The principle and application of picosecond laser

The advantages of laser ablation with ultrashort laser pulses have been proven in many applications, and until recently no industrial byproducts of these applications were found on the factory floor. In industrial applications, besides quality factors, reproducibility and cost per part are also important criteria.

The reliability of the laser system is of course directly linked to the laser technology used. The cost per part is basically directly linked to the repetition rate of the ultrashort laser pulses. High added value, high repetition rate productivity, reliable technology, etc. All these requirements have only recently been met with diode-pumped picosecond lasers.

The first application of picosecond lasers in mass production was reported at The Photonics West 2005. Pulses of 1 μJ with a pulse duration of 20 ps were focused onto a thin steel foil and a series of concentric rings were removed in the direction of the pulse beam. The different concentric rings formed a disk and the different disks of material removed formed a conical hole that served as the nozzle for injecting ink in a professional high-quality print head. In the past few years, most ultrafast laser manufacturers have focused on the development of femtosecond lasers. These lasers require a complex CPA technology to keep the peak power density within a certain amplification step below the damage threshold. In this way, pulse energies in the mJ range can be generated, but the repetition rate is limited to a few kHz. Comparatively high pulse energies of several hundred μJ are advantageous for drilling or cutting thick materials. The drilling of injection nozzles for diesel engines is an example, where 1 mm thick steel sheet material has to be precisely perforated so that no further cleaning is required.

LUMERA LASER has developed the picosecond laser STACCATO for this application. Because its pulse time is in the picosecond range, the STACCATO laser system does not require CPA technology. The laser working material Nd:YVO4 allows direct pumping with a laser diode, giving it high pulse energy and high repetition rate. The STACCATO laser system outputs an average power of 10W at 1064nm within a pulse duration of 10ps. Its laser beam is close to the diffraction limit and can be ideally focused.

Compared to femtosecond lasers, very high repetition rates up to 100 kHz ensure high productivity. Many materials have a relatively high linear absorption coefficient in the UV spectral region, which is very beneficial for material removal. The infrared laser radiation emitted by STACCATO can be converted into shorter laser wavelengths of 532 nm, 355 nm and 266 nm due to its very high peak power density. When a 100 μJ light pulse generated by a STACCATO laser hits the material, cold ablation is triggered, but the rapidly spreading plasma may still cause heating of the material. Therefore, for a good result, not only the generation of ultrashort laser pulses but also the appropriate process technology is important. It has been shown that processes such as hole drilling are not only important for micromachining results, but also polarization control, appropriate use of auxiliary gases and vacuum nozzles. Figure 2 shows high-quality small holes in steel and ceramics. Heat-induced cracks and liquid points can be avoided by using appropriate processing strategies. Figure 2. Left and right are holes machined in steel and ceramics with picosecond lasers. The intense plasma formation associated with high pulse energies during material removal raises the question of whether the commercially available ultrafast lasers are ideally designed for precision surface machining. The following considerations concern the ideal parameters for ultrafast lasers for precision removal of material close to the surface. General experience in many experiments has shown that the best precision can be achieved with a power density selected at 5-10 times the threshold ultrashort laser pulse if reasonable removal rates are achieved. For metals, this means that the energy density of a 10PS pulse should be 1J/cm2. If the micromachining laser beam is focused to a typical spot diameter of 10μm, the optimum pulse energy is 1μJ. In this case, the typical thickness of material melted per laser pulse is 10-100nm, plasma formation is minimal, and the quality is very good, allowing microstructures with 0.1μm precision to be machined.

After 100 pulses of the STACCATO picosecond laser, the material is removed in a defined manner, so that the profile of the laser beam is mapped onto the material. Promising practical applications are the surface processing of metal parts designed for mechanics, chemistry, and fluids. Picosecond laser processing allows almost all high-productivity structure processing. The structure is produced by focusing the 532 nm radiation of the STACCATO laser at 50 kHz to a spot with a diameter of 25 μm. Properly designed structures on the surface of turbine blades can significantly reduce friction, fuel consumption and environmental damage.

As a summary of the above, surface processing lasers should have pulse durations in the picosecond range, moderate pulse energies of a few μJ and very high repetition rates. Such a laser has recently been marketed by LUMERA under the name RAPID. Due to its relatively simple design, the RAPID laser is small and compact, and consists of a picosecond laser oscillator, an amplifier and a very fast Pockels core, so that it can provide repetition rates of up to 500 kHz. Micromachining with ultrafast lasers requires various processing parameters. The repetition rate of RAPID can be controlled externally. Single pulse operation, burst mode or any externally defined pulse sequence can be triggered by TTL pulses. This enables various processing strategies and provides control of the beam path. The beam spot quality is very good and stable, with almost no influence in different environments at various repetition rates, which is very remarkable. The RAPID laser beam is almost close to the diffraction limit and therefore shows good focusability. Its pulse duration is about 10ps, and the pulse energy is enough to exceed the threshold energy for metal removal even at high repetition rates and conversion to 532nm, 355nm, 266nm. The pulse-to-pulse energy variation is less than 1%rms. In

the technical design of the RAPID laser, its output laser parameters, controllable performance and other indicators indicate that the laser is an ideal tool for metal surface processing, such as metal surface drilling, memory repair, and marking in the semiconductor industry.