mJ Pulse Research Articles

High-energy and high-average-power two-channel Yb:YAG amplifier with passive coherent combining

A two-channel laser amplifier with passive coherent combining based on Yb:YAG active elements with bright diode end pumping was developed. The pulse energy limits of a one-channel scheme are investigated, and the ability to double pulse energy in the two-channel scheme is demonstrated. 10 mJ pulse energy at a repetition rate of 1.5 kHz and 5 mJ at 6 kHz simultaneously with high gain and perfect beam quality at coherent combining efficiency over 97% was achieved.

Failure Analysis of the 100 T Pulsed Magnet at the WHMFC

A triple-coil pulsed magnet aiming at 100 T (Tesla) was designed and tested at the Wuhan National High Magnetic Field Center. The magnet was powered by a 100 MJ pulse generator, a battery bank, and 25 kV capacitor banks in combination. The magnet failed at 83 T, while the whole structure remained intact. The failure process was analyzed by using the recorded waveforms of the voltage and the current, as well as disassembling and dismantling the failed magnet. It shows that the inner coil broke down first, and then the electrical breakdown occurred between the middle and outer coils. The possible reasons were also discussed. The buckling analysis results show that the inner and middle coils would buckle at 71 T and 83 T, respectively, which were consistent with the experimental results. The cross-ply reinforcement method is proposed to enhance the structural stability of high field magnets.

Delamination Detection in Concrete Plates using a Laser Ablation Impact Echo Technique

This study investigates the detection of delaminations in concrete plates using non-contact laser ablation instead of the conventional hammer excitation. Tests were performed on two specimens of which one had an artificial delamination. 5 mm to 15 mm diameter steel tip hammers were used as a reference source and compared to a 7 ns pulsed 1064 nm Nd:YAG laser with 150 mJ pulse energy. Signals were recorded by surface- mounted accelerometers and in air by microphones and a laser Doppler vibrometer. Frequencies up to 150 kHz are excited with a broad frequency spectrum and little energy compared to the narrower frequency spectra of the hammers. The characteristic thickness resonance frequency of the intact plates can be accurately detected by the hammer excitation. The laser can excite low frequency flexural vibration modes over a shallow delamination at 3 cm depth, but does not excite the thickness resonance frequency. The low frequency flexural vibrations are verified by numerical natural frequency analysis.

Tunable Pulsed Dysprosium Laser Within a Continuous Range of 545 nm Around 3 μm

We report a widely tunable pulsed dysprosium (Dy) -doped fluoride fiber laser within a 545 nm continuous wavelength range around 3 μm, where a home-made random Raman fiber laser (RRFL) at 1.690 μm acts as the pump, and a commercial InAs saturable absorber (SA) is adopted as the Q-switcher. The designed RRFL, excited by a 1.055 μm ytterbium-doped fiber oscillator, can provide the 8 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> order Stokes emission at 1.690 μm with a high spectral purity of 98.6% and a maximum output power of 14.4 W. Feeding it to a Dy cavity involving an InAs SA and a ruled diffraction grating in a Littrow configuration, stable Q-switching can be initiated and tuned within the continuous/total range of 2.775∼3.320 μm (545 nm)/2.765∼3.338 μm (573 nm), an extension of 50% over the prior 370 nm record [Opt. Lett., vol. 44, no. 9, pp. 2322-2325, 2019], representing the widest level to date from a pulsed rare-earth-doped laser at any wavelengths, to the best of our knowledge. When the wavelength is tuned to 2.992 μm, a maximum average power of 106 mW with 460 ns pulse width, 125 kHz repetition rate, and 0.85 μJ pulse energy is achieved. This work marks the first pulsed Dy-doped fluoride fiber laser pumped at ∼1.7 μm while exhibiting the utility of InAs in pulse generation at the wavelengths of 3∼3.4 μm for the first time, to the best of our knowledge. The demonstration highlights the great potential of ∼1.7 μm pumped Dy-doped fluoride fiber laser as a platform for developing widely tunable pulsed sources in the 3 μm region of the mid-infrared.

Optical and Electrical Considerations for Developing Pulsed High-Power LED for Volumetric Particle Tracking Velocimetry

Volumetric Particle Tracking Velocimetry (PTV) in volumes larger than 103 cm3 with 15 μm Air Filled Soap Bubbles (AFSB) requires a powerful pulsed laser source of few tenth of mJ pulse energy, depending on the optical arrangement of the experiment. Initial experiments in our lab with 15 μm AFSB and 1 mJ pulse energy from a 532 nm laser source showed that in order to image turbulence flow phenomena in volumes about 4x10^3 cm^3 at 1 m working distance and an f-number of 8, one would need a laser source with about 315 mJ pulse energy and 1260 W average power at 4 kHz repetition rate. Such high-power 532 nm lasers are not simple to build and are expensive and complicated to develop. By reducing the working distance and increasing the diameter of the camera aperture, the required laser power was reduced by a factor 18 to 17.5 mJ pulse energy and 70 W average power, which is still a very high power laser. As a possible alternative, we report here on the development of a collimated beam, high-power, pulsed 460 nm LED for volumetric PTV measurements of turbulence flow in a large volume seeded with AFSB p articles. We report of the optical and electrical consideration that are required in development of such an LED source. First, we combined a practical model for the output power of an LED with a model that estimates the signal level on a CMOS detector in a volumetric PTV experiment with AFSB particles. It predicted that our LED should be driven by 277 A in order to generate 1 mJ of optical pulse energy with 20 μs pulse duration. In order to avoid power drooping at high currents density in the LED, we chose an architecture of LEDs array where each LED unit of the array operates at 20 μs pulse duration, 4 kHz repetition rate and with peak current of 200 A. Each such LED will deliver 0.72 mJ pulse energy and 24 such LED units will supply the required optical density. Second, we report of the optical and electrical features of the collimated , high-power, pulsed 460 nm LED unit we built. We tested the LED in a PTV experiment where the LED operated with 0.72 mJ pulse energy, 1 kHz repetition rate and 20 A peak current. The measured signal current from a CMOS-pixel due to Mie scattering AFSB particles was in good agreement with the predicated signal level.

Picosecond Bessel Beam Fabricated Pure, Gold-Coated Silver Nanostructures for Trace-Level Sensing of Multiple Explosives and Hazardous Molecules.

A zeroth-order, non-diffracting Bessel beam, generated by picosecond laser pulses (1064 nm, 10 Hz, 30 ps) through an axicon, was utilized to perform pulse energy-dependent (12 mJ, 16 mJ, 20 mJ, 24 mJ) laser ablation of silver (Ag) substrates in air. The fabrication resulted in finger-like Ag nanostructures (NSs) in the sub-200 nm domain and obtained structures were characterized using the FESEM and AFM techniques. Subsequently, we employed those Ag NSs in surface-enhanced Raman spectroscopy (SERS) studies achieving promising sensing results towards trace-level detection of six different hazardous materials (explosive molecules of picric acid (PA) and ammonium nitrate (AN), a pesticide thiram (TH) and the dye molecules of Methylene Blue (MB), Malachite Green (MG), and Nile Blue (NB)) along with a biomolecule (hen egg white lysozyme (HEWL)). The remarkably superior plasmonic behaviour exhibited by the AgNS corresponding to 16 mJ pulse ablation energy was further explored. To accomplish a real-time application-oriented understanding, time-dependent studies were performed utilizing the AgNS prepared with 16 mJ and TH molecule by collecting the SERS data periodically for up to 120 days. The coated AgNSs were prepared with optimized gold (Au) deposition, accomplishing a much lower trace detection in the case of thiram (~50 pM compared to ~50 nM achieved prior to the coating) as well as superior EF up to ~108 (~106 before Au coating). Additionally, these substrates have demonstrated superior stability compared to those obtained before Au coating.

Energy scaling of carrier-envelope-phase-stable sub-two-cycle pulses at 1.76 µm from hollow-core-fiber compression to 1.9 mJ.

We present the energy scaling of a sub-two-cycle (10.4 fs) carrier-envelope-phase-stable light source centered at 1.76 µm to 1.9 mJ pulse energy. The light source is based on an optimized spectral-broadening scheme in a hollow-core fiber and a consecutive pulse compression with bulk material. This is, to our knowledge, the highest pulse energy reported to date from this type of sources. We demonstrate the application of this improved source to the generation of bright water-window soft-X-ray high harmonics. Combined with the short pulse duration, this source paves the way to the attosecond time-resolved water-window spectroscopy of complex molecules in aqueous solutions.

Continuous-wave and pulsed operation of Ho:YLF laser end-pumped by Tm:fiber laser

Continuous-wave and pulsed operation of a Ho:YLF laser was demonstrated. A Tm:fiber laser with 70 W output power was used for the pump source. In this experimental setup, the end-pumped gain medium with 0.5% and 1% Ho concentration in the optical resonators were used. In the continuous-wave regime, 14.2 and 12.8 W linearly polarized power with a concave-concave resonator for 1.0% and 0.5% Ho dopes were extracted, respectively. These amounts correspond to slope efficiencies of 51.9% and 71.3% and optical-to-optical efficiencies of 39.4% and 54.7% for absorbed pump power, respectively. To avoid optical damage, a U-shaped concave-concave resonator was installed, and the resulting spot size in the resonator was about 1.31 mm. The spot size changes about 10% along the total physical length of the resonator. In the pulsed operation regime, for 0.5% Ho concentration with 20 ms pump pulse duration and 10 Hz repetition rates, the free-running output energy was 172 mJ with 16 ms pulse duration. In addition, the absorbed pump energy was about 485 mJ, corresponding to the slope and the optical-to-optical efficiencies of 14.8% and 12.3%, respectively. Also, 27 mJ pulse energy with 29.7 ns duration was achieved for Q-switched operation. Finally, the angle of output beam divergence was measured at approximately 1.1 mrad, and the laser emission wavelength was 2.0568 µm with 0.0065 µm line width.

100 kHz Narrow-Linewidth Burst-Mode MOPA Laser System With Uniform Envelope

In this study, a 100-kHz narrow-linewidth burst-mode laser system combined with a master oscillator power amplifier (MOPA) structure was reported with a stable pulse burst profile over 2 ms duration. A theoretical model was established to characterize and compensate the variation characteristics of coefficient of variation (COV) within a burst envelope for the double-pass three-stage amplification system, in terms of different parameter factors. Simulated results yielded similar tendency with the measured ones. For a stable burst envelope with a COV of 2.72% and 2.93%, output pulse energy at a 100 kHz repetition rate was scaled up to 1.08 and 4.38 mJ in the case of an input average pulse energy of 8.6 and 116.51 nJ, respectively. Corresponding Mx2 and My2 values with 2.2 and 2.4 were endowed with 1.08 mJ output single-pulse energy and 2.7 and 2.1 for 4.38 mJ pulse energy. The spectral linewidth of amplified pulses was measured to be less than 83.1 and 67.9 MHz, with respect to output pulse duration of 10.3 and 96.1 ns, respectively.

Enhancement of gain and efficiency of an Ho:YLF energy booster through deep thermoelectric cooling

It is well known that the gain and efficiency of an amplifier with a quasi-three level medium such as Ho:YLF is strongly affected by the coolant temperature. By cooling the Ho:YLF crystals of a two-stage energy booster in a Ho:YLF Chirped Pulse Amplification laser with -20°C thermoelectric cooling rather than room temperature water, a 60 mJ pulse energy at 1 kHz repetition rate was achieved with 8.5 mJ input.

Studies on passively Q-switched ytterbium doped all-fiber laser using fiber optic ring resonator

We report our studies on ytterbium doped fiber laser passively Q-switched by fiber optic ring resonator in all-fiber format. The Q-switched laser characteristics viz. pulse duration, pulse profile, repetition rate and average power are easily varied not only by pump power but also by changing the polarization state of light inside the laser resonator with the help of polarization controllers. At 330 mW pump power, the laser generates 1.07 μs duration pulses with 0.68 μJ pulse energy at 83.6 kHz repetition rate. The Q-switched laser wavelength is tunable from 1059 to 1069.7 nm by changing the polarization state inside the cavity.

Linearly polarized 2044-nm pulsed laser generation in Ho:YAG active medium

We demonstrated the efficient generation of a linearly polarized Ho:YAG laser pumped by a thulium fiber laser. Independent of the pumping power and without utilizing any dispersive optical element, the output radiation remained stable with a high degree of linear polarization. The output with 2044 nm wavelength and 0.12 nm bandwidth was measured. In the free-running generation, with respect to the incident pump power, we recorded more than 9 W of output power with 59% slope efficiency. In addition, in the pulsed generation, we obtained more than 2.2 mJ pulses with 50 ns time duration, and 44 kW peak power.

Nonlinear pulse compression of a thin-disk amplifier and contrast enhancement via nonlinear ellipse rotation.

We demonstrate pulse compressibility from 840 fs to 38 fs of 10 mJ pulses from a thin-disk amplifier at a repetition rate of 3 kHz after nonlinear broadening in a multipass cell. In addition, the temporal-intensity contrast is enhanced via nonlinear ellipse rotation of more than a factor 50 with an optical efficiency of 56%. We believe this is the first published experimental combination of multipass cell-based nonlinear compression and nonlinear ellipse rotation-based contrast enhancement preserving both pulse compressibility and beam quality.

Carrier-envelope phase stable few-cycle laser system delivering more than 100 W, 1 mJ, sub-2-cycle pulses

Two-stage multipass-cell compression of a fiber-chirped-pulse amplifier system to the few-cycle regime is presented. The output delivers a sub-2-cycle (5.8 fs), 107 W average power, 1.07 mJ pulses at 100 kHz centered at 1030 nm with excellent spatial beam quality (M2 = 1.1, Strehl ratio S = 0.98), pointing stability (2.3 µrad), and superior long-term average power stability of 0.1% STD over more than 8 hours. This is combined with a carrier-envelope phase stability of 360 mrad in the frequency range from 10 Hz to 50 kHz, i.e., measured on a single-shot basis. This unique system will serve as an HR1 laser for the Extreme Light Infrastructure Attosecond Light Pulse Source research facility to enable high repetition rate isolated attosecond pulse generation.

Quantitative analysis of chlorine in cement pastes based on collinear dual-pulse laser-induced breakdown spectroscopy

On-line detection of chlorine in cement is the key to evaluate the corrosion of the reinforced concrete inside the structure. A collinear dual-pulse LIBS system based on two nanosecond lasers with a total energy of 30 mJ is developed to detect the chlorine in cement. The key parameters of the dual-pulse LIBS system are optimized to improve the detection sensitivity of trace chlorine element in cement pastes, and the optimal values of the parameters are obtained to be: 4 L/min helium gas flow rate, 2000 ns inter-pulse delay, 800 ns gate delay, 19 mJ/11 mJ pulse energy ratio, and 42.8 mm lens-to-sample distance. After the optimization of the dual-pulse system, the signal-to-noise ratio of the trace chlorine emission line at 837.6 nm has been improved from 1.75 to 2.68 for a sample containing 0.706 wt% chlorine. The temperatures of plasma are obtained based on Saha-Boltzmann plot for exploring the plasma radiation features in the dual-pulse system. The results show that the plasma temperature which is influenced by laser irradiance and parameters of the double pulse configuration, is closely related to the signal-to-noise ratio of chlorine spectrum line. Sixteen standard cement pastes made from a series of sodium chloride solutions with various concentration are used for LIBS calibration. Two calibration methods including internal standardization and partial least squares regression are adopted for determining chlorine concentrations within a series of standard cement pastes, and the limit of detection based on internal standardization model is calculated to be 103.4 ppm. The prediction performance of IS and PLSR is evaluated by Leave-One-Out Cross-Validation with the root mean square error of calibration of 0.0910 and 0.0859, respectively.

High-efficiency KYW acousto-optic Q-switch for a Ho:YAG laser.

A new, to the best of our knowledge, type of acousto-optic Q-switch was developed using slow shear acoustic mode in potassium yttrium tungstate (KYW) crystal. Two Q-switch configurations were created: one for vertical and one for horizontal light polarization, both providing over 50% diffraction efficiency at a wavelength of 2.1 μm and an RF driving power below 8 W. The laser-induced damage threshold of the KYW crystal was found to equal 650 MW/cm2. Operation of a nanosecond periodically pulsed Ho:YAG laser emitting 15 mJ pulses at 2.1 μm with the KYW Q-switch is reported.

Sub-50 fs pulses at 2050 nm from a picosecond Ho:YLF laser using a two-stage Kagome-fiber-based compressor

The high-energy few-cycle mid-infrared laser pulse beyond 2 μm is of immense importance for attosecond science and strong-field physics. However, the limited gain bandwidth of laser crystals such as Ho:YLF and Ho:YAG allows the generation of picosecond (ps) long pulses and, hence, makes it challenging to generate few-cycle pulse at 2 μm without utilizing an optical parametric chirped-pulse amplifier (OPCPA). Moreover, the exclusive use of the near-infrared wavelength has limited the generation of wavelengths beyond 4 μm (OPCPA). Furthermore, high harmonic generation (HHG) conversion efficiency reduces dramatically when driven by a long-wavelength laser. Novel schemes such as multi-color HHG have been proposed to enhance the harmonic flux. Therefore, it is highly desirable to generate few-cycle to femtosecond pulses from a 2 μm laser for driving these experiments. Here, we utilize two-stage nonlinear spectral broadening and pulse compression based on the Kagome-type hollow-core photonic crystal fiber (HC-PCF) to compress few-ps pulses to sub-50 fs from a Ho:YLF amplifier at 2 μm at 1 kHz repetition rate. We demonstrate both experimentally and numerically the compression of 3.3 ps at 140 μJ pulses to 48 fs at 11 μJ with focal intensity reaching 10 13 W / cm 2 . Thereby, this system can be used for driving HHG in solids at 2 μm. In the first stage, the pulses are spectrally broadened in Kagome fiber and compressed in a silicon-based prism compressor to 285 fs at a pulse energy of 90 μJ. In the second stage, the 285 fs pulse is self-compressed in air-filled HC-PCF. With fine-tuning of the group delay dispersion (GDD) externally in a 3 mm window, a compressed pulse of 48 fs is achieved. This leads to a 70-fold compression of the ps pulses at 2050 nm. We further used the sub-50 fs laser pulses to generate white light by focusing the pulse into a thin medium of YAG.

Sum-frequency generation of 133 mJ, 270 ps laser pulses at 266 nm in LBO crystals.

We demonstrate the generation of high-energy (133 mJ) and sub-nanosecond (∼270 ps) deep ultraviolet (DUV) pulses at 266 nm by sum-frequency mixing in LiB3O5 (LBO) crystals. The highest 133 mJ pulse energy ever reported corresponds to a peak power of 0.49 GW and an energy conversion efficiency of 13.3% from the infrared at 1064 nm to DUV at 266 nm. This is the highest output energy ever reported for the DUV sub-nanosecond pulses to the best of our knowledge. Higher energy efficiency of 25.7% can be achieved from 1064 nm to 266 nm when the fundamental energy was reduced to 346 mJ. Furthermore, the DUV generations using LBO and typical β-BaB2O4 (BBO) crystals were compared regarding the energy efficiency, and the effects of the nonlinear absorption are discussed.

Cavity-dumped mode-locked Alexandrite laser oscillator with 100 mJ pulses stabilized by using a double trigger system.

We report a mode-locked Alexandrite single pulse laser with cavity dumping. Mode locking was achieved by using an AOM and an EOM was used for Q-switching and cavity dumping. The instability of the single pulse laser energy output was reduced down to a tenth of that of the conventional single trigger system by introducing a novel double trigger system. The single pulse laser energy and pulse width were 100 mJ and 475 ps in multiple mode and 12.5 mJ and 275 ps in single mode, obtained without a laser amplifier.

Spatio-spectral couplings in saturated collinear OPCPA.

Ultrafast laser pulses featuring both high spatio-temporal beam quality and excellent energy stability are crucial for many applications. Here, we present a seed laser with high beam quality and energy stability, based on a collinear optical parametric chirped pulse amplification (OPCPA) stage, delivering 46 µJ pulses with a 25 fs Fourier limit at 1 kHz repetition rate. While saturation of the OPCPA stage is necessary for achieving the highest possible energy stability, it also leads to a degradation of the beam quality. Using simulations, we show that spectrally dependent, rotationally symmetric aberrations dominate the collinear OPCPA in saturation. We experimentally characterize these aberrations and then remove distinct spatial frequencies to greatly improve the spectral homogeneity of the beam quality, while keeping an excellent energy stability of 0.2 % rms measured over 70 hours.