Laser

Résumé

thumb|250px|Lasers rouges (660 & ), verts (532 & ) et bleus (445 & ). thumb|250px|Rayon laser à travers un dispositif optique. thumb|250px|Démonstration de laser hélium-néon au laboratoire Kastler-Brossel à l'Université Pierre-et-Marie-Curie. Un laser (acronyme issu de l'anglais light amplification by
stimulated emission of radiation qui signifie « amplification de la lumière par émission stimulée de radiation ») est un système photonique. Il s'agit d'un appareil qui produit un rayonnement lumineux spatialement et temporellement cohérent reposant sur le processus d'émission stimulée. La lumière du laser est aussi appelée lumière cohérente. Descendant du maser, le laser s'est d'abord appelé « maser optique ». Une source laser associe un amplificateur optique à une cavité optique, encore appelée résonateur, généralement constituée de deux miroirs, dont un au moins est partiellement réfléchissant, c'est-à-dire qu'une partie de la lumière sort de la cavité et l'autre partie est r

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https://fr.wikipedia.org/wiki/Laser

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The common configuration for deflecting laser beams in two dimensions is based on two single-axis galvanometric scanners placed perpendicular to each other. Over recent years, these fast scanners have proved to be both very reliable and very accurate. They are widely available in the marketplace and have wide-ranging technical specifications. However, such a configuration is rather bulky and can not satisfy the needs of industry which seeks compact solutions for an easy integration of these scanning heads into their production lines. In addition, two separate mirrors leads to optical aberrations when these scanning heads are used in conjunction with scanning lenses, which are specially designed for a single entrance pupil. Therefore, a compact fast-steering tip/tilt laser scanner with a single mirror would act as a key component in diverse applications, e.g. in material processing, astronomy, intersatellite laser communications, imaging systems, bio-medical and ophthalmologic applications. The compact two-axis tilt laser scanner presented in this work can perform a variety of functions such as image tracking, beam stabilization and alignment, line-of-sight pointing, and scene scanning. Designing such a scanner required a multi-disciplinary approach involving optics, mechanics, electromagnetics, sensors and control theories. The scanner is composed of one single mirror with a large active area and has a single point of rotation for the two axes of tilt. Moreover, the center of gravity coincides with the center of rotation to reach optimum dynamic behavior. The scanner's compact dimensions allow an easy integration in the various optical systems. Furthermore, the mirror is easily interchangeable which gives greater flexibility in the choice of the laser source. Another major advantage of the proposed design is its ability to be scaled down to miniaturized versions. The scaling depends mainly on the active surface of the mirror, which is generally specified by the application. The larger the mirror, the higher its inertia and the larger are the actuators in order to guarantee high performance. Two pairs of push-pull linear electromagnetic actuators are used to drive the mirror of the scanner. The mirror suspension is based on a sliding bearing composed of a cone-ball contact with optimized friction and wear behavior. A magnetic pre-load force is used to hold the mirror and to give sufficient rotational stiffness. Furthermore, a position transducer based on a photodetector and a miniature laser pen is integrated in the module and is used for closed loop feedback positioning. The mirror can be tilted around both axes by ± 61mrad (± 3.5°) with an accuracy better than 50μrad. Moreover, a differential resolution in the order of 2.5μrad, and settling times for maximum tip and tilt deflections of 10ms and 14ms respectively are achieved. The overall volume of the scanner is 30 x 40 x 50 mm3, and its total weight does not exceed 90g. Design, simulations and experimental investigations were carried out in this work in order to optimize the different components of the tip/tilt scanner. These components are: the motors, the bearing system, the position transducer and the feedback control loop. An analytical model that allows an optimization of the geometrical parameters for an electromagnetic actuator based on rectangular coils and magnets is proposed. This model proved to be very reliable and can be extended to the design of a variety of electromagnetic driving systems. In addition, an extensive analysis of the beam path distortions generated when a single mirror is pivoted around two axes is covered. Furthermore, correcting equations used to compensate for the systematic errors resulting from this configuration are proposed. The tip/tilt scanner is suited to a variety of applications in industrial material laser processing. These applications are not just restricted to conventional processes like welding, cutting, drilling, and marking, but also to new technologies such as micro-manipulation of parts by laser. In this new approach, bending and shrinkage of the material result due to laser irradiation, allowing fast adjustment and positioning of micro-mechanical and opto-mechanical components with sub-micron accuracy. Furthermore, local annealing of shape memory alloys, leading to stiffness variation in monolithic parts such as micro-grippers, represents a very innovative application in smart materials. On the other hand, the scanner is also a key component in bio-medical and ophthalmologic applications. The first measurements carried out with the scanner for optical tomography of the retina appeared very promising. Furthermore, an optical tweezers set-up has been realized using the scanner, allowing easy and flexible manipulation of laser beam trapped cells and molecules.

EPFL2000

Chargement

On isotopically selective collisional energy transfer and infrared multiple photon absorption by vibrationally highly excited molecules

Richard Bossart

This study focuses on an interesting and important phenomenon that was employed in a new approach to laser isotope separation that has been recently proposed and developed in our laboratory for highly selective separation of carbon isotopes. This approach consists of pre-exciting CF3H molecules with the desired isotope to a low vibrational overtone of the CH-stretch with a subsequent selective infrared multiple photon dissociation (IRMPD) of only the pre-excited molecules by a CO2-laser pulse. Significant isotopic shifts of the employed CH overtone bands already allow high selectivity at the pre-excitation step. This selectivity, however, can be further greatly increased by increasing the pressure of the sample gas and/or the delay between the two laser pulses; that is, by increasing the number of molecular collisions during the process. At first glance, the observed effect contradicts the general expectations that the isotopic selectivity of such a process should drop with an increase in the number of collisions because of vibrational energy transfer between different isotopic species. In this work we have studied this phenomenon and found its physical origins. We propose the contribution of two different mechanisms to the observed enhancement of isotopic selectivity by collisions. First, the vibrational collisional relaxation itself is isotopically selective, that is vibrationally excited 12CF3H relax on the bath of cold, 12CF3H, molecules faster than the excited 13CF3H do on the same bath. The primary reason for such a selectivity could be a significant isotopic shift of the vibration (CF-stretch, we believe) that mediates the energy transfer. A second mechanism arises from the difference between the IRMPD probabilities of the two isotopic species by the CO2 laser tuned to a particular wavelength that enhances the dissociation yield of the targeted, carbon-13, species. As collisional deactivation of both species increases the number of photons they have to absorb to be dissociated, this difference in dissociation probability increases as well, yielding an additional isotopic selectivity of the process. We perform a set of experiments and numerical simulations to investigate these two mechanisms. Experimentally we find that, indeed, collisional vibrational deactivation of CF3H is isotopically selective. This is, perhaps, the first direct observation of isotopically selective collisional relaxation of highly excited medium-sized molecules. At low pressures and increased time-delay between the lasers both suggested mechanisms contribute equally to the enhancement of isotopic selectivity (with a slight dominance of the IRMPD step). We also perform numerical simulations of vibrational energy transfer (VET) between highly excited and cold CF3H of both isotopic species. The model we employ includes V-V energy transfer due to long-range dipole-dipole interactions and V-V',T,R energy transfer due to head-on collisions between two molecules. The results reproduce well the measured isotopic selectivity in vibrational energy transfer. At the next step, we propose a model for the simulation of the laser isotope separation process. We solve the master equations including vibrational energy transfer and absorption and stimulated emission of IR photons. The main improvement of our calculations with respect to existing models is that we introduce a dependence of the absorption/emission rates on the frequency of the laser, the internal energy of the molecule and its isotopic species. We are able to reproduce our experiments numerically and thus gain information on the laser isotope separation process. In particular, we find that at high sample pressures the mechanism of isotopically selective IRMPD rates prevails over the mechanism of different collisional relaxation rates.

EPFL2005

Chargement

Additive manufacturing (AM) is a group of processing technologies which has the potential to revolutionize manufacturing by allowing easy manufacturing of complex shapes and small series. One AM-method which is of high interest for processing of metals is laser powder bed fusion (LPBF), where a powerful laser is used to melt powder in a layer-by-layer fashion into a consolidated part.Many defects can form during LPBF processing, but copper, copper alloys, and similar metals are particularly prone to the formation of pores.The reason why these metals tend to have these defects is the high reflectivity decreasing the effective energy input, and the high thermal conductivity transporting the absorbed energy away. Several ways of decreasing the pore formation tendency are known: increasing the power, increasing the absorptivity, and alloying to decrease thermal conductivity.The relative effectiveness of these strategies are however not known, making it difficult to choose the correct strategy for the different industrial applications of these metals.In this thesis a simple analytical model for predicting the occurrence of these defects is proposed, allowing a lowest possible processing power given the material properties to be estimated. The model agrees well with experiments, and can be used to predict the transition from so called balling mode processing to conduction mode processing, and for qualitative analysis of different processing strategies. Two strategies of decreasing the porosity are proposed and tested: coating of the powder with an absorbing layer and processing with a green laser, which is mor e strongly absorbed by copper than the conventional IR-lasers.The copper powder was coated with thin layers of tin and nickel using a simple and cheap immersion deposition method, and it is shown that the amount of porosity is decreased more than can be expected of the alloying alone, showing both that the method is working and that laser interaction with the solid is important for the heat balance of the system.The interaction with the solid is shown through computational fluid dynamics simulations to arise from periodic fluctuations of the molten metal which cause laser light to be reflected forward, pre-heating the powder bed.The preheating is larger the higher the solid absorptivity is, explaining the porosity decrease when using coated powders, and shows that processing using a green laser increase the laser coupling more than the absorptivity of the liquid would.Single line tracks made from fine 5 µm pure copper with a green laser source show that the desired conduction mode melting can be achieved at a power below 70 W, an order of magnitude lower than what is needed for processing of the conventionally used 45 µm powder with an IR-laser.

EPFL2021

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On isotopically selective collisional energy transfer and infrared multiple photon absorption by vibrationally highly excited molecules

Richard Bossart

EPFL2005

EPFL2021

EPFL2000