Total Pulse Energy Research Articles

Bistable dependence of the efficiency of second harmonic generation on the total energy of femtosecond soliton pulses propagating in an optical fiber

Taking into account the effect of cubic nonlinearity on the process of second harmonic generation by soliton-like femtosecond pulses results in a hysteresis dependence of the generation efficiency, the pulse durations, and the mismatch of the wave numbers of the interacting waves on their total energy. Color solitons with earlier unknown shapes are formed at a high efficiency of the frequency conversion. Varying the phase mismatch makes it possible to “switch” the soliton duration and the conversion efficiency without a change in the total energy of the waves or the soliton shape. It is emphasized that the soliton corresponding to the middle branch of the hysteresis dependence is stable against small perturbations. It is significant that the soliton shape remains unchanged in the considered range of energies.

High intensity pulsed electric fields applied for food preservation

Preservation of liquid foods by high intensity pulsed electric fields (PEF) is an interesting alternative to traditional techniques like thermal pasteurization. Based on the underlying mechanism of action, in this paper the crucial process parameters electrical field strength, total pulse energy input and treatment temperature were investigated experimentally. Inactivation studies were performed with three bacteria ( E. coli, Bacillus megaterium, Listeria innocua) and one yeast ( Saccharomyces cerevisiae). Stainless steel and carbon electrodes have been tested to investigate their applicability as electrode material. Simulating the influence of cell size and orientation as well as the presence of agglomerations or insulating particles indicated that the applied field strength has to be increased above the critical one to achieve product safety. It was found that temperatures higher than 40 °C can strongly increase the lethality of the PEF process. In this way also small cells like Listeria are easily affected by pulsed fields even at a field strength as low as 16 kV cm −1. In addition, heating of the product prior to PEF has the advantage that most of the required process energy can be recovered using heat exchangers. Exemplary, such a process is analyzed by an enthalpy balance.

On the possible effect of pedestal pulse on material removal by ultrahigh intensity laser pulses

The results of the interaction of 200 fs and 4 ns laser pulses produced by a Ti : Sapphire commercial laser system with various materials are reported. The drilling rates were measured as a function of laser pulse energy and material thickness. Saturation of the material removal rate was observed for high laser pulse energy. The dependence of the material removal rate on the sample thickness was measured. The results of numerical simulation of laser drilling with femtosecond and nanosecond laser pulses using a theoretical model which includes phase transition at the critical temperature are reported. The computed drilling rates are in agreement with the data of measurements. The effect of a suspected nanosecond pedestal pulse containing approximately 30% of total pulse energy on the rate of drilling with femtosecond laser pulses is numerically simulated and the computed results are discussed.

Comparative studies of eye-safe intracavity and extracavity optical parametric oscillators with an unstable telescopic cavity

We have experimentally studied the lasing characteristics of an eye-safe optical parametric oscillator (OPO) with an unstable telescopic cavity when it is placed inside (intracavity OPO) and outside (extracavity OPO) the plane-parallel cavity of a pulsed, nearly single-mode KGW:Nd pump laser. We used a KTP crystal as the nonlinear medium for the OPO. We have shown that the intracavity OPO has the higher lasing efficiency. We have observed that the distribution of nonlinear losses introduced by the intracavity OPO, nonuniform over the cavity cross section, leads to an increase in the diameter and divergence of the radiation beam from the pump laser and a dependence of its temporal lasing dynamics on the transverse beam coordinate. We propose a physical model qualitatively explaining the spatial and temporal lasing dynamics of a radiation source with an intracavity OPO. Both OPO versions generate beams of radiation with about the same divergence. When the KGW:Nd laser has an electrical pumping energy of 7.3 J and a cavity length of 77 cm, the intracavity OPO and the extracavity OPO emit pulses with energies of 14.5 mJ and 12.0 mJ and duration 18 nsec and 13 nsec respectively. The divergence of the eye-safe radiation (λ = 1.578 µm) at 86.5% of the total pulse energy is no greater than 5.5 mrad for an OPO output beam diameter of = 2 mm.

Laser-induced ablation of a steel sample in different ambient gases by use of collinear multiple laser pulses

The sensitivity of laser-induced breakdown spectroscopy of solid samples depends on the number of ablated and excited analytes. Laser ablation of solid samples can be enhanced by using collinear multiple laser pulses, for example double or triple pulses, rather than single laser pulses with the same total laser pulse energy. The ablation rates and the plasma conditions are affected by the ambient gas. In this study laser ablation was examined by varying the interpulse separation of the multiple pulses, within double and triple-pulse bursts, and the gas mass density at constant gas pressure. Different ambient gases and gas mixtures consisting of argon, oxygen, and nitrogen were used to study their effect on ablation rates. In a pure argon atmosphere (99.999% v/v Ar) the ablation burst number required to penetrate a steel plate of thickness 100 microm is reduced by a factor of approximately six by use of triple-pulse bursts with a symmetric interpulse separation of 15 micros rather than single pulses with the same total burst energy of 105 mJ. For double and single pulses the factors are 1.6 for Ar and 2.8 for synthetic air. Analyte lines are 4 to 8 times more intense if an argon atmosphere, rather than air, is used.

Coherent Control of THz Wave Generation in Ambient Air

Our study of THz wave generation in the pulsed laser induced air plasma with individually controlled phase, polarization, and amplitude of the optical fundamental wave (omega) and its second harmonic (2omega) indicates that the third-order nonlinear optical process mixing the omega and 2omega beams in the ionized plasma is the main mechanism of the efficient THz wave generation. The polarity and the strength of the emitted THz field are completely controlled by the relative phase between the omega and 2omega waves. The measured THz field amplitude is proportional to the pulse energy of the fundamental beam and to the square root of the pulse energy of the second-harmonic beam once the total optical pulse energy exceeds the plasma formation threshold. The optimal THz field is achieved when all waves (omega, 2omega, and THz waves) are at the same polarization in the four-wave-mixing process.

Pulse Shape Influence on the Accuracy of Z-scan Measurements

Although the laser pulses with durations ranging from nanoseconds to femtoseconds and various pulse shapes are utilized for the Z-scan measurements, the influence of the temporal pulse shape on the measurement results is often neglected. In this paper, we tried to differentiate the influence of the temporal pulse shape on the common Z-scan technique with a small on-axis aperture in two cases: when the pulse peak intensity at the beam waist is known (for relatively long pulses), or when the total pulse energy and full width at half maximum (FWHM) of the correlation functions or FWHM of pulse durations are known (for short pulses).

High-energy quasi-phase-matched optical parametric oscillation in a 3-mm-thick periodically poled MgO:LiNbO_3 device

We have demonstrated high-energy quasi-phase-matched optical parametric oscillation in a 3-mm-thick periodically poled 5-mol. % MgO-doped LiNbO3 device with a 32.1-microm grating period and a 30-mm length. With a large-spot-size pump laser of 2.2-mm diameter, we obtained a total output pulse energy of 22 mJ for both the signal (wavelength 1.82 microm) and the idler (2.56 microm) waves at an input pump energy of 46 mJ.

Drilling of steel and HgCdTe with the femtosecond pulses produced by a commercial laser system

The results of interaction of single and multiple 200 fs laser pulses with thick stainless steel and HgCdTe samples are reported. The threshold laser energy density required to produce surface melting is measured. The melt dynamics and evolution of surface morphology are observed for different pulse energies and number of laser pulses. It is observed that, as with a long laser pulse interaction, a layer of melt can be produced at the sample surface. Melt ejection in the radial direction towards the periphery of the interaction zone is observed when the pulse energy is increased. The observed melt dynamics resembles the evaporation recoil melt removal typical of the laser interactions in the range from nanoseconds to continuous wave (CW). The observed melt ejection is attributed to a nanosecond component of the laser pulse with an estimated energy of approximately 25% of the total laser pulse energy. The measured melting threshold energy density for stainless steel is comparable with the published theoretically predicted threshold for nickel computed using a two-temperature model.

Analysis of passively Q-switched lasers with simultaneous mode locking

Simultaneous Q-switching and modelocking in a diode-pumped Nd:YAG/Cr/sup 4+/:YAG laser is experimentally demonstrated. A general recurrence is derived for the analysis of the temporal shape of a single Q-switched envelope with mode locked pulse trains. With the developed model, the modelocked pulse energy and the total Q-switched pulse energy can be calculated. Excellent agreement was found between the present results and detailed theoretical computations.

Excitation and ionization of sodium clusters by a strong electromagnetic field

This paper reports on a study of the excitation and ionization of small sodium clusters by femtosecond light pulses with a maximum intensity of 5×1012–1×1014 W/cm2 and photon energy from 1 to 3 eV made in terms of the density functional theory and jellium model through direct numerical solution of the Kohn-Sham time-dependent equation. The dependence of the degree of ionization on the intensity, duration, and frequency of the light pulses, as well as on the cluster size, is studied. The efficiency of the processes is shown to be determined primarily by the field intensity rather than by the total pulse energy.

The “Katran” CO2 Laser with High Specific Output Power and Stable Parameters

The design and parameters of the UV-preionized discharge module “Katran” are described. A particular feature of the scheme is a high-voltage pulse formation technique for sharp discharge current ignition to stabilize the self-sustained glow discharge. The free-running laser based on the discharge module allows one to obtain high specific laser power exceeding 145 MW/liter in the P(20) line for the 10-μm band for an active volume of 3 liters. Duration of the first spike of generation is 30 ns FWHM and energy content is about 65% of the total pulse energy. The high reliability and reproducibility of the module's operation for a wide range of parameters ensures laser suitability for different scientific and technical applications.

Effects of pump pulse temporal structure on long-pulse multi-order stimulated Raman scattering in optical fiber

We present an experimental and theoretical investigation of the effects of pump pulse temporal structure on cascade Raman generation in the quasi-continuous wave regime. The growth and saturation of the Stokes pulse energies depend very strongly on the pump pulse temporal envelope and substructure. Experiments and simulations on the growth of the total Stokes pulse energy and the energy in a narrow central slice highlight the importance of knowing the detailed pump pulse temporal structure. Experimentally measured growth curves for different Stokes orders could be transformed by a simple scaling to lie on a universal curve.

Excimer laser beam homogenizer with low divergence

A new beam homogenizer device which leaves the beam parameter product almost unaffected is presented. Excimer laser beams usually show a nearly Gaussian beam profile along the short beam axis whereas along the long beam axis a nearly flat-top profile is provided. The new beam homogenizer efficiently converts the short beam axis into a top-hat profile featuring nearly the same divergence as the incoming laser beam. Depending on the output beam profile of the excimer laser the homogenizer presented provides a beam profile with a homogeneity of ±5% to ±10% within an area covering 65% to 70% of the total pulse energy.

Synchronous, dual-wavelength, injection-seeded amplification of 5-ns pulses in a flash-lamp-pumped Ti:sapphire laser

The outputs of two cw diode lasers are coupled into a flash-lamp-pumped Ti:sapphire laser cavity for regenerative amplification. Slices of each seed beam are simultaneously trapped in the cavity and amplified for approximately 33 round trips. The output is a 4.7-ns pulse, the total pulse energy is 110 mJ, and linewidths are only 50% greater than the Fourier-transform limit. The timing jitter between the pulses at the two wavelengths is less than +/-75 ps , and their relative energy is controlled by adjustment of the seed power of the cw diode lasers. Gain competition between the two frequencies does not lead to instabilities.

Noise behavior of pulsed vertical-cavity surface-emitting lasers

We determine the pulse energy fluctuations of vertical-cavity surface-emitting lasers by measuring the number of photoelectrons produced when a laser pulse is incident upon a photodetector. We obtain probability distributions for the number of photoelectrons produced by the total pulse energy, as well as distributions produced by the energy in each of the two orthogonal laser polarizations. We find that the noise of the laser increases when each new laser mode comes above threshold and that the individual polarization outputs are noisier than the total output, which indicates negative correlations between the energies in the two polarizations. We also find that, while the statistics of the total output are Gaussian, this is not always true of the individual polarizations.

Waveforms of external defibrillators: analysis and energy contribution

Background and objective: Defibrillation is the most important therapy for terminating ventricular fibrillation in cardiac arrest patients. In addition to performing defibrillation at the earliest possible time, appropriate pulse energy and optimal waveform seem to be crucial for success. Emergency medical service personnel use different defibrillators and rely on their similarity of energy content. This study examined the true pulse energy content and waveform of 17 commonly used defibrillators. Methods and results: Defibrillation energies were selected to be 30, 200 or 360 J and defibrillators were discharged into test resistors, simulating transthoracic impedances of 25, 50 or 100Ω. Pulse energy deviated by up to +23% or −29% from the selected energy. Pulse energy within the initial 8 ms ranged from 90 to 30% of total pulse energy. Fourteen defibrillators utilising damped sinusoidal waveforms produced a monophasic pulse when discharged into resistances of 50Ω and 100Ω. Conclusions: Defibrillators used at the same energy settings do not necessarily produce the same defibrillation pulse energy. All but one defibrillator actually use monophasic waveforms, leaving the potential advantage of biphasic waveforms unused. Energy accuracy of defibrillators needs to be improved, and biphasic waveforms should be used more.

Ultrafast Spectroscopy of Semiconductor Devices

In this work we present an experimental technique for investigating ultrafast carrier dynamics in semiconductor optical amplifiers at room temperature. These dynamics, influenced by carrier heating, spectral hole-burning and two-photon absorption, are very important for device applications in information processing at high data transfer rates. The technique is based on single pulse propagation through the device, with pulse duration tunable from 150fs to 11ps, and the characterization of the pulse at the output of the device by measuring the total pulse energy, the real time profile and the frequency spectrum. Results on a InGaAsP bulk amplifier are shown as an example.

Laser lithotripsy of difficult bile duct stones: results in 60 patients using a rhodamine 6G dye laser with optical stone tissue detection system

Introduction—Laser lithotripsy of bile duct stones has become a widely accepted endoscopic treatment modality for giant, impacted, or very hard stones. The procedure is usually carried out under direct endoscopic...

Modeling of Erbium: YAG Laser-Mediated Explosive Photovaporization: Implications for Vitreoretinal Surgery

Motivated by the potential for highly precise tissue removal, investigators are exploring fiberoptic microsurgical maneuvers with the erbium:YAG (Er:YAG) laser. Tissue is disrupted and removed by direct ablation and the acousto-mechanical sequelae of explosive vaporization of the tissue water. The authors investigated the scaling laws for photoablative and photodisruptive interactions and interpreted these results to optimize energy delivery for vitreoretinal surgical maneuvers. A model for laser-generated-bubble expansion is presented based on energy principles and adiabatic gas expansion. Comparisons are made with the authors' previous studies of ablation rate and thermometry. The authors' modeling studies generated predictions similar to experimental data. The maximum bubble diameter increases as the cube root of the pulse energy. At constant radiant exposure, the maximum bubble diameter increases as the probe tip diameter raised to the two-thirds power. The authors demonstrated that tissue ablation depends on radiant exposure (J/cm2), whereas temperature increases, bubble size, and peak pressure depend on total pulse energy. Mechanical injury should be minimized and efficient ablation preserved by delivering low-pulse energy through small-diameter probe tips at high repetition rates.