• English

The Application of Ultrafast Laser in Fine Machining

DATE:2016-10-27READ:


1. introduction
Since the birth of the first ruby laser in 1960, the exploration of laser technology has never stopped. Because of its superior characteristics more and more known, more applications will be unearthed, especially for high-tech sensitive industry. Can be said that the development of human society at this stage, industrial demand is the most powerful driving force for social development. Laser used in industrial processing has been more than 20 years of history, the application of laser in the industrial field of the depth and breadth have reached an unprecedented stage.
In this case,
2. The development of laser processing industry
In the beginning of the birth of the first laser, low power, the system as a whole large, in fact, can not be directly applied to industrial processing. In the following decades, with the development of laser technology, the emergence of new laser medium, laser technology bottlenecks breakthrough, in terms of depth or breadth of the laser has made great progress. There are many laser applications in the industry, such as: laser printing, recording, marking, carving, welding, cutting, hair, resistance, welding and so on, and great potential.
In this case,
From the 80s of last century, from the depth point of view, the development of laser processing industry has gone through three main stages:
In this case,
The first stage is the nanosecond laser application stage. The peak power of nanosecond pulse obtained by Q-switched technology is much higher than the average power, and it can realize the high instantaneous power density that can not be achieved by the continuous laser, and instantly exceeds the material destruction threshold to realize the etching effect.
In this case,
The second stage is the beginning of this century, SESAM (semiconductor saturable absorption mirror) so that the rapid development of picosecond laser technology, and soon used in industry. Picosecond lasers have traditionally been locked by dyes, but dyes need to be recycled and often bleach to affect the stability of the mold. SESAM can not only replace the dye for mold clamping, but also to achieve self-starting. In this context, the rapid emergence of a number of commercial-grade industrial picosecond lasers.
In this case,
Compared to nanosecond lasers, picosecond lasers can achieve finer processing results with shorter pulse widths and higher peak power. Therefore, a time, fine processing become a hot topic.
However, the real fine processing is achieved in the femtosecond laser, which is what we said the third stage. We first look at the interaction between the laser and solid mechanism:
In this case,
(1) The laser first excites the electrons of the solid, and in 100 femtoseconds the electrons absorb the energy of the photon and jump to the high energy level.
In this case,
(2) Since the temperature of electrons relative to the lattice is higher, it is in a non-equilibrium state. In order to achieve equilibrium, electrons will transfer energy to the lattice in 1 picosecond time;
In this case,
(3) Within 10 picoseconds, these energies will be gradually transferred to the interior of the material.
In this case,
Thus, for picosecond laser processing of about 10 picoseconds, the material has enough time to transfer heat to its interior, and then the etching takes place, so that the thermal effect is practically unavoidable. For femtosecond lasers, the pulse action time is actually less than 1 picosecond, and the electrons do not have enough time to transfer the energy to the lattice. Thereby generating a large number of plasmas on the surface of the material, and the energy is dissipated along with the removal of the material, so that a strong etching effect occurs. That is, when the laser pulse width is far less than the lattice heating time, the ablation time does not depend on the laser pulse width.

In this case,
From the above three stages, we can see that the development of laser processing and laser development is basically synchronous, a new technological breakthrough, it may usher in a leap in industrial applications of the laser, of course, will bring new questions :
In this case,
First, since the picosecond laser can not completely avoid thermal effects during machining, does it have any meaning to fine processing, or has it lost its value and necessity?
In this case,
of course not. Although the picosecond laser can not achieve the fine cold working, but the relative femtosecond laser, the price is lower, and the structure is relatively simple, the power is higher. Therefore, for such an ultrashort pulse technology over-products, how to make full use of it is worth thinking about. At present, more worthy of reference is the picosecond laser triple frequency, the use of ultraviolet light in the material of the "light etching" effect, high-energy photons through the "cold" directly undermine the chemical bonds, thereby reducing the thermal effect of processing , To achieve "cold" processing. At present, the system for the processing of transparent material has its greater advantage. Many of these lasers, such as Photonics industry's 15ps 355nm laser.
In this case,
Second, femtosecond lasers can achieve non-thermal etching, which is not the shorter the pulse width, the more sophisticated processing it? In fact, the etching effect is related to six main factors: average power, pulse width, wavelength, spectral width, single pulse energy, frequency.
In this case,
In general, the shorter the pulse width is, the more difficult it is to obtain a high average power, so in this respect the short pulse is at the expense of the average power.
In this case,
In addition, the narrower pulse width, spectral width will be greater, therefore, color will affect the processing effect.
In this case,
The second is the effect of wavelength, for the general material, the wavelength of the etching effect has little effect, only in the transparent material (such as SiO2) when processing to consider, and should consider the wavelength problem. As mentioned earlier, the transparent material has a strong absorption of ultraviolet light, and for the visible and infrared light transmittance is high. Therefore, triple-frequency light of an ultra-fast laser is usually used for processing. For tens of femtosecond lasers, the spectral width of 20nm, or even 50nm, so the tripling efficiency is very low. While the picosecond laser is able to obtain a higher triple frequency output.
Frequency will affect the processing speed. Generally speaking, the higher the better, but usually only the oscillator is about 80MHz, while the oscillator is too low a single pulse of energy, so the need to amplify, the amplifier frequency is generally from 1KHz to several hundred KHz range.
In this case,
Therefore, for ultra-fast laser processing, can not blindly pursue ultrashort pulses, must see the actual demand. And the pulse of a very short and high-power femtosecond lasers, expensive. For today's fine processing needs, lasers with hundreds of femtosecond and hundreds of KHz frequencies should be sufficient to meet the usual fine processing requirements such as JENOPTIK's D2.fs industrial lasers (4W, 400fs, 300KHz, 1025nm, 20 μJ @ 200 kHz). And the laser using fiber laser oscillator, can do long-life, maintenance-free, this is a bright spot. Indicators, price, stability, comprehensive cost-effective point of view, the laser is very good.
In this case,
The past two years, femtosecond laser technology has been rapid increase in China's economy has developed steadily, more foreign advanced products poured into. Such as the 12fs oscillator and peak power TW-class amplifiers from KMlabs, femtosecond fiber lasers from menlosystems, picosecond lasers from Lumera and the photonics industry, are among the more successful commercial ultrafast lasers.
In this case,
3. Laser fine processing
In this case,
For a laser fine processing system, in addition to the laser, the whole system platform to build, software control is also an important issue. Any one aspect of the shortcomings, are sufficient to affect the performance of the entire system.
In this case,
Software, the ordinary marking software has been unable to meet the demand, in the nanosecond pulse marking need not consider many problems, how to overcome the first pulse, corner leakage and so placed in front of us.
In this case,
Hardware, because the best galvanometer positioning accuracy can only reach 10 micron level, it is difficult to do more sophisticated processing, therefore, usually choose the slower and high precision displacement platform to form the system, high-precision displacement The platform can achieve precise control at the nanometer level. At the same time, we must consider the optical system and focus system design issues, correction of aberration and color to achieve a smaller focus spot.
In this case,
In addition, to meet the different processing needs, but also consider the fixture, process and other issues. For example, the processing of cardiovascular stents because of the cylindrical etching, so the software and fixture has a different plane processing requirements, and more difficult.
Ultrafast laser
Laser processing of cardiovascular stents
In this case,
Another example is to adjust the focal length so that the central focal spot of the focal spot just reaches the multiphoton ionization threshold of the material, the energy absorption and action during processing are limited to a small fraction of the volume at the center of the focal spot, not the entire focused spot area. In this way, it is possible to realize a processing dimension much smaller than the focused spot.
 
4. Conclusion

As China's economic and industrial development, fine processing has become a hot spot in the field of laser processing, at present, China's fine processing is still in the embryonic stage, therefore, has two aspects:
In this case,
First, the embryonic stage means immature, at present there is no mature commercial femtosecond fine processing system, we need to explore more, perhaps in the exploration will detour, will pay a price, but for the future The possible return is worth it. In order to avoid as much as possible detours, reduce unnecessary losses, should pay more attention to the method, a more comprehensive consideration of the problem. The problem of fine processing is not only the problem of the laser, but the problem of optimizing the system in order to achieve higher precision. The etching effect is the ultimate goal of our pursuit. Foreign countries have more mature products, and domestic yet, the difference between each other is not just a laser, as well as technology issues. Process research is a complex science, a complex system, the need for a team of unremitting efforts, such as software parameter settings and process effects attempt. Although lasers are the key components, it is our fatal injury to limit the vision to lasers while ignoring the integration and optimization of the overall system.
In this case,
Second, the initial stage of China's fine processing areas, because there is no mature femtosecond fine processing system, so the market has great potential, who will be the first person to eat crab it?