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Coherent anti-Stokes Raman spectroscopy (CARS) is a powerful tool in combustion diagnostics [1] . It requires three beams, the pump, Stokes and probe beam with at least two different wavelengths. For high pressure applications, the use of fs-pulses is of additional benefit, as the CARS signal is free from distortions from molecular collisions, if the excited Raman level is probed within a few ps [2] . On the other hand, too short pulses (~100-500 fs) result in poor spectral resolution [3] . In this study, we present an optical parametric amplifier (OPA), providing a maximum pulse energy of ~200 µJ, at 700-900 nm and a pulse duration of ~1 ps. Using a pump source at 1030 nm, this setup may serve as a customized fs-CARS system for high pressure gas analysis.

Due to ageing populations the number of tumors is increasing worldwide. Successful surgical treatment requires complete resection of tumors to reduce recurrence rates. To reach this goal, novel methods combining in vivo tumor and tumor margin detection with low invasive precision surgical tools are needed. Coherent anti-Stokes Raman scattering (CARS) imaging is a highly promising optical tool for visualizing tumors based on characteristic changes in tissue morphology and molecular composition, while fs-laser ablation is to date the most precise surgical tool established in ophthalmology. In this contribution, CARS imaging has been combined with fs-laser ablation as a new approach for image-guided precision surgery for the first time. CARS guided fs-ablation has been applied to ablate brain, liver, skin, muscular and vascular tissues with μm-precision using sub-100 fs pulses of μJ level. We demonstrate superior imaging performance and contrast as well as detection of tissue margins by coherent Raman microscopy in comparison to laser reflectance imaging. The combination of CARS-image-guided tissue ablation is a promising tool for minimally invasive surgeries particularly in the vicinity of functional structures in the future.

Femtosecond (fs)-CARS is a promising approach for gas spectroscopy under high pressure and temperature conditions, as it allows probing molecular states on a time scale which is significantly shorter than the typical decay time induced by interfering collisions. Usually, fs-CARS is performed in a three beam setup, which requires maintaining spatial and temporal overlap of the pulses at the focal point. This is a challenging task, especially in harsh environments such as in a combustion chamber. In this study, we present an alternative approach, which uses two beams in a collinear configuration. An ultra-broadband, sub 7 fs laser pulse acts as pump and Stokes pulse, and a ∼500 fs pulse is used for probing. We show that this configuration is suitable for measuring the gas temperature and concentration. Furthermore, possible single shot measurements of the gas temperature are evaluated.

Background and Objective To investigate femtosecond (fs)-laser patterns within ocular media for ophthalmic surgery. Methods Vitreous and crystalline lens tissue from porcine eyes were treated with 2-dimensional fs-laser patterns and inspected under the optical microscope. Time resolved pump-probe experiments were conducted on vitreous tissue and gelatin, which should act as a model for crystalline lens tissue. Results Within crystalline lens tissue, pulse overlap leads to the formation of large bubbles, which is caused by subsequent energy input from the surrounding plasma channel. This effect can be used for bubble size control. Vitreous tissue behaves similar to water under fs-laser treatment, but it still allows fs-laser cutting. Conclusion Bubble size control by laser bursts may reduce optical side-effects of fs-laser treatment. Furthermore, fs-laser treatment could be used for vitreoretinal applications. Lasers Surg. Med. 45:589–596, 2013. © 2013 Wiley Periodicals, Inc.

International audience; Ultrashort pulse lasers enable reliable and versatile high precision ablation and surface processing of various materials such as metals, polymers and semiconductors. However, when modifications deep inside the bulk of transparent media are required, nonlinear pulse material interactions can decrease the precision, since weak focusing and the long propagation of the intense pulses within the nonlinear media may induce Kerr self-focusing, filamentation and white light generation. In order to improve the precision of those modifications, simultaneous spatial and temporal focusing (SSTF) allows to reduce detrimental nonlinear interactions, because the ultrashort pulse duration is only obtained at the focus, while outside of the focal region the continuously increasing pulse duration strongly reduces the pulse intensity. In this paper, we review the fundamental concepts of this technology and provide an overview of its applications for purposes of multiphoton microscopy and laser materials processing. Moreover, numerical simulations on the nonlinear pulse propagation within transparent media illustrate the linear and nonlinear pulse propagation, highlighting the differences between conventional focusing and SSTF. Finally, fs-laser induced modifications in gelatine are presented to compare nonlinear side-effects caused by conventional focusing and SSTF. With conventional focusing the complex interplay of self-focusing and filamentation induces strongly inhomogeneous, elongated disruptions. In contrast, disruptions induced by SSTF are homogeneously located at the focal plane and reduced in length by a factor >2, which is in excellent agreement with the numerical simulations of the nonlinear pulse propagation and might favor SSTF for demanding applications such as intraocular fs-laser surgery.

International audience; We report new experimental and numerical results on supercontinuum generation at ultraviolet/visible wavelengths produced by the propagation of infrared femtosecond laser pulses in air. Spectral broadening is shown to similarly affect single filaments over laboratory distance scales, as well as broad beams over long-range propagation distances. Numerical simulations display evidence of the crucial role of third harmonic generation in the build-up of UV-visible wavelengths, by comparison with current single-envelope models including chromatic dispersion and self-steepening.

International audience; In recent years, femtosecond (fs)-lasers have evolved into a versatile tool for high precision micromachining of transparent materials because nonlinear absorption in the focus can result in refractive index modifications or material disruptions. However, when high pulse energies or low numerical apertures are required, nonlinear side effects such as self-focusing, filamentation or white light generation can decrease the modification quality. In this paper, we apply simultaneous spatial and temporal focusing (SSTF) to overcome these limitations. The main advantage of SSTF is that the ultrashort pulse is only formed at the focal plane, thereby confining the intensity distribution strongly to the focal volume and suppressing detrimental nonlinear side effects. Thus, we investigate the optical breakdown within a water cell by pump-probe shadowgraphy, comparing conventional focusing and SSTF under equivalent focusing conditions. The plasma formation is well confined for low pulse energies

Objective: In this feasibility study, we investigate possible femtosecond laser thrombolysis. Background data: Because of low pulse energies, femtosecond laser surgery inherently minimizes side effects on the surrounding tissue. Moreover, current femtosecond laser sources as well as fiber technology allow consideration of catheter-based treatments. Methods: Two femtosecond laser systems (?=800 nm, ?=1030 nm) along with a three dimensional (3D) scanner system (NA 0.1) were used in this study. In vitro experiments were performed on porcine thrombi and blood vessels. Ablation thresholds were determined in air, by determining the pulse energy at which single shot ablation was visible under the optical microscope. Ablation rates were determined in physiological saline. Additionally, ablation of thrombi and blood vessels was monitored by means of a fiber spectrometer. Results: Depending upon the scan velocity, typical ablation rates for thrombi were 0.04 mm3/sec. Ablation thresholds of thrombi and blood vessels differ by factors of 3 and 1.5 at laser wavelengths of 800 and 1030 nm, respectively. At a distance of 5 mm above the surface, second harmonic generation was observed in blood vessels, but not within thrombi. Conclusions: The results show that a typical thrombus volume can be destroyed within a reasonable time frame. Because of the higher threshold difference of thrombi and blood vessels, the use of a laser wavelength of 800 nm is preferable. Furthermore, the detection of the second harmonic could provide a feedback mechanism to protect the vascular wall from mechanical and laser damage.

Femtosecond lasers are a versatile tool to process transparent materials like glasses, polymers or ophthalmic tissue. However, when focusing pulses of several μJ into the material, the high intensity near the laser focus leads to undesired nonlinear side effects like self-focusing and filamentation, resulting in an increased length of the induced plasma or the fragmentation of the breakdown volume. To overcome this limitation, we studied the influence of simultaneous spatial and temporal focusing (SSTF) on the laser induced optical breakdown (LIOB) in water. For this purpose, the incoming laser pulse is spectrally separated by a grating stretcher setup and recompressed by the focusing optics. Due to the increased pulse duration outside of the laser focus, the nonlinear laser-material interaction is confined to the focal region. We investigated the formation of the plasma and the resulting disruption in water by shadow imaging. With conventional focusing (τ = 70 fs, NA = 0.1) self-focusing, filamentation and breakup of the disruption volume was observed for pulse energies > 2 μJ, leading to a breakdown length of ~ 800 μm at a pulse energy of 8 μJ. With SSTF the axial length of the breakdown is significantly reduced by a factor of ~ 2. Plasma formation and the resulting disruption stay within the focal region. No self-focusing could be observed for pulse energies up to 8 μJ. Therefore, SSTF appears to be a promising tool to induce photodisruptions in transparent materials even with low numerical aperture, e.g. for precise fs-laser surgery within the posterior segment of the eye.

We demonstrate a femtosecond fiber laser system delivering >5-μJ, sub-400-fs pulses at a pulse repetition rate of 200 kHz. At constant average power the pulse repetition rate of this Watt-level femtosecond laser can be adjusted up to several MHz. The laser is monolithically integrated from the oscillator to the booster amplifier stage. The system was applied for structuring metallic as well as transparent media as e.g. biological tissues in ophthalmology.

International audience; Lightning research needs on-demand lightning strikes, because of the random character of natural lightning. Lasers have been proposed as alternatives to the current technique using rocket-pulled wires, because they would expectedly provide more flexibility. However, high-energy, nanosecond lasers cannot provide long connected plasma channels. In contrast, we recently reported the triggering of electric events in thunderclouds using ultrashort laser pulses. Further improvements of the laser pulse sequence and experiment geometry are discussed.

International audience; We investigated the possibility to trigger real-scale lightning using ionized filaments generated by ultrashort laser pulses in the atmosphere. Under conditions of high electric field during two thunderstorms, we observed a statistically significant number of electric events synchronized with the laser pulses, at the location of the filaments. This observation suggests that corona discharges may have been triggered by filaments.

We show that laser filamentation can be initiated and propagate through strong extended turbulence well above the typical atmospheric values. We suggest that the effect of turbulence on filamentation is characterized by the product of the structure parameter for the refractive index Cn2 and the length L of the turbulence region. Half of the filaments are transmitted for Cn2L⩽4.4×10−10m1∕3. Moreover, the surviving filaments keep their key spectral properties including correlations inside the white-light continuum.

Due to potential of applications, self-trapping of a peak-power laser pulse in a so called filament became an intensively investigated phenomenon. In this paper we demonstrate experimentally advantages of using filaments for the remote laser induced plasma spectroscopy (LIBS). This novel approach can increase effective range of conventional LIBS system up to single kilometers. We also show that Fourier-limited pulse does not optimize LIBS signal, opening the perspective for the pulse shaping techniques in a break-down spectroscopy.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In this paper, ultrashort laser pulses while propagating in air undergo filamentation which results from a dynamic balance between Kerr lens focusing and defocusing on laser-induced micro plasma. Such equilibrium leads to a self-guided ionized channel, or filament, with a diameter on the order of 100 /spl mu/m, extending over long distances up to several hundreds of meters. Using the Teramobile laser system providing 250 mJ pulses of 100 fs duration, centered at 800 nm, we have shown that the beam propagation for multi-TW pulses is driven by the interplay between random nucleation of small-scale cells and relaxation to long waveguides. We compare experimental results with simulations. Also, a white-light supercontinuum is generated by self-phase modulation, and mixing with the generated third harmonic generation (THG) extends the spectrum down to 230 nm in the infrared, providing a promising light source for multiparameter remote sensing by Lidar.

International audience; We demonstrate that the capacity of ultrashort high-power laser pulses to trigger and guide high-voltage discharges can be significantly enhanced by a subsequent visible nanosecond laser pulse. The femtosecond pulse induces a bundle of filaments, which creates a conducting channel of low density and cold plasma connecting the electrodes. The subsequent laser pulse photodetaches electrons from O2- ions in the electrode leader. The resulting electrons allow efficient heating by Joule effect in a retroaction loop, resulting in a 5% reduction of the breakdown voltage.

We report experimental and numerical results on supercontinuum generation at ultraviolet/visible wavelengths produced by the long-range propagation of infrared femtosecond laser pulses in air.

Optical side-effects of fs-laser treatment in refractive surgery are investigated by means of a model eye. We show that rainbow glare is the predominant perturbation, which can be avoided by randomly distributing laser spots within the lens. For corneal applications such as fs-LASIK, even a regular grid with spot-to-spot distances of ~3 µm is sufficient to minimize rainbow glare perception. Contrast sensitivity is affected, when the lens is treated with large 3D-patterns.

The authors demonstrate optimal control of the propagation of ultrashort, ultraintense (multiterawatt) laser pulses in air over distances up to 36m in a closed-loop scheme. They optimized three spectral ranges within the white-light continuum as well as the ionization efficiency. Optimization results in signal enhancements by typical factors of 2 and 1.4 for the target parameters. The optimization results in shorter pulses by reducing their chirp in the case of white-light continuum generation, while they correct the pulse from its defects and set the filamentation onset near the detector as far as air ionization is concerned.

Femtosecond coherent anti-Stokes Raman scattering (fs-CARS) is an ideal approach for non-invasive temperature measurement under high temperature and high pressure conditions [1], which is essentially important for applications in gasification or combustion [2]. In this study, the important combustion species hydrogen (H 2 ), nitrogen (N2) and carbon dioxide (CO 2 ) are used for temperature determination. Using a ∼ 7 fs ultra-broadband (∼ 650 nm – 1100 nm, 200 kHz) beam from an OPCPA laser as the pump/Stokes beam, and a ∼ 1.5 ps narrowband (∼ 516 nm, 200 kHz) beam as the probe, we show that our two-beam fs-CARS setup enables a wide simultaneously detectable range for multiple molecules, covering Raman shift up to ∼ 4160 cm−1. Moreover, we present that in a H 2 /N 2 /CO 2 gas mixture, the temperature can be determined without considering the non-resonant signal simultaneously from a single spectrum of H 2 /CO 2 fs-CARS within ∼ 3 ps probe delay [3], and also from the decay rate of N 2 fs-CARS signal in the same measurement. Temperature measurements have been implemented up to 1000 K and at pressures up to 20 bar. This provides high flexibility when targeting temperature determination under different environments. The modelling for fs-CARS gas temperature determination is described in detail in [4].

We investigated the laser induced plasma in air by means of coherent Anti-Stokes Raman scattering. The generation and decay of N 2 + was studied for different pulse energies.