Ultra Short Pulse Laser Processing
Laser processing and machining offer a variety of advantages to medical device manufacturers across a number of sectors in the industry. The next generation of laser systems is improving upon these technologies by providing enhanced results in a fraction of the time. This enables even greater precision with less impact on the material being used.
Three of the most important goals in advanced medical device manufacturing are miniaturization, no undesirable impact on material and minimal post laser processing. Short pulse lasers are used as a tool for achieving these (and other) goals. In particular, laser processing can be fast and cost effective. It can also generate features smaller than other competitive technologies with better edge quality. The flexibility of lasers allows them to be easily used on a wide variety of materials. Additionally, with the proper software, it is possible to generate extremely small, high quality features with minimal material impact.
|Figure 1A: Kapton cut using 355 nm laser with 30 ns pulse
length; B: Kapton cut using 266 nm laser with 30 ns pulse
length; C: Kapton cut using 355 nm laser with 12 ps pulse
In order to get to the regime where laser processing is truly wavelength independent, it requires a laser pulse shorter than 100 fs (femtoseconds, or 10-15seconds). These lasers exist, but they tend to be very low in pulse energy, very expensive, and complicated to operate. Therefore, lasers with pulse lengths of hundreds of femtoseconds, and even into the picosecond (ps) range, are being commercialized. These lasers allow short pulses to be obtained, but with usable pulse energies (hundreds of microJoules per pulse or more) and simpler operation.
For example, the Lumera Super Rapid laser outputs about 18 Watts in the fundamental; the doubled (532 nm) and tripled (355 nm) wavelengths are also available with a simple changeover. The pulse length of this laser is about 12 ps, so wavelength dependence can still be seen in most materials—as a rule of thumb, the shorter the wavelength, the “better” the part quality. Also, as a simple rule of physics, smaller spot sizes are obtainable with shorter wavelengths, so for miniaturization, the higher frequencies are desirable. It is pretty straightforward to get a 10 to 15 micron spot size on target, which, coupled with the short pulse length, gives a very high peak power intensity.
Figure 1 shows a feature made in Kapton—the kerf width is approximately 75 microns. Figure 1A was made using a 355 nm laser with 30 ns pulse length, 1B was made using a 266 nm laser with 30 ns pulse length, and 1C was made using a 355 nm laser with 12 ps pulse length. These images clearly illustrate how a shorter pulse can give better quality on target.
One of the most interesting applications for these new laser sources is in the manufacture of bioabsorbable stents. Because the material is specifically developed to be heat sensitive and dissolve in the body over the period of a few weeks, the heat affected zone from traditional laser processing is too large. Using ultra short pulse lasers, even heat sensitive devices can be laser machined. Figure 2 shows a stent being manufactured using a Raydiance fs laser. An interesting demonstration of the technology involves the machining of features into a kitchen match without causing ignition.
|Figure 2: Bioabsorbable stent manufactured with Raydiance 800