kHz Repetition Rate Research Articles

Pulse Burst Generation and Diffraction with Spatial Light Modulators for Dynamic Ultrafast Laser Materials Processing.

A pulse burst optical system has been developed, able to alter an energetic, ultrafast 10 ps, 5 kHz output pulse train to 323 MHz intra-burst frequency at the fundamental 5 kHz repetition rate. An optical delay line consisting of a beam-splitting polariser cube, mirrors, and waveplates transforms a high-energy pulse into a pulse burst, circulating around the delay line. Interestingly, the reflected first pulse and subsequent pulses from the delay line have orthogonal linear polarisations. This fact allows independent modulation of these pulses using two-phase-only Spatial Light Modulators (SLM) when their directors are also aligned orthogonally. With hybrid Computer Generated Holograms (CGH) addressed to the SLMs, we demonstrate simultaneous multi-spot periodic surface micro-structuring on stainless steel with orthogonal linear polarisations and cylindrical vector (CV) beams with Radial and Azimuthal polarisations. Burst processing produces a major change in resulting surface texture due to plasma absorption on the nanosecond time scale; hence the ablation rates on stainless steel with pulse bursts are always lower than 5 kHz processing. By synchronising the scan motion and CGH application, we show simultaneous independent multi-beam real-time processing with pulse bursts having orthogonal linear polarisations. This novel technique extends the flexibility of parallel beam surface micro-structuring with adaptive optics.

Localized nitrogen-vacancy centers generated by low-repetition rate fs-laser pulses

Among hundreds of impurities and defects in diamond, the nitrogen-vacancy (NV) center is one of the most interesting to be used as a platform for quantum technologies and nanosensing. Traditionally, synthetic diamond is irradiated with high-energy electrons or nitrogen ions to generate these color-centers. For precise positioning of the NV centers, fs-laser irradiation has been proposed as an alternative approach to produce spatially localized NV centers in diamond. However, most of the studies reported so far used high-repetition rate fs-laser systems. Here, we studied the influence of the irradiation conditions on the generation of NV$^-$. Specifically, we varied pulse fluence, laser focusing, and the number of pulses upon irradiation with 150 fs pulses at 775 nm from a Ti:sapphire laser amplifier operating at 1 kHz repetition rate. Optically Detected Magnetic Resonance (ODMR) was used to investigate the produced NV centers, revealing a sizeable zero-field splitting in the spectra and indicating the conditions in which the lattice strain produced in the ablation process may be deleterious for quantum information applications.

Dual-loop diode-tuned Fourier domain harmonically mode-locked opto-electronic oscillator with over 50 dB side-mode spur reduction.

We propose and demonstrate a dual-loop harmonic Fourier domain mode-locked optoelectronic oscillator (FDML-OEO) for drastically reducing the side-mode spurs. The frequency domain mode-locking is achieved by synchronizing the scanning period of the filter to an integer fraction of the round trip times of the two loops with a self-made low cost diode-tuned RF filter. We found, for the first time to the best of authors' knowledge, that the frequency scanning bandwidth (FSBW) of the mode-locked output is strongly affected by the length mismatch between the two fiber loops. By using the phase noise of FDML OEO's delayed self-heterodyne signal as a performance indicator, we found experimentally that both the locking bandwidth and the FSBW of the device are inversely proportional to the length mis-match of the two loops. Finally, with dual-loop fiber lengths of 2041m and 2449.2m, including 2039m common fiber loop, we successfully obtained linearly chirped microwave signals around 9GHz with a phase noise of -127dBc/Hz at 10kHz offset from the 9GHz carrier, a FSBW of 0.4GHz, and a chirp rate of 200THz/s at 500.38 kHz repetition rate. More impressively, the side-mode spur ratio of the linear frequency modulated (LFM) signal is reduced to less than -83dB, the lowest ever achieved for a FDML OEO to the best of authors' knowledge, which is more than 50dB improvement over that achieved with a single loop FDML OEO reported previously.

Quantitative fluorescence-based imaging of in-cylinder fuel film thickness at kHz frame rates in a production-type direct-injection spark-ignition engine

Incomplete evaporation of liquid fuel films inside the combustion chamber is a major source of soot and particulate emissions in internal combustion engines. In this study, quantitative imaging of liquid fuel films on the cylinder wall is performed by laser-induced fluorescence (LIF) in a single cylinder engine with optical access to the cylinder liner. A four-component surrogate fuel mixed with 0.5% anisole as the fluorescent tracer was directly injected in a single injection into the motored engine. Fuel impinging on the cylinder wall is excited at 266 nm at kHz repetition rates and imaged by a high-speed camera coupled with an image intensifier. Simultaneously, lubricating oil doped with 0.01 mmol/l pyrromethene 567 is excited at 532 nm and imaged with a second camera. The fuel film thickness on the cylinder wall is compared for two different injections timings, 285 and 240°CA before top-dead center. The initial fuel film thickness is similar for both, but evaporation occurs faster for the later injection, such that the film thickness is similar at the time the piston rings reach the fuel films. When the rings scrape the fuel off the wall, it accumulates on top of the first piston ring and is partly transported downwards to the second and third ring. In the simultaneous oil image, washout of lubricating oil is seen.

A high throughput rapid scanning dispersive spectrometer for longwave infrared absorption spectroscopy

A rapid-scanning spectrometer was constructed to capture several hundred wavenumber wide spectral scans at greater than 1 kHz repetition rate in the longwave infrared. The system, made from commercial-off-the-shelf components, exceeds scan width and repetition rate limitations present in other infrared diagnostics while maintaining better than 10 cm−1 spectral resolution through its high throughput (f/2) design. Instrument design and construction is discussed in detail, and validation of the spectrometer’s ability to capture time resolved spectra in the 800–1400 cm−1 region at 1.2 kHz was demonstrated through the measurement of organophosphorus chemical nerve agent simulant destruction inside high explosive fireballs.

Time-resolved 2D angular scattering of soot particles in atmospheric turbulent flames

A better understanding of the formation of soot particles is essential to improve combustion-related processes. For this purpose, this work reports for the first time the measurement of the size of soot particles under formation in a turbulent atmospheric flame in a planar configuration. This is ensured by the detection of the elastic scattering of a laser sheet thanks to two cameras positioned at different scattering angles (45° and 135°). The size information is then used to interpret the scattering signal collected by an additional camera positioned at 90° in order to evaluate the soot number concentration. This last step relies on a calibration ensured by soot volume fraction and primary particle size previously determined by autocompensating laser-induced incandescence in the same flame. Moreover, the use of a 1 kHz repetition rate nanosecond laser and three high speed CMOS cameras enables the access to these physical parameters with a high temporal resolution. The 2D and time-resolved soot characterization is of interest because it enables the observation of the temporal and spatial modifications of soot structures when propagating in such turbulent flames. In particular, in this flame, the transport and mixing of soot pockets are clearly observed and the impact of these processes on the size and number concentration is quantified.

Multi-gigahertz femtosecond pulses from linear and nonlinear propagation of a phase-modulated laser.

We propose and demonstrate a non-mode-locking approach to generating multi-gigahertz repetition rate, femtosecond pulses in burst mode by shaping a continuous-wave (CW) seed laser in an all-fiber configuration. The seed laser at 1030 nm is first phase modulated and de-chirped to low-contrast, ∼2 ps pulses at a 17.5 GHz repetition rate, then carved to bursts at a 60 kHz repetition rate, and finally shaped to <2 ps clean pulses by a Mamyshev regenerator. This prepared high-quality picosecond source is further used to seed an Yb-doped fiber amplifier operating in the highly nonlinear regime, delivering output pulses at 23 nJ/pulse and $20\,\mathrm{\mu}$J/burst, compressible to ∼100 fs level. The system eliminates the need for mode-locked cavities and simplifies conventional ultrafast electro-optic combs to using only one phase modulator, while providing femtosecond pulses at multiple gigahertz repetition rate, enhanced pulse energy in burst mode and the potential of further power/energy scaling.

Design and construction of passively pulse compressor using PM–Mach–Zehnder interferometers

Abstract In line fiber Mach–Zehnder inferometer (MZI) pulse compression was designed three different lengths of single mode-polarization maintaining fiber with (8, 16, 24) cm after splicing them between two single mode fibers (SMF-28e) with (23 and 13) cm and applying different weights on splicing region and the cross sectional area of SM-PM fiber, the designed performance of the in line fiber compressor system was studies in terms of compressor factor. Two minima pulse compression factor were obtained, one is 1.13 with FWHM 251.584 pm, centered wavelength 1547.394 nm, 52 cm interferometer length and 5 g was applied on the micro-cavity splicing region, and the second is equal 1.10 with FWHM 259.730 pm, centered wavelength 1547.120 pm and, 68 cm interferometer length and 10 g was applied on the cross sectional area of the second PMFs, in the case of single and cascaded interferometers, respectively. The input of the all interferometers was pulsed laser source with peak power 1.2297 mW, 286 pm spatial FWHM, 10 ns temporal FWHM, 3 kHz repetition rate and centered at 1546.7 nm.

Effects of Initial Phase on Silicon Nanoparticles Formation in Femtosecond Laser Ablation

We report on the ultrafast (femtosecond) laser ablation of monocrystalline Si (100), polycrystalline Si, and Si (100) capped with a SiO2 layer. The target material was ablated using femtosecond laser pulses (~50 fs duration, 1 kHz repetition rate, and 800 nm wavelength) with an input energy of ~100 μJ in acetone medium to fabricate Si Nanoparticles (NPs). The average size of NPs produced by Si (100) was found to be less than that of the particles produced by poly Si. Ablation of Si caped with SiO2 resulted in bigger Si NPs together with a low concentration of SiO2 NPs. NPs were found to be of polycrystalline in all three cases irrespective of the initial phase.

Low divergence proton beams from a laser-plasma accelerator at kHz repetition rate

Proton beams with up to 100 pC bunch charge, 0.48 MeV cut-off energy and divergence as low as a $3^{\circ}$ were generated from solid targets at kHz repetition rate by a few-mJ femtosecond laser under controlled plasma conditions. The beam spatial profile was measured using a small aperture scanning time-of-flight detector. Detailed parametric studies were performed by varying the surface plasma scale length from 8 to 80 nm and the laser pulse duration from 4 fs to 1.5 ps. Numerical simulations are in good agreement with observations and, together with an in-depth theoretical analysis of the acceleration mechanism, indicate that high repetition rate femtosecond laser technology could be used to produce few-MeV protons beams for applications.

Improving the accuracy of high-repetition-rate LIBS based on laser ablation and scanning parameters optimization.

Laser-induced breakdown spectroscopy system based on high-repetition-rate microchip laser (HR-LIBS) has been widely used in elemental analysis due to its high energy stability, good portability and fast spectral acquisition speed. However, repeated ablation on powder pellets like soil and coal using HR-LIBS system encounters the problem of serious decline in measurement accuracy. In this work, the relationship between laser ablation and scanning parameters, their correlation with spectral intensity, as well as the optimization approach were fundamentally studied. The correlations among the crater overlapping rate, crater depth and spectral intensity were obtained. An HR-LIBS system with microchip laser (4 kHz repetition rate, 100 µJ laser pulse energy) to perform repeated scanning ablation was established. A theoretical model of the ablation crater morphology for repeated scanning ablation was developed. By taking soil pellets as the experimental samples, the linear fitting curves of crater depth and the spectral intensity ratio were established with the R2 of 0.90∼0.99. The experimental results showed that as the crater depth developed during repeated ablation, the Si-normalized spectral intensity decreased, and thus the spectral repeatability decreased. It was found that by optimizing the overlapping rate to form a flat crater bottom, the confinement effect of the crater on the plasma could be avoided. As a result, the spectral repeatability was significantly improved. The relative standard deviation (RSD) of Si-normalized spectral intensity was improved from 5% to 0.6%. Finally, repeated ablation was performed with the optimized overlapping rate on soil pellets. The R2 of calibration curves of Fe, Mg, Ca, and Al were all above 0.993, and the average RSDs were between 0.5% and 1%. This study provides a fast, accurate, and stable method for the analysis of the samples consisting of various materials with high heterogeneity.

Dual-output kilohertz pump laser for high-energy picosecond OPCPA.

A dual-output thin-disk picosecond laser operating at 100 W with 1 kHz repetition rate is reported in this Letter. By electronically adjusting the amplitude of the optical seed pulses that are injected into the laser cavity, the energy extracted from the gain medium can be shared between two pulses. Amplified double pulses are subsequently spatially separated into two independent beams by a fast Pockels cell, compressed in one common compressor, and frequency-doubled with ∼70% efficiency. This approach significantly decreases strain on the optics, as well as nonlinear effects, and is advantageous for power scaling.

Electron temperature and density measurement by Thomson scattering with a high repetition rate laser of 20 kHz on LHD

Thomson scattering measurements with a high-repetition-rate laser have commenced in the Large Helical Device. As an example of the fast phenomena captured by this diagnostic system, measurements at a 20 kHz repetition-rate in hydrogen pellet-injected plasmas are presented. Signal processing methods for this measurement have been developed and electron temperature profiles with almost 70 spatial points were evaluated at time intervals of 50 upmus. After Raman scattering calibration, electron density profiles were derived. Fast changes in the electron temperature and density profiles within 1 ms were observed.

Controlling LIPSS formation on Ni surface using near-infrared laser beam and its low-order harmonics

Ultrafast laser ablation of metal surfaces is a common physical process used to generate periodic surface structures with tuneable multifunctionality for various applications in science and technology. In this work, the formation of femtosecond laser-induced periodic surface structures (LIPSSs) is investigated with 50 kHz repetition rate laser of 40 fs pulse duration and a central wavelength () along with its second () and third ) harmonic components. The variation in the LIPSS periodicity is demonstrated by changing the wavelength of the ablating laser pulses. LIPSS with periodicity ∼0.85 m at the fundamental ablating pulses, ∼0.42 m periodicity at the second harmonic, and ∼0.35 m periodicity at the third harmonic were observed. The third harmonic radiation was generated in an air-filament with minimal temporal delay with respect to the fundamental driving pulses to structure Nickel surfaces at ambient conditions. The approach demonstrates the potential for generating very fine LIPSS on materials’ surfaces using the efficient high-order harmonics of an infrared fundamental laser beam.

Hybrid Integrated Frequency-Modulated Continuous-Wave Laser With Synchronous Tuning

We report a wavelength widely-tunable hybrid integrated external cavity laser (ECL) with synchronous tuning to achieve a large frequency chirp bandwidth and high chirp linearity. Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> Vernier microring resonators (MRRs) in the ECL function as an optical bandpass filter to offer strong wavelength selectivity and fast frequency tunability. The wavelength tuning range is 56 nm, the maximum output optical power is 22.5 mW, the minimum intrinsic linewidth is 2.9 kHz, and the static frequency sweep bandwidth is 56 GHz. A frequency-modulated continuous-wave (FMCW) signal is generated under synchronous modulation with a chirp bandwidth of 7.68 GHz at a 1 kHz repetition rate, much larger than that produced by modulating the phase shifter or the Vernier filter alone. A chirp linearization method based on a non-uniform sampling algorithm and an iterative learning algorithm is employed to reduce the chirp nonlinearity to 1.02×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> , allowing for a detection resolution of 2.47 cm.

Ti:Sa Crystal Geometry Variation vs. Final Amplifiers of CPA Laser Systems Parameters

In this paper, the different Ti:Sapphire crystal configurations of the final amplifiers, depending on the Chirped Pulse Amplification laser system parameters, such as the repetition rates and pulse energy, are discussed. Restrictions placed on the final Ti:Sa amplifiers with a high repetition rate are discussed. The repetition rate of these systems is limited due to the crystal overheating, which leads to stress fracturing or significant beam distortion. The heating density threshold leading to stress fracturing was calculated and taken as the limit of the upper level of the possible pump average power. On the basis of these calculations, the highest repetition rates and corresponding thermolens focal distances were estimated for conventional crystal geometry of the most suitable thicknesses. It was demonstrated that conventional crystal shapes, such as a thin disc, can be used for systems with repetition rates below a few hundred Hz if several Joules of the output pulse energy are required. The rectangular thin crystal plate geometry was more suitable for Ti:Sa amplifiers with repetition rates above 1 kHz. Finally, the parameters of rectangular thin crystal plate Ti:Sa laser amplifier with an output energy above 3 J per pulse for a laser system with more than 100 TW pulse power and 1 kHz repetition rates are presented.

Self-cascaded noncritically-phase-matched KTP optical parametric oscillator for generating orthogonally polarized multi-wavelength emissions at 3 μm

Mid-infrared self-cascaded noncritically-phase-matched (NCPM) KTP optical parametric oscillator (OPO) was developed, generating multiple wavelengths at 2.95, 3.29 and 3.37 μm. The polarization state of 2.95 μm is orthogonal to that of the other two wavelengths. Under 1064 nm intracavity pumping, the first NCPM KTP-OPO can generate signal and idler waves at 1.57 and 3.29 μm, respectively. Then the 1.57 μm is further used as pump light for the second NCPM OPO in the same KTP, generating signal and idler waves at 2.95 and 3.37 μm. Broadband spectral feedback condition is established between the OPO cavity mirrors, enabling the two NCPM OPOs to operate simultaneously in the same KTP and OPO resonator. With regard to 2.95 μm, the achieved maximum average output power and minimum pulse width are respectively 1.18 W and 5 ns, corresponding to a pulse peak power of 56.9 kW with 4.2 kHz repetition rate. As for the dual-wavelength at 3.29 and 3.37 μm, the maximum total average output power is 0.22 W. Our work provides an effective scheme to obtain multiple wavelength laser output in 3 μm region with orthogonal polarization state.

Temperature-dependent electrical and structural characterization of laser-induced graphitic microwires in CVD diamond

An array of 2500 vertical graphitic microwires was fabricated within a single-crystal diamond plate with the purpose of creating distributed electrically conductive structures intended for the development of electronic devices operating at high temperatures. To this end, the structural, morphological, and electrical properties of the diamond/graphite system were investigated as a function of temperature up to 550 °C, by means of optical and secondary electron microscopy (SEM), micro-Raman spectroscopy and current-voltage measurements. The vertical microstructuring of the diamond bulk was obtained by a laser-induced phase transition from diamond to graphite by means of ultra-short laser pulses (100 fs duration, 800 nm wavelength, 1 kHz repetition rate). As inferred from SEM micrographs, the graphitic wires display a high-aspect-ratio with length of approximately 200 μm and diameter of about 10 μm. The electrical resistivity of the single microwire is estimated to be 0.49 ± 0.15 Ω cm at room temperature, then decreasing linearly with temperature with a coefficient of approximately −1 × 10−2 K−1. Raman spectroscopy results point out the absence of structural alterations after high-temperature operations.

Microstructure, guiding performance and micro-spectroscopy characterization of Ce:LuAG crystal induced by direct femtosecond laser writing

Dual-line waveguides in Ce:LuAG crystal are fabricated by femtosecond laser using 1030 nm, 450 fs pulses at a 100 kHz repetition rate. The microstructure and composition of femtosecond-laser induced waveguides are studied in detail using SEM, TEM and EDX elemental composition analyses. Efficient single-mode guidance is obtained in the type II waveguide structures with relatively low propagation loss along both TE and TM polarization at 632.8 nm. Based on confocal micro-Raman spectroscopy, the strain-affected regions and the lattice-damaged tracks are demonstrated. The contribution of each effect to the refractive-index distribution are quantitatively determined in combination with beam propagation calculations. The generation of yellow-green fluorescence emission is demonstrated under excitation at 405-nm, retaining well luminescent performance in the Ce:LuAG waveguide.

Digital single-photon-avalanche-diode arrays for time-of-flight Thomson scattering diagnostics.

The collection optics of Thomson scattering systems for plasma devices are designed with maximum possible étendue to keep the required laser energy low. If the spatial resolution along the laser beam is performed by a time-of-flight method, then the detectors, in addition to a large sensitive area, must offer a high frequency bandwidth. Up until now, only microchannel-plate photomultipliers meet these requirements. Here, we investigate the potential use of digital avalanche photodiode arrays operated in the Geiger mode as alternative detectors. In this mode of operation, each array will serve as a fast, sensitive detector. The use of these detectors will lead to significant improvements of the Thomson scattering diagnostic. Most important of these will be a better spatial resolution, down to about 2cm without deconvolution. Furthermore, the lifetime of the detectors will be increased; the detectors will cover the whole blue wing of the scattered spectrum when using a single wavelength laser, and this will enable measurements of electron temperature and density profiles at kHz repetition rates.