Ultrafast lasers have recently been used in micromachining applications, producing micron-sized features in a variety of materials. Using a femtosecond Ti:sapphire system, a group at the University of Michigan`s Center for Ultrafast Optical Science (CUOS, Ann Arbor, MI) have successfully ablated holes into metal and polymer substrates. They believe the short pulse durations minimize thermal diffusion effects, leading to higher-quality machining than obtainable with longer-pulse systems.
Laser ablation occurs when sufficient optical energy is absorbed by a material to bring it to vaporization temperature. Because pulse fluence decreases radially outward from spot center, the central region of a spot can deliver high enough energy density to cause ablation, while material illuminated by the outer zones is unaffected. Holes much smaller than the spot size can, in principle, be ablated; however, thermal diffusion effects can distort and enlarge features beyond the region of ablation.
Led by research scientist Xinbing Liu, the CUOS group has shown that holes machined with nanosecond or picosecond pulses suffer from thermally induced damage. For sufficiently short pulses, however, thermal diffusion is limited and the holes are not degraded; in particular, femtosecond pulses can machine crisp, high-quality features.
System design and results
The chirped pulse amplification system used for laser machining consists of a Ti:sapphire regenerative amplifier seeded with sub-100-fs pulses from an oscillator. Prior to injection into the amplifier, pulses selected from the oscillator pulse train are stretched to 150 to 200 ps. After amplification to energies of 1.5 mJ, the pulses can be used as is (picosecond regime) or compressed to somewhere between 150 fs and 10 ps by a grating pair. Q-switched pulses from the unseeded amplifier were used for nanosecond-regime investigations.
A waveplate/polarizer combination controls output pulse energy. The waveplate rotates the polarization and the polarizer transmits some portion of the light that depends on the angle of rotation. This output is relayed to a focusing objective that concentrates the beam to the appropriate geometrical spot size (larger than the region of ablation).
The group has performed machining experiments on metals and plastics for 3M Corp. (St. Paul, MN), studies that could serve as test beds for laser cutting or marking applications. Future work may include laser machining of exotic or unmachinable materials such as single crystal metals.