High Pulse Repetition Rate Research Articles

High-repetition rate, mid-infrared, picosecond pulse generation with µJ-energies based on OPG/OPA schemes in 2-µm-pumped ZnGeP2.

We report on two different concepts to generate µJ-level mid-infrared laser pulses with sub-10 ps pulse duration via nonlinear parametric generation at high pulse repetition rates. Both schemes rely on the recent development of compact and efficient CPA-free, Ho:YLF-based 2-µm laser sources pumping the highly nonlinear crystal ZnGeP2 used for parametric amplification. The first concept comprises a simplified OPG/OPA scheme efficiently producing signal and idler radiation at fixed wavelengths of 3.0 and 6.5 µm, respectively. In the second concept, we demonstrate a wide spectral tunability of the picosecond, µJ pulses over the entire tuning range between 2.5 and 12 µm maintaining a constant bandwidth of sub-20 cm-1 by integrating a two-color pumping scheme and a spectral shaper into the setup. The presented compact and efficient mid-IR picosecond radiation sources offer a way towards low-cost mid-IR material processing and spectroscopic applications.

Single-frequency Q-switched Er:YAG laser with high frequency and energy stability via the Pound-Drever-Hall locking method.

We present an injection-seeding Q-switched 1645 nm Er:YAG ceramic laser with high frequency stability and energy stability by combining the injection-seeding technique and Pound-Drever-Hall technique. 10.31 mJ single-frequency pulses with optimal frequency stability (525 kHz) and relative energy stability (0.52%) at a high pulse repetition rate of 1 kHz are obtained. To the best of our knowledge, this is the highest frequency stability and energy stability so far in a high pulse repetition frequency, large pulse energy, single-frequency Q-switched Er-doped solid laser. This single frequency laser with high stability provides an excellent light source for a coherent lidar system.

Sub-two-cycle octave-spanning mid-infrared fiber laser

Compact and powerful ultrafast light sources at high pulse repetition rates, based on mode-locked near infrared fiber lasers, are now widely available and are being used in applications such as frequency metrology, molecular spectroscopy, and laser micro-machining. The realization of such lasers in the mid-infrared has, however, remained a challenge for many years. Here we report a record-breaking three-stage fiber laser system that uses an Er-doped fluoride fiber as gain medium, delivering W-level few-cycle pulses at 2.8 µm at a repetition rate of 42.1 MHz. A fiber-based seed oscillator, cavity dispersion-managed by a pulse-stretcher, generates near-100-fs mid-infrared pulses with > 110 n m spectral bandwidth. These pulses are amplified to an average power of ∼ 1 W in a chirp-engineered fiber amplifier, and then compressed to ∼ 16 f s in a short length of highly nonlinear ZBLAN fiber, resulting in a more-than-octave-wide spectrum reaching from 1.8 µm to 3.8 µm with a total power of 430 mW.

Mid-infrared high repetition mode-locked laser based on cross-band all-optical injection modulation

High speed pulse generation with adjustable output characteristics in the mid-infrared (MIR) wavelength is a challenge due to the lack of high speed optoelectronic modulator operating at this band. In this paper, we proposed a MIR actively mode-locked laser based on cross-band all-optical injection modulation. The proposed laser is modulated by injecting 1.55-μm pump pulses into the specially-designed cross-band group-velocity-matched fiber. By numerical simulation, we studied in detail the evolution mechanism of the pulses and the influence of the cavity parameters on the output pulse features. It is shown that the laser is suitable for high repetition rate pulse generation up to 40 GHz at 2.771 μm, and the pulse shape can be flexibly adjusted. The proposed MIR laser has great potential in MIR free-space optical (FSO) communication.

Laser processing of dielectrics using spatiotemporally tuned ultrashort pulses

The authors report on the theoretical and experimental studies of laser-induced optical breakdown on the surface of fused silica to elucidate the influence of time delay and spatial separation between two ultrashort pulses on the position and size of the modification. Carriers involved in the damage formation including free electrons in the conduction band and self-trapped excitons (STEs) are investigated. The relationship between damage morphology and time delay shows that the seeding carriers (free electrons and STEs) generated from the first pulse are found to play a significant role for the second pulse—which is temporally and spatially separated from the first pulse—in creating the critical electron density needed for an optical breakdown. Consequently, processing outcomes, such as accuracy (position of the hole) and resolution (size of the hole), depend on the interplay of various laser-induced physics that can be tailored for specific goals. As a demonstration, laser lithography with resolution below the diffraction limit is achieved by exploiting multipulse induced physics. This work is a step toward repeatable laser processing of dielectrics beyond the diffraction limit and provides insights into ultrafast laser-matter interaction under the condition of an extremely high pulse repetition rate.

Performance scaling of a 10-GHz solid-state laser enabling self-referenced CEO frequency detection without amplification.

A simple and compact straight-cavity laser oscillator incorporating a cascaded quadratic nonlinear crystal and a semiconductor saturable absorber mirror (SESAM) can deliver stable femtosecond modelocking at high pulse repetition rates >10 GHz. In this paper, we experimentally investigate the influence of intracavity dispersion, pump brightness, and cavity design on modelocking with high repetition rates, and use the resulting insights to demonstrate a 10.4-GHz straight-cavity SESAM-modelocked Yb:CALGO laser delivering 108-fs pulses with 812 mW of average output power. This result represents a record-level performance for diode-pumped femtosecond oscillators with repetition rates above 10 GHz. Using the oscillator output without any optical amplification, we demonstrate coherent octave-spanning supercontinuum generation (SCG) in a silicon nitride waveguide. Subsequent f-to-2f interferometry with a periodically poled lithium niobate waveguide enables the detection of a strong carrier-envelope offset (CEO) beat note with a 33-dB signal-to-noise ratio.

Third and fifth harmonics generation in air and nanoparticle-containing plasmas using 150-kHz fiber laser

High-pulse repetition rate lasers allow significant enhancement in the average power of the low-order harmonics generation in isotropic media. We report on the third (343-nm) and fifth (206-nm) harmonics generating in air and different nanoparticle-containing laser-produced plasmas by applying Yb-doped fiber laser delivering 37-fs, 100-kHz, 1030-nm, 0.5-mJ pulses. Different characteristics of these processes (power and density dependences, influence of impeding processes, role of synthesized nanoparticles, variations of the spectral modulation of driving pulses, etc.) are analyzed. We compare the formation of the plasmas on the surfaces of aluminum, carbon, manganese, and titanium bulk species, as well as quantum dots of metal sulfides as the media for low-order harmonics generation, using 250-fs (1030-nm) and 5-ns (1064-nm) heating pulses at different delays between the heating and driving pulses. Average power of about 20 mW was estimated for the third harmonic emission from metal plasmas, with 3 × 1016 UV photons (343 nm) emitted per second.

Concept and Performance Evaluation of a Novel UAV-Borne Topo-Bathymetric LiDAR Sensor

We present the sensor concept and first performance and accuracy assessment results of a novel lightweight topo-bathymetric laser scanner designed for integration on Unmanned Aerial Vehicles (UAVs), light aircraft, and helicopters. The instrument is particularly well suited for capturing river bathymetry in high spatial resolution as a consequence of (i) the low nominal flying altitude of 50–150 m above ground level resulting in a laser footprint diameter on the ground of typically 10–30 cm and (ii) the high pulse repetition rate of up to 200 kHz yielding a point density on the ground of approximately 20–50 points/m2. The instrument features online waveform processing and additionally stores the full waveform within the entire range gate for waveform analysis in post-processing. The sensor was tested in a real-world environment by acquiring data from two freshwater ponds and a 500 m section of the pre-Alpine Pielach River (Lower Austria). The captured underwater points featured a maximum penetration of two times the Secchi depth. On dry land, the 3D point clouds exhibited (i) a measurement noise in the range of 1–3 mm; (ii) a fitting precision of redundantly captured flight strips of 1 cm; and (iii) an absolute accuracy of 2–3 cm compared to terrestrially surveyed checkerboard targets. A comparison of the refraction corrected LiDAR point cloud with independent underwater checkpoints exhibited a maximum deviation of 7.8 cm and revealed a systematic depth-dependent error when using a refraction coefficient of n = 1.36 for time-of-flight correction. The bias is attributed to multi-path effects in the turbid water column (Secchi depth: 1.1 m) caused by forward scattering of the laser signal at suspended particles. Due to the high spatial resolution, good depth performance, and accuracy, the sensor shows a high potential for applications in hydrology, fluvial morphology, and hydraulic engineering, including flood simulation, sediment transport modeling, and habitat mapping.

Silicon wafer crack detection using nonlinear ultrasonic modulation induced by high repetition rate pulse laser

In this study, a high repetition rate laser ultrasonic system is developed for noncontact detection of silicon wafer crack. A high repetition rate pulse laser is used to generate two narrowband ultrasonic waves on a target silicon wafer, whereas a high-speed laser Doppler vibrometer is used to measure the corresponding ultrasonic responses. Nonlinear interaction occurs between the two generated ultrasonic waves due to crack formation, producing nonlinear ultrasonic modulation at the sum and difference of the two excitation frequencies. Outlier analysis is performed on the amplitudes of the nonlinear modulation components, and a fatigue crack is identified when the amplitudes of the nonlinear modulation components exceed a user specified threshold. The uniqueness of this study is as follows. (1) A noncontact, high-speed, and scanning laser ultrasonic system is developed using a high repetition rate laser for ultrasonic generation and a laser Doppler vibrometer for sensing as well as scanning the two laser beams simultaneously in a pitch-catch mode. (2) Two narrowband ultrasonic waves up to 1 MHz are simultaneously generated from a single laser pulse. (3) Micro cracks with widths of less than 10 µm are successfully detected by extracting the crack induced nonlinear ultrasonic modulation components. (4) The probability of detection of the proposed crack detection technique is evaluated. The performance of the proposed technique is experimentally validated using eight silicon wafer specimens with varying crack lengths. Results show that the proposed technique yields a 90% probability of detection in silicon wafers when the crack length is over 3.77 mm.

Effects of pulsed and continuous-wave laser radiation on the fabrication of tissue-engineered composite structures

We studied the formation of a composite from an aqueous dispersed medium with albumin and carbon nanotubes under the action of laser radiation in continuous wave (CW) mode and pulsed mode with a repetition rate of 10 Hz and pulse duration of 16 ns. During the experiments, the temperature was monitored at the site of exposure, as well as its distribution in the liquid. Pulsed solid-state Nd:YAG laser and CW diode laser with an irradiation power of ∼500 mW were used as radiation sources. However, a three-dimensional composite was formed only with constant exposure. The effect of pulsed laser radiation with an intensity corresponding to nonlinear interaction with water dispersion led only to its enlightenment. Thus, it is important not only the energy parameters of radiation but also the frequency of energy portions exposure for the fabrication of tissue-engineered structures (composites). As a result, it was found that the curing of the dispersion and the composite formation occurs under the action of continuous or pulsed (with a high pulse repetition rate) laser radiation at a temperature in the range from 45°C to 50°C; in case the pulse repetition rate is insufficient, composite formation is not observed even under the action of high intensity radiation and heating occurs only to a temperature of ∼40 ° C. This formation process can be generated both in the visible 532 nm and in the infrared 810-nm wavelength ranges. In this case, one of the main conditions is the absence of albumin or cells absorption at these wavelengths so that absorption occurs mainly with single-walled carbon nanotubes. Studies of the surface and internal structure of the composite made it possible to demonstrate the binding of nanotubes to each other. This happened under the influence of laser radiation. This led to high hardness values of the composites. The average value of hardness was 0.26 ± 0.02 GPa.

Towards high power longwave mid-IR frequency combs: power scalability of high repetition-rate difference-frequency generation

Frequency combs in the mid-IR wavelength are usually implemented by difference-frequency generation (DFG) that mixes pump pulses and signal pulses. Different from most optical parametric amplifiers that operate at a typical low repetition rate of <0.1 MHz, mid-IR frequency combs require that pump/signal pulse repetition rate must be at least as high as tens of MHz (normally >30 MHz). The DFG mixing high repetition rate (HRR) pulses limits the allowed pulse energy to prevent crystal damage. In this paper, we numerically investigate HRR DFG with a focus on the energy scalability of idler pulses. We show that HRR DFG-unlike optical parametric amplifiers-may operate in the linear regime, in which the idler pulse energy scales linearly with respect to the pump/signal pulse energy. Our simulation results suggest an efficient approach to energy scaling the idler mid-IR pulses in a HRR DFG: increase the signal pulse energy to the same level as the pump pulse energy. We also show that DFG seeded by pump/signal pulses at ∼2-µm range benefits from reduced group-velocity mismatch and exhibits better idler energy scalability. For example, 44.2-nJ pulses at 9.87 µm can be achieved by mixing 500-nJ, 2.0-µm pump pulses and 100-nJ, 2.508-µm signal pulses in a 2-mm-thick GaSe crystal. At the end of this paper, we show that such high-energy signal pulses can be derived from the pump pulses using a recently invented fiber-optic method. Therefore, implementation of high-power (>2 W) longwave mid-IR frequency combs is practically feasible.

High-repetition-rate pulsed fiber laser based on virtually imaged phased array

High-repetition-rate (HRR) pulsed fiber lasers have attracted much attention in various fields. To effectively achieve HRR pulses in fiber lasers, dissipative four-wave-mixing mode-locking is a promising method. In this work, we demonstrated an HRR pulsed fiber laser based on a virtually imaged phased array (VIPA), serving as a comb filter. Due to the high spectral resolution and low polarization sensitivity features of VIPA, the 30 GHz pulse with high quality and high stability could be obtained. In the experiments, both the single-waveband and dual-waveband HRR pulses were achieved. Such an HRR pulsed fiber laser could have potential applications in related fields, such as optical communications.

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.

Passively Q-switched laser using PtSe2 as saturable absorber at 1.3 μm

We report a passively Q-switched Nd:YVO4 laser at 1341.5 nm by using few-layer Platinum diselenide (PtSe2) as the saturable absorber (SA). The nonlinear saturable absorption properties of the PtSe2 at 1.3 μm were measured, showing a modulation depth of 7% and a saturation intensity of 25 GW/cm2, respectively. The maximum Q-switched pulse average power was 209 mW under the absorbed pump power of 3.62 W. The shortest pulse duration and the highest pulse repetition rate was 775 ns and 103.5 kHz, respectively. The instability (output powers, rms) of the Q-switched pulse was measured to be about 0.63% for one hour. The results indicated that the PtSe2 have a good performance as a potential SA in the 1.3 µm spectral range.

Nonlinear Optical Properties of Materials Based on Graphene Oxide: A Review

Background: Nonlinear optical properties of Graphene and Graphene Oxide have been widely used in industry and academia. Graphene oxide disperses easily in water and has easier interaction with other materials because of the presence of oxygen groups. So, this feature of Graphene oxide enables us to manipulate its nonlinear optical properties by combining it with other nanoparticles. Objective: We introduced recent advances in the nonlinear optical properties of materials based on Graphene oxide. Methods: Nonlinear optical properties and optical limiting of Graphene oxide and/or its composites with various nanoparticles, considering the wavelength and the incident pulse width, are investigated in this review. Conclusion: At low intensities and in all pulse regimes, saturation absorption seems to be the dominant mechanism of nonlinear absorption in Graphene oxide, while at higher intensities, the main mechanism is the reverse saturation absorption. In the regime of very short pulses of picoseconds and femtoseconds, the dominant mechanisms of two-photon and multiphoton absorption lead to reverse saturation. In the nanosecond pulse regime, long laser pulses and short pulses with high pulse repetition rates, excited-state absorption and nonlinear scattering due to thermal effects are causing the nonlinear process.

Thermally modified C-band acceleration unit for a high-repetition normal-conducting linac for free-electron lasers.

A high-repetition linear accelerator with multibeamline operation for X-rays allows users to perform multiple simultaneous experiments with an X-ray free-electron laser (XFEL) at lower actual cost per user while maintaining the availability of the laser. As the first step toward a higher pulse repetition rate (PRR), we developed radio frequency (RF) components at 5712 MHz and tested them for their feasibility at an RF PRR of 120 pulses per second (pps). To increase the RF PRR from the present value of 60 pps-at the SPring-8 Angstrom compact free-electron laser-to 120 pps, we re-examined the thermal designs of the present RF components for the XFEL. With the proposed design, the newly developed RF components worked well at 120 pps. The insulation-oil temperature in the high-voltage tank of the klystron modulator was below 70 °C, which is less than the operational temperature limit. At 5712 MHz, this is now the highest RF pulse repetition frequency yet to be achieved and represents a major step toward higher PRRs.

The pulsed X-ray radiation source based on a semiconductor opening switch with the focal point diameter of 0.5 mm and its application.

This investigation is devoted to developing a pulsed X-ray radiation source. The high-voltage pulse generator is based on an inductive energy storage unit and a semiconductor opening switch, and the latter ensures a high pulse repetition rate. A high-voltage pulse generator has the following parameters: the pulse voltage amplitude up to 120 kV, the pulse current up to 180 A, the current pulse duration at 50% of the peak height of about 20 ns, and the pulse repetition rate up to 5 kHz. The source works consistently when the pulse repetition rate is increased. We developed the designs of an anode and an explosive emission cathode for the pulsed X-ray tube with the focal point of 0.5 mm, used in the experiments. The setup which makes it possible to investigate demountable models of pulsed X-ray tubes is described in this paper. We believe there are advantages of using an X-ray source with the flare duration of about 20 ns and the pulse repetition rate up to 5 kHz over continuously working X-ray sources. The source is unique for some problems, such as studying high-speed processes. The experiments have shown that a pulsed X-ray source works consistently together with a digital detector, and the generation of X-ray flares is easily synchronized with the detector and the rotation system. In the article, we discuss the issues of reducing image blurring for computerized tomography purposes, caused by fast rotation of the source and the detector relative to the object of investigation and reducing the radiation exposure of the investigated object. We also set further objectives of the investigation and present perspectives and limitations of this source.

Femtosecond micro-machining of hydrogels: parametric study and photochemical model including material saturation

We report on the effects of various laser system parameters on the magnitude of phase change induced inside hydrogel-based contact lens materials in the two-photon absorption limit via a laser induced refractive index (LIRIC) technique. In comparison with near infrared writing at 1035 nm where four-photon absorption process dominates, blue writing at 405 nm allows for the achievement of a decent amount of phase change (one wave) with a low power and a fast scan speed due to a more efficient two photon absorption process. Efficacy of the LIRIC process could be further improved by taking advantage of intermediate repetition rate laser pulses instead of high repetition rate (> 60 MHz) pulses or low repetition rate (< 500 KHz) pulses. A generally applicable photochemical model based on multiphoton absorption mechanism and pulse overlapping effect in two dimensions is proposed to predict the scaling behavior of the induced phase change. A modified photochemical model incorporating a saturation factor is developed to account for the behavior at large phase shifts. The modified photochemical model also helps explain the inapparent dependence of the phase change on numerical aperture (NA) at low irradiation doses and the observed sub-linear inverse dependence on scan speed.

Estimating Regional Myocardial Contraction Using Miniature Transducers on the Epicardium

This paper describes an ultrasound system to monitor cardiac motion using miniature transducers attached directly to the epicardial surface. Our aim was to develop both a research tool for detailed studies of cardiac mechanics and a continuous, real time system for peri-operative evaluation of heart function. The system was tested on a porcine model. Two 3 mm diameter, 10 MHz ultrasound transducers were sutured to the epicardial surface. As the epicardial surface was the reference for the velocity and strain estimations, this procedure compensated for the motion of the heart. The short distance allowed for the use of high frequencies and pulse repetition rates. The system was driven in pulse-echo mode, using electronics developed for the application, and radio frequency (RF) lines were recorded at a pulse repetition rate of 2500 s−1. The endocardial border was detected using an algorithm based on fuzzy logic with filtration to reduce noise and remove outliers, and the myocardium was divided into four layers. Inside the myocardium, radial tissue velocity as a function of depth was calculated from the recorded RF signals, and the velocity estimates were used to estimate radial strain rate and strain and to track the motion of the myocardial layers. The scope of this paper is technical, giving a detailed description of system design, hardware electronics and algorithms, with examples of processed velocity patterns and myocardial strain curves. The results from this study on a porcine model demonstrate the system's ability to estimate myocardial velocity and strain patterns and to track the motion of the myocardial layers, thereby obtaining detailed information of the regional function of the myocardium.

The influence of processing parameters on X-ray emission during ultra-short pulse laser machining

During ultra-short laser material processing at high laser pulse repetition rates unwanted X-ray radiation can be generated in a quantity that may constitute a potential risk for health. An adequate X-ray radiation protection requires a thoroughly understanding of the influence of the laser processing parameters on the generation of X-ray radiation. In the present work, the generated X-ray dose during laser machining was investigated in air for varying beam scanning conditions at a pulse duration of 925 fs, a center wavelength of 1030 nm and a laser peak intensity of 2.6 × 1014 W/cm2. The X-ray radiation dose and the corresponding spectral X-ray emission were investigated in dependence on the laser’s pulse repetition rate and on the beam scanning speed. The results show a strong dependence of the X-ray emission on these laser processing parameters.