Micromachining with ultra-fast pulse lasers. (Seeing the Light).(Brief Article)

All things considered, the shorter the length of the laser pulse, the better the material-processing capability. Commercially available lasers have pulse lengths of anywhere from 10 nanoseconds (ns = [10.sup.-9] second) to a few hundred milliseconds (ms = [10.sup.-3] second). In the last few years, lasers have been developed that give pulses in the picosecond (ps = [10.sup.-12] second) and even down to the femtosecond (fs = [10.sup.-15] second) regime. Pulses this short give huge peak power output, even though the total average laser output may be only 1 W. This gives very interesting material/photon interaction characteristics resulting in the ability to machine any material, including inside transparent materials; reduce heat affected zone; minimize or eliminate damage to surrounding material; achieve wavelength independence; reduce or eliminate splatter; create submicron features; produce repeatable results, shot after shot; and increase efficiency as energy is deposited in the material before the plume f orms to mask it.

If you take a uniform material and expose it to a single laser pulse from a nanosecond laser, then repeat this process at different points on the material there will be wide variation in the shape and depth of the hole. Performing the same experiment with a femtosecond laser, however, shows that all of the "pits" are identical! A possible theory is that the ablation process occurs when electrons are stripped and the atoms are ionized. Due to momentum considerations, the electrons fly off first and at a higher velocity than the heavier ions. For fs lasers, free electrons that are in the material before laser interaction are amplified by the laser pulse and produce a cascade effect. Because free electrons are not distributed uniformly in the material, the on-target results of laser interaction vary. For fs lasers, the intensity is so high that electrons are stripped by the laser beam in very large numbers so the effect does not depend on the number of free electrons in the host material. Therefore, assuming a stable laser output and uniform material, every feature is identical. Figure 1 illustrates this effect. Nanosecond lasers give varying results on target regarding hole shape and size, while fs lasers produce uniform features.

Interesting phenomenon can be seen as a result. First, is essentially wavelength independence. In the ns regime, cleaner processing results are obtained with shorter wavelength light. In the fs regime, it does not appear to be necessary to use UV photons, which allows for greater flexibility in the design of the lasers. The other very important benefit is the almost complete absence of a Heat Affected Zone (HAZ). The pulses are so short and intense, that a high number of the photons are absorbed, and their energy used to affect material removal rather than losing the energy to the plume or surrounding material. This means that the quality of the features is outstanding with no HAZ, and typically minimal debris.

It also means that many materials can be processed that do not normally react well to laser light interaction, such as glass. Figure 2 shows a fs laser drilled hole on the left, and a fs drilled hole on the right. The hole diameter is 100 microns at the exit. The fs hole shows evidence of micro-cracking from the ...