of the molten pool is turbulent entertaining air and forming bubbles. The vaporization-induced keyhole in the molten pool becomes deeper and the sidewall becomes more inclined over time, accompanying with the “growing” of the recast hump. There are mainly three reasons causing the increase of recast height and recast width with the increased average laser power: (a) increase of recoil pressure; (b) increase of temperature gradient in the molten pool, enhancing the Marangoni convection; and (c) enlarging the molten pool and leading to more materials melting. With the decrease of the laser scanning speed, more liquid molten material is evaporated in the molten pool instead of being ejected outward to form recast humps. The increase of the laser pulse frequency decreases the peak laser intensity and hence results in the decrease of the recoil pressure. Besides, it also leads to the weakening of the heating ability of laser and lessening of the liquid molten material that could be ejected.

SRS converter – compressor of femtosecond ytterbium laser pulses

Stimulated Raman scattering (SRS) conversion of femtosecond pulses of an ytterbium laser at a wavelength of 1.03 μm is performed in compressed hydrogen under conditions of nonlinear phase modulation. Temporal compression of frequency-chirped Stokes pulses is carried out, as a result of which 400-fs laser pulses are converted into 27-fs Stokes pulses with a wavelength of 1.8 μm. An average Stokes power of 0.6 W is implemented at an average laser power of 6.3 W and a pulse repetition rate of 20 kHz.

Laser-slicing at a low-emittance storage ring.

Laser-slicing at a diffraction-limited storage ring light source in the soft X-ray region is investigated with theoretical and numerical modelling. It turns out that the slicing efficiency is favoured by the ultra-low beam emittance, and that slicing can be implemented without interference to the standard multi-bunch operation. Spatial and spectral separation of the sub-picosecond radiation pulse from a hundreds of picosecond-long background is achieved by virtue of 1:1 imaging of the radiation source. The spectral separation is enhanced when the radiator is a transverse gradient undulator. The proposed configuration applied to the Elettra 2.0 six-bend achromatic lattice envisages total slicing efficiency as high as 10-7, one order of magnitude larger than the demonstrated state-of-the-art, at the expense of pulse durations as long as 0.4 ps FWHM and average laser power as high as ∼40 W.

Electrical resistivity of pure copper processed by medium-powered laser powder bed fusion additive manufacturing for use in electromagnetic applications

Pure copper is an excellent thermal and electrical conductor, however, attempts to process it with additive manufacturing (AM) technologies have seen various levels of success. While electron beam melting (EBM) has successfully processed pure copper to high densities, laser powder bed fusion (LPBF) has had difficulties achieving the same results without the use of very high power lasers. This requirement has hampered the exploration of using LPBF with pure copper as most machines are equipped with lasers that have low to medium laser power densities. In this work, experiments were conducted to process pure copper with a 200 W LPBF machine with a small laser spot diameter resulting in an above average laser power density in order to maximise density and achieve low electrical resistivity. The effects of initial build orientation and post heat treatment were also investigated to explore their influence on electrical resistivity. It was found that despite issues with high porosity, heat treated specimens had a lower electrical resistivity than other common AM materials such as the aluminium alloy AlSi10Mg. By conducting these tests, it was found that despite having approximately double the resistivity of commercially pure copper, the resistivity was sufficiently low enough to demonstrate the potential to use AM to process copper suitable for electrical applications.

Mid-infrared Er:CaF2–SrF2 bulk laser Q-switched by MXene Ti3C2Tx absorber

We report a diode-pumped passively Q-switched Er:CaF2–SrF2 laser at 2.73 μm with MXene Ti3C2Tx as a saturable absorber (SA). A maximum average output power of the pulsed laser was measured at 286 mW, with a shortest duration of 814 ns and a repetition rate of 45.3 kHz. The corresponding single-pulse energy and peak power were calculated to be 6.32 μJ and 7.76 W, respectively. The results suggest the promising potential of MXene as an efficient SA for short pulse lasers in the mid-infrared spectral band.

Combined pulsed laser drilling of metal by continuous wave laser and nanosecond pulse train

In this paper, a new form of combined pulsed laser (CPL) consisting of continuous wave (CW) laser and nanosecond (ns) pulse train was proposed for drilling Q235B steel without high-pressure gas jet. The average power of CW laser ranged from 100 to 700 W, while the average power of the ns laser was fixed at 12 W. The experiments were carried out to study the influence of the average CW laser power on the drilling efficiency and quality of CPL. The results show that there were two completely different matching mechanisms between the CW laser and ns pulse train. Firstly, during the CPL drilling with the 100-200 W CW laser, the ns pulse train could be used to timely and periodically eject the central part of the molten metal, leaving a recast layer with a thickness of up to 200 μm. In addition, for the CPL drilling with the 550 W CW laser, a little part of molten metal was continuously ejected by the ns pulse train, and all of the remaining molten metal could be removed by the boiling of molten metal, thus leaving a recast layer with thickness less than 10 μm. The new CPL presented in this paper has an important guiding significance for improving the quality and making full use of the high efficiency of CW laser drilling.

Passively Q-switched Nd:GdYTaO4 laser based on two-dimensional MoS2 saturable absorber

We presented a LD-pumped passively Q-switched Nd:GdYTaO4 laser based on the two-dimensional (2D) MoS2 for the first time. The continuous-wave (CW) Nd:GdYTaO4 laser without the MoS2 was firstly investigated. The highest output power reached 5.24 W with the slope efficiency of 41%. Next, by using the 2D MoS2 nanosheet as the saturable absorber, the passively Q-switched Nd:GdYTaO4 laser was researched. The maximum average output power of the pulse laser was 112 mW at 1066 nm. The corresponding pulse width, repetition rate, pulse energy, and pulse peak power were 2.2 μs, 250 kHz, 0.39 μJ, and 0.18 W, respectively.

Optimized surface treatment of aerospace composites using a picosecond laser

In this work, a design of experiments (DoE) approach was used to determine optimum laser surface treatment conditions. Aerospace structural carbon fiber reinforced composites (Torayca® T800H/3900-2) were laser treated using a pulsed laser source with a wavelength of 355 nm, nominal pulse width of 10 ps, average power up to 7 W, and spot size of approximately 20 μm. The laser power, frequency, scan speed, and number of passes (1 or 2) were varied. The adherends were characterized using a handheld water contact angle device, scanning electron microscopy (SEM), optical profilometry, and laser induced breakdown spectroscopy (LIBS). Bonded specimens were mechanically tested using a modified double cantilever beam (DCB) method. The response models obtained from the DoE indicated that surface characteristics of adherends and failure modes of bonds were most sensitive to the average laser power. Below about 200 mW, adhesive failure was observed. For laser powers greater than about 600 mW, surface resin was largely removed from the fiber bed, and the failure mode was predominantly fiber tear. Cohesive failure (∼100%) was observed for a wide range of laser parameters indicating that laser treatment can be a robust means of surface preparation.

Thermoelectrically cooled, repetitively pulsed Fe : ZnSe laser

We report the investigation of a repetitively pulsed laser based on a Fe : ZnSe crystal, cooled to a temperature of 195 K by two thermoelectric modules with a maximum cooling power of 34 W. Two pulsed synchronised Er : YAG lasers, operating in the free-lasing regime, are used for pumping. It is shown that the average laser power in a series of short pulses may exceed 20 W. An average power of 3.1 W is attained in the stationary temperature regime. The lasing spectrum is centred at a wavelength of 4.38 μm, and the total divergence angle is 5 mrad.

Synchronized Optical and Acoustic Droplet Vaporization for Effective Sonoporation.

Inertial cavitation-based sonoporation has been utilized to enhance treatment delivery efficacy. In our previous study, we demonstrated that tumor therapeutic efficacy can be enhanced through vaporization-assisted sonoporation with gold nanodroplets (AuNDs). Specifically, the AuNDs were vaporized both acoustically (i.e., acoustic droplet vaporization, ADV) and optically (i.e., optical droplet vaporization, ODV). A continuous wave (CW) laser was used for ODV in combination with an ultrasound pulse for ADV. Although effective for vaporization, the use of a CW laser is not energy efficient and may create unwanted heating and concomitant tissue damage. In this study, we propose the use of a pulsed wave (PW) laser to replace the CW laser. In addition, the PW laser was applied at the rarefaction phase of the ultrasound pulse so that the synergistic effects of ADV and ODV can be expected. Therefore, a significantly lower laser average power can be expected to achieve the vaporization threshold. Compared to the CW laser power at 2 W/cm2 from the previous approach, the PW laser power was reduced to only 0.2404 W/cm2. Furthermore, we also demonstrate in vitro that the sonoporation rate was increased when the PW laser was applied at the rarefaction phase. Specifically, the vaporization signal, the inertial cavitation signal, and the sonoporation rate all displayed a 1-µs period, which corresponded to the period of the 1-MHz acoustic wave used for ADV, as a function of the relative laser delay. The increased sonoporation rate indicates that this technique has the potential to enhance sonoporation-directed drug delivery and tumor therapy with a lower laser power while keeping the cell death rate at the minimum. Photoacoustic imaging can also be performed at the same time since a PW laser is used for the ODV.

Rationale for creating detonation CO2 laser for radioactive surface decontamination

The laser decontamination method is based on the evaporation of oxide films under the influence of radiation. With the evaporation mechanism, laser radiation should heat the upper layer of the film to the boiling point during the pulse and evaporate it. It is relevant because of the growing environmental requirements in the world, which makes it possible to create a compact, energy-efficient laser installation. Unlike existing energy-efficient laser units, the detonation laser system will significantly affect and quickly decontaminate radioactive surfaces due to the evaporation of oxide films under the influence of radiation. Detonation technologies are critical and can be used for pulse detonation systems, such as pulse detonation engines, detonation lasers, magnetohydrodynamic generators with detonation combustion of fuel, volume explosion initiation systems. The introduction of these systems in armaments and military equipment can substantially change the scope of their application. The average laser power can exceed 100 kW and above. At the same time, the use of the mixture as a power source makes the system not only compact, but also light in weight with respect to the existing similar systems. The wavelength will be 10.6 μm due to radiation in the far infrared region. That is, combined power plants will provide not only actuation, but also electric power supply of machines. This will allow the creation of power detonation units with a periodic initiation frequency of at least 100 Hz, which will work on a liquefied mixture and insignificant use of oxygen in the incendiary portion.

Effect of average beam power on microstructure and mechanical properties of Nd: YAG laser welding of 304L and st37 steel

PurposeThe present research work aims to study the effect of average beam power (laser process parameters) on the overlapping factor, depth of penetration (DOP), weld bead width, fusion zone and heat affected zone (HAZ) in laser welding of 304L and st37 steel. Back side and top surface morphology of the welded joints have also been studied for varying average beam power.Design/methodology/approachLaser welding of austenitic stainless steel (304L) and carbon steel (st37) was carried out using Nd:YAG laser integrated with ABB IRB 1410 robot in pulse mode. The selection of laser process parameters was based on the specification of available laser welding machine. Dissimilar laser welding of 304L and st37 carbon steel for full depth of penetration have been performed, with varying average beam power (225-510W) and constant welding speed (5mm/s) and pulse width (5ms).FindingsRecrystallized coarse grains were observed adjacent to the fusion zone and nucleated grains were seen away from the fusion zone towards carbon steel. Overlapping factor and HAZ width st37 side increases with increase in average beam power whereas top weld bead width increases first, attains maximum value and then subsequently decreases. Bottom weld bead width increases with increase in average beam power. The mechanical properties namely microhardness and tensile strength of the welded joints have been investigated with varying average beam power.Originality/valueIn the recent development of the automobile, power generation and petrochemical industries the application of dissimilar laser welding of austenitic stainless steel (304L) and carbon steel (st37) are gaining importance. Very limited work have been reported in pulsed Nd:YAG dissimilar laser welding of austenitic stainless steel (304L) and carbon steel (st37) for investigating the effect of laser process parameters on weld bead geometry, microstructural characterization and mechanical properties of the welded joint.

Simulation of sub-nm carrier profiling by scanning frequency comb microscopy

A mode-locked laser focused on the tunneling junction of a scanning tunneling microscope (STM) superimposes a microwave frequency comb with hundreds of harmonics on the DC tunneling current. Each harmonic, at an integer multiple of the laser pulse repetition frequency, sets the present state-of-the-art for narrow linewidth at its frequency to enable low-noise measurements at an average laser power of several milliwatts. Measurements of the attenuation of the harmonics, which is caused by the spreading resistance, may be used to determine the resistivity of the sample. In Scanning Frequency Comb Microscopy (SFCM) feedback control of the tip-sample distance is based on the power at the harmonics. No DC bias voltage or DC tunneling current is required and the data rate is much higher than that with an STM. Simulations of the spatial distribution of the power dissipated in the sample show the feasibility of non-destructive true sub-nm resolution in the carrier profiling of semiconductors. With no DC bias voltage and no DC tunneling current band-bending and other changes to semiconductor samples in an STM are mitigated and there is a possibility for in vivo microscopy in biology and medicine.

Investigating the ablation depth and surface roughness of laser-induced nano-ablation of CVD diamond material

The CVD diamond material with high wear resistant has been frequently used as the material of micro mills. The laser-based pre-treatment of CVD diamond has been identified as a favorable technique for subsequent precision machining in terms of efficiency and surface quality. This study is aimed to investigate the effect of CVD diamond material on the ablation depth and surface roughness (Sa and Sz) in laser-based machining, so as to improve the overall machined surface quality. Controlling ablation depth and high surface finish are the key indicator for the follow-up accurate treatment. In addition, the energy density distribution (EDD) model was established to better explain the microstructural changes in CVD diamond material after performing the laser machining. Through EDD model, the variation in ablation depth and surface roughness were evaluated at different laser parametric levels. The results showed that the ablation depth and surface roughness of EDD model are basically consistent with the experimental values. Moreover, at the same total energy density, the ablation depth and surface roughness Sa and Sz values are lower under the condition of low average laser power, large filling pitch and small laser scanning speed. In a nutshell, minimum surface roughness and a controlled ablation depth are achieved on CVD diamond by adjusting laser parameters.

Low-dispersion low-loss dielectric gratings for efficient ultrafast laser pulse compression at high average powers

All-dielectric gratings are ideal for high average power laser pulse compressors due to their low loss and high diffraction efficiencies. However, they have not been utilized in petawatt-class lasers with pulse durations below 100 fs due to a variety of challenges. We present a resonance-free design for low-dispersion (1480 lines/mm) diffraction gratings made of dielectric materials resistant to femtosecond laser damage (SiO2/HfO2). A 14 cm diameter sample was fabricated resulting in a mean diffraction efficiency of 99.1% at λ = 810 nm with high spatial uniformity using equipment which can produce gratings up to 1 m diagonal. The implementation of these gratings in the compression of 30 fs pulses in an out-of-plane geometry can result in compressor efficiencies approaching 95%. The measured laser absorption is 500× lower than current ultrafast petawatt-class compressor gratings enabling a substantial increase in average power handling capabilities of these laser systems.

Disk laser heads based on Yb : YAG for multikilowatt average power lasers

The main factors affecting the cooling efficiency of Yb : YAG disk active elements have been investigated, and key technologies for designing disk lasers with high average power and efficiency have been developed. The main parameters of a disk active element operating in the cw regime are optimised using numerical calculation. Disk laser heads are fabricated based on calculations and developed technologies, and lasing with optical efficiency up to 50 % is demonstrated. An average power at the kilowatt level is obtained in a cavity with two laser heads, and a possibility of its further increasing is demonstrated.