Optically pump-induced athermal and nonresonant refractive index changes in the reference Cr-doped laser materials: Cr:GSGG and ruby

The refractive index of most ion-doped materials increases with the excited state population. This effect was studied in many laser materials, particularly those doped with 'Cr POT. 3+' and rare earth ions, using several techniques, such as interferometry, wave mixing, and Z-scans. This refractive index variation is athermal (has an electronic origin) and is associated with the difference in the polarizabilites of the 'Cr POT. 3+' ion in its excited and ground states, 'delta''alfa IND. p'. The 'Cr POT. 3+' optical transitions in the visible domain are electric-dipole forbidden, and they have low oscillator strengths. Therefore, the major contribution to 'delta''alfa IND. p' has been assigned to allowed transitions to charge transfer bands (CTBs) in the UV with strengths 'DA ORDEM DE' 3 orders of magnitude higher. Although this CTB model qualitatively explains the main observations, it was never quantitatively tested. In order to further investigate the physical origin of 'delta''alfa IND. p' in 'Cr POT. 3+'-doped crystals, excited state absorption (ESA) and Z-scan measurements were thus performed in Cr:'Al IND. 2''O IND. 3' (ruby) and Cr:GSGG. Cr:GSGG was selected because of the proximity of its 'ANTPOT. 2 E' and '4 ANTPOT. T IND. 2' emitting levels, and thus the possibility to explore the role of the spin selection rule in the ESA spectra and the resulting variations in polarizability by comparing low and room temperature data, which were never reported before.On the other hand, Cr:'Al IND. 2''O IND. 3'(ruby) was selected because it is the only crystal for which it is possible to obtain CTB absorption data from both ground and excited states, and thus for which it is possible to check the CTB model more accurately. Thanks to these more accurate and more complete data, we came to the first conclusion that the spin selection rule does not play any significant role in the variation of the polarizability with the 'ANTPOT. 2 E'-'4 ANTPOT. T IND. 2' energy mismatch. We also discovered that using the CTB model in the case of ruby would lead to a negative 'delta''alfa IND. p' value, which is contrary to all refractive index variation (including Z-scan) measurements

Effets de diffraction scalaires et vectoriels dans un laser a gaz

SIGLEAvailable from INIST (FR), Document Supply Service, under shelf-number : T 83307 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Kerr lens effect induced by a vortex LG0m laser beam

International audienceFor a long time, there was this idea that it is impossible to mode lock a laser on a pure high-order transverse mode. Recently, it has been demonstrated experimentally the generation of femtosecond pulses by a Kerr-Lens-Mode-Locking (KLM) laser operating on a vortex LG0m mode. At the best of our knowledge, there is no studies on lensing effect induced by a doughnut laser beam inside a Kerr medium which is fundamental for modelling accurately the behaviour of a KLM laser and optimising its parameters. We have shown that the doughnut Kerr lens can be view as a ring-lens, or eccentric lens, showing two combined lensing effects. The first one is equivalent to a pure converging lens, and the second one to a diverging axicon. Both lensing effects have been analytically modelled, and provide values for the best focus position which are in a good agreement with the focal position calculated numerically by using a diffraction integral

Simple intra-cavity beam shaping for generating a shape-invariant flat-top laser beam

International audienceFor laser users, the usual Gaussian beam (GB) may not always be the best intensity profile. Often, a preferred option is a Flat-Top (FT) beam which is generally obtained by using diffractive optics in a reshaping operation. In this case, the resulting FT profile occurs only in the focal plane of the focusing lens, and its vicinity. We consider here the generation of a shape-invariant FT laser beam resulting from the incoherent weighted (50%-50% in power) mixing of LG00 (GB) and LG01 (doughnut) beams. For that we consider the insertion inside the laser cavity of a diaphragm and an absorbing ring, and optimising their sizes makes possible the simultaneous oscillation on LG00 and LG01 modes allowing the generation of a shape-invariant laser beam

ABSTRACT Encoding a process algebra using the Event B Method. Application to the validation of user interfaces

This paper presents the use of the B technique in its event based definition. We show that it is possible to encode, using Event B, the models i.e. transition systems associated to a process algebra with asynchronous semantics. The Event B obtained encoding considers that the Event B model associated to the left hand side of a BNF rule defining the algebra expressions is refined by a model corresponding to the right hand side of the same rule. The translation rules of each operator of a basic process algebra are given. Then, an example illustrating each translation rule is given. This approach is based on a proof technique and therefore it does not suffer from the state number explosion problem occurring in classical model checking. The interest of this work is the capability to validate user tasks or scenarios when using a given system and particulary a critical system. Finally, we discuss the application of this approach for validating user interfaces tasks in the Human Computer Interaction (HCI) area

General design principle for structured light lasers

Using custom laser cavities to produce as the output some desired structured light field has seen tremendous advances lately, but there is no universal approach to designing such cavities for arbitrarily defined field structures within the cavity, e.g., at both the output and gain ends. Here we outline a general design approach for structured light from lasers which allows us to specify the required cavity for any selected structured light fields at both ends. We verify the approach by numerical simulation as well as by an unwrapped cavity experiment. The power of this approach is that the cavity can be designed to maximise the overlap with the available pump for higher powers, minimise thermal effects for higher brightness, and at the same time output a desired structured light field that may differ substantially from the gain-end profile. These benefits make this work appeal to the large laser communities interested in cavities for high brightness and/or customized output beams

A formal model for output multimodal HCI - An Event-B formalization

International audienc