In nonlinear optics, filament propagation is propagation of a beam of light through a medium without diffraction. This is possible because the Kerr effect causes an index of refraction change in the medium, resulting in self-focusing of the beam.
Filamentary damage tracks in glass caused by laser pulses were first observed by Hercher in 1964. Filament propagation of laser pulses in the atmosphere was observed in 1994 by Gérard Mourou and his team at University of Michigan. The balance between the self-focusing refraction and self-attenuating diffraction by ionization and rarefaction of a laser beam of terawatt intensities, created by chirped pulse amplification, in the atmosphere creates "filaments" which act as waveguides for the beam thus preventing divergence. Though sophisticated models have been developed to describe the filamentation process, a model proposed by Akozbek et al. provides a semi-analytical and easy to understand solution for the propagation of strong laser pulses in the air.
Re-focusing during the propagation of a focused short laser pulse
The filaments, having made a plasma, turn the narrowband laser pulse into a broadband pulse having a wholly new set of applications. An interesting aspect of the fillamentation induced plasma is the limited density of the electrons, a process which prevents the optical breakdown. This effect provides an excellent source for spectroscopy of high pressure with low level of continuum and also smaller line broadening. Another potential application is the LIDAR-monitoring of air.
Flat panel dicing using short laser pulses is an important application due to the fact that as the glass substrates become thinner it becomes more difficult to improve the process yield using conventional diamond blade dicing techniques. Using short pulses dicing speeds of over 400 mm/s has been successfully demonstrated on non-alkali glass and borosilicate glass, using a 50 kHz, 5W high-power femtosecond laser. The working principal developed by Kamata et al. is the following. The short pulse laser beam having a wavelength to which the work is transparent is directed to the front surface of the work toward the back surface and focused. A filament in the light beam traveling direction from the beam waist is formed by the auto-focusing action due to the laser beam propagation in the work is formed. The substance in the filament is decomposed by the laser beam and can be discharged from the back surface, and a cavity is formed in the channel. While forming the cavity, the laser beam is scanned, a machined surface is formed, and thereafter the work can be cut with a weak bending stress.
- Hercher, M. (1964). "Laser-induced damage in transparent media". Journal of the Optical Society of America 54: 563.
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- L. Wöstea, S. Freyb, J. Wolf, LIDAR-Monitoring of the Air with Femtosecond Plasma Channels, Advances In Atomic, Molecular, and Optical Physics, 2006, 53:413–441
- Kamata, M.; Sumyoshi, T.; Tsujikaula, S., & Sekita, H. (2008). Laser machining method, laser cutting method, and method for dividing structure having multilayer board, PCT Application, WO/2008/126742
- S. Mehdi Sharifi and Abdossamad Talebpour, Applications of Short Laser Pulses, cdn.intechopen.com