High power UV from a thin-disk laser system

High power UV from a thin-disk laser system S. M. Joosten 1, R. Busch 1, S. Marzenell 1, C. Ziolek 1, D. Sutter 2 1 TRUMPF Laser Marking Systems AG, Ausserfeld, CH-7214 Grüsch, Switzerland 2 TRUMPF Laser GmbH + Co. KG, Aichhalder Strasse 39, D-78713 Schramberg, Germany ABSTRACT The demand for laser systems for marking and micromachining using high power UV has created a significant growth of lasers in manufacturing. To further support this growth advanced and cost-efficient technologies are required. Using a cavity dumped laser system based on thin disk technology leads to very short pulses below 10 ns. In addition the pulse width is independent of the chosen pulse repetition rate. This is in contrast to conventional Q-switched lasers. The combination of high average power and short pulses leads to high peak powers, e.g. more than 20 kw at 100 khz. These short pulses are available even at high repetition rates up to 250 khz and enable both high quality and high speed marking and micromachining. Using the field proven disk technology allows easy scaling to even higher power while maintaining reasonable costs. In this paper we will present a laser system based on the thin disk technology suitable for both micro machining and precise material processing. Keywords: thin-disk laser, cavity dumping, UV, laser marking, laser micromachining 1. INTRODUCTION Over the last years the demand for lasers with high output powers has increased dramatically. On the one hand higher powers translate into shorter processing times and thus higher throughput in production which, in the end, saves cost. Splitting the beam into two or more parallel beams with each one having enough power and thus enabling parallel processing of more parts at the same time can increase productivity even further. On the other hand there are processes having high thresholds that require a suitable power level to be executed. Today, UV lasers are used in a variety of applications in the fields of marking and micromachining. These different applications each require specific features that ensure good process quality and reliability. These are (among others): High pulse peak power Stable laser output power Long lifetime of the UV components Easy maintenance/exchange of wear parts Rugged mechanical setup The most widespread lasers for marking and micromachining applications are diode pumped solid state lasers and fiber lasers, the latter increasing their market share continuously. Both types of lasers have drawbacks that limit their potential in high peak power applications, especially in the UV region. Diode pumped solid state lasers are limited in their maximum average power due to restrictions regarding their beam quality. In order to reach average infrared output powers of 100 W and above, oscillator-amplifier setups have often to be implemented. These setups tend to be complex and costly to achieve short pulse width, high peak power, and high average power at the same time. Fiber lasers on the other hand have typically no issues with beam quality but they suffer from their intrinsic limitation of peak power because of the nonlinear effects inside the fibers. Solid State Lasers XXIII: Technology and Devices, edited by W. Andrew Clarkson, Ramesh K. Shori, Proc. of SPIE Vol. 8959, 89590L 2014 SPIE CCC code: 0277-786X/14/$18 doi: 10.1117/12.2039414 Proc. of SPIE Vol. 8959 89590L-1

The goal is a combination of advantages of both types of lasers while omitting their drawbacks. In addition, this combination should provide a cost-efficient and reliable solution. One approach to overcome this conflict is to combine both technologies in a hybrid laser system 1. This kind of laser offers a very broad setting range at the expense of high complexity. In order to omit complex multi stage setups, we used a diode pumped thin-disk laser which can handle high peak and average powers while maintaining fundamental mode beam quality. In the scope of this paper we present a laser system comprising only one oscillator with an output power of >20 W at 343 nm based on TRUMPF s thin-disk laser technology. 2. THE LASER SYSTEM TRUMPF has gained a vast experience with thin-disk lasers over the past decade, implementing this technology into a variety of products from high power cw (constant wave) lasers for cutting and welding applications to pico- and femtosecond lasers with high pulse energies for micromachining. Tapping into this knowledge base, we used this technology to build up a robust, compact nanosecond laser source generating high average power levels of more than 100 W in the infrared. Power and repetition rate can be adjusted independently from each other without changing the pulse width. Using a thin-disk shaped Yb:YAG (Ytterbium doped Yttrium Aluminum Garnet) gain medium has many advantages, e. g. low thermal lensing and polarized output 2. The laser is pumped by laser diodes at a wavelength of 938 nm having an average power of 250 W. The pump light is imaged into the gain medium 48 times by using prisms and a parabolic mirror. This ensures an efficient absorption of the pump light within the thin active medium. Power scaling of the system is rather easy. The pump power can be increased by increasing the diameter of the pump spot accordingly. To maintain a good beam quality, the size of the resonator mode has just to be adjusted to the new pump spot. This kind of design flexibility is unique for diode pumped thin-disk lasers. Figure 1. Schematic view of the resonator. The light is coupled out through the polarizer when the pockels-cell rotates the polarization of the intracavity light. This process is triggered by a photo diode. Figure 1 shows a schematic view of the resonator. It has a so called V-shape geometry with the thin disk being the tip of the V, which means that the light in the resonator passes the disk 4 times per round trip. Pulse generation is realized via cavity dumping. Therefore both end mirrors of the resonator are high reflective for the laser wavelength and thus prevent light from being coupled out. A polarizer acts as an intracavity folding mirror that reflects one polarization state, while transmitting the other, thereby acting as a variable output mirror. The polarization is Proc. of SPIE Vol. 8959 89590L-2

1 controlled by an electro optic modulator, e. g. a pockels-cell. In our setup a single BBO (Beta Barium Borate) crystal is used as electro optic medium. It is worth noting that the pulse duration in a cavity dumped system only depends on the switching time, the level of the high voltage, and the resonator length 3. We achieved pulse durations of less than 10 ns constant over the entire range of repetition rates and pump powers. Of course, it is possible to adjust any or a combination of the three parameters mentioned above to adjust the pulse duration. The constant pulse duration leads to higher conversion efficiencies at higher repetition rates compared to classically Q-switched laser systems. There are new laser concepts available to achieve constant pulse duration at different repetition rates. These are based on fiber lasers and fiber amplifiers or are hybrid systems based on fiber lasers, fiber amplifiers, and bulk amplifiers. They offer a very wide setting range for the repetition rate but their maximum pulse peak power and pulse energy are limited. To achieve high UV power, these solutions are complex and not cost-efficient. 20-0.5-10- 15-10- 0.4, E >. 0.3 ED a c m Ñ 0.2 5-0.1-0....... t I t I 1 1 1 1 I 20 40 60 80 100 120 140 160 180 200 60-0.0 20 40 60 80 100 120 140 160 180 200 220 50 - ia3-3 a 30 - Y co N a a) 20 - En fl 40-0 I 1 I I 1 I I I I 1 I 20 40 60 80 100 120 140 160 180 200 220 Figure 2. Diagrams of the average power, the pulse energy and the pulse peak power versus repetition rate of the UV laser Figure 2 shows the measured average power, pulse energy and peak power at 343 nm of the cavity dumped system for different repetition rates. In order to achieve the best application results, it is important that all laser pulses in one pulse train have the same power. Cavity dumping provides stable pulse power inside a pulse train, but thin-disk lasers show a considerable variation of pulse power over the first few pulses. This could lead to unwanted results on the work piece, e. g. varying contrast or changes in the processing depth. To mitigate this problem, we use an acousto optical modulator (AOM) that is placed outside the resonator to adjust the laser power while keeping the resonator running at a constant power level. Proc. of SPIE Vol. 8959 89590L-3

All components required for the frequency conversion into UV are placed inside a sealed compartment. For the second harmonic generation noncritically phase matched LBO is used, which is placed inside a heating device. The third harmonic generation is realized in critically phase matched LBO. Both crystals are placed in close proximity within the Rayleigh range of a single lens. Degradation of the LBO crystal producing the third harmonic is a well known fact, especially at higher UV power levels. A newly developed crystal shifter enables long lifetimes of the LBO crystal by shifting the crystal periodically after a certain period of time. The bearings of the shifter suppress tilt of the crystal so that changing of positions can go on while the laser is running. This procedure has no effect on the application results. The UV compartment can be exchanged easily within a few minutes without having to replace the laser as a whole or even send it back to the factory. The remaining fundamental and second harmonic light is separated from the UV by dichroic mirrors. The beam is then collimated and exits the laser housing via a window. The average power can be monitored in real time to enable efficient process control. The laser resonator is designed for TEM 00 mode offering an M 2 of less than 1.1 at 1030 nm. Figure 3 shows a schematic - view of the complete laser system i i Figure 3. Schematic view of the overall setup. The light from the resonator is modulated by an AOM before it enters the UV compartment. The unwanted diffraction order is directed into a water cooled beam dump. In front of the wavelength separation and collimation a shutter is placed. 3. APPLICATIONS OF THE NEW LASER SYSTEM Lasers with high average power in the UV can serve a very wide range of applications. Their short wavelength light is absorbed very well by many materials, especially non metals like glass, silicon, plastics or epoxies used for PCB (printed circuit board). These materials are becoming increasingly important in today s fast growing electronics industry, for example in the production of smart phones and tablets. When processing these materials with UV light, the strong absorption reduces the necessary laser power which makes the process more efficient. Additionally, when the light is absorbed very well, the so called heat affected zone (HAZ) is reduced. Inside the HAZ, the processed material is altered by the heat generated by the absorbed laser power, but not fully processed. When dicing silicon wafers for example, the process yield is limited because there is an area on both sides of the cut, where the silicon is altered by the heat and has to be wasted. This makes UV the wavelength of choice when scribing or dicing silicon wafers. Another material becoming increasingly important is carbon fiber reinforced plastic. Over the last years it has spread into many areas of consumer goods, e.g. bicycles or the body of the latest BMW i3 4. In this growing field of use, efficient tools are needed to process this new kind of material. Figure 4 shows the differences between the results when drilling holes in carbon fiber reinforced plastic using ns pulsed IR or UV radiation, respectively. The left image shows the result obtained with IR light at 1064 nm. The laser used had an average power of 20 W and pulse duration of 20 ns. One can clearly see the melt and debris around the hole which is generated by the heat of the IR light. The right image shows the result when UV light at 355 nm is used. The average power in this experiment was 5 W at pulse duration of 9 ns. The edge of the hole is clean and sharp, no melt and debris is generated. Proc. of SPIE Vol. 8959 89590L-4

Figure 4. Left: Hole in carbon fiber reinforced plastic using IR laser with 20 W average power and 20 ns pulses. Around the hole melt and debris are accumulating. Right: The same material processed with a UV laser with 5 W average power and 9 ns pulses. There is no melt or debris, the edges are sharp. Cutting of hardened glass is another application gaining importance with the spreading of touch screen devices. The edges become sharper and show less cracks when using shorter wavelengths. This again results from the better absorption of the UV light, thus reducing thermal stress in the surrounding material. SUMMARY In this paper we present a new UV laser system based on diode pumped thin-disk laser technology. This new laser system has many advantageous features for material processing such as stable output power, very good beam quality. The short pulse width constant over the entire setting range enables high frequency conversion efficiency at high repetition rates. The thin-disk laser technology is a very versatile and cost-efficient platform for nanosecond beam sources with output powers exceeding 100 W at 1030 nm and 20 W at 343 nm. The laser is well suited for the challenging applications in today s industrial production environment. REFERENCES [1] Patel, R. S., Bopvatsek, J. M., High speed micromachining with high power UV laser, Proc. of SPIE 8608, Laserbased Micro- and Nanopackaging and Assembly VII (2013) [2] Weiler, S., Holzer, M., Graham, P., Stollhof, J., Havrilla, D., From multi kw continuous wave to multi MW femtosecond pulses: recent developments exploiting disk laser technology, Proc. of SPIE 7912, Solid State Lasers XX: Technology and devices (2011) [3] Stolzenburg, C., Voss, A., Graf, T., Larionov, M., Giesen, A., Advanced pulsed thin disk laser sources, Proc. of SPIE 6871, Solid State Lasers XVII: Technology and devices (2008) [4] http://en.wikipedia.org/wiki/bmw_i3 Proc. of SPIE Vol. 8959 89590L-5