Showing papers by "Gerard Mourou published in 1997"

Laser ablation and micromachining with ultrashort laser pulses

Abstract: The mechanisms of ultrashort-pulse laser ablation of materials are discussed, and the differences to that of long laser pulses are emphasized. Ultrashort laser pulses offer both high laser intensity and precise laser-induced breakdown threshold with reduced laser fluence. The ablation of materials with ultrashort pulses has a very limited heat-affected volume. The advantages of ultrashort laser pulses are applied in precision micromachining of various materials. Some femtosecond laser pulse micromachining results, including comparison with long pulses, are presented. Ultrashort-pulse laser micromachining may have a wide range of applications where micrometer and submicrometer feature sizes are required.

0.2-TW laser system at 1kHz.

Abstract: We have developed a 1-kHz repetition-rate Ti:sapphire laser system that can simultaneously generate high peak and average powers of 0.2 TW and 4 W, respectively. The laser system generates 4-mJ energy pulses with a 20-fs pulse width. We eliminated thermal lensing in the system by cooling the Ti:sapphire crystal to 125 K. The output 20-fs pulses were fully characterized by use of the new technique of transient-grating frequency-resolved optical gating. We demonstrate experimentally that the pulse duration at the output is limited only by fifth-order dispersion.

Photodisruption in the human cornea as a function of laser pulse width

Abstract: BACKGROUND We investigated the role of laser pulse width in determining fluence thresholds and efficiency for corneal photodisruption. METHODS A laser system that delivers a wide range of pulse energies and pulse widths was used to produce ablations at pulse widths from 100 femtoseconds (fs) to 7 nanoseconds (ns). The laser-induced breakdown fluence threshold at each pulse width was determined by monitoring individual plasma emissions. Using multiple shots, the photodisruption threshold and cutting depth at each pulse width were determined histologically. RESULTS Corneal breakdown thresholds decreased at a faster rate from 7 ns to approximately 10 picoseconds (ps), compared to further reductions in pulse width below 10 ps, where little variation was seen. Breakdown for pulse widths below 10 ps showed little intershot variability, resulting in highly reproducible fluence thresholds. Corneal tissue examined histologically showed similar fluence dependency. CONCLUSIONS Corneal tissue photodisruption thresholds demonstrate pulse width dependence. At pulse widths less than 10 ps and with fluences near the breakdown threshold, ablations are maximally precise and efficient. These findings suggest optimal laser parameters for corneal surgery.

The ultrahigh-peak-power laser: present and future

Abstract: to 1020-W/cm2 range. With some refinements and with superior energy storage materials, even higher peak power in the petawatt range should be possible from tabletop systems. In this paper we show the ultimate achievable power and intensity, as well as their applications in science and technology. Their applications cover a wide variety of fields, such as precision surgery, micromachining, coherent and incoherent X-ray generation, thermonuclear ignition, particle acceleration, and nonlinear quantum electrodynamics.

Time-resolved structures of macromolecules at the ESRF: Single-pulse Laue diffraction, stroboscopic data collection and femtosecond flash photolysis

Abstract: We review the time structure of synchrotron radiation and its use for fast time-resolved diffraction experiments in macromolecular photocycles using flash photolysis to initiate the reaction. The source parameters and optics for ID09 at ESRF are presented together with the phase-locked chopper and femtosecond laser. The chopper can set up a 900 Hz pulse train of 100 ps pulses from the hybrid bunch-mode and, in conjunction with a femtosecond laser, it can be used for stroboscopic data collection with both monochromatic and polychromatic beams. Single-pulse Laue data from cutinase, a 22 kD lipolic enzyme, are presented which show that the quality of single-pulse Laue patterns are sufficient to refine the excited state(s) in a reaction pathway from a known ground state. The flash photolysis technique is discussed and an example is given for heme proteins. The radiation damage from a laser pulse in the femto and picosecond range can be reduced by triggering at a wavelength where the interaction is strong. We propose the use of microcrystals in the range 25–50 μm for efficient photolysis with femto and picosecond pulses. The performance of circular storage rings is compared with the predicted performance of an X-ray free electron laser (XFEL). The combination of micro beams, a gain of 105 photons per pulse and an ultrashort pulse length of 100 fs is likely to improve pulsed diffraction data very substantially. It may be used to image coherent nuclear motion at atomic resolution in ultrafast uni-molecular reactions.

Compact directly diode-pumped femtosecond Nd:glass chirped-pulse-amplification laser system

Abstract: An all-solid-state longitudinally diode-pumped Nd:glass chirped-pulse-amplification laser system producing pulses of 50-MW peak power has been developed. The diode-pumped Nd:glass regenerative amplifier produces pulses with energies as great as 56 µJ at a 1-kHz repetition rate and pulse durations as short as 450 fs after compression in a compact single holographic-transmission-grating stretcher–compressor arrangement. Further, spectral gain shaping was shown to extend the bandwidth that was supported in the low-gain amplifier. To the best of our knowledge, this system provides the highest peak and average power obtained from a directly diode-pumped femtosecond laser.

Observation of the plasma channel dynamics and Coulomb explosion in the interaction of a high-intensity laser pulse with a He gas jet

Abstract: We report the first interferometric observations of the dynamics of electron-ion cavitation of relativistically self-focused intense 4 TW, 400 fs laser pulse in a He gas jet. The electron density in a channel 1 mm long and 30 μm in diameter drops by a factor of approximately 10 from the maximum value of ∼8×1019 cm−3. A high radial velocity of the plasma expansion, ∼3.8×108 cm/s, corresponding to an ion energy of about 300 keV, is observed. The total energy of fast ions is estimated to be 6% of the laser pulse energy. The high-velocity radial plasma expulsion is explained by a charge separation due to the strong ponderomotive force. This experiment demonstrates a new possibility for direct transmission of a significant portion of the energy of a laser pulse to ions.

Ultrahigh-intensity laser: physics of the extreme on a tabletop

Abstract: This paper reviews the development of ultrahigh-intensity laser technology from the early 1960`s to the present, explaining the obstacles to each increase in intensity and the technical means used to overcome them. These included the shortening of pulses, mode locking, and chirped pulse amplification (CPA). The particular technical advances that make CPA possible included the invention of matched pulse stretchers and compressors and the development of ultrabroadband gain media. The paper then discusses the generation of ultrashort pulses and their characteristics. It then moves on to the Petawatt laser, which incorporates the CPA technology. It then addresses the question of whether it is possible to forecast the ultimate peak power that can be achieved by a laser system of a given size. Applications of ultrahigh-intensity lasers in different physical regimes are discussed.

Ultrashort laser pulses tackle precision machining

Abstract: The nature of ultrashort pulse interaction with matter permits ultrafast lasers to beat the diffraction limit-cutting and drilling materials with feature sizes smaller than the wavelength of light.

Development of a subpicosecond large-dynamic-range x-ray streak camera

Abstract: A novel subpicosecond x-ray streak camera (called PX1) was developed by the INRS group for subpicosecond time resolved spectroscopy in x rays and X-UV range. Using the PX1 camera, we have measured keV x-ray pulses with a 950 fs FWHM and a 850 fs rise time. The camera has also been coupled to ultrafast photoconductive switches and tested in jitter-free mode as a signal averaging detector. This instrument allows to analyze ultrafast changes in short wavelength signals with an unlimited dynamic range.

High-dynamic-range laser-pulse-contrast measurement with a plasma-shuttered streak camera.

Abstract: The dynamic range of a picosecond visible streak camera has been improved by the combination of a plasma shutter and multishot averaging performed with a photoconductive switch sweep circuit. We use this technique to measure the contrast of a 100-fs laser pulse over 2 ns with a dynamic range of 7 orders of magnitude.

Temporal characterization of a self-modulated laser wakefield

Abstract: The temporal envelope of plasma density oscillations in the wake of an intense (I∼4×1018 W/cm2, λ=1 μm) laser pulse (400 fs) is measured using forward Thomson scattering from a copropagating, frequency doubled probe pulse. The wakefield oscillations in a fully ionized helium plasma (ne=3×1019 cm−3) are observed to reach maximum amplitude (δne/ne∼0.1) 300 fs after the pump pulse. The wakefield growth (4 ps−1) and decay (1.9 ps−1) rates are consistent with the forward Raman scattering instability and beam loading, respectively.

All-solid-state, directly diode-pumped chirped-pulse amplification laser system

Abstract: Recently there has been increased interest in regenerative amplification of short pulses in titanium-doped sapphire at wavelengths considerably shorter than 800 nm.'r2 The motivation is the use of the third harmonic of near IR radiation to seed KrF excimer laser amplifiers for the generation of ultrashort ultraviolet pulses with peak powers as high as 1 TW.' However, in the cases referenced above, the authors report difficulty in sustaining amplification at these wavelengths without physically impeding the cavity from lasing at 800 nm by means of a knife edge. In this paper we will present the details of a regenerative amplifier that not only oscillates unseeded at 745 nm, but has demonstrated tunability in its oscillation over a range from 735 nm to beyond 800 nm with one set of mirrors. A diagram of the cavity is shown in Fig. 1. The cavity design is based on a 2.5-m-long half symmetric confocal arrangement with the spherical mirror having a 5 m radius of curvature. All optics are designed for optimum performance at 745 nm. The prisms are SFlO and are placed with an apex separation of 28 cm to compensate the positive GVD introduced by the other intercavity elements. The tunability is achieved by the use of the prism pair in the cavity in combination with the spherical mirror in that end of the cavity. At the curved mirror, the spectrum is spatially dispersed as a result ofthe prism pair. Since the mirror has a finite radius of curvature, only a narrow band of wavelengths are retroreflected through the cavity and are therefore amplified. Experimentally, there is an optimum ratio of mirror radius R to overall cavity length L, which we find to be approximately 2. Higher ratios result in a loss of tunability, while a lower ratio results in severe spectral narrowing. Figure 2 shows the unseeded, Q-switched spectral output of the amplifier cavity. We realize that this method of tuning limits the pulse duration that may be amplified in this cavity as a result of spectral narrowing. However, since we are not interested in pulse durations shorter than -100 fs because of the finite bandwidth of excimer lasers, we are unaffected by this limitation. The amplifier is pumped with 25 mJ (+-0.2 mJ) from a frequency-doubled Nd:YAG laser, 60607-7059.

Ytterbium:glass CPA regenerative amplifier pumped by a free-running Ti:sapphire laser

Abstract: Chirped pulse amplification (CPA) has enabled efficient energy extraction from solid-state lasers.1

Efficient gain switched operation of a highly doped Yb:phosphate glass laser

Abstract: Ytterbium-doped phosphate glass with 15wt% doping is spectrally characterized and gain switched resulting with a maximum pulse energy of 37 mJ at 5 Hz.

Wavefront detection and correction for ultra-intense laser systems

Abstract: In order to correct wave front distortions, a technique has been developed based on adaptive optics used in a pre- compensation configuration. The system consists of a wave front sensor and an optically driven liquid crystal Spatial Light Modulator as a wave front corrector. The main advantage of the sensor is its ability to detect phase deformation of several tens of wavelengths with an accuracy around (lambda) /10. This approach appears to be a good candidate for phase shaping ultra-intense laser system exhibiting strong wave front distortions. The use of a optically addressed single large liquid crystal cell covering the entire beam aperture makes this device diffraction free. The choice of a pre-compensation geometry is discussed and preliminary results are presented.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Single-pulse Laue diffraction, stroboscopic data collection and femtosecond flash photolysis on macromolecules

Abstract: We review the time structure of synchrotron radiation and its use for fast time-resolved diffraction experiments in macromolecular photo-cycles using flash photolysis to initiate the reaction. The source parameters and optics for ID09 at ESRF are presented together with the phase-locked chopper and femtosecond laser. The chopper can set up a 900 Hz pulse train of 100 ps pulses from the hybrid bunch-mode and, in conjunction with a femtosecond laser, it can be used for stroboscopic data collection with both monochromatic and polychromatic beams. Single-pulse Laue data from Cutinase, a 22 kD lipolic enzyme, are presented which show that the quality of single-pulse Laue patterns is sufficient to refine the excited state(s) in a reaction pathway from a known ground state. The flash photolysis technique is discussed and an example is given for heme proteins. The radiation damage from a laser pulse in the femto and picosecond range can be reduced by triggering at a wavelength where the interaction is strong. We propose the use of microcrystals between 25–50 μm for efficient photolysis with femto and picosecond pulses. The performance of circular storage rings is compared with the predicted performance of an X-ray free electron laser (XFEL). The combination of micro beams, a gain of 105 photons per pulse and an ultrashort pulse length of 100 fs is likely to improve pulsed diffraction data very substantially. It may be used to image coherent nuclear motion at atomic resolution in ultrafast uni-molecular reactions.We review the time structure of synchrotron radiation and its use for fast time-resolved diffraction experiments in macromolecular photo-cycles using flash photolysis to initiate the reaction. The source parameters and optics for ID09 at ESRF are presented together with the phase-locked chopper and femtosecond laser. The chopper can set up a 900 Hz pulse train of 100 ps pulses from the hybrid bunch-mode and, in conjunction with a femtosecond laser, it can be used for stroboscopic data collection with both monochromatic and polychromatic beams. Single-pulse Laue data from Cutinase, a 22 kD lipolic enzyme, are presented which show that the quality of single-pulse Laue patterns is sufficient to refine the excited state(s) in a reaction pathway from a known ground state. The flash photolysis technique is discussed and an example is given for heme proteins. The radiation damage from a laser pulse in the femto and picosecond range can be reduced by triggering at a wavelength where the interaction is strong. We ...

Traveling-wave electro-optic modulator with a novel broadband velocity-matching structure

Abstract: This summary describes a novel GaAs electro-optic modulator that utilizes a resonant free, broadband, and velocity matching in order to achieve a reduced half-wave voltage and improved optical confinements. We have developed a technique to deposit, at room temperature, an amorphous hydrogenated silicon cover on the coplanar electrical waveguide of the electro-optic modulator.

High-dynamic-range Measurement Of Laser Pulse Contrast By Use Of A Plasmashuttered Jitter-free Streak Camera

Abstract: Recently there has been significant progress in the development of high-repetition-rate, highpeak-power Tisapphire systems.' Recent experiments in high harmonic generation (HHG) have demonstrated that with sufficiently short laser pulses (-20 fs), laser energies of 2-10 mJ can be used to generate very high-order harmonics, with photon energy up to 240 eV.2,3 However, to date, the lasers used to generate very high-order harmonics have typically been low-repetition-rate (10 Hz) systems. A kHz repetition-rate laser with sufficient peak power for high-harmonic generation would result in a 100 times increase in x-ray flux, with vastly improved signal-tonoise ratios. In addition, other processes such as low-order harmonic generation in gases4 become even more efficient with millijoules of laser energy. Thermal lensing is the main obstacle to scaling ultrafast Tisapphire systems to high average power, because the thermal time constant for sapphire is tens of milliseconds. If the focal length of the thermal lens is longer than the Rayleigh range in a focused beam, as it is in oscillators and low average power amplifiers ( < O S mJ compressed), the lensing effect is not ~ t r o n g . ~ However, with the larger beam sizes found in power amplifiers, thermal lensing compensation is critical. We have developed a Tisapphire amplifier system that generates a peak power of 0.125 TW, at 1-kHz repetition rate, with 2.5 mJ of energy, in a 20-fs pulse duration. The first stage is a ring multipass amplifier, which has been demonstrated previously.' It is pumped by 9 mJ from an intracavity doubled Nd:YLF laser (Quantronix 527). The second stage has a similar design to the first stage, and is pumped

A simple technique to remove thermal distortions in pulsed solid-state lasers

Abstract: A simple technique to remove thermal distortions allowing for an increase in the average power of a pulsed laser is discussed. The technique is implemented in a rotating Nd:glass hollow cylinder laser pumped by an alexandrite laser.

Single shot wave front measurement using a three wave interferometer

Abstract: A simple three wave interferometric technique is used to measure for the first time with an accuracy of λ/50 the wave front of femtosecond ultrahigh peak power pulses in a single shot.