Laser-material interactions with short-wavelength infrared lasers

A whole range of infrared lasers is currently available, several with high powers. Beside the CO2 laser (10.6 pm) and the Nd:YAG laser (1.06 pm), which are industrial, the CO laser (5.3 pm), the HI: laser (2.7 pm) and the iodine laser (1.3 pm) can deliver beams with high optical qualify and their power can easily be exlrapolated. Alter a short description of the diSjerent lasers, especially of those installed at ISL, this paper deals with the behaviour of materials irradiated by these lasers. For dielectrics, the absorption depth is the most relevant parameter and the interaction process is oflen wavelength-dependant. When the structure af dielectrics is heterogeneous, they exhibit a dijfuse reflection. When the absorption depth is significant, a boiling process occurs in plastics. On metals, the absorptivity of the loser radiation increases when the wavelength decreases. Owing ta the smaller focal spat and to the lower plasma absorption, the short wavelength lasers offers many advantages in laser processing (welding or cutting), especially of light metals; comparative investigations are currently carried out. 1 . m INFRARED HIGH-POWER GAS LASERS 1.1 Chamoions K O 2 and Nd:YAG) and challcneers (CO and HF) Two lasers arc currently well developcd for industrial purposes : the C02 laser and the Nd:YAG laser. Both can deliver relatively large focussed spots : the diflraction-limited spot of the C 0 2 laser is large due to the long wavelength, the Nd:YAG laser usually has a high-order mode cavity. Moreover, in spite of recent improvements, the scalability of this latter laser is difftcult due to the cooling problem of the YAG rod and the related limited dimensions, Thrce other Q e s of cw gas lasers have been developed in the meantime and have reached fairly high powers : the CO laser (5.3 pn), the 12/02 laser (1.3 pm) and the HF laser (2.7 pm). All present drawbacks in comparison with the C02 laser : the CO laser needs to be cooled in order to obtain a good efficiency whereas the HF as well as the 12/02 laser use a chemical reaction to obtain the population inversion, involving the use of expensive, corrosive gases and the production of hazardous gases. However, their smaller wavelengths offer four types of advantages in comoarison with the CO7 laser : the wavelengths-of the new lasers L e the half (for the CO laser), the quarter (for the HF laser) or 1/8 (for the 12/02 laser) of the C02 laser wavelength. This allows to obtain laser spots twice as small as, or 4 or 8 times smaller than, those of the C 0 2 laser, which means smaller heat affected zones, higher power dcnsitics, higher penetration depth andlor larger processing speeds; the plasma absorption coefficient is proportional to the square of the wavelength; the two shortwavelength lasers beams will therefore be far less absorbed by the keyhole plasma, which restrains the penetration depth in thc keyhole; for metals, the absorptivity increases when the wavelength decreases; this allows to perform surface treatment of metals without using an absorbing coating; moreover, the initiations of the melting pool and of the keyhole are obtained more easily, with smaller laser powers; with short wavelengths, the range of optical materials increases. For the 12/02 laser, the fiber optics currently used with Nd:YAG are suitable, for the 2.7 pm wavclength, fluorinated fibers present low losses and can carry medium power (kilowatt range) laser beams.