J. Fontaine

Control and measurement of ultrashort pulse shapes (in amplitude and phase) with femtosecond accuracy.

The performances of a tunable femtosecond dye laser are analyzed using accurate correlation techniques. The source is a passively mode-locked dye laser, of which both the frequency and frequency modulation are controlled by one or two intracavity prisms. Interferometric second-order autocorrelations, with a peak-to-background ratio of 8 to 1, are used simultaneously with the conventional intensity autocorrelation and the pulse spectrum to determine the pulse shape. The main advantages of the interferometric autocorrelations are that they provide phase information otherwise not available, and they are more sensitive to the pulse shape than the intensity autocorrelation. The phase sensitivity is demonstrated in an analysis of the Gaussian pulses with a linear frequency modulation. Analytical expressions for the envelopes of the interferometric autocorrelations of typical pulse shapes are provided for quick pulse shape identification. A numerical method is used to analyze the more complex pulse shapes and chirps that can be produced by the laser. A series of examples demonstrates the control of this laser over various pulse shapes and frequency modulations. Pulse broadening or compression by propagation through glass is calculated for the pulse shapes determined from the fittings. Comparisons of autocorrelations and cross correlations calculated for the dispersed pulses, with the actual measurements, demonstrate the accuracy of the fitting procedure. The method of pulse shape determination demonstrated here requires a train of identical pulses. Indeed, it is shown that, for example, a train of unchirped pulses randomly distributed in frequency can have the same interferometric autocorrelation than a single chirped pulse. In the case of the present source, a comparison of the pulse spectrum, with that of the second harmonic, gives an additional proof that pulse-to-pulse fluctuations are negligible.

Intracavity pulse compression with glass: a new method of generating pulses shorter than 60 fsec

The introduction of a glass prism in a ring dye laser is shown to provide simultaneous wavelength selection and pulse compression.

Chirp in a mode-locked ring dye laser

Measurements performed on the output of a mode-locked ring laser show the pulses to be chirped. The down-frequency modulation is evidenced by experiments of pulse compression in glass, for various concentrations of saturable absorbers.

Subpicosecond-time-domain reflectometry

We demonstrate a new method of optical time-domain reflectometry using subpicosecond pulses and nonlinear detection. The high-spatial-resolution ( approximately 15-microm) temporal resolution and sensitivity put new limits on the size and nature of defects that can be detected, localized, and analyzed in optical fibers.

Laser ablation acoustical monitoring

Short pulses laser generated shock waves in air during the ablation of ceramics have been monitored with acoustical techniques. We have found that on a given laser fluence domain the ablation rate can be related to the shock wave energy. This shock wave energy is measured both with the laser beam deflection technique and a microphone. We demonstrate that it is possible, after a calibration procedure, to simply use a microphone for a real time control of the ablation process.

Real-time determination of the amount of removed material during short pulses laser micromachining

Short pulses laser generated shock waves in the air during the photoablation of different ceramics have been monitored by laser beam deflection technique. Using a model, we have related the shock wave transit time over a fixed distance to the shock wave energy. We have found that the instantaneous ablation rate is a linear function of the shock wave energy in the considered range of fluence. This technique is proved to be suitable for monitoring in real-time the laser ablation process.

Imaging with short optical pulses

Abstract A new method of three-dimensional optical imaging based on range gating with subpicosecond pulses is being developed. As opposed to other techniques of medical imaging, this new method is characterized by both high contrast and resolution. Preliminary tests demonstrate a longitudinal resolution better than 10 μm. The computer algorithm needed to reconstruct the image from the attenuated backscattering and to extend to three dimensions the one-dimensional depth resolution, is discussed.

Multi-level diffractive optical elements produced by excimer laser ablation of sol-gel

Material ablation by excimer laser micromachining is a promising approach for structuring sol-gel materials as we demonstrate in the present study. Using the well-known direct etching technique, the behaviour of different hybrid organic/inorganic self-made sol-gel materials is examined with a KrF* laser. Ablated depths ranging from 0.1 to 1.5 microm are obtained with a few laser pulses at low fluence (< 1 J/cm(2)). The aim is to rapidly transfer surface relief multi-level diffractive patterns in such a substrate, without intermediate steps. The combination with the 3D profilometry technique of coherence probe microscopy permits to analyse the etching process with the aim of producing multi-level Diffractive Optical Elements (DOE). Examples of four-level DOEs with 10 microm square elementary cells are presented, as well as their laser reconstructions in the infrared.

Beneficial contribution of the arbuscular mycorrhizal fungus, Rhizophagus irregularis, in the protection of Medicago truncatula roots against benzo[a]pyrene toxicity

Arbuscular mycorrhizal fungi are able to improve plant establishment in polluted soils but little is known about the genes involved in the plant protection against pollutant toxicity by mycorrhization, in particular in the presence of polycyclic aromatic hydrocarbons (PAH). The present work aims at studying in both symbiotic partners, Medicago truncatula and Rhizophagus irregularis: (i) expression of genes putatively involved in PAH tolerance (MtSOD, MtPOX, MtAPX, MtGST, MtTFIIS, and MtTdp1α), (ii) activities of antioxidant (SOD, POX) and detoxification (GST) enzymes, and (iii) H2O2 and the heavy PAH, benzo[a]pyrene (B[a]P) accumulation. In the presence of B[a]P, whereas induction of the enzymatic activities was detected in R. irregularis and non-mycorrhizal roots as well as upregulation of the gene expressions in the non-mycorrhizal roots, downregulation of the gene expressions and decrease of enzyme activities were observed in mycorrhizal roots. Moreover, B[a]P increased H2O2 production in non-mycorrhizal roots and in R. irregularis but not in mycorrhizal roots. In addition, a lower B[a]P bioaccumulation in mycorrhizal roots was measured in comparison with non-mycorrhizal roots. Being less affected by pollutant toxicity, mycorrhizal roots did not activate any defense mechanism either at the gene expression regulation level or at the enzymatic level.

Defence mechanisms associated with mycorrhiza-induced resistance in wheat against powdery mildew

To develop a more sustainable agriculture using alternative control strategies, mechanisms involved in the biocontrol ability of the arbuscular mycorrhizal fungus Funneliformis mosseae to protect wheat against the foliar biotrophic pathogen Blumeria graminis f. sp. tritici were investigated under controlled conditions. B. graminis infection on wheat leaves was reduced by 78% in mycorrhizal plants compared with non-mycorrhizal ones (control). Wheat roots inoculated with F. mosseae revealed a systemic resistance in leaves to B. graminis, after a 6-week co-culture. Accordingly, this resistance was associated with a significant reduction of B. graminis haustorium formation in epidermal leaf cells of mycorrhizal wheat and an accumulation of phenolic compounds and H2O2 at B. graminis penetration sites. Moreover, gene expression analysis demonstrated upregulation of genes encoding for several defence markers, such as peroxidase, phenylalanine ammonia lyase, chitinase 1 and nonexpressor of pathogenesis-related proteins 1 in mycorrhizal wheat only in the absence of the pathogen. This study showed that protection of wheat obtained against B. graminis in response to mycorrhizal inoculation by F. mosseae could be interpreted as a mycorrhiza-induced resistance (MIR). Our findings also suggest that MIR-associated mechanisms impaired the B. graminis development process and corresponded to a systemic elicitation of plant defences rather than a primed state in wheat leaves.