Laser Pulse Waveform Research Articles

Control of laser pulse waveform in longitudinally excited CO2 laser by adjustment of excitation circuit

In a longitudinally excited CO2 laser that had a 45 cm-long discharge tube with a 1:1:2 mixture of CO2/N2/He gas at a pressure of 3.0 kPa, we realized the generation of a short laser pulse with a spike pulse width of about 200 ns and a pulse tail length of several tens of microseconds, control of the energy ratio of the spike pulse part to the pulse tail part in the short laser pulse, the generation of a long laser pulse with a pulse width of several tens of microseconds, and control of the pulse width in the long laser pulse, by using four types of excitation circuits in which the capacitance was adjusted. In the short laser pulse, the energy ratio was in the range 1:14–1:112. In the long laser pulse, the pulse width was in the range 25.7–82.7 μs.

Adiabatic-limit Coulomb factors for photoelectron and high-order-harmonic spectra

A momentum-dependent Coulomb factor in the probability for nonlinear ionization of atoms by a strong low-frequency laser field is calculated analytically in the adiabatic approximation. Expressions for this Coulomb factor, valid for an arbitrary laser pulse waveform, are obtained and analyzed in detail for the cases of linear and circular polarizations. The dependence of the Coulomb factor on the photoelectron momentum is shown to be significant in both cases. Using a similar technique, the Coulomb factor for emission of high-order harmonics by an atom in a bichromatic laser field is also calculated. In contrast to the case of a single-frequency field, for bichromatic fields the Coulomb factor depends significantly on the harmonic energy.

Spatiotemporal distributions of plasma and optical field during the interaction between ultra-short laser pulses and water nanodroplets

The transient changes of free electron density distribution and light field intensity during the interaction between the femtosecond Gaussian laser pulses and millimeter scale water droplets are studied. Based on the nonlinear Maxwell's equations and the ionization rate equation, a transient coupled model is proposed to describe the laser plasma produced in water droplet. The changes of electron density and light field with time are obtained by the finite element method. The calculation results show that the laser induced breakdown threshold in the droplet is about 2 TW/cm2, one quarter of that in a boundaryless water medium under the same condition. We find that the region of plasma generated in the droplet will move along the laser direction at first, however, when the incident laser intensity becomes larger, it will move in the direction opposite to the laser beam propagation and the plasma shielded effect becomes more obvious. The laser beam converged by the droplet focuses outside the droplet, and its power density is five times larger than that of the incident laser. There happen the laser pulse duration compression and waveform distortion at the focus point due to the plasma absorption, and the absorption energy increases with the laser intensity increasing and reaches a saturation finally. We expect the model and calculation results to be able to be used for the study of laser pulse propagation in cloud or rain, the precision control of droplet by laser or eye surgery by laser, and other laser technology applications.

The optimal control and dynamical analysis of high-order harmonic generation from atom

High-order harmonic generation (HHG) is a frequency up-conversion process in which an ultrafast femtosecond laser pulse at high intensity interacts with atom or molecule. HHG covers the wider wavelength spectral regions of extreme ultraviolet (XUV), soft X-ray and even hard X-ray. With the high photon flux, the HHG promises to be a attractive broadband tabletop light source, and a number of interesting applications of HHG have been demonstrated in many research fields. In this paper, we review some common methods and results on the optimal control of HHG to improve its low photon flux. HHG can now be controlled for different purposes, i.e., extending the cutoff energy, increasing the efficiency of the high-harmonic conversion process, and selecting single or range of harmonics, by combining the technologies of femtosecond temporal and spatial laser pulse shaping, waveform synthesizing and evolutionary algorithm, such as genetic algorithm and optimal control theory. Selective generation of a single harmonic can now be achieved both in hollow fiber, long gas cell and gas jet. We show that by using the newly developed waveform synthesis with two-color laser fields, the harmonic yield can be enhanced by more than one order without increasing the total laser energy, or the harmonic cutoff can be extended about two times without losing harmonic yield, even after considering the propagation effects. These progresses will exert a far-reaching impact on strong field physics. To optimize the HHG processes, a detailed understanding of the corresponding dynamics is essential. We also introduce a new time-frequency (TF) analysis technique, the synchrosqueezing transform (SST), which is used to reveal the quantum dynamics of HHG. Compared with the classical type of TF methods, such as the Gabor transform, the Morlet wavelet transform, the SST can be applied to explore the dynamical origin of near- and below-threshold harmonic emission.

Distortions of laser pulses in neodymium atomic vapor

An experiment on the simulation of the conditions for propagation of laser pulses in dense neodymium atomic vapor is proposed for the development of a technological setup for the laser separation of the 150Nd isotope from a kilogram sample of natural mixture. The comparison of waveforms of laser pulses in the presence and in the absence of absorption shows that the absorption predominantly takes place at the leading edge of the pulse. The experimental distortion of the waveform is related to the saturation of absorption. The effect is well described using the calculations based on the formalism of the rate equations.

Long-Term Stabilization of Carrier-Envelope Phase for Few Cycles Ti:Sapphire Laser Amplifier

We realize a long-term carrier-envelope phase (CEP) stabilization for a chirped pulse amplified Ti:sapphire laser by locking the oscillator and amplifier independently. Based on the measurement of CEP by employing f-to-2f interference technique between the octave-spanning spectrum which is generated from a rare gas filled hollow fiber, continuous locking time up to 7.2 h with 85 mrad fluctuation is demonstrated. Finely compensating the dispersion by a set of chirped mirrors, quasi-mono cycle pulses as shorter as 3.8 fs are obtained. Further experimental research on high harmonic generation dependence on CEP shown the waveform of laser pulses has been successfully controlled.

Molecular isomerization and fragmentation of polyatomic molecules controlled by inner-valence recollision-ionization

Control over various fragmentation reactions of a series of polyatomic molecules (acetylene, ethylene, 1,3-butadiene) by the optical waveform of intense few-cycle laser pulses is demonstrated experimentally. We show both experimentally and theoretically that the responsible mechanism is inelastic ionization from inner-valence molecular orbitals by recolliding electron wave packets.

Micro-welding of High Thermal Conductive Material Aluminum-Graphite Composite by Pulsed Nd:YAG Laser

The development of advanced materials with superior high thermal properties and high specific strength has led to new metal matrix composites (MMCs) as a great attractive material in electrical and electronic industries. In order to manufacture more practical component from MMCs, a tech-nique for joining MMCs to other similar composites or monolithic materials is strongly required. Therefore, the reliable and economic joining technique is investigated to increase the applications of MMCs. In this study, the overlap welding of pure aluminum and super thermal conductive (STC) aluminum-graphite composite was experimentally and numerically investigated by using a pulsed Nd:YAG laser. In order to discuss the welding of dissimilar materials with different thermophysical properties, the temporal change of heat input was controlled by arranging the laser pulse waveform. The porosities and bumps were observed as the remarkable weld defects in the welding process without a pulse control. On the other hand, the weld bead was largely free of defects, and a size of bump was relatively small with the appropriate controlled pulse waveform. It was clarified that the laser welded joint of an aluminum and a STC aluminum-graphite composite could be successfully achieved with the better weld penetration stability by the appropriate controlled pulse waveform.

Hidden identity in a generic Λ system: applications to coherent population trapping

A generic three-level Λ system is the source of a group of interesting quantum optical phenomena from coherent population trapping (CPT) to electromagnetic induced transparency. However, in contrast to its cousin, the two-level system, analytic solutions are difficult to acquire, except certain special cases. Here, we show that there exists a hidden identity in a generic Λ system as far as the rotating wave approximation is valid. This identity links the squares of the probability amplitudes in three states through the laser detuning phase (product of the detuning and time) and the Rabi phase. It is independent of the laser pulse wave forms and time delay. An application to CPT not only recovers all the previous asymptotic results, but also explains analytically why in the stimulated Raman adiabatic passage, the Stokes pulse must precede the pump pulse. More importantly, our results show that while a longer probe pulse is always favoured, a not too short probe pulse is still able to induce CPT because the time delay between the probe and coupling pulses can make up the difference in the formation of dark states. These can be directly tested experimentally.

(Sub-)femtosecond control of molecular reactions via tailoring the electric field of light

We review recent progress in the control over chemical reactions by employing tailored electric field waveforms of intense laser pulses. The sub-cycle tailoring of such waveforms permits the control of electron dynamics in molecules on sub-femtosecond timescales. We show that laser-driven electron dynamics in molecules has the potential to control chemical reactions. In the presence of strong fields, electron and nuclear motion are coupled, requiring models beyond the Born-Oppenheimer approximation for their theoretical treatment. Various mechanisms for the lightwave control of molecular reactions are described, and their relevance for the control of diatomic molecular reactions is discussed. Rapid experimental and theoretical progress is currently being made, indicating that attosecond controlled chemistry is within reach.

激光脉冲波形对铝合金焊接效果的影响

激光脉冲波形对铝合金焊接效果的影响

激光脉冲波形对铝合金焊接效果的影响

激光脉冲波形对铝合金焊接效果的影响

Attosecond-Recollision-Controlled Selective Fragmentation of Polyatomic Molecules

Control over various fragmentation reactions of a series of polyatomic molecules (acetylene, ethylene, 1,3-butadiene) by the optical waveform of intense few-cycle laser pulses is demonstrated experimentally. We show both experimentally and theoretically that the responsible mechanism is inelastic ionization from inner-valence molecular orbitals by recolliding electron wave packets, whose recollision energy in few-cycle ionizing laser pulses strongly depends on the optical waveform. Our work demonstrates an efficient and selective way of predetermining fragmentation and isomerization reactions in polyatomic molecules on subfemtosecond time scales.

Real-time measurement of ArF excimer laser corneal tissue ablation rates using cross-correlation of laser waveforms

The current popularity of excimer laser refractive surgery suggests a need for continued research and refinements to further improve clinical outcomes. A fundamental limitation of current clinical systems is the lack of real-time feedback specifically addressing the laser-tissue interactions as directly related to laser ablation rates. This paper reports data to assess the feasibility of a novel approach that holds promise as a real-time feedback scheme based on comparison of the incident and reflected laser pulse waveforms, as quantified using a cross-correlation algorithm. The approach is evaluated for ablation of bovine cornea over a range of clinically relevant laser fluences. A linear relationship was observed between several cross-correlation metrics and the directly measured corneal ablation rate, yielding an average RMS predictive error of 3.9% using a 25-shot average reflected waveform. Assessment of the cross-correlation approach for single-shot ablation data revealed a brief transient corresponding to the first few laser pulses, which is attributed to a slight hydration gradient near the surface of the de-epithelialized cornea. Clinical refractive data are necessary to assess the precision of this approach for actual refractive surgery.

Impact of picosecond laser pulse waveform on detection efficiency of gated-mode avalanche photodiodes for quantum key distribution

Detection efficiency is one of the key parameters that affect the performance of gated-mode avalanche photodiodes in quantum key distribution. In this paper, the impact of the waveform of faint picosecond laser pulses on detection efficiency is investigated both experimentally and analytically. Two kinds of laser pulses are attenuated to the single photon level to measure and plot single photon detection efficiency versus synchronous delay between the detector and the laser source. The results show that the narrower laser pulse with single peak can be detected with about 9 % higher detection efficiency and lead to a lower QBER than the pulse with subpeak and a broadened pedestal.

The influence of laser pulse waveform on laser–TIG hybrid welding of AZ31B magnesium alloy

By dividing laser pulse duration into two parts, three kinds of laser waveforms are designed, including a high power density pulse (HPDP) laser in a short duration set at the beginning of the laser waveform. This paper aims to find out the laser pulse waveform and idiographic critical values of HPDP, which can affect the magnesium penetration in laser–tungsten inert gas (TIG) hybrid welding. Results show that when the laser pulse duration of HPDP is not more than 0.4 ms, the welding penetration values of lasers with HPDP are larger than otherwise. Also, the welding penetration values of laser with HPDP have increased by up to 26.1%. It has been found that with HPDP, the laser can form the keyhole more easily because the interaction between laser and the plate is changed, when the TIG arc preheats the plate. Besides, the laser with high power density and short duration strikes on the plates so heavily that the corresponding background power can penetrate into the bottom of the keyhole and maintain the keyhole open, which facilitates the final welding penetration.

啁啾脉冲堆积宽带激光的时间与频谱特性分析

The dependence of the temporal wave-form and frequency spectrum of the broad-band laser pulse stacked by chirped pulses on the parameters of the sub-pulses is investigated theoretically and numerically.It is shown that,the temporal wave-form of the broad-band lasers stacked by the transform-limited pulses is usually smoothing,while that stacked by chirped pulses takes detrimental modulation.And the former consists of one spectrum line,while the latter consists of multiple spectrum lines.In addition,with the delay-time between neighboring sub-pulses increasing,the modulation riding on the broad-band laser pulse increases,and results in the spectrum lines of the output pulse to increase.The results can help in generating broad-band laser pulse of specific temporal wave-form and frequency spectrum by optimize parameters of the sub-pulses.

The LD end-pumped Q-switched solid-state laser design software based on the adaptive method

In the LD end-pumped acousto-optic (A-O) Q-switched solid-state laser, according to the requirements of users on the laser peak power, the pulse width, the repetition rate and the beam quality factor M 2, if the main parameters of the pumping source and the laser resonator can be pre-designed by a software, will effectively guide experiments, and save cost. A kind of laser design software is introduced in this paper, which is based on the adaptive adjustment method and using MATLAB/C mixed programming. According to the user requirements, a compact LD end-pumped A-O Q-switched Nd:YAG laser which peak power is 100-kW-class can be designed and actual outputs can be calculated based on the design results. Drawing the laser pulse waveform, analyzing of the thermal effect in laser gain medium and drawing the temperature distribution map, exporting and saving the data are the additional functions of this software.

Complete real-time temporal waveform characterization of single-shot few-cycle laser pulses

A method for complete characterization of the waveform of individual few-cycle laser pulses is presented. By analyzing the ``left'' and ``right'' asymmetries of high-energy photoelectrons along the polarization axis using the recently developed quantitative rescattering theory, we show that the carrier-envelope phase (CEP), pulse duration, and peak intensity of each single-shot pulse can be readily retrieved. By CEP tagging each laser shot, the method permits the study of waveform-dependent processes be extended to relativistic beams and to wavelengths, where CEP stabilization is not yet possible.

Nd3+- and Cr3+-Doped Yttrium Aluminum Garnet Ceramic Pulse Laser Using Cr4+-Doped Yttrium Aluminum Garnet Crystal Passive Q-Switch

A passively Q-switched Nd3+-and Cr3+-doped yttrium aluminum garnet (YAG) ceramic laser using Cr4+:YAG crystal as a Q-switch element is reported for the first time. Unique temporal waveforms of the output laser pulses were observed. A temporal waveform of a Q-switched laser pulse was compared with one in a free-running pulse. Laser pulses of 130–300 ns duration were generated. A maximum output laser energy of 1.1 J and an average peak power of 0.1 MW were obtained for multiple Q-switched pulses by a single shot, and a maximum output laser energy of 80 mJ was obtained for a single pulse. The evaluated maximum peak power was 0.6 MW as determined from measured pulse energy and duration. The effective saturation fluence and maximum stored energy were lower than those of Nd:YAG. However, a high peak power can be obtained using an output coupler with a low reflectivity owing to the high gain as in the case of Nd:YAG. The threshold of the Nd/Cr:YAG ceramic laser oscillator is reduced to less than 10% of that of a Nd:YAG oscillator. It was determined by calculation that the output laser energy of the Nd/Cr:YAG ceramic laser was close to 4 times that of Nd:YAG.