What Is Ultrafast Laser?

Definition

Ultrafast laser is a type of ultra-intense ultra-short pulsed laser with pulse width less than or within the picosecond level (10-12s), which is defined based on the energy output waveform. This definition is related to "ultrafast phenomena". Ultrafast phenomenon refers to a phenomenon that occurs in a physical, chemical or biological process that changes rapidly in the microscopic system of matter. In the atomic and molecular system, the time scale of the motion of atoms and molecules is on the order of picoseconds to femtoseconds. For example, the period of molecular rotation is on the order of picoseconds, and the period of vibration is on the order of femtoseconds. When the laser pulse width reaches the level of picosecond or femtosecond, it can largely avoid the influence on the overall thermal motion of molecules (the thermal motion of molecules is the microscopic essence of the temperature of matter), and the material is generated on the time scale of molecular vibration. Influence, so that while achieving the purpose of processing, the thermal effect is greatly reduced.

Types

There are many classification methods for lasers, among which there are four most commonly used classification methods, including classification by working substance, classification by energy output waveform (working mode), classification by output wavelength (color), and classification by power.

Among them, according to the energy output waveform, lasers can be divided into continuous lasers, pulsed lasers, and quasi-continuous lasers:

Continuous Laser

It is a laser that continuously outputs stable energy waveforms during working hours. It is characterized by high power and can process materials with large volume and high melting point, such as metal plates.

Pulsed Laser

It outputs energy in the form of pulses. According to the pulse width, it can be further divided into millisecond lasers, microsecond lasers, nanosecond shutdown devices, picosecond lasers, femtosecond lasers, and attosecond lasers; for example, if a pulse laser The pulse width of the output laser is between 1-1000ns, which we call nanosecond lasers, and so on. We call picosecond lasers, femtosecond lasers, attosecond lasers, and ultrafast lasers. The power of the pulsed laser is much lower than that of the continuous laser, but the processing accuracy is higher than that of the continuous laser, and in general, the narrower the pulse width, the higher the processing accuracy.

Quasi-CW Laser

It can repeatedly output relatively high-energy laser within a certain period, and it is also a pulse laser in theory.

The energy output waveforms of the above three lasers can also be described by the parameter "duty cycle". For a laser, the duty cycle can be interpreted as the ratio of the time of laser energy output relative to the total time within a pulse cycle.

CW laser duty cycle (=1) > quasi-CW laser duty cycle > pulsed laser duty cycle. Generally, the narrower the pulse width of the pulsed laser, the lower the duty cycle.

In the field of material processing, pulsed lasers were initially a transitional product of continuous lasers. This is because the output power of continuous lasers cannot be very high due to the influence of factors such as the bearing capacity of core components and the level of technology in the early stage, and the material cannot be heated to the melting point. The above achieves the purpose of processing. If certain technical means are used to concentrate the output energy of the laser on a single pulse, so that although the total power of the laser does not change, the instantaneous power at the time of the pulse is greatly increased, which satisfies the requirements of material processing. Later, continuous laser technology gradually matured, and it was found that pulsed laser has a great advantage in processing accuracy. This is because the thermal effect of pulsed laser on materials is smaller, and the narrower the laser pulse width, the smaller the thermal effect, and the smoother the edge of the processed material , the corresponding machining accuracy is higher.

Components

Two core demands of ultrafast lasers: high stability ultrashort pulse and high pulse energy. Generally, ultrashort pulses can be obtained by using mode-locking technology, and high pulse energy can be obtained by using CPA amplification technology. The core components involved include oscillators, stretchers, amplifiers, and compressors. Among them, the oscillator and amplifier technology are the most difficult, and they are also the core technology of an ultrafast laser manufacturing company.

Oscillator

In the oscillator, ultrafast laser pulses are obtained using a mode-locking technique.

Stretcher

The stretcher stretches the femtosecond seed pulses apart in time by different wavelengths.

Amplifier

A chirped amplifier is used to fully energize this stretched pulse.

Compressor

The compressor brings together the amplified spectra of different components and restores them to the femtosecond width, thus forming femtosecond laser pulses with extremely high instantaneous power.

Applications

Compared with nanosecond and millisecond lasers, although the overall power of ultrafast lasers is lower, because it directly acts on the time scale of material molecular vibrations, it realizes "cold processing" in the true sense, so the processing accuracy is greatly improved.

Due to different characteristics, high-power continuous lasers, non-ultrafast pulsed lasers and ultrafast lasers have great differences in downstream application fields:

High-power continuous lasers (and quasi-continuous lasers) are used for cutting, sintering, welding, surface cladding, drilling, 3D printing of metal materials.

Non-ultrafast pulsed lasers are used for marking of non-metallic materials, processing of silicon materials, precision engraving of metal surfaces, cleaning of metal surfaces, precision welding of metals, micromachining of metals.

Ultrafast lasers are used for cutting and welding of transparent materials such as glass, PET and sapphire and hard and brittle materials, precision marking, ophthalmic surgery, microscopic passivation and etching of materials.

From the point of view of usage, high-power CW lasers and ultrafast lasers have almost no mutual substitution relationship. They are like axes and tweezers, and their sizes have their own advantages and disadvantages. The downstream applications of non-ultrafast pulsed lasers have some overlap with continuous lasers and ultrafast lasers. From the actual results, under the same application, its power is not as good as that of continuous lasers, and its accuracy is not as good as that of ultrafast lasers. The more prominent is the cost performance.

Especially the nanosecond ultraviolet laser, although its pulse width does not reach the picosecond level, but the processing accuracy is greatly improved compared with other color nanosecond lasers, it has been widely used in the processing and manufacturing of 3C products. In the future, as the cost of ultrafast lasers decreases, it may occupy the nanosecond ultraviolet market.

Ultrafast lasers realize cold processing in a real sense and have significant advantages in precision processing. As the production technology of ultrafast lasers gradually matures, the cost gradually decreases. In the future, it is expected to be widely used in medical biology, aerospace, consumer electronics, lighting display, energy environment, precision machinery and other downstream industries.

Medical Cosmetology

Ultrafast lasers can be used in medical eye surgery equipment and cosmetic devices. Femtosecond laser is used in myopia surgery and is known as "another revolution in refractive surgery" after wavefront aberration technology. The eye axis of myopic patients is larger than the normal eye axis, so that in the state of eyeball relaxation, the focus of parallel light rays after refraction by the eye's refractive system falls in front of the retina. Femtosecond laser surgery can remove excess muscle in the axial dimension and restore the axial distance to normal. Femtosecond laser surgery has the advantages of high accuracy, high safety, high stability, short operation time, and high comfort, and has become one of the most mainstream myopia surgery methods.

In terms of beauty, ultra-fast lasers can be used to remove pigment and native moles, remove tattoos, and improve skin aging.

Consumer Electronics

Ultrafast lasers are suitable for hard and brittle transparent material processing, thin film processing, precision marking, etc. in the manufacturing process of consumer electronics. Mobile phone tempered glass and sapphire are representative hard, brittle and transparent materials in consumer electronics raw materials, especially sapphire, due to its high hardness and high brittleness, the efficiency and yield rate of traditional machining methods are very low; sapphire is now widely used It is widely used in smart watches, mobile phone camera covers, fingerprint module covers, etc.; nanosecond ultraviolet laser and ultrafast laser are the main technical means for cutting sapphire at present, and the processing effect of ultrafast laser is better than that of ultraviolet nanosecond laser. In addition, the processing methods used by camera modules and fingerprint modules are mainly nanosecond and picosecond lasers. For the cutting of flexible mobile phone screens (foldable screens) and the corresponding 3D glass drilling in the future, the mainstream technology will most likely be ultrafast lasers.

Ultrafast lasers also have important applications in panel manufacturing. Ultrafast lasers can be used for cutting OLED polarizers, peeling and repairing during LCD/OLED manufacturing.

For OLEDs, its polymer materials are particularly sensitive to thermal influences. In addition, the size and spacing of the cells currently made are very small, and the remaining processing size is also very small. The traditional die-cutting process like before is no longer suitable for today. The production needs of the industry, and now there are application requirements for special-shaped screens and perforated screens, which are beyond the capabilities of traditional crafts. In this way, the benefits of ultrafast lasers are reflected, especially picosecond ultraviolet or even femtosecond lasers, which have a small heat-affected zone and are more suitable for more flexible applications such as curve processing.

Micro Welding

For transparent solid media such as glass, various phenomena such as nonlinear absorption, melting damage, plasma formation, ablation, and fiber propagation will occur when ultrashort pulse laser propagates in the medium. The figure shows various phenomena that occur in the interaction between ultrashort pulse laser and solid material under different power densities and time scales.

Because ultra-short pulse laser micro-welding technology does not need to insert an intermediate layer, has high efficiency, high precision, no macroscopic thermal effect, and has relatively ideal mechanical and optical properties after micro-welding treatment, it is very suitable for micro-welding of transparent materials such as glass. For example, researchers have successfully welded end caps to standard and microstructured optical fibers using 70 fs, 250 kHz pulses.

Display Lighting

The application of ultrafast lasers in the field of display lighting mainly refers to the scribing and cutting of LED wafers. This is another example of ultrafast lasers being suitable for processing hard and brittle materials. Ultrafast laser processing has high cross-section flatness and significantly reduced edge chipping. Efficiency and accuracy are greatly improved.

Photovoltaic Energy

Ultrafast lasers have broad application space in the manufacture of photovoltaic cells. For example, in the manufacture of CIGS thin-film batteries, ultrafast lasers can replace the original mechanical scribing process and significantly improve the quality of scribing, especially for P2 and P3 scribing links, which can achieve almost no chipping and no cracks and residual stress.

Aerospace

In order to improve the performance and service life of the turbine blades, and then improve the performance of the engine, it is necessary to adopt air film cooling technology, which puts forward extremely high requirements for the air film hole processing technology. In 2018, Xi'an Institute of Optics and Mechanics developed the highest single pulse energy in China. The 26-watt industrial-grade femtosecond fiber laser, and developed a series of ultra-fast laser extreme manufacturing equipment, achieved a breakthrough in the "cold processing" of air film holes in aero-engine turbine blades, filling the domestic gap. This processing method is more advanced than EDM The accuracy of the method is higher, and the yield rate is greatly improved.

Ultrafast lasers can also be applied to the precision machining of fiber-reinforced composite materials, and the improvement of machining accuracy will help expand the application of composite materials such as carbon fiber in aerospace and other high-end fields.

Research Field

Two-photon polymerization technology (2PP) is a "nano-optical" 3D printing method, similar to light-curing rapid prototyping technology, and futurist Christopher Barnatt believes that this technology may become a mainstream form of 3D printing in the future. The principle of two-photon polymerization technology is to selectively cure photosensitive resin by using "femtosecond pulse laser". It sounds like photocuring rapid prototyping, the difference is that the minimum layer thickness and X-Y axis resolution that two-photon polymerization technology can achieve are between 100 nm and 200 nm. In other words, 2PP 3D printing technology is hundreds of times more accurate than traditional light-curing molding technology, and the printed things are smaller than bacteria.

At present, the price of ultrafast lasers is still relatively expensive. As a pioneer in the industry, STYLECNC is already producing ultrafast laser processing equipment and has achieved good market feedback. Laser precision cutting equipment for OLED modules based on ultrafast laser technology, ultrafast (picosecond/femtosecond) laser marking equipment, glass chamfering laser processing equipment for picosecond infrared display screens, and picosecond infrared glass wafers have been launched laser cutting equipment, LED automatic invisible dicing machine, semiconductor wafer laser cutting machine, glass cover cutting equipment for fingerprint identification modules, flexible display mass production lines and a series of ultra-fast laser products.

Pros & Cons

Pros

Ultrafast laser is one of the important development directions in the laser field. As an emerging technology, it has significant advantages in precision micromachining. The ultra-short pulse generated by the ultra-fast laser interacts with the material for a very short time, and will not bring heat to the surrounding materials, so ultra-fast laser processing is also called cold processing. This is because, when the laser pulse width reaches the picosecond or femtosecond level, the influence on the molecular thermal motion can be avoided to a large extent, resulting in less thermal influence.

For example, when we cut preserved eggs with a blunt kitchen knife, we often cut the preserved eggs into fine pieces. If you choose a cutting method with a particularly sharp knife edge that cuts the mess quickly, the preserved eggs will be cut evenly and beautifully. That's the advantage of being super fast.

Cons

High-end manufacturing industries such as integrated circuits and panels have extremely high requirements for laser processing equipment, and there is a risk of technological breakthroughs falling short of expectations.

The price of ultra-fast lasers is high, and switching to a new laser supplier has the risk of not being able to expand the market as expected for both laser equipment manufacturers and the most downstream users.