mJ Pulse Research Articles

Frequency-doubled Nd:YAG MOPA laser system with programmable rectangular pulses up to 200 microseconds

A compact frequency-doubled diode-pumped Nd:YAG master-oscillator power-amplifier laser system with programmable microsecond pulse length has been developed. Analog pulse shaping of the output from a single-frequency continuous-wave Nd:YAG oscillator, and subsequent amplification, allowed the generation of rectangular pulses with pulse lengths on the order of the Nd:YAG fluorescence lifetime. Temporally flat-top pulses of 1064 nm light with 520 mJ pulse energy, 2.6 kW peak power, and 200 μs duration, with linewidth below 10 kHz, were obtained at a repetition rate of 2 Hz. Second harmonic generation in a LBO crystal yielded pulses of 262 mJ and 1.3 kW peak power at 532 nm. The peak power can be maintained within 2.9% over the duration of the laser pulse, and long-term intensity stability of 1.1% was observed. The spatially flat-top beam at 1064 nm used in the amplifier is converted to a Gaussian beam at 532 nm with beam quality factor M2 = 1.41(14) during the second harmonic generation. This system has potential as a pump source for Ti:sapphire, dye, or optical parametric amplifiers to generate tunable high-power single-frequency radiation for applications in precision measurements and laser slowing.

Multipass cell for high-power few-cycle compression.

A multipass cell for nonlinear compression to few-cycle pulse duration is introduced composing dielectrically enhanced silver mirrors on silicon substrates. Spectral broadening with 388 W output average power and 776 µJ pulse energy is obtained at 82% cell transmission. A high output beam quality (${{\rm{M}}^2} \lt {1.2}$) and a high spatio-spectral homogeneity (97.5%), as well as the compressibility of the output pulses to 6.9 fs duration, are demonstrated. A finite element analysis reveals scalability of this cell to 2 kW average output power.

Effect of Er:YAG Laser and Association of Protocols on the Demineralized Enamel Microhardness.

Objective: The aim of this study was to analyze the microhardness of demineralized enamel following different treatments (fluoride varnish, Er:YAG laser, and Er:YAG laser associated with fluoride varnish). Methods: Forty-eight enamel blocks (4 × 4 × 7 mm) were divided into six groups (n = 8): (S) Sound; (DE) Demineralized; (DED) DE + Duraphat® 5% (fluoride varnish); (DEL20) DE + Er:YAG laser (20 mJ pulse mode; 0.20 W; 10 Hz; 60 sec; 1.18 J/cm2; 11.83 W/cm2); (DEL50) DE + Er:YAG laser (50 mJ pulse mode; 0.50 W; 10 Hz; 60 sec; 2.95 J/cm2; 29.58 W/cm2); (DEL20D) DE + Er:YAG laser (20 mJ) + Duraphat 5%. The irradiation was performed at 1 mm distance from the surface using a tip (AS7066X, L-14 mm, D-1.3 mm in diameter) in water/air spray refrigeration (level 6). The enamel blocks were submitted to pH cycling (4 h into DES solution +20 h into RE solution for 8 days and the solutions were changed every day). Knoop microhardness was measured (50 g/15 sec, six readings per sample) and data were analyzed by Kruskal-Wallis test at 5% significance. Results: After treatments, DF group showed higher microhardness values than all the groups. Also, DEL20D group showed similar results with H group according to the microhardness analysis (p < 0.05). Conclusions: It could be concluded that Duraphat 5% treatment showed better results when compared with all tested groups, however, the association of Er:YAG Laser 20 with Duraphat 5% also showed promising results.

Measurement of amplified binary-modulated chirped laser pulses generated by different acousto-optic pulse shaping algorithms

The paper belongs to the field of ultrafast optics. Acousto-optic pulse shaping has been studied experimentally for binary intensity modulation in a chirped pulse amplification laser system. Direct time-domain measurement of modulation rise/fall time with a picosecond streak camera was performed for a Ti:sapphire regenerative amplifier. It was discovered that the sign of the second order dispersion produced by the acousto-optic dispersion delay line affects the modulation rise/fall time. Two algorithms for synthesis of ultrasonic waveforms for feeding the delay line, dispersive Fourier synthesis and the Gerchberg-Saxton algorithm, were compared. Minimum modulation rise/fall time of 3.6 ps for 3 mJ pulses with the linear chirp of 6.2 ps/nm at the wavelength of 795 nm was obtained.

LED-pumped femtosecond Cr:LiSAF regenerative amplifier system.

We report on the first, to the best of our knowledge, LED-pumped femtosecond regenerative amplifier. It is based on a Cr:LiSAF crystal pumped by 2240 blue LEDs via a Ce:YAG luminescent concentrator. The amplifier was seeded by pulses from a Ti:sapphire oscillator at 835 nm temporally stretched from 90 fs to 100 ps. At the output of the regenerative amplifier, we obtain 1 mJ pulse energy at a 10 Hz repetition rate, given by the frequency of the LED-pumping module. After compression, we obtain 100 fs pulses with a spectral bandwidth of 10 nm at 835 nm.

Acousto-optic Q-switched Er:CaF2-SrF2 laser at 2.73 μm

A diode-end-pumped Er:CaF2-SrF2 laser actively Q-switched by a TeO2 acousto-optic crystal was demonstrated for the first time, to our best knowledge. By setting the repetition rate of radio frequency signal, 2.73 μm Q-switched lasers with repetition rates of 100–1000 Hz were obtained. With 100 Hz repetition rate, a 390 μJ pulse energy and a 176 ns pulse duration were employed at an absorbed pump power of 2.17 W, and the corresponding peak power was 2.2 kW. These results prove that Er3+-doped CaF2–SrF2 crystal is promising for actively Q-switched laser with high peak power in the mid-infrared region.

Effect of Geometric Parameters on Equivalent Load and Efficiency in Rectangular Bore Railgun

The electromagnetic launch system model provides a useful reference for the launch process and performance. Inductance gradient and resistance gradient of the rail, which reflect the equivalent load characteristics of the launcher, are related to shape, material of the rail, and other factors, making it difficult to characterize in the circuit model. Therefore, a method combining equivalent network with finite element analysis is proposed to solve this problem. A segmentation-based real-time trigger strategy is applied to the 4 MJ pulse power supply (PPS) system consisted of 28 modules to shoot the 0.15 kg armature on a 6 m long rail. The width, height of the rail, or separation between two rails increase individually from 20 to 180 mm. After computing the resistance and inductance gradients of the specific rail in ANSYS, the rapid system-level simulation is carried out in MATLAB-Simulink to deliver the railgun's performance. The results show that both the resistance gradient and the inductance gradient contribute to the launch efficiency. In order to obtain a larger inductance gradient, the width of the two rails should be narrower, the height should be shorter, and the separation between the two rails should be farther. Nevertheless, in the perspective of decreasing resistance gradient, the rail should be wider, taller, and farther. Although the resistance gradient increases with the decrease of height and width, the promotion in the inductance gradient has much more significant impacts on the electromagnetic force under the same geometric change. Therefore, for the sake of a higher launching efficiency, the rails should be narrower, shorter, and farther.

Femtosecond Pulse Compression With Pedestal Suppression in a Sagnac Interferometer Constructed of Anti-Resonant Hollow Core Fiber

In this paper, compression of high-power femtosecond pulses in Sagnac interferometers constructed of anti-resonant hollow core fibers (ARHCF) is investigated numerically and experimentally. By varying the types and pressures of gas filled in the hollow core fiber, the group velocity dispersion and the nonlinear pulse phase accumulation difference between clockwise and counterclockwise propagations could be tuned conveniently. In experiment, we demonstrate the pedestal suppression temporal pulse compression of 800 nm, 160 fs, 10 μJ pulses from a Ti: Sapphire laser at 1 kHz to 20 fs with maximum compression efficiency of 25% nearly and compression ratio of eight.

High-energy, narrow-linewidth, flat-top temporal profile nanosecond-pulses from ytterbium-doped tapered fiber amplifier with chirped-diode seeding

We present a narrow-bandwidth (167 pm) few-nanosecond fiber laser based on tapered fiber amplification and phase modulation to mitigate stimulated Brillouin scattering (SBS). We compare the use of chirped diode seeding and standard sinusoidal phase modulation. Below SBS appearance threshold, output temporal profiles can be accurately tailored. Typical results for flat-top profiles include 170 kW peak-power (for 3 ns duration) and 1.25 mJ pulse energy (for 10 ns duration).

Reduction Mechanism of Transition Metal Oxide Particles in Thermally Induced Nanobubbles during Pulsed Laser Melting in Ethanol.

Pulsed laser melting in liquid (PLML) is a technique to fabricate spherical submicrometer particles (SMPs) wherein nanosecond pulsed laser (several tens to several hundreds of mJ pulse-1 cm-2 ) irradiates raw particles dispersed in liquid. Raw particles are transiently heated above the melting point to form spherical particles, which enables pulsed heating of surrounding liquid to form thermally induced bubbles by liquid vaporization. These transient bubbles play an important role as a thermal barrier to rapidly heat the particle. Reduced SMPs are generated from raw metal-oxide nanoparticles by PLML process in ethanol. This reduction cannot be explained by high-temperature thermal decomposition, but by mediation of molecules decomposed from ethanol. Computational simulations of ethanol decomposition by pulsed heating for 100 ns at the temperature 1000-4000 K revealed that ethylene is generated as the main product. Gibbs free energies of oxide reduction reactions mediated by ethylene greatly decreased compared to those without ethylene mediation. This explanation can be applied to reductive SMP formation from various transition metal oxides by PLML.

1 kW, 10 mJ, 120 fs coherently combined fiber CPA laser system.

An ultrafast fiber chirped-pulse amplification laser system based on a coherent combination of 16 ytterbium-doped rod-type amplifiers is presented. It generates 10 mJ pulse energy at 1 kW average power and 120 fs pulse duration. A partially helium-protected, two-staged chirped-pulse amplification grating compressor is implemented to maintain the close to diffraction-limited beam quality by avoiding nonlinear absorption in air.

Evaluation of Er:YAG and Er,Cr:YSGG laser irradiation for the debonding of prefabricated zirconia crowns.

Reduced tooth structure in the pediatric and adolescent population is frequently restored with prefabricated zirconia crowns. On permanent teeth, these restorations may need to be removed and replaced with permanent restorations. To explore and compare the use of 2 high-powered erbium lasers for removing prefabricated zirconia crowns from molar teeth as a non-invasive alternative to rotary instruments. Twenty-five permanent molars were prepared to dentin and prefabricated all-ceramic zirconia crowns were fitted and cemented with resin modified glass ionomer (RMGI) cement. The teeth were randomly assigned into one of the 2 retrieval treatment groups: the erbium-doped yttrium, aluminum and garnet (Er:YAG) laser group (G1; n = 12) or the erbium, chromium-doped yttrium, scandium, gallium and garnet laser (Er,Cr:YSGG) laser group (G2; n = 13). The laser operating parameters for the Er:YAG laser were 300 mJ, 15 Hz, 4.5 W, and 50-microsecond pulse duration (SSP mode); for the Er,Cr:YSGG laser, they were 4.5 W, 15 Hz, 20 water/20 air, and 5 W, 15 Hz, 50 water/50 air, and 60-microsecond pulse duration (H mode). The experiment was repeated twice. The surface area and the volume of teeth and crowns were measured and the cement space was calculated. The retrieval time and temperature changes were tested and recorded. The data were analyzed with the t-test. The surfaces of the dentin and the crown from each group were further examined using scanning electron microscopy (SEM). The average time for crown removal using the Er:YAG laser was 1 min 32.7 s; for the Er,Cr:YSGG laser it was 3 min 13.9 s (p < 0.0001). The mean temperature changes were 1.41 ±1.36°C for the Er:YAG laser and 2.2 ±0.99°C for the Er,Cr:YSGG laser (p = 0.0321). The SEM examination showed no damage or major structural changes caused by treatment with either erbium-family laser. Both lasers are effective, non-invasive tools to remove prefabricated zirconia crowns cemented with resin cement and should be considered as viable alternatives to rotary instrumentation.

Analytical characterization of laser induced plasmas towards uranium isotopic analysis in gaseous uranium hexafluoride

To perform direct enrichment assay on gaseous uranium hexafluoride (UF6) with laser induced breakdown spectroscopy (LIBS), the dominant spectral-line features, evolution of the signal and background of the U II 424.437 nm line, and its Stark width and shift, were studied as a function of UF6 gas pressure and pulse energy of a nanosecond Nd:YAG laser. Vapor pressure of UF6 was found to be the most important parameter for LIBS analysis of gaseous UF6. Spectral congestion with numerous U lines of high excitation potential was observed and signal-to-background ratio (SBR) was low for measurements with 80 Torr UF6. Only when both UF6 vapor pressure and laser pulse energy were low, for example, less than 20 Torr pressure and 30 mJ pulse energy, the resultant LIBS spectra from gaseous UF6 resembled those obtained from solid U samples. The experimental data also suggest that U and F atoms recombine back to UF6 after the laser pulse. The U emission was found to decay fast with a persistent background signal, degrading SBR with delay time. Systematic positive biases were found for UF6 enrichment assays performed with the 235U–238U line pair at 424.412–424.437 nm, which was confirmed to be caused by self-absorption. Even with optimization of experimental parameters and incorporation of a self-absorption term into the spectral-fitting algorithm, to reduce and compensate for self-absorption, self-absorption is still a main factor limiting accurate UF6 enrichment assay. The use of another spectral window which contains no resonance lines is a prospective solution for the self-absorption issue.

Determination of Gum Arabic (&amp;lt;i&amp;gt;Acacia nilotica&amp;lt;/i&amp;gt;) Constituents Using Laser Induced Breakdown Spectroscopy

In this work, Laser Induced Breakdown Spectroscopy (LIBS) was used to determine the constituent of Gum Arabic (Acacia nilotica) collected from five different locations in Sudan. Gum samples were irradiated with 80 mJ pulse energy of Nd-YAG laser (1064 nm) and Atomic spectra Database was used for the spectral analysis of the plasma emitted from these samples. It was found that the samples contain the elements C, O, H, S, N, P, Na, Mg, Ca, Fe, Cr, Mn, Co with different amounts. Some elements like (Ti, Br, Ar, Th, Kr, Sc and Pr) are recorded here for the first time.

Initial results of Rayleigh scattering lidar observations at Zhongshan station, Antarctica

A Rayleigh scattering lidar for measuring the atmospheric density and temperature has been deployed at Zhongshan Station (69.4° S, 76.4° E), Antarctica. Lidar transmitter was a frequency doubled Nd:YAG laser with ~400 mJ pulse energy and 30 Hz repetition rate. A telescope with 0.8 m diameter pointing to the zenith direction served as the lidar receiver. This lidar was capable of profiling the density and temperature in the Upper Stratosphere and Lower Mesosphere (USLM) region. At the vertical resolution of 300 m and the temporal resolution of 30 min, the lidar measurement uncertainties, mainly due to the photon noise, were calculated to be within 1.5% and 1 K for density and temperature, respectively. Since March 2020, this lidar has been routinely operated at Zhongshan station for exploring the atmospheric density and temperature variations and wave propagation characteristics in the polar USLM region.

Compact wavelength tunable output around 440 nm pulsed laser for oceanic lidar application

A single-resonant BBO-based optical parametric oscillator (OPO) with 27 nm tuning range in deep-blue spectral band around 440 nm was developed. Pumped by a 355 nm ultraviolet laser with 4.9 mJ pulse energy at 100 Hz repetition rate, maximum pulse energy of 1.32 mJ at 441.4 nm wavelength, corresponding to more than 0.26 MW peak power, was successfully obtained from a plano-concave OPO cavity with an about 35% output coupler, the optical–optical​ conversion efficiency was up to 26.9%. The pulse width was around 5.0 ns, and the spectral linewidth was about 0.09 nm. When the output wavelength was tuned to the Fraunhofer lines of 440.5 nm and 438.4 nm, the pulse energy of 1.27 mJ and 1.15 mJ was obtained respectively with linewidth of both about 0.07 nm, well-matched to the corresponding Fraunhofer linewidth.

Disinfection of Root Canals with Laser-Activated Irrigation, Photoactivated Disinfection, and Combined Laser Techniques: An Ex Vivo Preliminary Study.

Objective: This study aimed to evaluate the efficacy of laser-activated irrigation using photon-induced photoacoustic streaming (PIPS®) and photoactivated disinfection (PAD) techniques and their combination to improve penetration and activation of toluidine blue in the endodontic space of teeth experimentally infected with Enterococcus faecalis. Materials and methods: Twenty-seven extracted single-root teeth were instrumented, sterilized, and infected with E. faecalis and divided into seven groups of three teeth each: Group A [sodium hypochlorite (NaClO) 5% hand irrigation], Group B [NaClO 5% hand irrigation+ethylenediaminetetraacetic acid (EDTA)+NaClO 5% activated by PIPS], Group C (EDTA+NaClO 5% activated by PIPS), Group D (toluidine blue activated by PAD), Group E (toluidine blue activated by PIPS and PAD), Group F (NaClO 5% hand irrigation+toluidine blue activated by PAD), and Group G (NaClO 5% hand irrigation+toluidine blue activated by PIPS and PAD). Finally, positive and negative group controls were prepared. The presence of biofilms after the treatments was assessed by the BioTimer assay. PIPS was performed with an Er:YAG laser (2940 nm, LightWalker, Fotona® d.o.o., Slovenia) at 20 mJ, 15 Hz, 0.3 W, and 50-μs pulse duration. PAD was performed with a 635 nm diode laser (Smart M, Lasotronix®, Poland) at 400 mW in continuous wave (CW). Results: When NaClO was used, significant decontamination (p ≤ 0.05) was obtained in all experimental groups with respect to the positive control, other than Group G. Irrigation with EDTA+NaClO activated by PIPS produced a higher level of decontamination than Group A (p ≤ 0.05). Significant results in reducing biofilm load compared with the control and Group A were observed when NaClO was coupled with toluidine blue activated by PAD (p ≤ 0.05). Conclusions: Disinfection of root canals can be obtained using a combination of different irrigants, photosensitizers, and activation protocols. EDTA+NaClO using the PIPS protocol and toluidine blue activated by PAD (both preceded by NaClO irrigation) can be considered effective tools. The possibility of replacing NaClO with toluidine blue, whatever the method of activation, should be further investigated.

High-energy diode-pumped alexandrite amplifier development with applications in satellite-based lidar

Efficient, wavelength-tunable diode-pumped alexandrite laser systems offer the potential for a more versatile, satellite-based lidar source compared to fixed wavelength Nd:YAG systems and non-space compliant lamp-pumped alexandrite. In this paper, we develop a strategy to enable the high-energy operation required for atmospheric lidar based on an efficient diode-pumped master oscillator power amplifier (MOPA) system design. A novel multipass “diamond” slab amplifier geometry is introduced alongside the experimental results of the world’s first diode-pumped alexandrite amplifier producing a gain of 2.13 in a demonstration system. A diode-pumped Q -switched alexandrite oscillator is presented with a record-highest pulse energy of 3.80 mJ. Detailed optimization of a two-stage amplifier design is studied numerically and maximized with temperature, wavelength, and pump pulse duration to produce 50 mJ pulse energy. This forms part of an optimized alexandrite MOPA design capable of high pulse energy, showing the future potential of diode-pumped alexandrite for satellite-based atmospheric lidar.

Hydrodynamic phenomena in optical discharges in liquids under self-focusing of periodic-pulse laser radiation

Quasi-stationary flows under the effect of focused periodic pulse femtosecond laser radiation were generated and observed in liquid solvents: water, heavy water, alcohols, ketones, chloromethanes. The mechanism inducing directional flows appears to be directional collapse of the gas bubbles produced by multiphoton dissociation in a focused laser beam. Laser pulses of 450 fs length, up to 220 μJ pulse energy at repetition rates up to 10 kHz have induced stationary flows of liquid originated from the laser beam waist directed along or transversely to the beam axis. The streams along the beam axis were observed under low pulse power (10-20 μJ), provided precise lens adjustment. Lens displacement transversely to the beam axis led to splitting beam waist in two astigmatic foci. Both foci generate the streams along the beam axis. Counter directed streams have collided in the gap between foci, forming the flow spreading transversely to the laser beam. The increase of the pulse energy was followed by formation of the filament of self-focusing. Repeating cycles of focusing and defocusing along the filament produced several beam energy dissipation zones, each one generating separate streams along the beam axis. Colliding of the counter directed streams gave rise to complex flow pattern transversely and upward with respect to the beam axis.

Passively Q-switched 0.24 mJ ring laser based on anisotropic tapered fiber

We report on a passively high-power Q-switched ring laser based on the active polarization-maintaining double-clad tapered Yb3+-doped fiber (APM-T-DCF) as gain media. It generates 0.24 mJ pulse trains which consist of Q-switched pulses each with the energy of about 48 µJ and up to 385 W of peak power. Q-switched regime was achieved by tuning the optical feedback polarization state and band pass filtering. For the best of our knowledge, for the first time, we demonstrate Q-switched fiber ring laser based on APM-T-DCF.