Showing papers on "Pulse duration published in 2004"

Generation of intense, carrier-envelope phase-locked few-cycle laser pulses through filamentation

Abstract: Intense, well-controlled light pulses with only a few optical cycles start to play a crucial role in many fields of physics, such as attosecond science. We present an extremely simple and robust technique to generate such carrier-envelope offset (CEO) phase locked few-cycle pulses, relying on self-guiding of intense 43-fs, 0.84 mJ optical pulses during propagation in a transparent noble gas. We have demonstrated 5.7-fs, 0.38 mJ pulses with an excellent spatial beam profile and discuss the potential for much shorter pulses. Numerical simulations confirm that filamentation is the mechanism responsible for pulse shortening. The method is widely applicable and much less sensitive to experimental conditions such as beam alignment, input pulse duration or gas pressure as compared to gas-filled hollow fibers.

Ablation of metals by ultrashort laser pulses

Abstract: Ablation of Fe by ultrashort laser pulses with durations 0.1, 1, and 5 ps were investigated experimentally. The laser fluence varied from the ablation threshold up to 100 J cm−2. Above 1 J cm−2, the ablation rate depended on the laser pulse duration, with the shortest pulse producing the highest value. A change in the ablation rate as the laser fluence increased was also observed. These results were analysed using molecular dynamics simulations. We show that the change in the ablation rate is connected to an overheating of the material above the critical point, which results in a steep rise of the pressure developed. Furthermore, due to the electron heat diffusion, the overheated volume increases and involves material located deeper than the skin depth. An increase in the pulse duration results in a decrease in the degree of overheating.

Nonlinear optics in the extreme ultraviolet

Abstract: Nonlinear responses to an optical field are universal in nature but have been difficult to observe in the extreme ultraviolet (XUV) and soft X-ray regions owing to a lack of coherent intense light sources. High harmonic generation is a well-known nonlinear optical phenomenon and is now drawing much attention in attosecond pulse generation. For the application of high harmonics to nonlinear optics in the XUV and soft X-ray regime, optical pulses should have both large pulse energy and short pulse duration to achieve a high optical electric field. Here we show the generation of intense isolated pulses from a single harmonic (photon energy 27.9 eV) by using a sub-10-femtosecond blue laser pulse, producing a large dipole moment at the relatively low (ninth) harmonic order nonadiabatically. The XUV pulses with pulse durations of 950 attoseconds and 1.3 femtoseconds were characterized by an autocorrelation technique, based on two-photon above-threshold ionization of helium atoms. Because of the small cross-section for above-threshold ionization, such an autocorrelation measurement of XUV pulses with photon energy larger than the ionization energy of helium has not hitherto been demonstrated. The technique can be extended to the characterization of higher harmonics at shorter wavelengths.

Basic Radar Signals

Abstract: Three basic radar signals are studied in this chapter: A constant-frequency pulse, a linear FM pulse (LFM) and a coherent train of identical, constant-frequency pulses. The ambiguity function (AF) is the main tool in this study. For these three signals closed-form expressions of the AF are developed. The role of an amplitude weight window is discussed with regard to reducing range sidelobes in the AF of the LFM pulse. Also studied is the mismatch loss caused by implementing the weight window only in the receiver.

The electron runaway mechanism in dense gases and the production of high-power subnanosecond electron beams

Abstract: New insight is provided into how runaway electrons are generated in gases. It is shown that the Townsend mechanism of electron multiplication works even for strong fields, when the ionization friction of electrons can be neglected. The non-local electron runaway criterion proposed in the work determines the critical voltage–pd relationship as a two-valued function universal for a given gas (p being the gas pressure, and d the electrode spacing). This relationship exhibits an additional upper branch as contrasted to the familiar Paschen's curves and divides the discharge gap into two regions: one where electrons multiply effectively, and the other which they leave without having enough time to multiply. Experiments on the production of electron beams with subnanosecond pulse duration and an amplitude of tens to hundreds of amperes at atmospheric pressure in various gases are addressed, and the creation of a nanosecond volume discharge with the high density of excitation power and without preionization of the gap by a supplementary source is discussed.

Long-wavelength monolithic mode-locked diode lasers

Abstract: A detailed study of the design issues relevant to long-wavelength monolithic mode-locked lasers is presented. Following a detailed review of the field, we have devised a validated travelling wave model to explore the limits of mode-locking in monolithic laser diodes, not only in terms of pulse duration and repetition rate, but also in terms of stability. It is shown that fast absorber recovery is crucial for short pulse width, that the ratio of gain to absorption saturation is key in accessing ultrashort pulses and that low alpha factors give only modest benefit. Finally, optimized contact layouts are shown to greatly enhance pulse stability and the overall operational success. The design rules show high levels of consistency with published experimental data.

High-power diode-pumped Yb3+:CaF2 femtosecond laser.

Abstract: We report what is believed to be the first demonstration of a high-power passively mode-locked diode-pumped femtosecond laser based on an Yb3+:CaF2 single crystal, directly pumped by a 15-W fiber-coupled laser diode With a 5-at % Yb3+-doped sample and prisms for dispersion compensation we obtained pulses as short as 150 fs, with 880 mW of average power and up to 14-W average output power, with a pulse duration of 220 fs, centered at 1049 nm The laser wavelength could be tuned from 1040 to 1053 nm in the femtosecond regime Using chirped mirrors for dispersion compensation, the oscillator provided up to 174 W of average power, with a pulse duration of 230 fs, corresponding to a pulse energy of 20 nJ and a peak power of 85 kW

Ultrashort pulse laser interaction with dielectrics and polymers

Abstract: Femtosecond laser micromachining has excited vivid attention in various industrial fields and in medicine owing to the advantages of ultrashort laser pulses compared to long-pulse treatment. These are mainly the reduction of the laser fluence needed to induce ablation and the improvement of the contour sharpness of the laser-generated structures. Recently, special attention was paid to femtosecond laser experiments on nonabsorbing inorganic dielectrics. This is due to the fact that optical damage in dielectric optical elements limits the performance of high-power laser systems. Despite the fact that a large variety of organic polymers can be machined with excimer lasers successfully, the involvement of thermal processes can lead to an unsatisfactory quality of the structures. Ultrashort, fs-laser pulses might be an alternative for the treatment of polymers. Therefore, femtosecond laser machining investigations of dielectrics and polymers are reviewed in this paper. Similarities and differences of the ablation behavior of both material classes are discussed. The influence of the bandgap on the ablation threshold in dependence on the pulse duration, the enhancement of the machining precision with a shortening of the pulse duration, incubation phenomena, and morphological features appearing on the surface after femtosecond laser treatment are mentioned. Possible applications, e.g., in medicine and biosensors, are described.

Electro-chemical micro drilling using ultra short pulses

Abstract: Electro-chemical machining (ECM) has been rarely applied in micro machining because the electric field is not localized In this work, ultra short pulses with tens of nanosecond duration are used to localize dissolution area The effects of voltage, pulse duration, and pulse frequency on the localization distance were studied High quality micro hole with 8 μm diameter was drilled on 304 stainless steel foil with 20 μm thickness Localization distance can be manipulated by controlling the voltage and pulse duration, and various hole shapes were produced including stepped holes and taper free holes

Ultrashort electrical pulses open a new gateway into biological cells

Abstract: Electrical models for biological cells predict that reducing the duration of applied electrical pulses to values below the charging time of the outer membrane causes a strong increase in the probability for electric field interactions with intracellular structures. For electric field amplitudes exceeding MV/m, such pulses are expected to cause electroporation of cell organelles, with the required electric field amplitude scaling linearly with the inverse of pulse duration. Experimental studies, where human cells were exposed to pulsed electric field of up to 300 kV/cm amplitude with duration as short as 10 ns, have confirmed this hypothesis. The observed effects include the breaching of intracellular granule membranes without permanent damage to the cell membrane, abrupt rises in intracellular free calcium levels, and enhanced expression of genes. At increased electric fields, the application of nanosecond pulses induces apoptosis (programmed cell death) in biological cells, an effect that has been shown to reduce the growth of tumors. The experimental studies require the use of nanosecond pulse generators with impedances in the range from 10 to 100Omega. Two typical bioelectrics pulse power sources are described

Nonlinear refraction in CS 2

Abstract: The nonlinear refractive index (γ) of CS2 was measured using the Z-scan technique and laser radiation of various (femto-, pico-, and nano-second) pulse durations. We observed the growth of γ with the increase of the pulse duration (from (3±0.6)×10-15 cm2 W-1 at 110 fs to (4±2)×10-14 cm2 W-1 at 75 ns) due to the additional influence of the molecular reorientational Kerr effect in the case of longer (picosecond and nanosecond) pulses. Acoustic wave induced negative nonlinear refraction was observed using wavefront analysis. We analyzed the simultaneous influence of both electronic and molecular processes leading to the positive nonlinear refraction and acoustic processes leading to the negative nonlinear refraction in carbon disulfide. Variations of the refractive index due to the thermal effect at high pulse repetition rates were also investigated.

Improvement of machining characteristics of micro-EDM using transistor type isopulse generator and servo feed control

Abstract: This paper describes the improvement of machining characteristics of micro electrical discharge machining (micro-EDM) using a newly developed transistor type isopulse generator and servo feed control. The RC generator is mainly applied in conventional micro-EDM even though the transistor type isopulse generator is generally more effective for obtaining higher removal rate, because the transistor type generator is unable to generate iso-duration discharge current pulses with small pulse duration (several dozen nano-seconds), which is the normal level for micro-EDM. A new transistor type isopulse generator was therefore developed using a current sensor with high frequency response. With the new transistor type isopulse generator developed, the pulse duration can be reduced to about 30 ns, which is equivalent to the pulse duration used in finishing by the conventional RC pulse generator for micro-EDM. In order to achieve stable machining and improve machining characteristics, a new servo feed control system for micro-EDM using average ignition delay time to monitor the gap distance was also developed. By integrating the transistor type isopulse generator with this new servo feed control system, we were able to obtain a removal rate of about 24 times higher than that of the conventional RC pulse generator with a constant feed rate in both semifinishing and finishing. The effectiveness of the servo feed control proved higher in finishing than in semifinishing, whereas the transistor type isopulse generator was more effective in semifinishing than in finishing.

Controlling HD+ and H+2 dissociation with the carrier-envelope phase difference of an intense ultrashort laser pulse.

Abstract: Carrier-envelope phase difference effects in the dissociation of the HD+ molecular ion in the field of an intense, linearly polarized, ultrashort laser pulse are studied in the framework of the time-dependent Schrodinger equation. We consider a reduced-dimensionality model in which the nuclei are free to vibrate along the field polarization and the electrons move in two dimensions. The laser has a central wavelength of 790 nm and a pulse length of 10 fs with intensities in the range 6x10(14) to 9x10(14) W/cm(2). We find that the angular distribution of dissociation to p+D and H+d can be controlled by varying the phase difference, generating differences between the dissociation channels of more than a factor of 2. Moreover, the asymmetry is nearly as large for H+2 dissociation.

Ultra-short pulses in linear and nonlinear media

Abstract: We consider the evolution of ultra-short optical pulses in linear and nonlinear media. For the linear case, we first show that the initial-boundary value problem for Maxwell's equations in which a pulse is injected into a quiescent medium at the left endpoint can be approximated by a linear wave equation which can then be further reduced to the linear short-pulse equation. A rigorous proof is given that the solution of the short pulse equation stays close to the solutions of the original wave equation over the time scales expected from the multiple scales derivation of the short pulse equation. For the nonlinear case we compare the predictions of the traditional nonlinear Schr\"odinger equation (NLSE) approximation which those of the short pulse equation (SPE). We show that both equations can be derived from Maxwell's equations using the renormalization group method, thus bringing out the contrasting scales. The numerical comparison of both equations to Maxwell's equations shows clearly that as the pulse length shortens, the NLSE approximation becomes steadily less accurate while the short pulse equation provides a better and better approximation.

Amplification of ultrashort laser pulses by a resonant Raman scheme in a gas-jet plasma

Abstract: Raman amplification of subpicosecond laser pulses up to 95 times is demonstrated at corresponding frequencies in a gas-jet plasma. The larger amplification is accompanied by a broader bandwidth and shorter pulse duration. Theoretical simulations show a qualitative agreement with the measurements, and the effects of the plasma conditions and laser intensities are discussed.

Generation of pure two-beam interference grating structures in an optical fiber with a femtosecond infrared source and a phase mask.

Abstract: Fiber Bragg gratings were fabricated in all-silica core fiber by focusing 125-fs 800-nm pulses with an 80-mm lens through a phase mask with 4.28‐µm pitch onto a fiber sample. When the phase-mask–fiber separation was 5 mm the observed structure was clearly the result of two-beam interference between the ±1 orders. The elimination of the remaining 9 orders is a consequence of the walk-off experienced by the mask orders and the short duration of the femtosecond pulse. This effect is unique to the fabrication of Bragg gratings with femtosecond sources and would not be observed with a longer pulse duration or incoherent UV sources.

Generation of 36-femtosecond pulses from a ytterbium fiber laser

Abstract: By optimizing the cavity dispersion map, 1.5-nJ, 36-fs pulses are obtained from a Yb-doped fiber laser. Higher-order dispersion currently limits the pulse duration.

Development of a mid-infrared laser for study of infrared countermeasures techniques

Abstract: Countermeasures against heat seeking missiles require access to efficient laser sources, which should emit wavelengths at band I, II and IV. Efficient diode pumped solid-state lasers, combined with efficient non-linear wavelength shifters, allow the development of practical tuneable mid-IR countermeasure sources. The paper describes the requirements and the development of a tabletop laser source for study of DIRCM techniques. Jamming laser systems must be able of creating pulse sequences in the frequency range between 100 Hz and 10,000 Hz, including the capability to mix and sweep the jam frequency. A Nd:YVO4 pump laser with maximum pump power of 3 Watt and pulse length of 10 ns, and a maximum modulation frequency of 100 kHz was selected. A linear single resonant OPO cavity with 30 mm long, 1mm thick PPLN crystals was build. With the tabletop laser system we were able to generate wavelengths from 1.5 to 4 micron. In band I, at 2 micron we can generate between 400-550 mW, and in band II, from 3-4 micron we can generate 130-160 mW laser jam power. The beam quality (M2) is approximately 2.5. The power efficiency for the idler was 8.8%, while the slope power efficiency was 15%. Jam patterns are generated by use of an acousto-optic modulator

Single sub − 50 − attosecond pulse generation from chirp-compensated harmonic radiation using material dispersion

Abstract: A method for obtaining a single sub-50-attosecond pulse using harmonic radiation is proposed. For the generation of broad harmonic radiation during a single half-optical cycle, atoms are driven by a femtosecond laser pulse with intensity above the saturation intensity for optical field ionization and hence experience a large nonadiabatic increase of the laser electric field between optical cycles. Although the chirped structure of the harmonic radiation imposes a limit on the minimum achievable pulse duration, we demonstrate that its positive chirp can be compensated by the negative group delay dispersion of an appropriately selected x-ray filter material, used also for the spectral selection, resulting in a single attosecond pulse with a duration less than 50 as.

Domain wall displacement induced by subnanosecond pulsed current

Abstract: We show that a single current pulse as short as 0.4 ns can trigger domain wall (DW) displacement in spin-valve stripes of 0.3 μm width inserted into a coplanar waveguide. The experiments were carried out with varying current pulse amplitude, duration, polarity, and applied static magnetic field. In zero field, DW displacement occurs in the same direction as the conduction electron current. In finite applied field, the direction of DW displacement is that favored by the field orientation. In both cases, the DW displacement occurs only above a critical current density jc of the order of 106 A/cm2. The distance traveled by the DW along the stripe increases with the current pulse amplitude and applied field strength, but it does not depend on the pulse duration between 0.4 and 2 ns.

Surface nanostructuring of metals by laser irradiation: effects of pulse duration, wavelength and gas atmosphere

Abstract: Surface modifications by nanostructuring present a new laser application for improvement of surface properties such as adhesion, mechanical characteristics or corrosion protection. In this study, we report the formation of nanoparticles by laser irradiation of a steel surface. The influence of laser parameters such as pulse duration (25–30 ns, 500 fs), wavelength (248 nm, 308 nm), and the background gas pressure (10 mbar-1 bar) on the formation of this back deposition layer composed of aggregated iron oxide nanoparticles were investigated. Scanning electron microscopy and atomic force microscopy were used to characterise the irradiated steel surface and the particle morphology deposited by backward flux. In the nanosecond laser ablation regime, films are formed by aggregated nanoparticles with well developed cauliflower like structures, the size and the morphology depending on the nature and pressure of the background gas. In the femtosecond regime, we observed the formation of micrometer sized structures at the steel surface. In particular, a non-conventional mechanism of nanocluster condensation and growth is revealed since two different ablation rates corresponding to two different predominant processes are observed. These analyses demonstrate the possibility of controlling the distribution and the size of particles by varying the laser parameters and the background gas pressure and nature.

Method and Apparatus for Laser Surgery of the Cornea

Abstract: A laser-based method and apparatus for corneal surgery. The present invention is intended to be applied primarily to ablate organic materials, and human cornea in particular. The invention uses a laser source which has the characteristics of providing a shallow ablation depth (0.2 microns or less per laser pulse), and a low ablation energy density threshold (less than or equal to about 10 mJ/cm 2 ), to achieve optically smooth ablated corneal surfaces. The preferred laser includes a laser emitting approximately 100-50,000 laser pulses per second, with a wavelength of about 198-300 nm and a pulse duration of about 1-5,000 picoseconds. Each laser pulse is directed by a highly controllable laser scanning system. Described is a method of distributing laser pulses and the energy deposited on a target surface such that surface roughness is controlled within a specific range. Included is a laser beam intensity monitor and a beam intensity adjustment means, such that constant energy level is maintained throughout an operation. Eye movement during an operation is corrected for by a corresponding compensation in the location of the surgical beam. Beam operation is terminated if the laser parameters or the eye positioning is outside of a predetermined tolerable range. The surgical system can be used to perform surgical procedures including removal of corneal scar, making incisions, cornea transplants, and to correct myopia, hyperopia, astigmatism, and other corneal surface profile defects.

4-ps passively mode-locked Nd:Gd0.5Y0.5VO4 laser with a semiconductor saturable-absorber mirror.

Abstract: We have demonstrated a passively mode-locked diode end-pumped all-solid-state laser, which is composed of a Nd:Gd0.5Y0.5VO4 crystal and a folded cavity with a semiconductor saturable-absorber mirror grown by metal-organic chemical-vapor deposition. Stable cw mode locking with a 3.8-ps pulse duration at a repetition rate of 112 MHz was obtained. At 13.6 W of the incident pump power, a clean mode-locked fundamental-mode average output power of 3.9 W was achieved with an overall optical-to-optical efficiency of 29.0%, and the slope efficiency was 38.1%.

Laser-based system for memory link processing with picosecond lasers

Abstract: A laser-based system for processing target material within a microscopic region without causing undesirable changes in electrical or physical characteristics of at least one material surrounding the target material, the system includes a seed laser, an optical amplifier, and a beam delivery system. The seed laser for generating a sequence of laser pulses having a first pre-determined wavelength. The optical amplifier for amplifying at least a portion of the sequence of pulses to obtain an amplified sequence of output pulses. The beam delivery system for delivering and focusing at least one pulse of the amplified sequence of pulses onto the target material. The at least one output pulse having a pulse duration in the range of about (10) picoseconds to less than (1) nanosecond. The pulse duration being within a thermal processing range. The at least one focused output pulse having sufficient power density at a location within the target material to reduce the reflectivity of the target material and efficiently couple the focused output into the target material to remove the target material.

Angular dispersion and temporal change of femtosecond pulses from misaligned pulse compressors

Abstract: A misaligned stretcher or compressor in a chirped pulse amplification laser introduces residual angular dispersion into the beam, resulting in temporal distortion of the pulse. We demonstrate that an imaging spectrograph is capable for measuring the angular dispersion of a laser beam by an accuracy of 0.2 /spl mu/rad/nm. Using this technique, the analytical expressions of residual angular dispersion of misaligned prism and grating compressors are experimentally proved. Temporal degradations of short pulses due to angular dispersion are studied by measuring the temporal stretch of 16-fs pulses, while the issues of contrast deterioration are also discussed. It is proved that the simultaneous measurement of angular dispersion and pulse duration offers the most precise alignment procedure of prismatic and grating compressors.

Femtosecond-laser-encoded distributed-feedback color center laser in lithium fluoride single crystals

Abstract: Laser-active F2 centers were produced in lithium fluoride (LiF) at a concentration of 2×1018 cm−3 by irradiating focused femtosecond (fs) laser pulses from a mode-locked titanium sapphire laser (wavelength ∼800 nm, emission pulse duration ∼100 fs). This technique was used to write waveguides embedded in LiF crystals. A refractive index change estimated from a guide propagation method was approximately +1% at a wavelength of 633 nm. Refractive index-modulated volume-type gratings were also encoded inside LiF crystals by a single interfered fs laser pulse. The distributed feedback laser structure was fabricated using the gratings thus encoded, which exhibited a room-temperature F2-color center laser oscillation at 707 nm. This demonstrates a DFB color center laser operating at room temperature utilizing photon written, permanent Bragg gratings.