Laser Applications Newsletter
22 November 1998
Laser Kinetics Inc.
Mtn. View, CA 94041
Issue 2 , Vol 2
FINALLY, ANOTHER NEWSLETTER
My nominal monthly schedule for this newsletter was severely disrupted by our workload this year. I will attempt to improve this situation in the future.
I just got back from ICALEO '98, held this year in Orlando, Florida. ICALEO is the premier laser applications conference in the world, and is the place where all the major laboratories and institutes that work with laser material processing present their latest developments. The conference gets bigger every year (over 200 papers in 1998), which has the unfortunate effect that one person can't hear all the papers. I tried to pick ones with interesting titles, but I'm sure I missed a lot of good ones. There were, however several obvious trends and I'd like to report on them.
Processing with ultrashort-pulse lasers
Ultrashort pulses are generally considered to be 1 ps or less; 100 femtoseconds is typical. To provide some sense of scale, a Q-switched Nd:YAG pulse is generally around 100 nanoseconds or 0.0000001 second. Since light travels 1 foot per nanosecond (my favorite non-SI unit), these pulses are about 100 feet long. A 100 femtosecond pulse (0.0000000000001 second if I counted my zeroes right) is 30 microns long.
Lately, it has become possible to build relatively small lasers that deliver these short pulses, and several workers have been using them for material processing. Materials react quite differently at femtosecond time scales than at longer ones. In metals, the electrons do not have time to transfer heat to the lattice; processing is essentially athermal. In dielectrics, the electrons are ionized by multi-photon absorption and are ejected from their atoms. The ionized atoms are dragged along with them to maintain electrical neutrality in the plume. For both metals and dielectrics, material is removed without transferring heat to the substrate. Ultrashort-pulse lasers are consequently ideal material removal tools.
At least, they could be ideal if they ran decently. Present units are fiendishly complicated, rather touchy to align and hard to keep running. The currently favored laser material, titanium-sapphire, requires another laser to pump it. Work is being done on other laser materials that can be pumped with diodes; ultrashort-pulse lasers made with these have a chance to be much simpler and more reliable than current units.
Martyn Knowles of Oxford Lasers provided an excellent counterpoint in his paper: In metals, reducing pulse duration 1000 times reduces the heat-affected zone by a factor of only 10. If you process metal with nanosecond pulses, you can get a 1 micron HAZ. Femtosecond pulses can give you 0.1 microns. In most real-life applications, a 1 micron HAZ is undetectably small, so it's not worth an enormous increase in complexity to make it smaller.
As I see it, ultrashort-pulse lasers will be extremely useful tools for material processing, but will not be widely adopted until they are much simpler and more reliable than the units that exist today. Until then, you can do more useful work with "long-pulse" nanosecond lasers because they run all the time.
Laser welding is fast and produces high-quality welds. One consequence of these characteristics is that laser welding is used in many high-production products that cannot tolerate defects. Two such product categories are airbag inflators and pacemakers. A defective weld in either of these products can kill. There is, therefore, a lot of interest in developing ways to insure that a weld is good, typically by in-process monitoring. Laser welds have been monitored acoustically, optically and electrically by dozens of investigators for the last 10 years. The big problem is that most laser welding processes are a lot better than the monitors: You can produce welds with 99.9% reliability, but the monitors are only about 90% reliable. This situation makes monitors totally useless.
Current thinking is to use a combination of detectors (acoustic + plasma + keyhole temperature) and poll them to see if they agree that something happened. In theory, this gives much better accuracy than any of the detectors alone. This approach is quite promising, but means that there will be a bunch of instruments all around the weld area, making it hard to build tooling or get parts in and out.
Heat treating with diode lasers
In the past, laser heat treating has never seen much popularity. This is probably because the only lasers with enough power were CO2's. This made the equipment very costly and required coating of the heat treat area with absorbent material.
The arrival of diode arrays has changed this drastically. Arrays are available with powers up to 2.5 kW, enough to do serious heat treatment. The short wavelength (800 to 980 nm) is absorbed by steel without a coating, and the beam coming out of the units is generally very flat-topped, which is ideal for heat treatment. Starting right now, laser heat treatment is economically competitive with other processes for localized transformation hardening, offering advantages in power density, elimination of induced electrical currents and precision in location.
Thick section cutting
In England, BOC gases has developed the LASOX process. Using this process, they have cut 2" thick steel with 1 kW of laser power. It is essentially oxy-fuel cutting using laser preheat. Edge quality is good, cut speed is around 200 mm/min. As the presenter (Prof. William O'Neill of the University of Liverpool) noted, we've been laser cutting for 30 years and still don't understand it.
You need to read the proceedings to get full information about the material presented during the conference. The proceedings of ICALEO '98 will be available in early 1999. To order your copy, contact LIA at (407) 380-1553 or look at their web site at http://www.laserinstitute.org
LASER RELATED EVENTS
High-Power Lasers and Applications
January 23-29, 1999
San Jose, CA
The laser applications part of Photonics West, SPIE's big lasers and optics symposium. The program on solid state, gas and free-electron lasers is of interest to workers in laser applications who want to understand the latest developments on laser sources. More directly related to applications is the program on microengineering/manufacturing, where we have a conference on laser applications in microelectronic and optoelectronic manufacturing. Important new developments in micromachining, laser ablation and electronic processing should be described here.
SPIE's phone no. is (360) 676-3290, fax is (360) 647-1445
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