Poster Lasers

Development and study of femtosecond lasers Alejandro A. Hnilo and Marcelo G. Kovalsky Centro de Investigaciones en Láseres y Aplicaciones (CEILAP) Instituto de Investigaciones Científicas y Técnicas de las Fuerzas Armadas (CITEFA) Zufriategui 4380 1603 Villa Martelli, Buenos Aires Argentina mail: [email protected] Abstract: The use of ultrashort laser pulses have been shown to be an attractive option for the high quality micromachining of many materials and their ability for minimal damage and precise processing has been thoroughly researched for many years. The increasing worldwide interest in femtosecond laser micromachining and the potential new markets being investigated have prompted the need to develop more powerful, simpler, rugged, and economic laser systems and this, in turn, has encouraged further work. The most efficient way to obtain femtosecond pulses is via Kerr lens mode locking. However the nonlinear effects that cause the pulse formation are also responsible for the instabilities always presents in this kind of lasers. In this work we describe the dynamics of the most widely used source of femtosecond pulses, the Ti:Sapphire laser. We also present the results obtained with a femtosecond laser based on a diode pumped Nd:Glass medium. Laser micromachining - the direct etching of solids by pulsed laser radiation – relies on the process of ablation1 . Laser solid interactions during ablation are complex, and depend both on the material and on the laser parameters (primarily the wavelength, pulse duration and intensity). Another factor of critical importance is the stability in the pulse train. Femtosecond lasers based on Kerr lens mode locking can display pulse to pulse instabilities2. A model based on iterative maps allows us to undestand3,4 and eventually control the chaotic behaviour .

Kerr effect Iterative maps + ABCD matrix

n(I) = n + n2 I

Kerr lens mode locking Lens

Responsible for the instabilities shown in the pulse train

Lens

Input beam

Output beam low power

high power

Self focusing

Self phase modulation The observed modes of operation are naturally obtained from a description based on an iterative map of five pulse variables, beam size, curvature, pulse duration, chirp and energy. The stability regions for each mode are obtained as well as its characteristic behaviors. On the other hand, the structurally stable properties of the system are experimentally found, observing a good agreement between the experiment and the model. It is experimentally shown that pulse to pulse instabilities in the output of the laser are usual and they can affect some of the pulse variables and not others. A simple way to detect and eliminate the instabilities is described. Experimental time series of the two pulsed model are obtained. From this series the chaotic low dimensional deterministic behavior is confirmed and the route to chaos in each case is established. The same attractors are obtained from the simulated series. As an application of the model, an algorithm of control of chaos that allows reach shorter pulse duration, close to 10 fs, is presented. control signal

Control signal ON

References: time

1.- N.H Rizvi,”Femtosecond laser micromachining:current status and applications”Riken Review N 50, January 2003.

Other lasers developed in our lab

2.- M. G. Kovalsky, A. A. Hnilo, C. González Inchauspe “Hidden instabilities in the Ti:sapphire Kerr lens mode-locked laser” Optics Letters, 24, p.1638 , November 15, 1999. 3.- M.G. Kovalsky, A.A. Hnilo “Stability and bifurcations in Kerr lens mode - locked Ti:Sapphire lasers” Optics Communications, 186, p.155, December 1, 2000.

• Nd:YAG Kerr lens mode locked: 4 ps, 800 mW

• Nd:YVO4 active Q-switch and SHG

• Nd:Glass active mode locking

250 mW @ 1064 nm

cheap ps source @1064 nm

50 mW @ 532 nm (20 ns, 10 KHz).

50mW @ 100MHz pulse duration 1ps

4.- M.G.Kovalsky, A.A. Hnilo, A. Libertun, M. C. Marconi “Bistability in Kerr lens mode - locked Ti:Sapphire lasers”, Optics Communications, 192, p. 333, June, 1, 2001.