High dynamic range, large temporal domain laser pulse measurement

Theoretical work has been previously reported in which the full thermo-mechanical response of an absorber to an incident laser pulse has been calculated. For a laser pulse of any energy or duration, the temperature rise, explosive bubble formation, and shock wave generation in the surrounding medium can be predicted. This work allows the assessment of danger to biological or opto-electronic systems from laser pulses. The work also allows the thermo-mechanical properties of micro and nano particle absorbers, such as the thermal expansion coefficient and bulk modulus, to be calculated from measurements of the pressure waves generated in the medium. Previous results assumed a temporal profile for the laser pulse that was a square wave with infinitely fast rise and fall times. We report on new calculations that use a more realistic gaussian-like temporal profile for the laser pulse. We compare how the resulting thermo-mechanical responses are altered compared to the idealized temporal square wave laser pulse.

Numerical simulations with all-optical signal processing, e.g. switching by nonlinear fiber Bragg gratings (FBGs) via the optimization of pulse and grating parameters are presented. Using numerical solutions of nonlinear coupled mode equations temporal and spectral pulse profile distortions caused by the modulation instability are investigated and reviewed. The possibilities of elimination of the distortions via different input pulse widths, intensities and grating parameters (e.g. grating materials, FBG types and lengths) are suggested. Optimization is performed considering the input/output temporal and spectral pulse profile. In conclusion a balanced alternative is shown.

年 第 47 卷 第 3 期 目前影响高次谐波广泛应用的一个主要瓶颈 是它的光通量很低, 对于常用的 Ti:sapphire 激光 (800 nm) 而言谐波的转换效率通常小于 10 -5 [25][26][27] .若使用波长更长的红外激光可以有效提高光子的最 大能量 -截止位置, 但是转换效率急剧下降 (单原 子 ∼ λ -5 ).从实际应用的角度来看, 高次谐波的产生 需要解决以下几个问题 [28] : 提高转换效率、扩展截 止位置、选择性增强和抑制某阶或某一范围的谐波 (见图 1).实验中观察到的高次谐波通常都是在稠密的气 体中产生, 它不仅与单个原子在激光场中的响应 (诱 导偶极矩) 有关 [29]

Serial femtosecond X-ray crystallography of protein nanocrystals using ultrashort and intense pulses from an X-ray free-electron laser has proved to be a successful method for structural determination. However, due to significant variations in diffraction pattern quality from pulse to pulse only a fraction of the collected frames can be used. Experimentally, the X-ray temporal pulse profile is not known and can vary with every shot. This simulation study describes how the pulse shape affects the damage dynamics, which ultimately affects the biological interpretation of electron density. The instantaneously detected signal varies during the pulse exposure due to the pulse properties, as well as the structural and electronic changes in the sample. Here ionization and atomic motion are simulated using a radiation transfer plasma code. Pulses with parameters typical for X-ray free-electron lasers are considered: pulse energies ranging from 10(4) to 10(7) J cm(-2) with photon energies from 2 to 12 keV, up to 100 fs long. Radiation damage in the form of sample heating that will lead to a loss of crystalline periodicity and changes in scattering factor due to electronic reconfigurations of ionized atoms are considered here. The simulations show differences in the dynamics of the radiation damage processes for different temporal pulse profiles and intensities, where ionization or atomic motion could be predominant. The different dynamics influence the recorded diffracted signal in any given resolution and will affect the subsequent structure determination.

Profile cutting of thin sheet bright metals can be achieved in a cost effective manner through the use of a pulsed fiber laser in conjunction with a galvanometric scanhead without co-axial assist gas. This is attractive where many batches of distinct small parts are manufactured in volume, for instance within the jewellery and the micro-electronics industries.In this paper the capabilities of a pulsed fiber laser are considered in detail when processing bright metals such as brass, copper and silver. The effect of key laser parameters such as peak power, temporal pulse shape and laser pulse frequency will be considered along with several process parameters such as pulse overlap and beam wobbling techniques. This is with the aim of producing parts at high speed and/or at high quality and to determine the maximum thickness that can be cut at effective speeds. The results will be compared and contrasted with other metals such as stainless steel and aluminium to determine the range of processing strategies needed to cut these materials.Profile cutting of thin sheet bright metals can be achieved in a cost effective manner through the use of a pulsed fiber laser in conjunction with a galvanometric scanhead without co-axial assist gas. This is attractive where many batches of distinct small parts are manufactured in volume, for instance within the jewellery and the micro-electronics industries.In this paper the capabilities of a pulsed fiber laser are considered in detail when processing bright metals such as brass, copper and silver. The effect of key laser parameters such as peak power, temporal pulse shape and laser pulse frequency will be considered along with several process parameters such as pulse overlap and beam wobbling techniques. This is with the aim of producing parts at high speed and/or at high quality and to determine the maximum thickness that can be cut at effective speeds. The results will be compared and contrasted with other metals such as stainless steel and aluminium to determine the range of processing strategies ne...

Only recently researchers started to investigate the frequency conversion of high power picosecond pulses produced by regenerative amplification1. The high power densities easily achieved with short, pulses allow efficient generation of parametric fluorescence. However, experiments show that the efficiencies achieved in these systems are not as high as one would expect from a simple calculation, due to the temporal and spatial pulse shape, which has to be considered in a reasonably accurate model2,3. We performed calculations on the efficiency of the parametric conversion for various temporal and spatial pulse profiles and for different arrangements of a picosecond optical parametric generator (OPG) and optical parametric amplifier (OPA) combination. In the numerical simulation and the experiments we used a KTP crystal pumped with the fundamental of an all-solid-state Nd:YAG regenerative amplifier.

Dealing with saturation of the laser‐induced fluorescence signal: An application to lead atoms

Optical trepanning provides a drilling mechanism by using an annular beam. This paper investigates the temperature distribution in the workpiece due to pulsed annular laser beams. The annular beams allow numerous irradiance profiles to supply laser energy to the workpiece and thus provide more flexibility in affecting the hole quality than a solid laser beam. Such profiles include half Gaussian with maximum intensities at the inner and outer radii of the annulus, respectively, and full Gaussian with maximum intensity within the annulus. In addition to this spatial beam shaping, the temporal profile of the laser pulse can be shaped to improve the hole quality. The Hankel and Laplace transforms have been used to obtain an analytic solution for the temperature distribution in a semi-infinite workpiece. The effects of the temperature distribution on laser drilling are analyzed to design an efficient drilling process.Optical trepanning provides a drilling mechanism by using an annular beam. This paper investigates the temperature distribution in the workpiece due to pulsed annular laser beams. The annular beams allow numerous irradiance profiles to supply laser energy to the workpiece and thus provide more flexibility in affecting the hole quality than a solid laser beam. Such profiles include half Gaussian with maximum intensities at the inner and outer radii of the annulus, respectively, and full Gaussian with maximum intensity within the annulus. In addition to this spatial beam shaping, the temporal profile of the laser pulse can be shaped to improve the hole quality. The Hankel and Laplace transforms have been used to obtain an analytic solution for the temperature distribution in a semi-infinite workpiece. The effects of the temperature distribution on laser drilling are analyzed to design an efficient drilling process.

Since biological response to photodynamic therapy (PDT) depends on the light fluence distribution and photosensitizer concentration in the tissue, these two variables should ideally be measured noninvasively in individual cases. This can be reduced to determining the optical absorption and transport scattering coefficients of the tissue because, if these two parameters are known, the fluence distribution can be calculated and the photosensitizer concentration can be deduced from its characteristic contribution to the absorption spectrum. The temporal spreading of a picosecond laser pulse as it propagates through tissue carries infonnation about both the scattering and absorption properties of the tissue. A mathematical model is presented which allows derivation of the interaction coefficients from the pulse shape, and preliminary experiments are reported which demonstrate the potential of these techniques in PDT dosimetry. Equivalent information can be obtained in the frequency domain by using modulated light sources andmeasuring thephase and modulation of the detected light. Analytical expressions are developed for these observable quantities in terms ofthe optical interaction coefficients. Particular auention is drawn to the potential oflow (less than 200 MHz) frequency measurements as these can be made with relatively simple instrumentation.

ABSTRACT. A developed temporal focusing-based multiphoton excitation system with additional patterned excitation and local laser control can provide high-throughput fabrication of three-dimensional gray-level biomicrostructures via two-photon cross-linking with rose bengal (RB) as the photoactivator. Multiple bovine serum albumin (BSA) structures of different concentrations were simultaneously achieved by selecting different pulse numbers in the designated regions with an appropriate femtosecond laser power within a few seconds. Intensity of the RB two-photon excited fluorescence (TPEF) is correlated to the concentration of the fabricated cross-linked BSA microstructure. Hence, the fabricated BSA microstructure can be monitored online by utilizing the RB TPEF as contrast agent. It suggests that this approach with its unique capability of high-speed, gray-level, and online-inspection fabrication meets the requirements of the biomedical researches involved in extracellular matrix.

Beam optimization of laser‐accelerated protons is required to progress towards the development of applications, e.g. for fast ignition of fusion targets or dense plasma radiography. For this, three areas of improvement need to be pursued: increasing the maximum proton energy, enhancing the laser‐to‐protons conversion efficiency and reducing the beam divergence. Here we report on the reasoning which led us to envision limited size target to improve protons acceleration. We point out that pre plasma leakage from the target front side is a critical issue to allow efficient acceleration from such limited size targets and show that with very high temporal contrast laser pulses proton acceleration is improved in such targets.

We present theory and numerical analysis which demonstrate laser-pulse compression in a collisional plasma under the weak-relativistic ponderomotive nonlinearity. Plasma equilibrium density is modified due to the ohmic heating of electrons, the collisions, and the weak relativistic-ponderomotive force during the interaction of a laser pulse with plasmas. First, within one-dimensional analysis, the longitudinal self-compression mechanism is discussed. Three-dimensional analysis (spatiotemporal) of laser pulse propagation is also investigated by coupling the self-compression with the self-focusing. In the regime in which the laser becomes self-focused due to the weak relativistic-ponderomotive nonlinearity, we provide results for enhanced pulse compression. The results show that the matched interplay between self-focusing and self-compression can improve significantly the temporal profile of the compressed pulse. Enhanced pulse compression can be achieved by optimizing and selecting the parameters such as collision frequency, ion-temperature, and laser intensity.

The temporal profiles of ultrashort laser pulses scattered in random media with different experimental geometries were measured and analyzed by using the photon flux and the photon density predicted by the diffusion theory. The scattered laser pulse profiles detected by an optical fiber inside an infinite random medium are found to be described by the photon density. For a semi-infinite medium, the scattered pulse profile emitted from the surface of the medium can be described by either the photon density or the photon flux. These results can be consistently explained by the diffuse intensity (radiance) of transport theory.

We develop a splicing technology of Ti:sapphire crystals for a high-energy chirped pulse amplifier laser system that can suppress the parasitic lasing to improve the amplification efficiency compared to a large-size single Ti:sapphire crystal amplifier. Theoretical investigations on the characteristics of the amplifier with four splicing Ti:sapphire crystals, such as parasitic-lasing suppression and amplification efficiencies, are carried out. Some possible issues resulting from this splicing technology, including spectral modulation, stretching or splitting of the temporal profile, and the sidelobe generation in the spatial domain (near field and far field), are also investigated. Moreover, the feasibility of the splicing technology is preliminarily demonstrated in an experiment with a small splicing Ti:sapphire crystals amplifier. The temporal profile and spatial distribution of the output pulse from the splicing Ti:sapphire crystal amplifier are discussed in relation to the output pulse from a single Ti:sapphire crystal amplifier.

The production of high-energy ultra-short laser pulses in gas-filled hollow fibres is now a well-established technique. Below a specific critical power, the smoothness of spatial and temporal profiles of input pulses is not perturbed during propagation. However, gas ionization and pulse filamentation occur above that critical power strongly disrupting the pulse profiles. Both propagation regimes in argon are numerically studied for two different propagation configurations: free gas and gas filled hollow fibre. Below critical power it is shown the pulse is spectrally broadened while maintaining smooth temporal and spatial profiles. Otherwise, these quantities show strong variations.