A theoretical time-dependent analysis of a high-average power copper HyBrID laser is proposed, pointing out the time-varying properties of beam quality parameters and brightness. Numerical data are compared to experimental measurements performed on a 80 W average power copper HyBrID laser. A significant improvement of the beam quality with time is put in evidence.
We propose an active and adaptative optics device dedicated to programmable femtosecond beam shaping, based on the use of an optically addressed light valve. A theoretical investigation of the system is presented. The experimental set-up incorporating an active beam shaping device, is depicted. Results are then described and discussed.
In the field of biophotonics the main goals are the control and processing of in vivo biological tissues and the monitoring of biomolecule dynamics. Two particular “pitfalls” are present: the dynamic multiscale organization and the photostress of the medium. Until now the state of the art of the pico-femtosecond systems designed to these applications shows that the changing laser technology has been only used as an add-on. Our approach is based on a bottom-up procedure and on the medium-centered knowledge. The range of neurobiological applications of ultrafast photonics extends from TRP (time-resolved propagation) to linear and non-linear TRE (time-resolved emission). The device combines a one kilohertz chirp pulse amplification laser system and a single shot streak camera. For discrete wavelength applications (TRE), the set-up is a SHG/OPG/OPA3/SHG design. In the case of TRP, the beam is focused into pure water to generate a white light continuum. After propagation through tissue, a single-shot streak camera with single photo-electron counting capability performs the picosecond time-resolved spectroscopy of the collected photons. Depending on the acceptable level of photostress, the integration time can extend from 33ms up to several minutes with a real-time control of the jitter and time drifts. The meaning of the TRE spectro-temporal image is particularly detailed in the 450-480nm excitation window in regards to the contributions of mitochondrial flavoproteins. This optical system fulfills the reliability and the sensitivity, conditions required for measuring opto-electronic quantities from freely moving animal at low irradiation.
Due to the rapid development of ultrashort lasers, quality of the machining is of prime interest for several applications. For instance deep marking of various materials. In this case, the depth can be controlled, knowing the ablation rate for the corresponding material. The evolution of ablation rates of Al, Cu and Ni are given in relation to the energy density. In metals the effect of thermal diffusion has to be taken into account to control collateral effects and especially the heat affected zone.
The advent of table top high repetition rate regeneratively amplified femtosecond lasers has opened the way to many recent and fast developments towards applications of economical interest. The most well known are microprocessing, thin film elaboration, waveguide photoinscription, surface treatment, dentistry, ophthalmology. Recent studies on microprocessing and laser-matter interaction using femtosecond lasers are reported. This is done using largely presently performed work in Saint-Etienne including investigations on Heat Affected Zone (HAZ), plume expansion characterization, and thin film elaboration. Indeed specific characters appear as compared to what is obtained using multipicosecond/nanosecond laser pulses : Typical submicronic HAZ lengths have been evidenced and particle energies of plasma plume ranging up to a few KeV (carbon target) using typical pulse energies of 1 mJ (150 fs, 800 nm, 1 KHz), creating specific conditions for deposition. The concept of the often vocabled 'athermal' interaction is discussed. Emphasis on actual microprocessing capability of the existing sources to approach industrial applications are questioned in terms of energy per pulse, timewidth, repetition rates and the need for further source development and control beam improvement stressed. A brief review of the progresses under way in these fields and their capability to answer to actual large scale commercial applications are given.
Several works on laser-matter interaction has shown the differences in sizes for the Heat Affected Zone (HAZ) obtained with nanosecond and femtosecond regimes in laser cutting or drilling. To understand more clearly the basic phenomena that occur in femtosecond regime during the absorption of light by matter, and specially in the case of metals, we have developed both an experimental and a theoretical approach. We use a new method aimed at quantifying the dimensions of the HAZ, using thin-down samples which are micro-drilled and then observed by a transmission electronic microscopy (TEM) technique. The grain size in the samples is analysed near the micro-holes. According to theoretical studies, the thermal diffusion is due to the smaller value of the electron specific heat compared to the lattice one. The thermal diffusion length is found to be a few hundred of nanometers in the case of metals. We use a thermal model to describe the heat diffusion in the sample in order to obtain a theoretical estimation of the HAZ. Holes are drilled in Aluminum using nanosecond and femtosecond laser pulses and characterized by Transmission Electronic Microscopy (TEM). The method for quantifying the dimensions of the heat affected zone (HAZ) surrounding micro-holes is based on the analyze of the grain size evolution. The experiments are using the same Ti-Sapphire laser source (1 kHz, 800 nm). The regeneratively amplified ultra-short pulses (150 fs) are utilized at a low fluence regime (typically 0.01-0.5 mJ/pulse), while the longer pulses (ns) are obtained from the regenerative amplifier without oscillator seeding (0.5 mJ,τ approximately 7-8 ns). The main conclusion is that a 40 micrometers wide HAZ is induced by nanosecond pulses, whereas the femtosecond regime does not produce any TEM observable HAZ. It has to be noticed that the width of the femtosecond HAZ is roughly less than 2 micrometers , which is our observation limit. These results are in agreement with theoretical predictions.
Pulsed laser ablation is a well-known technique used for thin film deposition, extending from oxydes to hard and wear resistant Diamond-Line Carbon (DLC) films. Most of the previous studies devoted to DLC thin films elaboration have used pulsed duration in the nanosecond range. The present study concerns femtosecond (10-15 s range) laser ablation of a graphite target for the elaboration of Diamond-Like Carbon. Compared to conventional nanosecond laser ablation, femtosecond laser pulses allow the production of high energy (up to a few keV) ions in the plasma, which may strongly affect the structure and properties of the deposited films. DLC films have been deposited under vacuum onto (100) p-type silicon substrates at room temperature, by ablating graphite targets with femtosecond laser pulses. The nature and properties of the film have been characterized by various techniques, including Raman, XPS and AFM. Discussion will be focused on the comparison between present results obtained using femtosecond laser pulses, with previously published results related to DLC films deposited using nanosecond laser pulses. Especially, Raman spectra of DLC films obtained by nanosecond laser ablation always show the two well-known D and G bands (located respectively at around 1350 cm-1 and 1550 cm-1), whereas some DLC films obtained when using femtosecond laser pulses exhibit an intense peak at 1140 cm-1, which may be attributed to nanocrystalline diamond.
Holes drilled in Aluminum using nanosecond and femtosecond laser pluses are characterized by Transmission Electronic Microscopy (TEM). Hence we present a method for quantifying the dimensions of the heat affected zone (HAZ) surrounding micro-holes by analyzing the grain size evolution. Drilled samples investigations are performed after electrolytic thinning down to 100 nm. The experiments require a real time imaging system to shot close to the located thinner zone with an accuracy in the micrometers range. Thin Al samples are drilled both in nanosecond and femtosecond regimes using het same pulses number and the same Ti-Sapphire laser source. The regeneratively amplified ultra-short pulses are utilized at a low fluence regime, while the longer pulses are obtained from the regenerative amplifier without oscillator seeding. The main conclusion is that a 40 micrometers wide HAZ is induced by nanosecond pulses, whereas the femtosecond regime does not produce any TEM observable HAZ. It has to be noticed that the width of the femtosecond HAZ is roughly less than 2 micrometers , which is our observation limit.
Our purpose is to spectrally probe the main brain absorbers. The determination of their spatial distribution remains a challenge. According to anatomical data, the proposed 3D model of the rat pial-cortical vascular networks is divided into three parts: (1) the pial vessels could be approximated by a dense layer of around 250 micrometers depth; (2) the penetrating vessels repartition is described as periodic hexagonal prisms with three modules; (3) the capillary network is modelized using a periodic tiling of polyhedron with a density of 817mm.mm-3 and a branching pattern of 10000mm-3. With anaesthetized rats under stereotaxic conditions, in vivo time-resolved brain spectroscopy experiments are presented. The setup is designed to allow broadband time-resolved spectroscopy using a streak camera. A femtosecond white light continuum is produced by focusing 800nm pulses (0.5mJ, 1kHz, 150fs) in an adapted third order non linear medium. In the case of water, the spectrum expands over 380-780nm with an efficiency of 20 percent. Mathematical homogenization techniques could be applied to the radiative transfer equation with this geometrical vascular architecture and might be useful to analyze in depth time-resolved spectroscopy of such complex media.
Ultrashort duration laser sources are considered as a promising tool for new micromachining applications: precise microdrilling and microcutting on various materials. As an illustration of the non thermal micromachining, we also paid attention to precious wood. Cutting is achieved without burning, and the cut surface remains undamaged. However, until now, only low-average-power sources are available. An average power level of 10 W appears to be the lowest limit for this type of laser to really become industrial. We are presently developing such a source based on the use of a 15kHz, 100W copper HyBrID laser as a pump laser. Thus, we intend to reach in a near future a typical drilling rate of one mm per second, for instance, in stainless steel compared with a 50 microns per second drilling rate obtained with presently available kHz and low-average-power sources. Micromachining obtained with out a 1 kHz source will be presented and discussed.