Pulsed Laser Source Research Articles

Energetic few-cycle pulse compression in gas-filled hollow core fiber with concentric phase mask**Project supported by the National Natural Science Foundation of China (Grant No. 61521093), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB1603), the International Science and Technology Cooperation Program of China (Grant No. 2016YFE0119300), and the Program of Shanghai Academic/Technology Research Leader, China

The compression of high-energy, linearly polarized pulses in a gas-filled hollow core fiber (HCF) by using a concentric phase mask is studied theoretically. Simulation results indicate that using a properly designed concentric phase mask, a 40-fs input pulse centered at 800 nm with energy up to 10.0 mJ can be compressed to a full width at half maximum (FWHM) of less than 5 fs after propagating through a neon-filled HCF with a length of 1 m and diameter of with a transmission efficiency of 67%, which is significantly higher than that without a concentric phase mask. Pulses with energy up to 20.0 mJ can also be efficiently compressed to less than 10 fs with the concentric phase mask. The higher efficiency due to the concentric phase mask can be attributed to the redistribution of the transverse intensity profile, which reduces the effect of ionization. The proposed method exhibits great potential for generating few-cycle laser pulse sources with high energy by the HCF compressor.

Improving the homogeneity of diffraction based colours by fabricating periodic patterns with gradient spatial period using Direct Laser Interference Patterning

This study focuses on the development of a strategy to produce periodic structures with a variable spatial period for increasing the homogeneity of structural colours by means of direct laser interference patterning. Using a four-beam interference configuration, hole-like periodic arrays are produced on stainless steel with a 70 ps pulsed laser source operating at 532 nm laser wavelength. The laser processing parameters are optimised for obtaining patterns with the highest possible diffraction efficiency and thus showing the highest possible colour intensity. A model for calculating the required spatial period to obtain a defined colour under specific conditions of illumination and observation angles is presented. A very good agreement between the captured structural colour spectrum and the real visible spectrum of light was obtained. In addition, a strategy for mixing holographic colours, in particular for obtaining the white colour is developed. Finally, the developed model is successfully integrated into machine software, in order to automatically process images that exhibit required colours at certain viewing conditions. The produced patterns are characterised using confocal microscopy and the efficiency of the first diffraction order was measured by optical spectroscopy.

WS2-based Q-switched laser generation from Nd:YAG ridge waveguides fabricated by combination of swift heavy ion irradiation and laser ablation

We report on the operation of passively Q-switched waveguide lasers in Nd:YAG ridge waveguides based on WS2 as a saturable absorber (SA). The ridge waveguides are fabricated by swift Kr ion irradiation and femtosecond (fs) laser ablation. The 670 MeV Kr8+ ion irradiation creates a planar waveguide layer of ∼50-μm thickness in the crystal, and the following laser ablation patterns two parallel air grooves on the planar structure for ridge waveguide construction. Under the optical pump at 808 nm, the Q-switched waveguide laser operation at 1064 nm is realized with WS2-based SA, reaching a maximum average output power of 119 mW and a slope efficiency up to 34.5%. The laser pulses are with duration of 209 ns, and with 870 kHz repetition rate corresponding to single-pulse energy of 25 nJ. This WS2-based waveguide laser system is a compact pulsed laser source, which may have potential applications in a number of areas.

High precision laser macrodrilling of carbon fiber reinforced plastics with a new nanosecond pulsed laser—Optimized toward industrial needs

Due to their outstanding strength-to-weight ratio, carbon fiber reinforced plastics (CFRP) are of high interest for lightweight construction. Within industrial fields requiring lightweight design and energy efficiency, the demand for CFRP increases continuously. Until now, machining of CFRP, as a crucial step in the production chain of composite parts, is mainly performed by conventional techniques such as drilling, milling, and abrasive water jet cutting. These techniques come with known drawbacks, including force input, handling of auxiliaries, and tool wear. The laser cutting of CFRP has already shown its high potential to be a practical alternative due to wear-free and contactless processing. However, if sufficient cutting rates are required, laser cutting has to overcome the challenge of either nonorthogonal cutting edges when using short pulsed high power lasers or heat input into the material when using continuously emitting (continuous wave) high power singlemode laser sources, as shown by Staehr et al. [J. Laser Appl. 28, 022203 (2016)]. Within this investigation, the machining with a novel short pulsed high power laser source by Trumpf Laser GmbH emitting at λ = 1030 nm will be examined. The lab-state laser source provides nanosecond pulses at tp = 20 ns, at a maximum pulse energy of Ep = 100 mJ, and a maximum average power of Pavg = 1.5 kW, while maintaining a very good beam quality allowing for small focus diameters down to df = 84 μm with the focusing optics used. It was shown that processing with small focus diameters results in narrow cutting kerfs contributing to the cutting edge angle. In addition, the investigations revealed a minimized thermal load resulting from the short pulses. At the same time, challenges resulting from shielding effects when specific parameters are used will be displayed and discussed. Different processing strategies for adjusting the cutting edge angle toward orthogonality will be presented in this paper. This optimization procedure aims at the drilling of macroscopic holes with a diameter of Ø > 4 mm in CFRP laminates with a thickness of s ≥ 2 mm at very high precision.

Spatial and temporal pre-alignment of an X-ray split-and-delay unit by laser light interferometry.

We present a novel experimental setup for performing a precise pre-alignment of a hard X-ray split-and-delay unit based on low coherence light interferometry and high-precision penta-prisms. A split-and-delay unit is a sophisticated perfect crystal-optics device that splits an incoming X-ray pulse into two sub-pulses and generates a controlled time-delay between them. While the availability of a split-and-delay system will make ultrafast time-correlation and X-ray pump-probe experiments possible at free-electron lasers, its alignment process can be very tedious and time-consuming due to its complex construction. By implementing our experimental setup at beamline P10 of PETRA III, we were able to reduce the time of alignment to less than 3 h. We also propose an alternate method for finding the zero-time delay crossing without the use of X-rays or pulsed laser sources. The successful demonstration of this method brings prospect for operating the split-and-delay systems under alignment-time-critical environments such as X-ray free electron laser facilities.

Picosecond Pulsed Laser Ablation of Liquid Covered Stainless Steel: Ef-fect of Liquid Layer Thickness on Ablation Efficiency

Under liquid laser ablation is a material removal technique in which a focused laser beam passes through a liquid layer on top of the surface of a sample to be processed. When compared to laser ablation without a liquid layer, material (re)deposition around ablated regions, as well as the size of the Heat Affected Zone (HAZ) is decreased. In addition, the ablation efficiency of the process, in terms of the amount of material removed per pulse, can be optimized by careful variation of the height of the liquid layer: a liquid layer height variation as small as a few tens of millimeters already has a measurable effect on the amount of ablated material. In studies reported in existing literature, the required liquid layer height is typically realized by pouring a pre-defined amount of liquid on top of the sample surface. Surface tension, however, causes the air-liquid interface at the boundaries of the domain to deviate from the planar interface away from the boundaries, which affects the accuracy with which the liquid layer height can be determined. To the best of our knowledge, these accuracy issues have not been studied in previous research. Therefore, an experimental set-up is proposed which circumvents the issues of a curved free surface. Next, a 7 picosecond pulsed laser source (M2≤1.3) at a wavelength of 515nm was employed to study the efficiency of laser ablation of stainless steel for a range of liquid layer heights. Our findings provide a more detailed quantification of crater depth as a function of liquid layer height than is available through existing literature.

A dispersion engineered silica-based photonic crystal fiber for supercontinuum generation in near-infrared wavelength region

We report computational modeling of photonic crystal fibers in silica glass for near-infrared supercontinuum generation. Optical parameters like chromatic dispersion, nonlinearity and the effective area of the fundamental mode have been computed and tailored by using a full vectorial finite element method. At pump wavelength of 1300 nm, a flat-top dispersion profile has been obtained with very low dispersion value + 0.6402 ps/nm/km in the anomalous region. The reported structure offers high nonlinear coefficient 26.27W−1. km−1with effective mode area 4.78 μm2 at the pump wavelength. An ultra-broadband supercontinuum spanning, from 0.67 μm up to 2.4 μm can be easily achieved by the proposed fiber of 37 cm length using a 63 fs secant laser pulse source with 8 kW peak power in the near-infrared wavelength region. Such type of fiber can be advantageous in the field of near-infrared spectroscopy, optical communication, gas sensing, medical, frequency metrology, optical coherence tomography, and food quality control.

Study on fabrication of PCD micro-milling tool by picosecond pulsed laser

For the purpose of processing some forms of diamond, such as polycrystalline diamond (PCD) and chemical vapor deposition diamond, picosecond pulsed laser (PSPL) has the decisive advantage of wear-free material removal. This availability of picosecond pulsed laser sources enables the efficient applications of cutting diamond to generate sharp and regular cutting edge. Therefore, in this study, picosecond pulsed laser is used to fabricate polycrystalline diamond (PCD) micro tool. In order to optimize the picosecond pulsed laser processing, the effects of PSPL cutting parameters including the pulse energy, pulse pitch, and grain size of PCD on the cutting results are investigated. The results indicate the grain size has the greatest impact on the cutting edge radius and pulse energy has great effect on the processed surface of PCD. The results prove that PSPL is a better choice for precision fabrication of PCD micro tool. Finally, the fabrication of PCD micro tools by PSPL has been demonstrated.

2D reproduction of the face on the Turin Shroud by infrared femtosecond pulse laser processing.

Femtosecond pulse laser processing concentrates a huge quantity of light energy in extremely short pulses of a few tens to hundreds of femtoseconds, enabling superficial laser machining or marking of any kind of materials, with a reduced or insignificant heat affected area. A digitized paper printed image of the face on the Turin Shroud was used to monitor a scan head intercalated between a femtosecond pulsed laser source and a linen fabric sample, enabling the direct 2D reproduction of the image of the face with a laser beam size corresponding to one pixel of the digitized image. The contrast in the marked image was controlled by adjusting the energy density, the number of superimposed pulses per pixel, and the distance between successive impacts. The visual aspect of the laser-induced image is very similar, at naked eye, to the source image. The negative photograph of the marked linen fabric reveals a face remarkably close to the well-known negative picture of the face on the Turin Shroud. Analyses by infrared spectroscopy, Raman spectroscopy, and scanning electron microscopy were performed to characterize the laser marked areas.

Design of a hybrid chalcogenide-glass on lithium-niobate waveguide structure for high-performance cascaded third- and second-order optical nonlinearities.

Dispersion engineering for efficient supercontinuum generation (SCG) is investigated in a hybrid nonlinear photonic platform that allows cascaded third- and second-order optical nonlinearities in transverse-electric (TE) guided modes. The highly nonlinear chalcogenide waveguides enable SCG spanning over 1.25 octaves (from about 1160nm to more than 2800nm at 20 dB below maximum power), while the TE polarization attained is compatible with efficient second-harmonic generation in a subsequent thin-film lithium niobate waveguide integrated monolithically on the same chip. A low-energy pump pulsed laser source of only 25pJ with 250fs duration, centered at a wavelength of 1550nm, can achieve such wideband SCG. The design presented is suitable for the f-to-2f carrier-envelope offset detection technique of stabilized optical frequency comb sources.

Burst Machining of Copper and Stainless Steel with Picosecond Lasers

AbstractGiven the very low ablation thresholds in metals when processing with picosecond pulse laser sources, it is important to carefully manage the applied laser fluence to maximize material removal efficiency while at the same time minimizing detrimental heat effects. Burst machining of copper and stainless steel can be a flexible and cost‐effective alternative to other techniques such as spatial beam shaping and ultra‐high speed beam scanning for optimizing these processes.

Near-THz bursts of pulses – Governing surface ablation mechanisms for laser material processing

Laser operation in bursts of pulses has recently attracted a lot of attention, as high average powers and pulse energies have become available. Bursts of pulses have become a mean to achieve different laser-matter interaction regimes with a single laser source. We have used an ultra-short pulse laser source in combination with an external array of birefringent crystals to generate near-THz bursts of single-picosecond pulses. Variability of the setup and high single pulse energy were exploited to generate bursts of up to 16 pulses at different delays between consecutive pulses. The experimental setup was applied for surface processing of different industrially relevant materials, including metals, ceramics, and polyimides.The goal of this study was to find out if the reduced ablation thresholds using near-THz bursts of pulses as reported in previous studies of other group could be utilized for increased ablation efficiency. Our findings show that near-THz bursts of pulses interact with materials in a similar fashion as a single, albeit longer, pulse would. We observed a clear trend on all the tested materials, showing a declining ablation efficiency with increasing total burst duration at a fixed pulse repetition frequency. Findings contradict the results of other groups concluding on an improved efficiency of laser to material energy transfer using bursts at near-THz intra-burst repetition rates. A simple model describing ablation efficiency decline with pulse prolongation was used to compare the experimental data with theoretical predictions. Detailed analysis of surface ablation effects on metallic materials also revealed a presence of melting processes typically associated with laser pulses above 10 ps in duration as opposed to original 1.5 ps pulses.

Towards all-fiber structure pulsed mid-infrared laser by gas-filled hollow-core fibers

We report here on a diode-pumped pulsed mid-infrared laser source based on gas-filled hollow-core fibers (HCFs) towards an all-fiber structure by the tapering method. The pump laser is coupled into an acetylene-filled HCF through a tapered single-mode fiber. By precisely tuning the wavelength of the diode to match different absorption lines of acetylene near 1.5 μm, mid-infrared emission around 3.1–3.2 μm is generated. With 2 m HCFs and 3 mbar acetylene gas, a maximum average power of 130 mW is obtained with a laser slope efficiency of ∼24%. This work provides a potential scheme for all-fiber mid-infrared fiber gas lasers.

Numerical solution of photocounting statistics inverse problem for few-photon pulsed laser sources

We present a numerical procedure for recovering of photon statistics of few-photon pulsed laser sources. It provides controlled level of mean error for both of photon and photocounting statistics. It is shown that procedure is correct for model data with average error level of 10−4and fidelity 0.9998.

Laser texturing of stainless steel under different processing atmospheres: From superhydrophilic to superhydrophobic surfaces

Wettability plays a major role in a variety of surface related phenomena, as corrosion, heat transfer or tissue adhesion on implants. Consequently, great research effort is being devoted to control the wetting degree of functional surfaces. Pulsed laser texturing at micro/nanometric level has been widely used for that purpose as a precision/time efficient technique. This work studies the role of the processing atmosphere in controlling the wettability of commercial AISI 304 by laser texturing. A pulsed laser source (λ = 532 nm) working at the nanosecond regime was employed and five different atmospheres were tested (i.e. O2, Air, CO2, N2, Ar). The results show clear differences in the wetting behaviour depending solely on the processing environment, ranging from hydrophilicity (31°) to hydrophobicity (125°). Those differences in wettability were found to be a consequence of changes in surface chemistry between samples processed under the various gases. The laser processing parameters showed a capability to tune the final wetting behaviour by controlling the topography and modulating the chemical composition given by the processing environment. It is demonstrated how the effects of the atmosphere can be exploited to tailor the wettability of the untreated surfaces (θ = 88°) up to the desired value, ranging from superhydrophilicity (θ = 0°) to superhydrophobicity (θ = 152°).

Increasing the peak power of a pulsed laser source using optical delay lines

We consider the implementation of a patented source of pulsed laser radiation, which allows economical use of power owing to the original structure of a parallel set of optical delay lines and the addition of the amplitudes of the pulses of the input laser radiation. The results of a computer experiment for different numbers of switching channels with the determination of the predicted output peak power of the laser source are presented. To assess the radiation source, the concept of system efficiency is introduced as the ratio of the output peak power to the number of channels of delay lines. The results of the calculations are presented in the form of efficiency graphs for different numbers of channels, considering losses in the optical fiber lines of the optical delay. The proposed source of pulsed laser radiation can potentially be used in optical communication lines, laser data systems, and medical devices.

All-fiber passively Q-switched erbium fiber laser implementing erbium-ytterbium-thulium co-doped saturable absorber fiber

We demonstrate a passive Q-switched fiber laser using an erbium-ytterbium-thulium (Er-Yb-Tm) doped fiber. The length of the Er-Yb-Tm co-doped fiber is varied from 0.4 m to 1.4 m to study the effect of this passive gain medium for pulse laser performance. The proposed laser operates in the pulse operation regime within a certain range of pump powers, which is dependent on the length of the Er-Yb-Tm co-doped fiber. A longer fiber employed in the laser cavity results in a higher laser pump power threshold, but a wider pump power range of pulse laser operation. The experimental outcome shows that a 1.0 m Er-Yb-Tm co-doped fiber offers the best performance in this work with a Q-switched pump power threshold of 45.1 mW, the widest pulse energy output range of 0.0439 μJ from 0.0167 μJ to 0.0606 μJ, with the supplementary advantage of 120.7 mW Q-switched laser operating range. This fiber gain medium analysis contributes to the generation of short pulse laser source without employing any chemical materials and complex laser setup.

Hybrid multi-wavelength nonlinear photoacoustic sensing and imaging.

Multi-wavelength photoacoustic (PA) imaging has been studied extensively to explore the spectroscopic absorption contrast of biological tissues. To generate strong PA signals, a high-power wavelength tunable pulsed laser source has to be employed, which is bulky and quite expensive. In this Letter, we propose a hybrid multi-wavelength PA imaging (hPAI) method based on the combination of a single-wavelength pulsed laser source and multi-wavelength continuous-wave (CW) laser sources. By carefully controlling the laser illumination sequence (pulse-CW-pulse) and extracting the PA signal difference before and after the heating of CW lasers, the optical absorption property of multi-wavelength light illumination could be obtained. Compared with conventional PA imaging, the proposed hPAI shows a much lower system cost due to the usage of single-wavelength pulsed lasers and multiple inexpensive CW lasers. As the preliminary results show in this Letter, hPAI imaging has the potential to provide another pathway for high spectroscopic optical absorption contrast in PA imaging.

MoTe2-Based Broadband Wavelength Tunable Eye-Safe Pulsed Laser Source at <inline-formula> <tex-math notation="LaTeX">$1.9~\mu$ </tex-math> </inline-formula>m

In this letter, molybdenum ditelluride (MoTe2), a new member of the transition metal dichalcogenides family, has been characterized and fabricated as an optical modulator for the development of broadband wavelength tunable laser using a Tm3+-doped CaYAlO4 crystal as a laser gain medium. The maximum average output power reaches 0.75 W with the shortest pulse width of about $0.69~\mu \text{s}$ and repetition rate of 70.9 kHz, which leads to a pulse energy of about $10.58~\mu \text{J}$ and peak power of 15.3 W. Keeping the MoTe2 optical modulator inside the laser resonator and at the same time introducing an undoped YAG for wavelength tuning, wavelength tunable passively Q-switched laser operation with a wavelength range of about 112 nm has been demonstrated. This letter experimentally verifies the potential of the novel 2D material MoTe2 as an optical modulator. At the same time, this letter paves the way toward the realizations of simple and compact wavelength tunable pulsed laser sources based on broadband 2D materials saturable absorbers.

Sub-100 fs pulse generation from a Kerr-lens mode-locked Yb:Lu2O3 ceramic thin-disk laser.

In this Letter, we demonstrate, to the best of our knowledge, the first Kerr-lens mode-locked (KLM) ceramic thin-disk laser. Ceramic laser gain media have many advantages for a high-power laser such as high fracture toughness, size scalability, low cost, and short time fabrication. By using the Yb:Lu2O3 ceramic thin disk, the pulse duration of 98fs with the output power of 3.7W was demonstrated. The shortest pulse duration was 86fs with the decreased output power of 0.36W. From our results, it was proven that a KLM thin-disk laser with ceramic gain media can become a new option for high-power, sub-100fs pulse laser sources.