Pulsed Laser Operation Research Articles

Energy and mass assessment of the laser Directed Energy Deposition process (DED-LB) for reduced environmental impact

ABSTRACT Additive Manufacturing (AM) processes have the capability to promote a more rational use of resources, reduce generated waste, as well as repair damaged components to extend their lifespan. With the aim of contributing to a more efficient and environmentally friendlier manufacturing process, this research work investigates the energy consumption of the Laser Directed Energy Deposition process (DED-LB) through the introduction of new parameters in its characterisation, which are energy and mass efficiency. First, the energy efficiency of the laser generator itself has been characterised. Based on the results obtained, it was possible to conclude that pulsed laser operation strategies allow a reduction in consumption of up to 9% compared to a continuous laser operation mode. Second, a design of experiments was carried out to relate the energy and mass efficiency of the deposition process to the input parameters: laser power, feed rate, and mass rate. The parameter that most influences the energy efficiency of the process is the mass rate, where high rates are advisable to maximise its value. On the other hand, it is concluded that an increase in the mass rate or a reduction in the feed rate results in an increase in the mass efficiency

Relevant parameters for laser surgery of soft tissue

In recent years, the laser has become an important tool in hospitals. Laser surgery in particular has many advantages. However, there is still a lack of the understanding of the influence of the relevant parameters for laser surgery. In order to fill this gap, the parameters pulse frequency, use of an exhaustion system, air cooling, laser power, laser scan speed, laser line energy and waiting time between cuts were analysed by ANOVA using inter-animal variation as a benchmark. The quality of the cuts was quantized by a previously published scoring system. A total of 1710 cuts were performed with a CO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ ext {CO}_2$$\\end{document} laser. Of the parameters investigated, laser power and scan speed have the strongest influence. Only the right combination of these two parameters allows good results. Other effects, such as the use of pulsed or continuous wave (CW) laser operation, or air cooling, show a small or negligible influence. By modulating only the laser power and scan speed, an almost perfect cut can be achieved with a CO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ ext {CO}_2$$\\end{document} laser, regardless of the external cooling used or the laser pulse duration or repetition rate from CW to nanosecond pulses.

Generation of pulse bursts with controllable number of pulses and of spectral broadening directly at the laser output from a km-long Yb-doped all-fiber laser

In this work, we study the pulse burst regime of a fiber laser, which allows generating packets of multiple pulses whose number can be precisely controlled by modifying the polarization state by means of two retarder plates (QWR, HWR). The proposed experimental scheme is a highly energetic Yb-doped all-fiber ring pulsed laser operating in the 1064 nm wavelength region and using a nonlinear polarization rotation (NPR)-based passive mode locking technique. Raman scattering effects can be found by increasing pump power to achieve the formation of up to four Stokes, resulting in a spectral broadening over more than 200 nm.

Integrating 2D Materials and Plasmonics on Lithium Niobate Platforms for Pulsed Laser Operation at the Nanoscale

AbstractThe current need for coherent light sources for integrated (nano)photonics motivates the search for novel laser designs emitting at technologically relevant wavelengths with high‐frequency stability and low power consumption. Here, a new monolithic architecture that integrates monolayer MoS2 and chains of silver nanoparticles on a rare‐earth (Nd3+) doped LiNbO3 platform is developed to demonstrate Q‐switched lasing operation at the nanoscale. The localized surface plasmons provided by the nanoparticle chains spatially confine the gain generated by Nd3+ ions at subwavelength scales, and large‐area monolayer MoS2 acts as saturable absorber. As a result, an ultra‐compact coherent pulsed light source delivering stable train pulses with repetition rates of hundreds of kHz and pulse duration of 1 µs is demonstrated without the need of any voltage‐driven optical modulation. Moreover, the monolithic integration of the different elements is achieved without sophisticated processing, and it is compatible with LiNbO3‐based photonics. The results highlight the robustness of the approach, which can be extended to other 2D materials and solid‐state gain media. Potential applications in communications, quantum computing, or ultra‐sensitive sensing can benefit from the synergy of the materials involved in this approach, which provides a wealth of opportunities for light control at reduced scales.

Passively Q-switched Erbium-Doped Fibre Laser Based on Graphene-Chitin Saturable Absorber

Graphene is a carbon allotrope composed of densely packed sp2-assembled carbon atoms in a honeycomb-like crystal lattice. It is widely used in various fields of materials science to develop sensors, optoelectronic devices, nanocomposites, and others due to its excellent electronic, mechanical, optical, and thermal properties. Graphene offers several advantages, including ease of fabrication, ultrafast recovery times, low fabrication cost, and low saturation intensities. Its unique zero bandgap property makes it the only material with wavelength-independent absorption, which is advantageous in generating pulsed lasers using saturable absorbers. This study investigates the performance of graphene with a chitin bio-host polymer as a saturable absorber in a 1.5 µm erbium-doped fibre laser. Graphene embedded in chitin was fabricated and utilized in a pulsed laser cavity operating in the wavelength region of 1.5 µm to enable Q-switched pulsed laser operation. The study achieved a pulse energy of approximately 19.83 nJ and a peak power of 10.72 mW. Increasing the pump power from 30.67 mW to 174.28 mW resulted in the highest repetition rate of 63.40 kHz, corresponding to the shortest pulse width of 1.74 µs. These findings have significant implications for applications requiring ultrafast precision and accuracy, such as medical laser equipment and flexible sensors.

Tunable optical nonlinearities and surface plasmon resonance in Fe3O4 nanoparticles-decorated Ti3C2 MXene composite for ultrafast fiber lasers

In this work, the Ti3C2 MXene was modified with Fe3O4 nanoparticles to obtain a composite with in-plane optical anisotropy and surface plasmon properties. Based on a side-polished D-shaped fiber (DSF), this Fe3O4/Ti3C2 MXene (F/M) composite exhibits a tunable nonlinear optical (NLO) absorption response under 1.5 μm laser excitation with different polarization states. The modulation depths can be obtained from 0.9% to 12.1%. As an application, the ultrafast erbium-doped fiber laser (EDFL) with multiple pulsed laser operations was demonstrated using the F/M-DSF device as a saturable absorber (SA). Both stable Q-switching and mode-locking operations were demonstrated in the same all-fiber laser cavity, which generated pulsed laser with microsecond and sub-picosecond durations respectively. In addition, the bound state (BS) of solitons in harmonic mode-locked lasers with the pulse time interval of 3 ps and the highest order of 30th was also realized. This versatile and simple-structured all-fiber laser has great potential in various applications, such as optical communication. With the numerical simulations, the dynamic of F/M composite-based soliton mode-locking was also explored, illustrating the mechanism of soliton formation and propagation. This work not only expands the advanced applications of F/M composite in ultrafast photonics but also provides a new perspective for designing and optimizing its ultrafast modulating performance.

Worst-Case X-ray Photon Energies in Ultrashort Pulse Laser Processing.

Ultrashort pulse laser processing can result in the secondary generation of unwanted X-rays if a critical laser irradiance of about 1013 W cm-2 is exceeded. Spectral X-ray emissions were investigated during the processing of tungsten and steel using three complementary spectrometers (based on CdTe and silicon drift detectors) simultaneously for the identification of a worst-case spectral scenario. Therefore, maximum X-ray photon energies were determined, and corresponding dose equivalent rates were calculated. An ultrashort pulse laser workstation with a pulse duration of 274 fs, a center wavelength of 1030 nm, pulse repetition rates between 50 kHz and 200 kHz, and a Gaussian laser beam focused to a spot diameter of 33 μm was employed in a single pulse and burst laser operation mode. Different combinations of laser pulse energy and repetition rate were utilized, keeping the average laser power constant close to the maximum power of 20 W. Peak irradiances I0 ranging from 7.3 × 1013 W cm-2 up to 3.0 × 1014 W cm-2 were used. The X-ray dose equivalent rate increases for lower repetition rates and higher pulse energy if a constant average power is used. Laser processing with burst mode significantly increases the dose rates and the X-ray photon energies. A maximum X-ray photon energy of about 40 keV was observed for burst mode processing of tungsten with a repetition rate of 50 kHz and a peak irradiance of 3 × 1014 W cm-2.

High repetition frequency and high energy Pr:YLF pulsed laser operating in single-longitudinal-mode

In this article, a single-longitudinal-mode pulsed laser with a center wavelength of 639.7 nm was developed by using a 444 nm laser diode end face continuously pumped Pr:YLF crystal, with the method of inserting the Q-switching pre-lase technology and a Fabry-Perot etalon set in the resonant cavity. Experimental results show that by optimizing the parameters of the inserted components in the laser cavity, the problem of single-longitudinal-mode oscillation during high energy injection being unstable can be solved. With the proposed laser, a Q-switching pre-lase single-longitudinal-mode pulsed laser output has been achieved with 10.25 μJ energy and 77.66 ns pulse duration at a repetition of 20 kHz. The spectral linewidth of the single-longitudinal-mode pulsed laser was approximately 18 MHz.

First Principles Study and Experimental Investigation of Graphene-Molybdenum Disulphide Nanocomposites Based Passive Saturable Absorber

Research on hybrid graphene with other two-dimensional materials has gained considerable attention owing to their potential applications beyond single components. Through our first principles analysis via density functional theory, graphene-molybdenum disulphide (MoS2) demonstrated a band gap opening by 2 meV, from gapless graphene when MoS2 layer is introduced into the structure. The simulated graphene-MoS2 has a direct band gap situated at K point of Brillouin zone with preserved Dirac properties of graphene. The experimental studies on graphene-MoS2 also have been performed by preparing graphene-MoS2-chitin nanocomposite through facile liquid-phase exfoliation method. Apart from energy gap using Tauc relation, the physical morphology and nonlinear properties of the material were systematically characterized. Graphene-MoS2-chitin exhibits a modulation depth of 10.5%, which is lower than individual graphene but higher than individual MoS2. Further investigation on the material’s performance was done by integrating the fabricated film into Erbium-doped fiber laser. Stable nanosecond pulse laser operation was realized with graphene-MoS2-chitin hybrid saturable absorber. The pulse width was measured to be 156.4 ns with repetition rate of 1.89 MHz, corresponding to a peak power of 56.13 mW and pulse energy of 8.78 nJ.

Enhanced X-ray Emissions Arising from High Pulse Repetition Frequency Ultrashort Pulse Laser Materials Processing.

The ongoing trend in the development of powerful ultrashort pulse lasers has attracted increasing attention for this technology to be applied in large-scale surface engineering and modern microfabrication. However, the emission of undesired X-ray photon radiation was recently reported even for industrially relevant laser irradiation regimes, causing serious health risks for laser operators. In the meantime, more than twenty influencing factors have been identified with substantial effects on X-ray photon emission released by ultrashort pulse laser processes. The presented study on enhanced X-ray emission arising from high pulse repetition frequency ultrashort pulse laser processing provides new insights into the interrelation of the highest-contributing parameters. It is verified by the example of AISI 304 substrates that X-ray photon emission can considerably exceed the legal dose rate limit when ultrashort laser pulses with peak intensities below 1 × 1013 W/cm² irradiate at a 0.5 MHz pulse repetition frequency. The peak intensity threshold value for X-ray emissions decreases with larger laser spot sizes and longer pulse durations. Another key finding of this study is that the suction flow conditions in the laser processing area can affect the released X-ray emission dose rate. The presented results support the development of effective X-ray protection strategies for safe and risk-free ultrashort pulse laser operation in industrial and academic research applications.

Ultra-short optical pulse generation in micro OLEDs and the perspective of lasing

We report experimental and theoretical investigations with an in-house fabricated tris(8‐hydroxyquinoline)‐aluminum (Alq3)-based high-speed micro-organic light-emitting diodes (μ-OLEDs) and demonstrate very promising optical pulse responses as short as 400 ps. With a model for an electrically pumped OLED, we simulate the emission of sub-nanosecond optical pulses. The model includes field-dependent (Poole-Frenkel) Langevin recombination and reabsorption of photons by singlets (Stokes-shifted), and by triplets (TA). The good agreement between the measurement and the simulation is the basis for further study of the prospects on the ps time scale, i.e. stimulated emission and conditions for laser operation. For an Alq3-based micro-OLED with high-Q optical cavity, we predict pulsed laser operation with damped relaxation oscillations in the GHz regime and several orders of magnitude linewidth narrowing, but only during 3 ns at most. The simulated current density threshold values vary from 10 kA cm−2 for Q= 500 to less than 1 kA cm−2 for Q = 1000. No continuous-wave (CW) lasing is found due to the accumulation of triplet excitons for t > 5 ns, which has two deteriorating effects, (a) suppression of the gain-providing singlet excitons due to singlet-triplet absorption and (b) suppression of the photon density due to photon absorption by triplets (TA). Prospects for CW-lasing with other organic molecules are discussed.

Controllable Dynamic Single‐ and Dual‐Channel Graphene Q‐Switching in a Beam‐Splitter‐Type Channel Waveguide Laser

AbstractDirect inscription of structures by femtosecond‐laser pulses facilitates the flexible fabrication of diverse optical channel waveguides in dielectric laser gain media. Among them, beam‐splitter‐type waveguide lasers have been recently studied; however, there is a lack of investigations on the effect of unique characteristics of such waveguides, such as controlled splitting ratios of output powers and selectable excitation of individual channels, for pulsed laser operation. Here, dynamic single‐ and dual‐channel graphene Q‐switching of an Yb:YAG waveguide consisting of one input and two output channels is demonstrated by controlling the power‐splitting ratio. Single‐channel Q‐switched operation, exhibiting typical Q‐switched pulses, is achieved by interaction between the graphene saturable absorber and the desired one of the two channels. When both channels exceed the Q‐switching threshold, dual‐channel Q‐switching generates a pulse train that simultaneously combines the separately Q‐switched pulses induced from each channel under excitation with a single pump source. At a fixed power‐splitting ratio, the pulsed mode can be dynamically switched between the single‐ and dual‐channel Q‐switched operations by varying the pump power. The beam‐splitter‐type waveguide laser demonstrated in this study can be further developed for multiple‐channel Q‐switching, high‐repetition‐rate mode‐locking, and on‐chip dual‐comb sources advantageous for diverse applications in optical communication, metrology, and sensing.

Multifunctional Optical Device with a Continuous Tunability over 500 nm Spectral Range Using Polymerized Cholesteric Liquid Crystals.

We report that polymerization makes a robust, practically applicable multifunctional optical device with a continuous wavelength tunable over 500 nm spectral range using UV-polymerizable cholesteric liquid crystals (CLCs). It can be used as a circular polarizer generating an extremely high degree of circularly polarized light with |g| = 1.85~2.00. It can also be used for optical notch filters, bandwidth-variable (from ~28 nm to ~93 nm) bandpass filters, mirrors, and intensity-variable beam splitters. Furthermore, this CLC device shows excellent stability owing to the polymerization of CLC cells. Its performance remains constant for a long time (~2 years) after a high-temperature exposure (170 °C for 1 h) and an extremely high laser beam intensity exposure (~143 W/cm2 of CW 532 nm diode laser and ~2.98 MW/cm2 of Nd: YAG pulse laser operation for two hours, respectively). The optical properties of polymerized CLC were theoretically analyzed by Berreman’s 4 × 4 matrix method. The characteristics of this device were significantly improved by introducing an anti-reflection layer on the device. This wavelength-tunable and multifunctional device could dramatically increase optical research efficiency in various spectroscopic works. It could be applied to many instruments using visible and near-infrared wavelengths.

>25 W pulses from 1.5 μm double‐asymmetric waveguide, 100 μm stripe laser diode with bulk active layer

The experimental characterization of a high-power pulsed semiconductor laser operating in the eye-safe spectral range (wavelength around 1.5 μm), with an asymmetric waveguide structure, a 100 μm wide stripe, and a bulk active layer positioned very close to the p-cladding, is reported. An anti-reflection/high reflection coated laser with a stripe width of 100 μm exhibits a single-facet output power over 25 W at a pumping current amplitude of 100 A.

17-GHz waveguide lasers modulated by a heterostructure layered material

In this work, a thin-film MoS2/WS2/MoS2/WS2 heterostructure is employed as a saturable absorber (SA) material for efficient ultrafast pulsed laser operation, achieving a 17.54 GHz Q-switched mode-locked Nd:GGG waveguide laser fabricated by femtosecond laser direct-writing (FsLDW). The mode-locked pulse duration is measured to be as short as 31 ps. The maximum laser slope efficiency and the average output power are determined to be 29.27% and 310 mW, respectively. Such a high-performance compact pulsed laser exhibits promising applications of crystalline waveguide structures and layered heterostructures in ultrafast integrated photonics. This work represents the very first experimental demonstration of a pulsed laser based on FsLDW Nd:GGG waveguides and the very first demonstration of using a MoS2/WS2/MoS2/WS2 heterostructure as an efficient SA.

Multimode oscillation Q-switched mode-locking fiber laser based on gain equalized few-mode Er-doped fiber

We propose and demonstrate experimentally multimode Q-switched mode-locking (QML) fiber laser signal generation based on a long few-mode Er-doped fiber (FM-EDF). The erbium-doped profile of the fiber has been optimized to equalize the modal gain. The fiber supporting six modes has a total length of 5.5 m, a part of the fiber is used as a gain medium, and the unpumped part of the fiber is used as a saturable absorber. Multimode oscillation lasers in the state of the continuous wave laser operation, Q-switching, and QML state pulse laser operation were observed and measured at different pump powers by adjusting the polarization controller. A 1570-nm multimode QML pulse train was obtained, when the pump power was about 2.4 W. The width of the Q-switched envelope was 19 μs, and the repetition rate was 17.4 kHz. The stable mode-locked pulses repetition rate in Q-switched envelopes was 13.18 MHz. These temporal pulses can also be observed under the case of partly blocking the beam profile, and the part of the spectral signal gradually disappeared. The mode number in the laser was checked by a homemade mode selection photonic lantern. The results showed that the fiber laser had multimode oscillation, and the modes in generated signal were ≥5. The multiple transverse modes of the generated lasers are all in QML state at the same time. To the best of our knowledge, this is the first experimental demonstration of a multimode QML fiber laser using a long FM-EDF.

Straightforward Patterning of Functional Polymers by Sequential Nanosecond Pulsed Laser Irradiation.

Laser-based methods have demonstrated to be effective in the fabrication of surface micro- and nanostructures, which have a wide range of applications, such as cell culture, sensors or controlled wettability. One laser-based technique used for micro- and nanostructuring of surfaces is the formation of laser-induced periodic surface structures (LIPSS). LIPSS are formed upon repetitive irradiation at fluences well below the ablation threshold and in particular, linear structures are formed in the case of irradiation with linearly polarized laser beams. In this work, we report on the simple fabrication of a library of ordered nanostructures in a polymer surface by repeated irradiation using a nanosecond pulsed laser operating in the UV and visible region in order to obtain nanoscale-controlled functionality. By using a combination of pulses at different wavelengths and sequential irradiation with different polarization orientations, it is possible to obtain different geometries of nanostructures, in particular linear gratings, grids and arrays of nanodots. We use this experimental approach to nanostructure the semiconductor polymer poly(3-hexylthiophene) (P3HT) and the ferroelectric copolymer poly[(vinylidenefluoride-co-trifluoroethylene] (P(VDF-TrFE)) since nanogratings in semiconductor polymers, such as P3HT and nanodots, in ferroelectric systems are viewed as systems with potential applications in organic photovoltaics or non-volatile memories.

Multiple mode-locked regimes of an Er/Yb double-clad fiber laser based on NPR

In this paper, we experimentally demonstrate generation of diverse structures of robust optical pulses obtained with a ring cavity passively mode-locked fiber laser based on a double-clad erbium-ytterbium doped fiber, and operating at net-anomalous cavity dispersion. A half-wave and a quarter-wave retarder plates allow adjustable saturable absorbing action by the nonlinear polarization rotation technique. Pulsed laser operation in quasi- continuous wave, molecules of solitons, and single-wavelength noise-like pulses (NLPs) regimes is obtained by properly adjusting the wave retarders. In the soliton and NLPs operating regimes, the central laser wavelength lies in the 1540 nm region. The obtained NLPs exhibit narrow peak coherence of ∼231 fs.

In situ and tunable structuring of semiconductor-in-glass transparent composite.

SummarySemiconductor-in-glass composites are an exciting class of photonic materials for various fundamental applications. The significant challenge is the scalable elaboration of composite with the desirable combination of tunable structure, high semiconductor loading ratio, and excellent transparency. Here we report that the topological engineering strategy via hybridization of the glass network former enables to surmount the aforementioned challenge. It not only facilitates the in situ precipitation of (Ga2-xAlx)O3 domains with continuously tunable composition but also allows to simultaneously refine the grain size and enhance the crystallinity. In addition, the composites exhibit excellent transparency and can host various active dopants. We demonstrate the attractive broadband optical response of the composite and achieve the pulse laser operation in mid-infrared waveband. The findings are expected to provide a fundamental principle of in situ modification in hybrid system for generation of high-performance semiconductor-in-glass composites.

Near‐Infrared All‐Optical Switching Based on Nano/Micro Optical Structures in YVO4 Matrix: Embedded Plasmonic Nanoparticles and Laser‐Written Waveguides

Nonlinear interactions between light and matter give rise to a wide range of applications for both fundamental and applied research. Herein, near‐infrared switching of the optical response based on embedded Ag nanoparticles and laser‐written optical waveguides within yttrium vanadate (YVO4) crystal matrix is demonstrated. Using broadband transient absorption spectroscopy with a combination of femtosecond Z‐scan spectroscopy, the plasmon‐enhanced features of the third‐order optical nonlinearity in Ag:YVO4 nanocomposite at the near‐infrared band are elucidated. Meanwhile, an optical‐lattice‐like microscale waveguide is fabricated with well‐preserved photoluminescence properties by femtosecond laser writing. Taking advantage of the ultrafast on–off switching behavior of Ag:YVO4 saturable absorber, a Q‐switched pulsed laser operation in the waveguide platform, delivering 1 μm light pulses with high peak power of 298 mW, is demonstrated. This work indicates a possible path for the development of chip‐scale ultrafast photonic devices.