Micromachining of optical materials by femtosecond laser pulses

Keywords: Optical waveguide writing, Femtosecond laser, Micromachining

Description:
This activity is aimed at the local modification of the refractive index in transparent optical materials using tightly focussed femtosecond laser pulses. Through the processes of non-linear absorption and avalanche ionization the light pulses from a Ti:sapphire laser in the near infrared (~800 nm) can provide refractive index modifications in optical materials without optical damage. The region modified is smaller than the focal volume and the relative movement of the sample with respect to the laser beam gives the capability of realizing various photonic devices at arbitrary depth inside the bulk substrate. Typical integrated devices that can be fabricated are: gratings, waveguides, waveguide splitters, waveguide directional couplers, laser arrays and waveguide amplifiers. Great flexibility and 3D structuring of the photonic devices are the main advantages of this technique, which can be applied to various materials: passive and active glasses, crystals and polymers. Laser ablation is also possible with femtosecond laser pulses, with resolution below the focussing capability thanks to the nonlinear absorption process. Fabrication of photonic bandgap structures (PBG) with this technique is thus possible and extremely promising.

Main techniques:
The laser-assisted micromachining of optical materials consists in focussing a femtosecond laser beam to dimensions of a few microns inside or on the surface of the substrate. Multiphoton absorption and avalanche ionization can cause a material modification in a very small volume and this “hot spot” can be moved to create different patterns of modified refractive index. The absence of any photolithographic process and the high processing speed allowed by stretched cavity oscillators make this fabrication technique very interesting also in view of industrial applications.

Projects:
Development and application of compact mode-locked laser oscillators – DACO (CRAFT Project – V FP, Tematic Priority: Growth)
Advanced laser micro/nano processing, material synthesis and modification- ADLAS (Integrated Project VI FP, submitted)
FIRB-MIUR: “Miniaturized Systems for Electronics and Photonics”

Staff:
Dr. Roberto Osellame - +39 02 2399 6115 – roberto.osellame@polimi.it
Prof. Paolo Laporta - +39 02 2399 6096 – paolo.laporta@fisi.polimi.it
Prof. Roberta Ramponi - +39 02 2399 6150 – roberta.ramponi@fisi.polimi.it
Prof. Giulio Cerullo - +39 02 2399 6164 – giulio.cerullo@fisi.polimi.it
Dr. Stefano Taccheo - +39 02 2399 6167 – stefano.taccheo@fisi.polimi.it
Dr. Marco Marangoni - +39 02 2399 6156 – marco.marangoni@polimi.it
Ing. Giuseppe Della Valle - +39 02 2399 6098 – giuseppe.dellavalle@fisi.polimi.it
Ing. Massimo Orio - +39 02 2399 6163 – massimo.orio@fisi.polimi.it

Collaborations:
Institut fur Angewandte Physik, Friedrich-Schiller-Universitat Jena, Germany.
Optotec S.p.A., Milano, Italy.
Dip. Ingegneria Nucleare, Politecnico di Milano, Italy.
High Q Laser Production GmbH, Hohenems, Austria.

Main publications:
R. Osellame, S. Taccheo, G. Cerullo, M. Marangoni, D. Polli, R. Ramponi, P. Laporta, S. De Silvestri, “Optical gain in Er-Yb doped waveguides fabricated by femtosecond laser pulses”, Electron. Lett., 38(17), pp. 964-965 (2002).

G. Cerullo, R. Osellame, S. Taccheo, M. Marangoni, D. Polli, R. Ramponi, P. Laporta, S. De Silvestri, “Femtosecond micromachining of symmetric waveguides at 1.5 mm by astigmatic beam focusing”, Opt. Lett., 27(21), pp. 1938-1940 (2002)

Roberto Osellame, Stefano Taccheo, Marco Marangoni, Roberta Ramponi, Paolo Laporta, Dario Polli, Sandro De Silvestri, Giulio Cerullo, “Femtosecond writing of active optical waveguides with astigmatically shaped beams”, J. Opt. Soc. Am. B, in press (2003)

G. Cerullo, R. Osellame, “Apparato per la fabbricazione di dispositivi fotonici con impulsi laser ultrabrevi e relativo metodo di fabbricazione”, National Patent n° MI2002A 001717, 31/7/2002

R. Osellame, S. Taccheo, G. Cerullo, M. Marangoni, G. Sorbello, D. Polli, S. De Silvestri, P. Laporta, and R. Ramponi, “1.5-micron enhancement in active waveguides fabricated with femtosecond laser pulses”, SPIE Photonics Fabrication Europe, October 28-November 1, 2002, Brugge (Belgium)

S. Taccheo, R. Osellame, G. Cerullo, G. Sorbello, M. Marangoni, D. Polli, G. Jose, Sandro De Silvestri, R. Ramponi and P. Laporta, “Erbium-Ytterbium Doped Waveguide Made by Femtosecond Pulse Laser”, European Conference on Optical Communications, September 8-12, 2002, Copenhagen (Danmark)

	Green fluorescence of a waveguide realized by femtosecond laser pulses in an Erbium doped tellurite glass, pumped at 980nm.
	AFM image of holes fabricated with a single femtosecond pulse each in a phosphate glass
	Left column: optical microscope images of the exit face of waveguides written with a focused astigmatic beam for increasing values of the astigmatic difference z0. Right column: corresponding simulated electron density profiles.