there are two kinds of removal methods in the material micromachining with laser. the first is the photo-thermal effect, the high energy laser is used as the heat source to quickly heat the material and make it vaporized to remove the material. the second is photo-chemical, and the laser has high peak energy and enough to instantly destroy the molecular of the material surface to remove material without accumulation. it is a way of "cold" processing. compared with nanosecond laser, the picosecond laser has the characteristics of shorter pulse width and higher peak power rate, especially when the pulse width is around 10ps, the ultra-fast laser pulse can be used to achieve very ideal laser "cold" processing effect. picosecond uv laser has shorter wavelength and higher photon energy. therefore, picosecond ultraviolet can be used to obtain smaller processing dimensions and higher machining accuracy.
picosecond ultraviolet laser is obtained on the basis of picosecond infrared and the sum frequency technique of nonlinear optical crystals, as shown in figure 1.
figure 1
the low power picosecond seed source (pico seed) obtains high power picosecond infrared laser through the high performance amplifier (amplifier), and then through the frequency doubling crystal (shg) and frequency crystal (thg), the high power picosecond ultraviolet output is finally obtained.
under the condition of small signal approximation, the intensity of the three frequency doubling of the fundamental frequency and the frequency doubling light and frequency parameters can be expressed as:
d is the frequency doubling coefficient, l is the three frequency doubling crystal length, p1p2 is fundamental frequency and frequency doubling light power, a is the spot area of three frequency doubling, and it is the phase matching factor of the three frequency doubling crystal. through this formula, it can be seen that the uv optical efficiency is determined by many factors, such as basic frequency (ir 1064nm), frequency doubling crystal, frequency crystal, sum frequency crystal and phase matching, so it is very difficult to obtain high power and stable output of picosecond uv laser.
after a long period of experimental test, bellin laser developed a picosecond uv laser (amber uv-10~30) with output power of 10w~30w at the end of 2017. the maximum conversion efficiency from infrared light to ultraviolet is up to 50%.
at present, picosecond uv laser has a very bright advantage in the processing of organic polymer film and metal film, and most of its applications can be controlled within 20μm. through the following processing examples, we can see preliminarily the effect of picosecond uv processing.
1、cvl is protective film for printed circuit board. picosecond uv and nanosecond uv are different when cutting cvl. (as shown in below table)
when cvl is cut by nanosecond uv laser, the heat affected zone is bigger, and the black carbon dust is more, which needs to be wiped.
after cvl is cut by picosecond uv laser, the heat affected zone is small and dust vaporized and no need for wiping.
2、cop (cyclic olefin polymer, cyclic olefin polymer) is an optical plastic. it is suitable for making lens, liquid crystal light guide plate (lgp, light guiding plate), optical film and so on. using picosecond uv cutting effect, as shown in figure 2, the heat affected zone is only 12μm.
figure 2
3、pe film, that is, polyethylene film, is compared with nanosecond uv and picosecond uv after cutting and drilling . as shown in figure 3 and figure 4, figure 3 is a nanosecond uv effect, and the processing heat affected zone is obvious; the figure 4 is picosecond uv laser is effective, the edge is neat, and the heat affected zone is very small.
figure 3 figure 4
4、in metal film materials, picosecond uv can be used to achieve more satisfactory processing results. as shown in figures 5 and 6, the diameter of the hole is about 100μm on the copper foil and aluminum foil. it can be seen that the edge of the circular hole is smooth, and there is hardly any thermal effect.
figure 5
figure 6
as the technology continues to be mature and perfect, we have reason to believe that picosecond uv laser as a "weapon" will bring further improvements to the material micro processing; we will continue to advance in the r & d field of picosecond uv laser and help to forward the “china made”!
