Parallel Laser Research Articles

Angular-dependent asymmetries of above-threshold ionization in a two-color laser field

We measure photoelectron momentum spectra from above-threshold ionization of Xe atoms in a linearly polarized two-color laser field with comparable intensities. The spectra show distinct forward-backward asymmetries along the laser polarization direction, which depend sensitively on the electron emission angle and the relative phase between the two-color components. We find that with increasing the emission angle relative to the laser polarization direction, there is a phase shift in the asymmetry curve as a function of the two-color relative phase. Using a semiclassical model, we disentangle the relative contributions of the nonscattering and the rescattering trajectories on the photoelectron angular distributions of the above-threshold ionization. We show that the angular-dependent asymmetry comes from different contributions of the nonscattering and rescattering trajectories at different emission angles. By tuning the relative phase of the parallel two-color laser fields, the relative contributions of the nonscattering and the rescattering trajectories can be precisely controlled.

A review on measurement techniques of apparent thermal conductivity of nanofluids

Thermal conductivity of nanofluids has been extensively studied for a number of years because it is a very first evaluation of the heat transfer performance of nanofluids. However, not the single theoretical model predicts thermal conductivity of nanofluids accurately. Hence, different measurement techniques have been used to measure thermal conductivity of nanofluids. This paper focuses on different measurement techniques of thermal conductivity of nanofluids. The working principle, limitation and advantages of different measurement techniques have been discussed. The measurement techniques discussed in this paper included transient hot wire, transient plane source, 3-omega technique, steady-state parallel method, thermal comparator and laser flash method. Eventually, some suggestions have been made for improving the reliability of the measurement of thermal conductivity.

850 nm Vertical-Cavity Surface-Emitting Laser Arrays With Enhanced High-Speed Transmission Performance Over a Standard Multimode Fiber

The functionality of novel parallel and series high-speed vertical-cavity surface-emitting laser (VCSEL) arrays, which can greatly relax the tradeoff between output power and modulation speed, is demonstrated. Both types of array structure allow improvement in the output power with no degradation in their maximum modulation speed as compared to a single reference unit. The observed invariant electrical–optical bandwidth shown for these array structures is mainly due to the effective reduction of the differential resistance and parasitic capacitance, arising from the Zn-diffusion and oxide-relief apertures fabricated in the VCSEL unit. This in turn minimizes the degradation in the RC -limited bandwidth with the VCSEL arrays. Furthermore, the dense packing of single VCSELs, with Zn-diffusion apertures for optical mode control, minimizes the coupling loss between the output from the array into a standard multimode fiber (MMF). Compared with the single VCSEL unit, the parallel VCSEL array shows a significant enhancement in transmission performance over a standard OM4 MMF, which includes a larger eye margin, a higher signal-to-noise ratio, as well as a higher error-free data rate (48 versus 44 Gbit/sec). The device modeling technique is used to perform device analysis. From the results we can conclude that the improvement with the parallel array is because the value of the internal impedance is closer to the 50-Ω signal source, which minimizes the microwave reflection induced timing jitter in the eye-patterns.

Measurement of six degree motion error of linear stage

In this paper, we propose measurement system of the six degree of motion errors which is based on distance measurement by the laser interferometer. The system has six parallel laser beams and six corner cube mirrors on the linear stage, which reflect the corresponding laser beams. The error of axial direction is measured with the ordinary distance measurement method by laser interferometer. The vertical errors to the axial direction and the roll errors are measured by tilted beams using the wedge prism. The pitch and yaw errors are measured by the difference between distance of two corner cube mirrors. The experimental layout of the corner cube mirrors and the other optical devices are shown. As a result, the resolution of 66.5 nm for error of the axial direction, the resolution of 383 nm for the vertical errors to the axial direction, the resolution of 1.23 arc-sec for the roll error and the resolution of 0.429 arc-sec for the pitch and yaw errors are obtained in this system.

Parallel femtosecond laser light sheet micro-manufacturing based on temporal focusing

We present a parallel depth resolved laser fabrication technique for micro- and nano-machining metal substrates based on temporal focusing. In this system, the spectrum of a femtosecond laser pulse is first spatially separated by a digital micromirror device, which simultaneously serves as a diffraction grating and a programmable binary mask. After collimation and beam flattening, an objective lens recombines the spectrum to the focal region, forming a high-intensity, depth resolved light sheet for laser micromachining. The light sheet technology enables parallel fabrication of highly uniform micro-structures of close to diffraction-limited resolution. Experimental results demonstrate high-resolution (~800nm) direct area patterning on various metal substrates, e.g., nickel and copper, over an area of ~100×60µm2 within tens of laser pulses. The relationship among material removal rate, surface flatness, laser power, and number of pulses have been experimentally studied; the results suggest the application of higher power with fewer number of laser pulses produce microstructures of better surface quality. The light sheet technology substantially improves the throughput of ultrafast laser machining, enabling direct area patterning without compromising the resolution.

Body growth and life history in wild mountain gorillas (Gorilla beringei beringei) from Volcanoes National Park, Rwanda.

Great apes show considerable diversity in socioecology and life history, but knowledge of their physical growth in natural settings is scarce. We characterized linear body size growth in wild mountain gorillas from Volcanoes National Park, Rwanda, a population distinguished by its extreme folivory and accelerated life histories. In 131 individuals (0.09-35.26 years), we used non-invasive parallel laser photogrammetry to measure body length, back width, arm length and two head dimensions. Nonparametric LOESS regression was used to characterize cross-sectional distance and velocity growth curves for males and females, and consider links with key life history milestones. Sex differences became evident between 8.5 and 10.0 years of age. Thereafter, female growth velocities declined, while males showed increased growth velocities until 10.0-14.5 years across dimensions. Body dimensions varied in growth; females and males reached 98% of maximum body length at 11.7 and 13.1 years, respectively. Females attained 95.3% of maximum body length by mean age at first birth. Neonates were 31% of maternal size, and doubled in size by mean weaning age. Males reached maximum body and arm length and back width before emigration, but experienced continued growth in head dimensions. While comparable data are scarce, our findings provide preliminary support for the prediction that mountain gorillas reach maximum body size at earlier ages compared to more frugivorous western gorillas. Data from other wild populations are needed to better understand comparative great ape development, and investigate links between trajectories of physical, behavioral, and reproductive maturation.

Tomographic imaging of reacting flows in 3D by laser absorption spectroscopy

This paper describes the development of an infrared laser absorption tomography system for the 3D volumetric imaging of chemical species and temperature in reacting flows. The system is based on high-resolution near-infrared tunable diode laser absorption spectroscopy (TDLAS) for the measurement of water vapour above twin, mixed fuel gas burners arranged with an asymmetrical output. Four parallel laser beams pass through the sample region and are rotated rapidly in a plane to produce a wide range of projection angles. A rotation of 180° with 0.5° sampling was achieved in 3.6 s. The effects of changes to the burner fuel flow were monitored in real time for the 2D distributions. The monitoring plane was then moved vertically relative to the burners enabling a stack of 2D images to be produced which were then interpolated to form a 3D volumetric image of the temperature and water concentrations above the burners. The optical transmission of each beam was rapidly scanned around 1392 nm and the spectrum was fitted to find the integrated absorbance of the water transitions and although several are probed in each scan, two of these transitions possess opposite temperature dependencies. The projections of the integrated absorbances at each angle form the sinogram from which the 2D image of integrated absorbance of each line can be reconstructed by the direct Fourier reconstruction based on the Fourier slice theorem. The ratio of the integrated absorbances of the two lines can then be related to temperature alone in a method termed, two-line thermometry. The 2D temperature distribution obtained was validated for pattern and accuracy by thermocouple measurements. With the reconstructed temperature distribution, the temperature-dependent line strengths could be determined and subsequently the concentration distribution of water across the 2D plane whilst variations in burner condition were carried out. These results show that the measurement system based on TDLAS can be used for 2D temporal or 3D volume imaging of temperature and chemical species concentration in reacting flows.

Eye-safe Nd:LuAG ceramic lasers

Diode-pumped continuous-wave Nd:LuAG ceramic lasers at the 1.4-μm eye-safe spectral region are reported. A simultaneous dual-wavelength laser at 1418 nm and 1442 nm is achieved with a maximum output power of 1.37 W and a slope efficiency of 13.5% using a plane-concave cavity configuration. Laser generation at another emission line of 1432 nm is also realized with a maximum output power of 0.82 W with the help of an intracavity etalon. To improve the laser beam quality, a parallel plane laser cavity operating still at 1418 nm and 1442 nm is adopted in free-running mode.

Exploration of strong-field double ionisation of a A2B molecule in parallel and perpendicular laser fields

ABSTRACTBy using classical ensemble method, we investigate the double ionisation of a A2B model molecule in parallel and perpendicular laser fields, respectively. We presented three two-electron model of A2B molecule with different bond angles, respectively. The numerical results show that the double ionisation probability of A2B model molecule is the same in parallel and perpendicular laser fields with bond angle π/3, but for the 2π/3 case, the double ionisation of the A2B molecule in parallel field is larger than that in perpendicular case, and the difference becomes even larger of A2B molecule with π case. Besides, the ‘finger-like’ structure along x-axis at about 150 TW/cm2 is obvious with bond angle 2π/3 and π cases in the perpendicular electric field. The corresponding momentum distribution of A2B molecule with different bond angles also presented in parallel and perpendicular laser fields, respectively. The above phenomenon are also demonstrated by analysing the energy distributions of double-ionised electrons versus time and corresponding trajectories.

Volumetric Generation of Optical Vortices with Metasurfaces

Recent advances in metasurfaces, i.e., two-dimensional arrays of engineered nanoscale inclusions that are assembled onto a surface, have revolutionized the way to control electromagnetic waves with ultrathin, compact components. The generation of optical vortex beams, which carry orbital angular momentum, has emerged as a vital approach to applications ranging from high-capacity optical communication to parallel laser fabrication. However, the typically bulky elements used for the generation of optical vortices impose a fundamental limit toward on-chip integration with subwavelength footprints. Here, we investigate and experimentally demonstrate a three-dimensional volumetric optical vortices generation based on light–matter interaction with a high-efficiency dielectric metasurface. By employing the concepts of Dammann vortex gratings and spiral Dammann zone plates, a 3D optical vortex array with micrometer spatial separation is achieved from visible to near-infrared wavelengths. Importantly, we show that...

Evaporation of Gasoline-Like Sprays from an Outwards-Opening Injector Studied with LIEF

The design and operation of the fuel injection system plays an important role to achieve a suitable fuel distribution in the cylinder. The liquid fuel should evaporate and mix well with air, and avoid overly rich or lean zones as well as wall and piston wetting. Spray imaging in a constant-volume, constant-pressure chamber, provides a well-controlled environment for detailed characterization of spray development under different operational conditions. In this study planar laser-induced exciplex fluorescence is used since it enables separate imaging of both vapor and liquid phase of fuel simultaneously. As a surrogate fuel for gasoline n-hexane is used with fluorobenzene and diethylmethylamine added as exciplex-forming tracers. Fluorescence excitation is carried out with a parallel laser sheet from the fourth harmonic light of a Nd: YAG-laser (266 nm). Exciplex fluorescence images from liquid phase and monomer fluorescence images from vapor phase can be acquired by a single UV-sensitive CMOS camera equipped with a stereoscope having filters selectively transmitting monomer fluorescence at 295 nm and exciplex fluorescence at 355 nm. Sprays from an outwards-opening injector with an initial hollow-cone shape were investigated at gas pressures above atmospheric. The expanding conical fuel sheet breaks up and a torus-shaped vortex structure is formed with the vortices continuing to expand after the end of injection, which is the typical development using this type of injector. Fuel vapor is firstly observed at the same locations as the liquid drops, and is then accumulated into the center of the vortices. The fuel evaporation was found to be very slow at room temperature, whereas the vapor fraction was significant already at the end of injection above 40°C, and an even faster evaporation was observed above 55°C.

Gas dynamics and combustion wave expanding velocity of laser induced plasma

Fused silica is an indispensable basic element in a laser system and the weakest link in all components. When the laser interacts with fused silica, the target absorbs the laser energy so that its own temperature rises, and then it melts and vaporizes. The vaporization of the target gasification further absorbs the laser energy and produces a low density ionization reaction, resulting in the laser supported combustion wave (LSCW) phenomenon. In this paper, taking into account the effects of temperature residual, change in target morphology, distribution of splash material, and distribution of target surface airflow condition, we model and simulate the process of LSCW in stages. The laser energy transfer process, including the inverse bremsstrahlung radiation, thermal radiation, heat conduction and convection processes, is simulated by establishing a two-dimensional axisymmetric gas dynamic model. In addition, the LSCW in the visible light band has a strong radiation characteristic, which is significantly different from the laser induced target melting and vaporization phenomenon. The LSCW is easily received and displayed by high-speed camera. Therefore, a shadow system is established to measure the expanding velocity of the combustion wave in the process of fused silica damaged by laser, and the evolution process image of the combustion wave is obtained. The results show that under the action of parallel laser beam, the propagation of the combustion wave is in a steady-state and the gas dynamic behavior is stable. For the pulse widths of 1 ms and 3 ms, the average propagation velocity of the LSCW is calculated to be about 24 m/s, which is consistent with the experimental result in the literature available. This verifies the correctness of our theoretical model. For the pulse width of 3 ms, the average velocity of the flow field near the wavefront is calculated to be about 200 m/s. The numerical relationship between the velocity of the flow field and the propagation velocity of the LSCW is also basically consistent with the theoretical derivation result. Under the action of focused laser beam, the propagation of the combustion wave is unsteady. For the pulse widths of 1 ms, the laser intensity at the front of the plasma decreases gradually and the beam radius becomes larger. For the pulse width of 1.8 ms, both a similar pattern of mushroom cloud in the combustion wave and turbulence are observed, which is basically consistent with the evolution process of the combustion wave appearing in our experiment. The simulation results are in good accordance with the experimental results, and also provide a theoretical and experimental basis for studying the LSCW of fused silica.

Thermoluminescent response of C-modified Al2O3 thin films deposited by parallel laser ablation plasmas

Aluminium oxide thin films modified with different amounts of carbon were prepared using a parallel laser ablation plasmas configuration. The effect of the amount of carbon incorporated in the films on their compositional, morphological, structural, and thermoluminescent properties was studied. The results showed that films with different C content, from 11 to 33 at. %, were obtained. The structural characterization revealed the growth of an amorphous material. Surface morphology of the obtained thin films showed smooth surfaces. The films were exposed to UV and gamma radiation (Co-60) in order to study their thermoluminescence response. The results tend to indicate that carbon incorporation into the alumina favours the increase of a high temperature TL peak.

METHOD FOR DETERMINATION OF FOCAL PLANE LOCATION OF FOCUSING COMPONENTS

Mass-production of different laser systems often requires utilization of the focal spot size method for determination of output laser beam spatial characteristics. The main challenge of this method is high accuracy maintenance of a CCD camera beam profiler in the collecting lens focal plane. The aim of our work is development of new method for placing of photodetector array in the collecting lens focal plane with high accuracy. Proposed technique is based on focusing of several parallel laser beams. Determination of the focal plane position requires only longitudinal translation of the CCD-camera to find a point of laser beams intersection. Continuous-wave (CW) diode-pumped laser emitting in the spectral region near 1μm was created to satisfy the requirements of the developed technique. Designed microchip laser generates two stigmatic Gaussian beams with automatically parallel beam axes due to independent pumping of different areas of the one microchip crystal having the same cavity mirrors. It was theoretically demonstrated that developed method provides possibility of the lenses focal plane determination with 1 % accuracy. The microchip laser generates two parallel Gaussian beams with divergence of about 10 mrad. Laser output power can be varied in the range of 0.1–1.5 W by changing the pumping laser diode electrical current. The distance between two beam axes can be changed in the range of 0.5–5.0 mm. We have proposed method for determination of positive lens focal plane location by using of CCDcamera and two laser beams with parallel axes without utilization of additional optical devices. We have developed CW longitudinally diode pumped microchip laser emitting in the 1-μm spectral region that can be used in the measuring instrument that doesn’t require precision mechanical components for determination of focal plane location with 1 % accuracy. The overall dimensions of laser head was 70 × 40 × 40 mm3 and maximum power consumption was 7W per one laser beam.

激光同向切割单层碳纤维复合材料的温度场模拟

激光同向切割单层碳纤维复合材料的温度场模拟

Comparative analysis of serial and parallel laser patterning of Ag nanowire thin films

Ag nanowire (AgNW) films solution-coated on a glass substrate were laser-patterned in two different ways. For the conventional serial process, a pulsed ultraviolet laser of 30kHz repetition rate and ∼20ns pulse width was employed as the laser source. For parallel patterning, the film was directly irradiated by a spatially-modulated Nd:YAG laser beam that has a low repetition rate of 10kHz and a shorter pulse width of 5ns. While multiple pulses with energy density ranging from 3 to 9J/cm2 were required to pattern the film in the serial process, a single pulse with energy density of 0.16J/cm2 completely removed AgNWs in the parallel patterning. This may be explained by the difference in patterning mechanism. In the parallel process using short pulses of 5ns width, AgNWs can be removed in their solid state by the laser-induced thermo-elastic force, while they should be evaporated in the serial process utilizing a high-repetition rate laser. Important process parameters such as threshold energy density, speed, and available feature sizes are comparatively discussed for the two patterning

Optical turbulence in confined media: part I, the indoor turbulence sensor instrument.

Optical system performances can be affected by local optical turbulence created by its surrounding environment (telescope dome, clean room, atmospheric surface layer). We present our new instrument INdoor TurbulENce SEnsor (INTENSE) dedicated to this local optical turbulence characterization. INTENSE consists of using several parallel laser beams separated by non-redundant baselines between 0.05 and 2.5m and measuring the angle of arrival fluctuations from spot displacements on a CCD. After introducing the theoretical background, we give a description of the instrument including a detailed characterization of instrumental noise and, finally, give the first results for the characterization of the turbulence inside clean rooms for optical systems studies.

Massively parallel femtosecond laser processing.

Massively parallel femtosecond laser processing with more than 1000 beams was demonstrated. Parallel beams were generated by a computer-generated hologram (CGH) displayed on a spatial light modulator (SLM). The key to this technique is to optimize the CGH in the laser processing system using a scheme called in-system optimization. It was analytically demonstrated that the number of beams is determined by the horizontal number of pixels in the SLM NSLM that is imaged at the pupil plane of an objective lens and a distance parameter pd obtained by dividing the distance between adjacent beams by the diffraction-limited beam diameter. A performance limitation of parallel laser processing in our system was estimated at NSLM of 250 and pd of 7.0. Based on these parameters, the maximum number of beams in a hexagonal close-packed structure was calculated to be 1189 by using an analytical equation.

Intensity and angle-of-arrival spectra of laser light propagating through axially homogeneous buoyancy-driven turbulence.

Frequency spectra obtained from the measurements of light intensity and angle of arrival (AOA) of parallel laser light propagating through the axially homogeneous, axisymmetric buoyancy-driven turbulent flow at high Rayleigh numbers in a long (length-to-diameter ratio of about 10) vertical tube are reported. The flow is driven by an unstable density difference created across the tube ends using brine and fresh water. The highest Rayleigh number is about 8×109. The aim of the present work is to find whether the conventional Obukhov-Corrsin scaling or Bolgiano-Obukhov (BO) scaling is obtained for the intensity and AOA spectra in the case of light propagation in a buoyancy-driven turbulent medium. Theoretical relations for the frequency spectra of log amplitude and AOA fluctuations developed for homogeneous isotropic turbulent media are modified for the buoyancy-driven flow in the present case to obtain the asymptotic scalings for the high and low frequency ranges. For low frequencies, the spectra of intensity and vertical AOA fluctuations obtained from measurements follow BO scaling, while scaling for the spectra of horizontal AOA fluctuations shows a small departure from BO scaling.

Design and fabrication of a diffractive beam splitter for dual-wavelength and concurrent irradiation of process points

We report on a dual-wavelength diffractive beam splitter designed for use in parallel laser processing. This novel optical element generates two beam arrays of different wavelengths and allows their overlap at the process points on a workpiece. To design the deep surface-relief profile of a splitter using a simulated annealing algorithm, we introduce a heuristic but practical scheme to determine the maximum depth and the number of quantization levels. The designed corrugations were fabricated in a photoresist by maskless grayscale exposure using a high-resolution spatial light modulator. We characterized the photoresist splitter, thereby validating the proposed beam-splitting concept.