Marco A. F. Scarparo

Positive and negative chirping of laser pulses shorter than 100 fsec in a saturable absorber

We present a calculation of the chirp generated in laser pulses shorter than 100 fsec on propagation through a saturable absorber (DODCI in ethylene glycol). The calculation takes into account the absorber saturation and the solvent nonlinear refractive index. At pulse energies greater than 10 nJ the chirp tends to be predominantly positive, and it increases rapidly as the pulse duration becomes shorter than 50 fsec. At pulse energies in the 1–7-nJ range the chirp is mostly negative for pulses longer than 30 fsec.

Study of resin based materials using CO2 laser stereolithography

Abstract We present an experimental and theoretical analysis of i.r. (thermal) sintering of thermosetting resins used for stereolithography. In the experimental work an epoxy resin was chosen. The optimum mix (which was quite critical) was composed of 10 parts (by weight), 1.4 parts diethylene triamine and 0.7 parts of silica powder. Using a differential scanning calorimeter (d.s.c.) we were able to determine reaction rates as a function of temperature as well as the enthalpy ( ΔH ≅ 368 J g −1 ) involved in the phase transition and the activation energy (21.7 ± 0.9 kJ mol −1 ) of the sintering process. The solution of the heat equation simulating this process was in general agreement with our previous observations of the stereolithographic results. It was also found that the minimum radius of lateral confinement was limited to ∼0.3 mm due to heat conduction. The absorption depth should be approximately the same (0.2 mm in this case) at the frequency of the laser. It was also noticed that the epoxy degrades rapidly at temperatures above ∼280°C.

Pump power and saturable absorber effect on a colliding pulse-modelocked dye laser emitting pulses with 47 fs

Abstract We present a quantitative experimental study of the effects of pump power and saturable absorber concentration on the performance of a colliding pulse modelocked dye laser. Pulses as short as 47 fs were generated without the use of any additional (intracavity or extracavity) pulse compression scheme. The laser has a well defined stability region in the pump power × saturable absorber concentration plane, and exhibits a strong bistable behavior in the limits of this stability region.

Mechanisms of carbon dioxide laser stereolithography in epoxy‐based materials

We present in this article the use of infrared laser radiation to achieve localized curing in thermosensitive epoxy resin compounds. In stereolithography, the objective is to cure a localized region in a material by precisely confining the laser energy to the area that is to be cured. Industry already uses ultraviolet laser radiation at 352 nm to fabricate three-dimensional structures. Via infrared laser curing, we demonstrate the viability of a completely thermal localized curing process. In our experiment, we have focused the beam from a carbon dioxide (CO2) laser onto a sample composed of epoxy resin, diethylene triamine, and silica powder. Such resins typically cure, or solidify, when heated to moderately high temperatures, and our results show that we can confine the heating of the material, and, therefore, its curing in all three dimensions. We present a physical and a chemical model to describe the process and measure the curing rate as a function of temperature. In order to model the flow of heat in our sample as a result of infrared laser irradiation, we solved the time-dependent heat equation in cylindrical coordinates using the Crank-Nicholson finite-difference method. The results allow us to predict the curing behavior of the sample as a function of laser irradiation conditions, and we find good agreement with our preliminary experimental observations.

Optical properties of dye molecules adsorbed on fractal structures

Abstract We present preliminary measurements of the optical properties of Rhodamine B molecules adsorbed or trapped in porous matrices such as Vycor 7930 glass, silica xerogels and aerogels. The electronic transfer between donor and acceptor molecules adsorbed on such fractal structures is also discussed; a fast sub-picosecond resolution experiment is proposed to study this process.

New infrared stereolithography: control of the parameters of the localized curing thermosensitive materials

In normal practice, stereolithography has been used for photosensitive resins where ultraviolet light (HeCd laser λ=O.352μm) initiates the curing process for prototypes construction. In this work we developed a new aspect of stereolithography, using CO2 laser (λ=1O.6μm) in thermosensitive materials where no shrinkage and post-cure treatment was observed. The bulk curing process in resins, epoxy unlike, has been proved useful in a new tethnique for fabrication of prototypes. Control of the laser parameters appears as an important tool for localized cure in the material. Our resin basic sample is composed by the thermosensitive epoxy resin proved to be (in weight) 10 parts, 1,4 parts diethilene triamine (the curing agent) and 0.7 parts silica powder. Silica play an important hole in the curing process Tests with fumed silica and non treated fumed silica showed considerable difference in the obtained final product We also developed a physical and chemical models of the setting process determine the time delay for onset of curing as a function of temperature and the profiles of the isotherms in the sample. In order to model the flow of the heat in laser-induced cunng we used theoretical approach to solve the time dependent heat equation in cylindrical coordinator. The data show the curing rate as function of temperature. Activation energy results were derived from differential scanning calorimetry (d.s.c.).

Optical bistability and hysteresis in a colliding pulse mode locked femtosecond dye laser

Abstract We describe the observation of optical bistability in a colliding pulse mode locked dye laser. Two lasing states can occur, one at 612 nm and with femtosecond pulse formation and the other at 570 nm and with a c.w. output. An hysteresis loop can be obtained plotting the output power as a function of the pump laser power. Control of the hysteresis can be accomplished by changing the beam focusing over the saturable absorber.

Picosecond duration laser pulse generation by simultaneous active-passive mode-locking in Ar+ ion and dye lasers

We use a rate equation model to analyse a technique for the generation of picosecond duration laser pulses in argon pumped dye lasers. The system is based on passive mode-locking of the Ar+ laser by saturable absorption of the dye which is inserted in the Ar+ laser cavity. Simultaneously, the dye is forced to oscillate in a regime of synchronously pumped mode-locking. The system is relatively simple, does not require the use of an acousto-optical light modulator and can be easily implemented in commercially available lasers. Pulses shorter than 100 ps and having an average power of 20 mW were obtained at a repetition rate of 110 MHz. Good agreement between the model and experimental results was also obtained.

Experimental and theoretical study of local curing on thermosetting resins using a CO2 laser

We present a theoretical and experimental study of thermosetting resins used in thermal stereolithography. In usual practice, stereolithography makes use of photosensitive resins where HeCd (0.352 micrometers ) laser ultraviolet laser initiates the curing process. In this work we study the process of local curing through the application of infrared radiation, which has proved to be useful in a new technique for the making of prototypes by means of selective heating with a CO2 laser (10.6 micrometers ). The sample consists of a thermosetting resins (epoxy) with the curing agent (diethylene triamine) and a filler (silica). The ideal composition of the thermosetting resins has proved to be 10 parts epoxy, 1.4 part diethylene triamine (the curing agent) and 0.7 part silica powder. A physical theoretical model is applied for control of the parameters which influence the confinement of the curing in the irradiated bulk. A mathematical model was developed through the solution of the time dependent heat conduction equation in cylindrical co-ordinates, which enables the determination fo the behavior of curing in terms of irradiation conditions. An experimental analysis has determined the temperature range at which the curing process starts and the optimum silica concentration for efficient curing.

Thermal three-dimensional lithography in thermoset resins using CO2 laser

This work presents a new method to produce 3D structures, using a CO2 laser and thermosensitive resins. This method is called thermal stereolithography and is based on the spatially selective thermal curing of a resin, a filling material and a catalyst. The results obtained with a physical model for the resin system indicates that filling material must be added to control a pressure wave that is thermally induced. A resolution of 0.1 mm was achieved for epoxy and polyester resins. The successive layering of a laser scanned spot can produce 3D solid structures out of a high viscosity liquid.