14 Décembre – Thesis defense - Nicolas Valero

14 h30 Amphi A - building A29

Development of a versatile picosecond laser source for cosmetic dermatology.

This thesis reports on a versatile picosecond laser development for use in aesthetic dermatology and particularly for a tattoo removal application, which is an increasingly common application in France and around the world. In this context of multidisciplinary work, the following manuscript has several parts.
In the first part, in order to understand the tattoo removal using a pulsed light, we study the physical effects brought into play during interaction between ink contained in biological tissues and the laser field. To that aim, we will give answers concerning the possible physical mechanisms at play, by studying in particular, the impact of the following parameters: ink particle size, wavelength and irradiance of notoriously efficient lasers used for tattoo removal.
The second part of this manuscript is an important laser development work leading to the realization of a pulsed light source for aesthetic dermatology. We start with mW level continuous wave oscillators development. First, these oscillators have wavelength tunability throughout the Ytterbium emission bandwidth, typically from 1020 to1060 nm. Also, they have several temporal coherence properties. That’s why an important part of this section is dedicated to the study of the temporal coherence, which varies for the four studied sources, from the purest spectral source (monochromatic) to the most chaotic one (ASE source: Amplified Spontaneous Emission). We also study the ability to generate light around 976 nm wavelength using Ytterbium. Then, starting from these continuous wave oscillators, we generate high average power pulsed trains of average power larger than 10 W. To that end, we use an innovative prototype of pulsed amplifier (MOPA: Master Oscillator Fiber Amplifier) based on picosecond pulses generation by pulse gating a continuous light train. With this technology we are able to generate pulse trains of tunable duration from 45 ps to few ns, and a repetition rate tunable from 5 MHz to 1 GHz.  Again, we  study carefully the coherence properties of the generated pulses. We have notably developed a non-linear method to characterize stochastic properties of the source and so the pulses’ coherence time.
The last part of this work consists in the generation of visible wavelengths using optical non-linear effects into non-linear media. We will particularly focus on four wave mixing effect in microstrcutured non-linear optical fibers. In the context of the selective photothermolysis of polychromatic tattoos, we look for wavelengths between 500 and 800 nm. Finally, we also study the impact of light coherence on non-linear processes efficiency. In that way, we develop a high-power amplified spontaneous emission pulsed system with tunable coherence for generation of efficient non-linear processes. A proof of principle of this device is demonstrated for the generation of second harmonic non-linear effect.

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