Rectangular Laser Pulse Research Articles

Development of Figure‐of‐Nine Laser Cavity for Mode‐Locked Fiber Lasers: A Review

AbstractArtificial saturable absorbers (SA) are nonlinear optical devices widely employed in mode‐locked lasers. Among the artificial SAs are nonlinear polarization evolution (NPE) and nonlinear amplifying loop mirrors that can work in either figure‐of‐eight (Fo8) or figure‐of‐nine (Fo9) laser cavities. The NPE technique is highly sensitive to environmental perturbations. Both Fo8 and Fo9 laser cavities exhibit a higher environmental stability than the NPE technique. Here the recent advances of the Fo9 laser cavity, with a focus on the pulse formation mechanisms and the role of different cavity parameters that can enable ultrafast mode‐locking analyses are discussed. Besides, the recent development of using the Fo9 laser cavity to generate high‐energy rectangular laser pulses through either dissipative soliton resonance or noise‐like pulse regimes is also reviewed. In conclusion, the current issues and challenges of pulse formation through the Fo9 laser cavity are highlighted and recommendations for future research directions are proposed. This review is expected to provide a deeper insight into the Fo9 laser cavity as the next generation of artificial SA with interesting cavity structures, laser features, and output performances.

A scalar field theory of 1+1-dimensional laser wakefield accelerators

Abstract A relativistic non-linear scalar field theory is developed from a 2+2-dimensional decomposition of the cold plasma field equations, and the theory is used to investigate a 1+1-dimensional description of a laser wakefield accelerator. The relationship between the properties of a compact laser pulse and its wake is explored. Non-linear solutions are sought describing a regular (i.e. unbroken) wake driven by a prescribed rectangular circularly-polarised laser pulse. An upper bound on the dimensionless amplitude a 0 of the laser pulse is determined as a function of the phase speed v of the wake. The asymptotic behaviour of the upper bound on a 0 as v → c is shown to agree with well-established, but approximate, results obtained using the conventional encoding of the plasma degrees of freedom. Our approach leads to a closed-form expression for the upper bound on a 0 which is exact for all values of the phase speed of the wake, unlike conventional results that are applicable only when v is sufficiently close to c.

Photothermal defect imaging in hybrid fiber metal laminates using the virtual wave concept

This study presents photothermal imaging results of subsurface material defects within fiber metal laminates utilizing the virtual wave concept. Therefore, we theoretically analyze the propagation of the virtual wave signal in a hybrid composite laminate via the method of images. For provoking local material damage, the hybrid composite sample is subjected to a defined impact loading. The results obtained from photothermal defect imaging, utilizing rectangular laser pulse excitation, are compared with results obtained from 3D x-ray computed tomography. To sum up, we demonstrate a fast, non-invasive, and easily interpretable reconstruction of defects within macroscopic hybrid composite laminates based on the virtual wave concept.

Determination of Rate Constants and the Mechanism of Phototransformations of Metalloporphyrins by Solving the Inverse Photokinetic Problem

Fluorescence kinetics of some substituted Zn- and Mg-porphyrins in solid polymeric films upon excitation by two-step-wise rectangular laser pulses and at 293 K are studied by methods of the inverse kinetic problem. The experimental nonmonotonic kinetic curves are approximated by simulated ones within the framework of a six-level energy scheme describing reversible interconversions of two complexes. The maximum correspondence between experimental and simulated curves is obtained by iterative optimization using the Nelder–Mead algorithm. Based on statistically estimated values of rate constants and parameters of the considered models, an interpretation of the observed kinetics is presented and a conclusion is drawn that the interconversions are caused by the process of reversible extraligation of the central metal ion; the process occurs in the excited triplet T1 state.

Определение констант скоростей и механизма фотопревращений металлопорфиринов методом решения обратной фотокинетической задачи

The fluorescence kinetics of some substituted Zn- and Mg-porphyrins in solid polymer films upon excitation by two-step-wise rectangular laser pulses and 293 K were investigated by the inverse kinetic problem methods. The experimental non-monotonic kinetic curves were approximated by the simulated ones within the framework of a six-level energy scheme describing the reversible interconversions between the two complexes. The best fit between the experimental and simulated curves was obtained by iterative optimization using the Nelder-Mead algorithm. Based on the statistically estimated values ​​of the rate constants and parameters of the considered models, an interpretation of the observed kinetics was given and the conclusion drawn that the interconversions are caused by the process of reversible extra-liganding of the central metal ion, which occurs in the excited triplet T<sub>1</sub> state.

Characterization of Thermal Damage Due to Two-Temperature High-Order Thermal Lagging in a Three-Dimensional Biological Tissue Subjected to a Rectangular Laser Pulse.

The use of lasers and thermal transfers on the skin is fundamental in medical and clinical treatments. In this paper, we constructed and applied bioheat transfer equations in the context of a two-temperature heat conduction model in order to discuss the three-dimensional variation in the temperature of laser-irradiated biological tissue. The amount of thermal damage in the tissue was calculated using the Arrhenius integral. Mathematical difficulties were encountered in applying the equations. As a result, the Laplace and Fourier transform technique was employed, and solutions for the conductive temperature and dynamical temperature were obtained in the Fourier transform domain.

Anapole: Its birth, life, and death.

Despite the recent extensive study of the nonradiating (anapole) mode in the resonant light scattering by nanoparticles, the key questions, about the dynamics of its excitation at the leading front of the incident pulse and collapse behind the trailing edge, still remain open. We answer the questions, first, by direct numerical integration of the complete set of the Maxwell equations, describing the scattering of a rectangular laser pulse by a dielectric cylinder. The simulation shows that while the excitation and the collapse periods, both have the same characteristic time-scale, the dynamics of these processes are qualitatively different. The relaxation to the steady-state scattering at the leading front is accompanied by high-amplitude oscillatory modulations of the envelope of the basic electromagnetic oscillations, while behind the trailing edge the decay of the envelope is monotonic. Then, we present the general arguments showing that this is the case for the anapole excited in any classical system. Next, we introduce a simple, exactly integrable yet accurate, physically transparent model describing the dynamics of the anapole. The model admits generalization to a broad class of resonant phenomena and may be regarded as a compliment to the commonly used Temporal Coupled-Mode Theory.

Three-dimensional biological tissue under high-order effect of two-temperature thermal lagging to thermal responses due to a laser irradiation

Using the lasers and heat transfer through biological skin tissue is essential for the processes of the medical treatment. Thus, two-temperature bio-heat transfer equations were introduced and used to discuss the variation of temperature in a laser-irradiated biological tissue. As a result, the Laplace and Fourier transform techniques have been applied, and the solutions of the conductive temperature have been represented in figures. The two-temperature parameter, the penetration depth parameter, the dimensions of the rectangular laser pulse, and the power density of laser irradiation parameter have significant effects on the thermal waves through the skin tissue.

Time-domain multiplexed high resolution fiber optics strain sensor system based on temporal response of fiber Fabry-Perot interferometers.

We developed a multiplexed strain sensor system with high resolution using fiber Fabry-Perot interferometers (FFPI) as sensing elements. The temporal responses of the FFPIs excited by rectangular laser pulses are used to obtain the strain applied on each FFPI. The FFPIs are connected by cascaded couplers and delay fiber rolls for the time-domain multiplexing. A compact optoelectronic system performing closed-loop cyclic interrogation is employed to improve the sensing resolution and the frequency response. In the demonstration experiment, 3-channel strain sensing with resolutions better than 0.1 nε and frequency response higher than 100 Hz is realized.

Photoinduced diffusion molecular transport

We consider a Brownian photomotor, namely, the directed motion of a nanoparticle in an asymmetric periodic potential under the action of periodic rectangular resonant laser pulses which cause charge redistribution in the particle. Based on the kinetics for the photoinduced electron redistribution between two or three energy levels of the particle, the time dependence of its potential energy is derived and the average directed velocity is calculated in the high-temperature approximation (when the spatial amplitude of potential energy fluctuations is small relative to the thermal energy). The thus developed theory of photoinduced molecular transport appears applicable not only to conventional dichotomous Brownian motors (with only two possible potential profiles) but also to a much wider variety of molecular nanomachines. The distinction between the realistic time dependence of the potential energy and that for a dichotomous process (a step function) is represented in terms of relaxation times (they can differ on the time intervals of the dichotomous process). As shown, a Brownian photomotor has the maximum average directed velocity at (i) large laser pulse intensities (resulting in short relaxation times on laser-on intervals) and (ii) excited state lifetimes long enough to permit efficient photoexcitation but still much shorter than laser-off intervals. A Brownian photomotor with optimized parameters is exemplified by a cylindrically shaped semiconductor nanocluster which moves directly along a polar substrate due to periodically photoinduced dipole moment (caused by the repetitive excited electron transitions to a non-resonant level of the nanocylinder surface impurity).

DEVELOPMENT OF LONG PULSED ND:YAG LASER SYSTEM

The Nd:YAG laser with long pulse duration can be produce by using an appropriate pumping scheme. The purpose of this study is to construct a high voltage power supply for laser system. In this attempt multiple-mesh pulse forming technique was performed to obtain electrical pump pulses with a more rectangular shape and long normal-mode laser pulses at constant power. The flashlamp driver was designed with variable input energy. The developed flashlamp driver composes of five major electronic circuits. There are comprised of signal controller device, simmer power supply (SPS), trigger pulse ignition circuit, capacitor charging power supply (CCPS) and multiple-mesh LC pulse forming network (MPFN). The construction of the flashlamp driver is started by designing a signal controller. The controller generated a small voltage to activate the electronic components such as silicon controlled rectified (SCR) and transistor. The ignition circuit was used to ignite xenon gases which responsible to form ionized spark streamer between the two electrodes of flashlamp. A Low dc current was induced by the simmer power supply to sustain the flashlamp in simmering mode. The capacitor charging power supply was used to supply electrical power to capacitor bank within specific time. Nd:YAG laser oscillator was aligned and pumping by the new developed flashlamp radiation. As a result Nd:YAG laser beam was generated having fundamental wavelength of 1064 nm and 650 microsecond of pulse duration with maximum output energy of 250 mJ.

Influence of the thickness and absorption coefficient of a copper oxide film on the ignition delay of PENT by a laser pulse

Numerical modeling of PETN ignition by a copper oxide film absorbing laser radiation has been performed. The calculation results showed the presence of a minimum in the curve of the dynamic delay of PETN ignition by a rectangular laser pulse versus thickness of the absorbing film. This effect is due to the fact that when the film thickness is commensurate with the reciprocal of the absorption coefficient, the amount of heat generated in the thin film due to the multiple reflection of the light flux is proportional to its thickness. Therefore, the smaller the film thickness, the more time is required to heat it to the ignition temperature of PETN. In the case of a thick film, additional energy and time are required to heat its cold part to the ignition temperature of PETN.

Investigation on the laser-induced shock pressure with condensed matter model

In this paper, based on the basic shock wave relations and the stiffened gas equation of state, the laser-induced shock pressures, which are generated with homogenous rectangular laser pulse at power density of several GW/cm2, are proposed theoretically and experimentally. The laser-induced shock pressures are affected by the laser power density as well as the shock velocities, initial densities and adiabatic exponents of the target material and confined water which are considered to be condensed matter other than “solid” or “perfect gas” in the previous work. Performed on Al-2024 alloys, the laser-induced shock wave was measured with the poly(vinylidene fluoride) transducer and recorded by the oscilloscope. The laser-induced shock pressure derived from the proposed method agrees much better with the experimental results than that using the “solid” or “perfect gas” theory.

High fidelity state mapping performed in a V-type level structure via stimulated Raman transition

It is proved that a qubit encoded in excited states of a V-type quantum system cannot be perfectly transferred to the state of the cavity field mode using a single rectangular laser pulse. This obstacle can be overcome by using a two-stage protocol, in which the fidelity of a state-mapping operation can be increased to nearly one.

Arbitrary pulse shaping in Er-doped fiber amplifiers—Possibilities and limitations

Abstract A temporal deformation of nanosecond laser pulses occurring during an amplification process in a two-stage erbium-doped fiber amplifier operating in saturation is presented. We discuss distortion of rectangular laser pulses with duration ranging from 5 ns to 300 ns, generated at 20 kHz repetition rate. Calculations of a suitable peak power scaling factor for distorted pulses are presented. A simple compensation method of temporal pulse distortion, based on the direct current modulation with the use of arbitrary function generator (AFG), is also reported. An appropriate input pulse shape modification leads to obtaining an output amplified pulse with any desired shape. The main limitation of pulse shaping in a fiber amplifier seeded by a narrow linewidth laser diode turns out to be stimulated Brillouin scattering (SBS). However, the pulse shaping method can also improve the amplifier performance by increasing the power threshold of SBS. SBS phenomenon in the context of arbitrary pulse shaping have been also discussed.

The effect of intense short pulse laser shapes on generating of the optimum wakefield and dissociation of methane molecule

AbstractThe optimum convolution of dual short pulse for producing the maximum wakefield and the highest dissociation probability of CH4has been investigated. By using three fundamental shapes of pulses though four different arrangements, the generated wake are considered in plasma. It is found that when the first and second pulses were rectangular–triangular and sinusoidal pulse shapes, respectively, the resultant wakefield amplitude is the highest. This effect opens up a new novel way by pulse shaping mechanism in the photo dissociation dynamics of molecules and controlling of chemical reactions in the desired channels by short pulse intense lasers for reducing the computation time of genetic algorithm model. Using field assisted dissociation model, the dissociation probability for a CH4+molecule exposed to a 100 femtosecond 8 Jcm−2Ti:Sapphire laser pulse is calculated. Here, the highest possible dissociation probability of the methane ion is calculated by the gradient optimization method in which the gradient of a function should be in the direction of the local extremes. The C-H molecular bond of CH4+ion is assumed to be in the same direction as the electric field component of the laser pulse. These results show that there is an excellent match with experimental data. The remarkable feature of this work is that the sensitivity of the dissociation probability of the initial bond lengthq, is studied and the desired product channel is controlled by variation of the laser intensity and it's time evolution by introducing a characteristic vectored space for intensity and duration of two tailored rectangular femtosecond laser pulses.

Design and Experiment of PVDF Transducer for Laser Ultrasound Detecting

The design of PVDF transducer is finished and its designing idea and structure frame are presented. The laser ultrasound in nano A1P/SiO2 is detected by the PVDF transducer and the acoustic signal curve is obtained. The ultrasound waveform thermal elastically generated by a rectangular laser pulse is theoretically presented. The ultrasound pulse waveform obtained in the experiment is in accordance with the one in theory. It is shown that this design method is simple, practical and easy to implement.

Carrier-envelope phase dependence of the duration of generated solitons for few-cycle rectangular laser pulses propagation

We investigate the nonlinear propagation of few-cycle rectangular laser pulses on resonant intersubband transitions in semiconductor quantum wells using an iterative predictor–corrector finite-difference time-domain method. An initial 2π rectangular pulse will split into Sommerfeld–Brillouin precursors and a self-induced transparency soliton during the course of propagation. The duration of generated soliton depends on the carrier-envelope phase of the incident pulse. In our case, not only the near-resonant frequency components but also the low frequency components could contribute to the generation of the soliton pulse when the condition of multi-photon resonance is satisfied. The phase-sensitive property of the solitons results from the phase-dependent distribution of high and low frequency sidebands of few-cycle rectangular pulses.

Laser-heated boundary of transparent and absorbing materials

Temperature profiles near the boundary of an absorbing and a transparent media under laser heating of the absorbing medium, which is in contact with the transparent medium, are calculated. An exact solution for the heat conductivity equations with constant coefficients for rectangular laser pulses is obtained. The results of calculations are presented as a family of curves with dimensionless parameters which are used to recover the temperature-distribution profiles near the interface of any pair of materials, one of which is transparent for laser emission. As an example, laser heating of a graphite-diamond interface is considered.

Heating of a sample with a laser pulse

The transient temperature associated with the bulk absorption of a rectangular laser pulse in a solidstate sample of finite size is calculated analytically and analyzed. Radiation is incident on the frontal surface with an arbitrary surface thermal conductivity. The opposite surface is thermostatically controlled and maintained at a constant equilibrium temperature. The general solution is obtained for pulses of arbitrary duration. The pulse duration is determined with respect to the relaxation time of the nonstationary thermal diffusion (it is the characteristic time of the problem). The limiting cases of the adiabatic insulation and isothermal contact at the frontal surface are considered, and the criteria for surface and bulk light absorption are derived for both cases. The temperature distributions are numerically simulated and examined for long and short pulses, as well as for different values of the light absorption coefficient and the surface thermal conductivity of the frontal surface.