Ultrahigh Laser Research Articles

Modeling of thermal evolution and fluid flow during synchronous induction-assisted laser wire deposition

Introducing induction heating into laser deposition, which is referred to as synchronous ultra-high frequency induction-assisted laser deposition (SUHF-LD), not only alleviates the defects of stress concentration and microcracks, but also utilizes the electromagnetic stirring effect to refine grain size for improvement in mechanical property of the deposited components. Given that the incorporation of induction heat into laser deposition make the thermal evolution and flow behavior more complex. To this end, a numerical model coupled with multi-physical fields (such as electromagnetic/ temperature/ flow field) is developed to reveal the impact of Lorentz force on the heat transfer and fluid flow in the molten pool. Results show that the adjunction of the induction heat in laser deposition accelerates the flow velocity and decreases the maximum temperature of the molten pool due to the enhanced thermal convection in molten pool caused by Lorentz force. Investigation on the varying induction heat parameters indicates that the flow velocity of the molten pool is increased respectively by 23.8 % and 33.3 % under the condition of 800 kHz/1000A and 900 kHz/1400A compared to laser deposition. The SUHF-LD hybrid deposition simulation demonstrates that the temperature distribution and macro-geometry of the deposited track agree well with the experimental results. Furthermore, the multi-layer single-pass deposited tracks fabricated by SUILD hybrid deposition exhibits excellent tensile performance and wear resistance, indicating the hybrid deposition process can be widely applied in industry fields.

Analysis of thermal behavior and mass transport for manufacturing deep-small hole by ultra-high frequency induction-assisted laser wire deposition

Synchronous ultra-high frequency (UHF) induction-assisted laser deposition is an effective approach to tackle the extreme heat input and common defects that originate from laser direct deposition. The added auxiliary induction heat source can reduce the energy input of the laser heat source which guarantees the geometric integrity of the embedded tube and decreases temperature gradient. Meanwhile, the effective preheating of the induction heat source assures the metallurgical bonding among the substrate, deposited track and embedded tube. Experiments showed that the effective bonding can be formed among substrate-deposited track and deposited track-embedded tube under suitable combination parameters of the laser heat and induction heat. To gain insight into the hybrid deposition, a 3D numerical model coupled with multi-physical fields was established to explore the thermal process and flow behavior with assistance of induction heat. Results indicated that the electromagnetic force produced by induction coil promotes the flow velocity, which increases heat convection. Investigation on the induction heat parameter demonstrated that raising current intensity can accelerate the flow velocity from 0.07 m s-1 to 0.09 m s-1 while the maximum temperature of the molten pool declined from 2610 K to 2440 K owing to the reinforced heat convection. The experimental cross-section of the deposited tracks and the detected element distribution in transition areas are well tested against the numerical simulation results. The grain size in the top region of the deposited track is refined with increasing current intensity, which is experimentally and numerically verified by observed microstructure and G*R value. Meanwhile, the fabricated deposited track with average friction coefficient of 0.445 can be obtained when the laser and induction parameters are 1000 W and 400A, respectively.

Ultrahigh sensitivity fiber laser integrated biosensor for TNF-α detection in human blood sample

A label-free fiber biosensor based on Erbium-doped fiber laser (EDFL) integrated singlemode – tapered quad-core -singlemode fiber (STQS) structure is proposed and demonstrated for the detection of TNF-α. An excessive level of TNF-α in the body is an indicator for a number of serious health conditions. The sensor head structure is constructed by splicing and tapering a short section quad-core fiber (length 20 mm, diameter 5.45 µm) between two single-mode fibers. Antibodies of tumor necrosis factor-α (TNF-α) are then immobilized on the surface of the STQS structure to realize specifical binding with TNF-α. An EDFL is integrated to the sensor system to improve sensor resolution due to its high Quality (Q) factor of laser. Experimental result shows that the EDFL integrated STQS biosensor can detect TNF-α in solution with a concentration as low as 10 pg/mL (diluted by phosphate buffered saline) with an average wavelength shift of 0.59 nm. Human blood samples from three patients (TNF-α concentrations vary from 10 to 30 pg/mL) have been tested using the developed biosensor and the results agree well with standard method of flow immunofluorescence luminescence assay used in a hospital setting.

Effect of WC content on the microstructure and properties of AlCoCrFeNi2.1 eutectic high entropy alloy coating prepared by ultra-high speed laser cladding technology

AlCoCrFeNi2.1 eutectic high entropy alloy coating with different WC content were prepared on 304 stainless steel surface by ultra-high speed laser cladding technology. Microstructure, phase distribution, Microhardness, friction, and wear properties were analyzed to investigate the influence mechanism of WC on the properties of alloy coatings. The results indicate that the addition of WC results in refined crystalline strengthening. The decomposition of WC leads to the formation of W2C and Cr23C6, which contribute to solid solution strengthening and second-phase strengthening. With increasing WC content, a noticeable improvement in microhardness and wear resistance properties is observed.

Photon emission and radiation reaction effects in surface plasma waves in ultra-high intensities

Manipulating and harnessing plasmonic phenomena in the ultra-relativistic regime reveal promising prospects for the use of surface plasma waves (SPW) to create high-energy particle and radiation sources in the next generation of multi-petawatt lasers. Indeed, relativistic high-charge electron bunches can be produced by SPW excited by ultra-high intensity femtosecond lasers impinging on a periodically modulated solid-density target. In this regime, there is good evidence that SPW excitation survives and that the produced electron bunches experience strong acceleration, thus emitting large amounts of electromagnetic radiation. Therefore, extending the study to ultra-high laser intensities (I>1021 W/cm2), the use of a resonant grating for SPW generation represents an interesting alternative to light sources, as the energy lost by electrons due to radiation emission is transferred to high-energy γ photons. In addition, we show that using a laser with wavefront rotation coupled with a tailored blazed grating improves photon emission in the ultra-relativistic regime of interaction.

Ultrahigh precision laser nanoprinting based on defect-compensated digital holography for fast-fabricating optical metalenses.

The 3D structured light field manipulated by a digital-micromirror-device (DMD)-based digital hologram has demonstrated its superiority in fast-fabricating stereo nanostructures. However, this technique intrinsically suffers from defects of light intensity in generating modulated focal spots, which prevents from achieving high-precision micro/nanodevices. In this Letter, we have demonstrated a compensation approach based on adapting spatial voxel density for fabricating optical metalenses with ultrahigh precision. The modulated focal spot experiences intensity fluctuations of up to 3% by changing the spatial position, leading to a 20% variation of the structural dimension in fabrication. By altering the voxel density to improve the uniformity of the laser cumulative exposure dosage over the fabrication region, we achieved an increased dimensional uniformity from 94.4% to 97.6% in fabricated pillars. This approach enables fast fabrication of metalenses capable of sub-diffraction focusing of 0.44λ/NA with the increased mainlobe-sidelobe ratio from 1:0.34 to 1:0.14. A 6 × 5 supercritical lens array is fabricated within 2 min, paving a way for the fast fabrication of large-scale photonic devices.

MAS:Eu-YAG:Ce composite phosphor converters with excellent thermal performance and enhanced color rendering index for high-power white LDs

Phosphor converters with excellent thermal performance and high color rendering index (CRI) demonstrate vast application prospects in the field of laser diodes (LDs) lighting. Nevertheless, the significant heat accumulation within the converters and the absence of red spectral components pose challenges in achieving both a high saturation threshold and a high CRI for Y3Al5O12:Ce phosphor converters destined for white LDs. In this work, Mg2Al4Si5O18:Eu/Y3Al5O12:Ce (MAS:Eu/YAG:Ce) composite phosphor converters with excellent thermal performance and enhanced CRI were designed and fabricated for reflective laser lighting. Impressively, MAS:Eu/YAG:Ce phosphor-glass-ceramic composites (PGCC) retained a remarkable 97.8 % of the room temperature emission intensity at 150 °C. The reasons for this could be attributed to the strong thermal quenching resistance exhibited by MAS:Eu phosphors, as well as the ultra-thin glass bonding layer of about 150–200 μm. Meanwhile, by integrating the MAS:Eu/YAG:Ce PGCC with a 450 nm laser source, white LD devices operating in a remote excitation mode were fabricated. These white LDs demonstrated an improved CRI of 76 and maintained a low surface temperature of 131.7 °C under an ultra-high laser power density of 47.8 W/mm2 (laser power ∼ 4.70 W). Moreover, the maximum luminous flux and luminous efficiency of the white LDs were 484 lm and 103 lm/W, respectively. Therefore, the design of MAS:Eu/YAG:Ce composite phosphor converters offers a new approach for the development of high-power white LDs.

A new path towards ultra-high efficient laser power converters: Silicon carbide-based multijunction devices

Current high power laser transmission technology faces two major limitations to improve the efficiency of the photovoltaic receivers: the intrinsic entropic losses associated to low bandgap materials (such as GaAs) and the series resistance losses that degrade the device performance at high power densities. The use of high bandgap materials and new architectures for laser power converters (LPC) have been pointed out as alternatives to overcome these limitations. In this work, three silicon carbide polytypes (3C, 4H and 6H) are proposed as base materials for the standard horizontal laser power converter (hLPC) architecture and the Vertical Epitaxial Hetero-Structure Architecture (VEHSA). 3C SiC based hLPCs outperform the power converters based on the other two polytypes, achieving a maximum efficiency of 84.6% at ▪, but suffer from series resistance losses, that deteriorate their efficiency, at higher laser power densities. This issue is solved with 3C SiC 4 cells VEHSAs that demonstrated increasing efficiency with the input power, reaching a maximum of 87.4% at ▪. The VEHSA reduced number of cells minimize the risks of efficiency losses due to current mismatch between cells. These results support the feasibility of a new generation of LPCs capable of efficiently convert ultra-high laser power densities.

Research on thermal efficiency and weld forming coefficient prediction of ultra-high strength steel welded joint under different energy inputs

As an ultra-high strength steel, Al–Si coated 22MnB5 hot stamping steel is widely used in the manufacturing of body-in-white structural parts. Compared with traditional welding technology, laser welding is widely used in the field of automobile body-in-white manufacturing due to its high energy density, small heat affected zone, and high weld quality. For ultra-high strength steel laser welded joints, the weld forming coefficient is an important index to reflect the welding quality. In order to investigate the influence of weld forming coefficient on the Al content and the thermal efficiency, laser welding experiments of ultra-high strength steel under different laser energy inputs are carried out. The melting of the base metal and coating under different laser energy inputs is also considered. Particle swarm optimization (PSO) algorithm is introduced into radial basis function (RBF) neural network model to optimize the central parameters as the RBF neural network model alone is not sufficient in predicting the outcomes. The results shows that if the laser power density is constant (7 × 105 W/cm2), when laser energy inputs are 400 J/cm to 1200 J/cm, the Al content in welded joints is 1.458%–1.886%, the melting efficiency of welded joints is 0.23–0.26, the energy conversion efficiency of welded joints is 0.45–0.46. When the laser energy inputs are lower than 522 J/cm, the Al content of the laser welded joint increases gradually. When the laser energy inputs are higher than 522 J/cm, the Al content of the laser welded joint decreases. The root mean square error(RMSE) of PSO-optimized RBF neural network testing sets prediction results is 0.1551, R-square (R2) is 0.771 and mean absolute percentage error (MAPE) is 2.758%. The prediction accuracy of the PSO-optimized RBF neural network model for the upper surface weld width, waist weld width, and lower surface weld width is 93.9%, 91.1%, and 92.2%.

A synthetic pulse interaction for modelocking at ~100 GHz

Harmonic modelocking allows high repetition rates but has limited controllability and scalability. We propose a synthetic pulse-to-pulse interaction, acting like a saturable absorber, but imparting loss as a function of temporal delay, as a route to ultrahigh frequency fiber lasers.

Research on constellation attitude synchronous tracking control for space gravitational wave measurement

Space based gravitational wave detectors require a precision relative attitude pointing control in science mode. Their configuration drifts sensitively under the influence of perturbations. The spacecraft and telescope optical assembly attitude control loops need to respond to this change in real time to maintain an ultra-high precision laser pointing. To implement precision gravitational wave scientific measurement, distributed detector systems need to coordinate the relative attitude pointing between spacecrafts in the constellation. This article proposes a synchronization coordination control method based on state estimation feedforward and finite frequency domain optimization. The simulation results show that the synchronization and coordination tracking of the relative attitude between satellites can be achieved under configuration perturbations, and the jitter in the ultra-low frequency range can be effectively reduced. Especially in the frequency range below 10−3 Hz, the stability of the line of sight attitude pointing is improved from 10−6 rad/Hz1/2 to 10−9 rad/Hz1/2. The absolute attitude pointing performance and stability are greatly improved.

Spatio-temporal characterization of tightly focused femtosecond laser fields formed by paraboloidal mirrors with different F-numbers.

The focusing spatiotemporal property of a femtosecond laser pulse is presented under tight focusing conditions by using the frequency-resolved incident electric field and vector diffraction formulas with the wavefront correction term. In the ideal case, the focused laser intensity reaches its maximum at the F-number of ∼0.35 due to the strong diffraction effect under extremely tight focusing conditions. In spatio-temporal coupling distortion cases, their spatiotemporal Strehl ratios show a trend of improvement as the F-number decreases and this phenomenon is mainly concentrated along the y-direction. Based on the numerical calculation method used in this work, the precise information of tightly focused ultra-intense femtosecond laser fields can be obtained, which is crucial for assessing a focused intensity and describing the motion of charged particles under an extremely strong electric field. Moreover, the evolution law of focal fields with spatiotemporal distortions found in this paper can offer some theoretical guidance for realizing ultrahigh laser intensity in the near future.

In-Situ Observation and Analysis of the Evolution of Copper Aluminum Composite Interface

To study the micromorphology and dynamic evolution law of copper aluminum composite interface evolution, ultra-high temperature laser Confocal microscopy (CLSM) was used to observe and analyze the evolution of copper aluminum interface in situ, and then SEM, EDS and other advanced material analysis methods were used to observe the micromorphology of the composite layer, and study the composition of the interface layer and the formation process of the copper aluminum composite interface. The results indicate that the formation of the copper aluminum composite interface layer is mainly related to the mutual diffusion of copper aluminum atoms and the interface reaction between copper and aluminum. The bonding of the copper aluminum composite interface is mainly related to the melting of the metal surface of the interface layer and the mutual diffusion of copper aluminum atoms, which is the main mechanism of the copper aluminum composite interface bonding. The intermetallic compound is mainly Al2Cu. In situ, observation of copper aluminum composite interface shows that there is a clear and relatively flat boundary between copper and the interface layer, while the boundary between aluminum and the interface layer is not straight, which is caused by the difference in thermal expansion coefficient, Lattice constant and hardness between intermetallic compounds and matrix and between intermetallic compounds. At the same time, it was found that there is a certain relationship between the visual changes of the copper aluminum composite interface image and reaction-diffusion migration during in-situ observation using a confocal laser scanning high-temperature microscope. Moreover, under no pressure, the oxide layer and interface inclusions can seriously affect the interface bonding.

Optimum design of double-layer target for proton acceleration by ultrahigh intense femtosecond lasers considering relativistic rising edge.

Advances in laser technology have led to ever-increasing laser intensities. As a result, in addition to the amplified spontaneous emission and pedestal, it has become necessary to accurately treat the relativistic rising edge component. This component has not needed much consideration in the past because of its not relativistic intensity. In the previous study, a thin contamination layer was blown away from the target by the rear sheath field due to the relativistic rising edge component, and the target bulk was accelerated by the sheath field due to the main pulse. These indicated that the proton acceleration is not efficient in the target normal sheath acceleration by the ultrahigh intense femtosecond laser if the proton-containing layer is as thin as the contamination layer. Here we employ a double-layer target, making the second (rear) layer thick enough not to be blown away by the rising edge, so that the second layer is accelerated by the main pulse. The first layer is composed of heavy ions to reduce the total thickness of the target for efficient proton acceleration. We investigate an optimal design of a double-layer target for proton acceleration by the ultrahigh intense femtosecond laser considering the relativistic rising edge using two-dimensional particle-in-cell simulations. We also discuss how to optimize the design of such a double-layer target and find that it can be designed with two conditions: the first layer is not penetrated by hole boring, and the second layer is not blown away by the rising edge.

Radiation-dominated injection of positrons generated by the nonlinear Breit–Wheeler process into a plasma channel

Plasma acceleration is considered a prospective technology for building a compact multi-TeV electron–positron collider in the future. The challenge of this endeavor is greater for positrons than for the electrons because usually the self-generated fields from laser–plasma interaction are not well-suited for positron focusing and on-axis guiding. In addition, an external positron source is required, while electrons are naturally available in the plasma. Here, we study electron–positron pair generation by an orthogonal collision of a multi-PW laser pulse and a GeV electron beam by the nonlinear Breit–Wheeler process. We studied conditions favorable for positron deflection in the direction of the laser pulse propagation, which favors injection into the plasma for further acceleration. We demonstrate using the OSIRIS particle-in-cell framework that the radiation reaction triggered by ultra-high laser intensity plays a crucial role in the positron injection. It provides a suppression of the initial transverse momentum gained by the positrons from the Breit-Wheeler process. For the parameters used in this work, the intensity of at least 2.2×1023 W/cm2 is needed in order to inject more than 1% of positrons created. Above this threshold, the percentage of injected positrons rapidly increases with intensity. Moreover, subsequent direct laser acceleration of positrons in a plasma channel, using the same laser pulse that created them, can ensure a boost of the final positron energy by a factor of two. The positron focusing and guiding on the axis is provided by significant electron beam loading that changes the internal structure of the channel fields.

Spectrum Extreme Purification and Modulation of DBR Fiber Laser With Weak Distributed Feedback

Highly coherent lasers with spectral hyperpurity can further advance the fields of science and engineering. Especially, a light source with controllable frequency domain parameters can be adapted to a variety of emerging application scenarios. Herein, we report on an implementation of a novel ultra-high spectral purity distributed Bragg reflector (DBR) all-fiber laser based on weak distributed feedback, and its spectrum can be modulated in a certain extent by precisely controlling the intensity of the distributed feedback signal. Eventually, an ultra-high spectral purity laser with a spectral signal-to-noise ratio of 64 dB, a side mode suppression ratio (SMSR) of 83 dB, an output Lorentz linewidth of 115 Hz and a relative intensity noise of less than -122 dB/Hz is successfully obtained under normal conditions. Also, the frequency noise limit of the fiber laser with weak distributed feedback in the high frequency white noise flat region is 4.8 Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Hz, corresponding to a fundamental linewidth of ∼15.1 Hz. In particular, the SMSR and Lorentz linewidth of the laser can be continuously adjusted from 53 dB to 83 dB and 115 Hz to 8.2 kHz, respectively. In addition, we also investigated the effect of different pumping configurations on the performance of the DBR fiber laser. The proposed controllable mechanism of laser spectrum also provides a new perspective for extreme regulation of other laser parameters.

Microstructure and tribological performance of ultra-high speed laser-cladded AlCrCoFeNi2.1-xTiB2 (x = 0, 0.5, 1.0, 1.5 and 2.0 wt%) high-entropy alloy coatings

AlCoCrTeNi2.1- xTiB2 (x = 0, 0.5, 1.0, 1.5 and 2.0 wt%) high-entropy alloy (HEA) composite coatings were prepared on 304 stainless steel by ultra-high speed laser cladding. The microstructure, microhardness and tribological performance of HEA composite coatings were studied in detail. The results showed that HEA composite coatings all mainly consisted of BCC and FCC solid solution phase, and the addition of TiB2 didn’t change the phase structure of HEA composite coatings. The addition of TiB2 improves the microhardness of HEA composite coating. When x = 2.0 wt%, the microhardness of HEA composite coating reaches the maximum value (331.3 ± 5.0 HV), which is nearly 1.5 times that of 304 stainless steel. Accordingly, the wear resistance of the AlCoCrTeNi2.1-xTiB2 HEA composite coatings with TiB2 addition were improved, and the wear rate of AlCoCrTeNi2.1- xTiB2 HEA composite coatings showed a decreased tendency with the increase of x. However, the wear mechanism of HEA composite coatings are the same, mainly oxidation wear, adhesive wear and fatigue wear.

Mitigating fast thermal instability by engineered laser sweep in AlN soliton microcomb generation

Transient thermal instability represents a significant challenge in generating soliton microcombs. Fast laser sweep can be an efficient method to mitigate thermal instability, but it requires an ultrahigh laser sweep rate for crystalline microresonators with fast thermal relaxation. Here, we engineer a laser sweep waveform to generate AlN-on-sapphire soliton microcombs with an intermediate sweep speed (&lt;30 GHz/μs). Two laser sweep methods with backward plus forward tuning or two-step backward tuning added after the fast forward laser sweep were demonstrated to stabilize solitons. Reducing the soliton number is found to be useful to stabilize solitons in fast laser sweep. The effectiveness of the methods was numerically verified. Our measurements and simulations also reveal the impacts of different thermal relaxation processes occurring at quite different time scales on thermal instability. The requirement of the laser sweep protocols is discussed.

Polarization-isolated stretched-pulse mode-locked swept laser for 10.3-MHz A-line rate optical coherence tomography.

Stretched-pulse mode-locked (SPML) lasing based on a chirped fiber Bragg grating (CFBG) has proven to be a powerful method to generate wavelength-swept lasers at speeds of tens of megahertz. However, light transmitted through the CFBG may lead to undesirable laser oscillation that disrupts the mechanism of the laser active mode locking in the theta ring cavity. In this Letter, we demonstrate a simple and low-cost approach to suppress the transmitted light and achieve an effective duty cycle of ∼100% with only one CFBG and no need for intra-cavity semiconductor optical amplifier (SOA) modulation, extra-cavity optical buffering, and post amplification. By utilizing polarization isolation of the bi-directional CFBG, a swept laser centered at 1305 nm, with repetition rate of 10.3 MHz, optical power of 84 mW, and 3 dB bandwidth of 109 nm, is demonstrated. Ultrahigh speed 3D optical coherence tomography (OCT) structural imaging of a human palm in vivo using this swept laser is also demonstrated. We believe that this ultrahigh speed swept laser will greatly promote the OCT technique for industrial and biomedical applications.

Novel approach to TNSA enhancement using multi-layered targets—a numerical study

In the context of ion acceleration driven by ultra-high contrast lasers using thin foils, there is a clear trend towards increasing ion energy when the target thickness is reduced. However when the target is too thin and the prepulse strength is not negligible, this trend is reversed due to degradation of the target mainly caused by prepulse-induced shocks, among other effects (thermal plasma expansion, early onset of transparency, etc). In this paper, we propose and motivate the use of multi-layered targets for the purpose of enhancing the target normal sheath acceleration mechanism by means of attenuating the shock waves inside the target. It is demonstrated through hydrodynamic simulations that multi-layered targets, composed of alternating layers of plastic and gold, can significantly delay the time of shock wave breakout, reducing the shock energy that breaks out of the target and shortening the plasma scale-length. This approach paves the way for enhanced laser-driven ion acceleration using thinner targets even for relatively low contrast lasers.