Lithuanian and Taiwanese scientists have achieved their first 'victory' in collaboration: in autumn 2022, National Sun Yat-sen University (NSYSU) in Taiwan demonstrated the first prototype of a high-medium power thin disk laser developed in their country, to which experts from Lithuania have contributed significantly.
The university has grown the Yb:YAG (Ytterbium-doped yttrium aluminum garnet) crystal for the thin disk laser itself. The crystals were then cut, polished and coated with the necessary dielectric coatings at a Lithuanian laser company. The optical characteristics of the crystals were measured at Center for Physical Sciences and Technology (FTMC), and Dr. Paulius Mackonis, a researcher in the Department of Laser Technologies, shared his experience in developing so-called solid-state lasers during a long-term visit to Taiwan.
In thin disk lasers, the most important feature of the technology is the geometry of the amplifying medium: the laser crystal is a disk (a thin cut-out plate), which is typically 150 to 200 micrometres (thousandths of a millimetre!) thick. The large surface area (relative to the volume) ensures rapid one-way heat removal from the crystal, which allows high power (more than 500W) light pulses to be delivered.
(A prototype of the thin disk laser being developed by Taiwanese and Lithuanian scientist. Photo: FTMC)
"This would enable faster, more efficient and more productive work in materials processing, electronic components, crystal cutting, etc. This technology opens up new possibilities for laser production and applications. A lot depends on who is working with it and what technology is around the laser. Optical components, for example, usually cannot cope with high powers or pulse energies, but they are also improving.
It is like a vicious circle. It is not that I cut a crystal and the laser will automatically work better. There are many things that make it work. So the limits of this technology are, in principle, entirely dependent on how fast the whole industry develops.
But there is no doubt that the development of the thin disk laser is already a major step forward for Taiwan," says Dr. Paulius Mackonis.
(Dr. Mitch Ming Chi-Chou. Phpto: Crystal.nsysu.edu.tw)
"This project is a great success and is a good example of international cooperation (involving academia and industry) between Taiwan and Lithuania," says Dr. Mitch Ming Chi-Chou, Vice President of NSYSU University and Director of the Center for Crystal Researches. According to the scientist, the thin disk laser will be very useful in industry:
"Single crystals are one of the most important groups of materials due to their uniform and highly-ordered structure which enables them to possess outstanding properties, therefore crystals have always been regarded as an important strategic material of the country. In view of this, Center of Crystal Research (CCR) at National Sun Yat-sen University plays a key role on developing novel crystalline materials and growth technologies.
In past ten years, CCR has put lots of efforts to develop Yb:YAG laser crystal for high power thin disc laser. It has been realized as promising light source for application, such as cutting or welding. That’s due to its nature of high repetition rate operation. In the future, the thin disk laser with the average output power of 50W will be our target."
(Dr. Paulius Mackonis and PhD student Augustinas Petrulėnas standing by the TERRA laser system they are developing. Photo: FTMC)
Thin object - big benefit
How does a thin disk laser work? To get an idea, let's take a trip to the Solid-State Lasers Laboratory of FTMC. There, Dr. Mackonis, together with PhD student Augustinas Petrulėnas, is working on a laser system called TERRA which uses a solid-state laser they have developed based on a technology similar to the one currently being developed at NSYSU.
Various components - lenses, mirrors, wires, electronics, etc. - are mounted on an optical table. We can also see a brown copper heat exchanger - a "room" in which a 2 cm long, rectangular crystal is placed. Its "task" is to collect infrared light, which enters the crystal from the side.
By collecting the light, the crystal begins to glow (laser) on its own, and eventually (together with other components) makes it much more powerful, by "shooting" a laser pulse. The laser light itself is infrared, so invisible to the naked eye. However, we can see a speck of that light, for example with infrared binoculars.
(The TERRA laser system in FTMC Solid-State Lasers Laboratory: a heated crystal shines in a brown heat exchanger. With the naked eye, you can see green light it emits, while smartphone's camera shows the 'true' colour. Photo: FTMC)
What is the difference between FTMC and NSYSU versions of the lasers? It is the thickness of the crystal. If we are describing a crystal rod here, the Taiwanese researchers have tested a laser that incorporates a thin 'slice' of that rod - which is called a thin disk. However, it still needs to be coated and attached to a heat exchanger.
"Then the more complex science begins, how to attach it properly to the heat exchanger, how to ensure good thermal contact, how to prevent the crystal from breaking up due to deformation, etc. However, when done properly, the thin disk allows much higher light pulse powers.
The first disk lasers (from 1994 onwards), which were first developed, emitted just a few watts, then several hundred watts, and today you can't keep track of those numbers. Sometimes a few kilowatts or even tens of kilowatts are achieved. With such lasers, you can cut metal like butter," says Dr. Mackonis.
(A 2 cm-long Yb:YAG (Yttrium-doped aluminium garnet) crystal, which is needed to create solid-state lasers, is placed in a special container. A thin "slice" of such a crystal was cut for NSYSU University. Photo: FTMC)
The crystal - whether 2 cm or "sliced" - must be cooled. Two blue tubes are therefore connected to its "house", a copper heat exchanger, where water flows through it to cool it down. If this were not the case, the crystal would simply explode, giving off a strong dose of continuous light! This cooling is also necessary to prevent unwanted thermo-optical phenomena. Here again, the thin disk laser has a better chance.
"It has the advantage of a high surface-to-volume ratio. The surface can therefore be used to remove the heat that accumulates in the crystal very efficiently. Otherwise (as we and the Taiwanese have seen), when designing lasers of hundreds of watts, with a high pulse repetition rate, not doing so degrades the efficiency of the laser, the quality of the beam and so on. On the other hand, mechanical stresses due to the high heat content of the laser element can eventually cause the crystal to disintegrate," says the scientist.
(Meeting of FTMC and NSYSU scientists. FTMC delegation from left: Dr. Paulius Mackonis, Dr. Mindaugas Kamarauskas and Dr. Vytautas Jakštas. Photo: FTMC)
A complex system
The thin disk is the 'core' of the laser, but it is far from the only thing. Simply cutting the crystal will not make it lase, and it takes a lot of optics and electronics to make the laser shine - and shine the way you want it to. That's why Dr. Mackonis visited NSYSU in February 2023 to bring his knowledge and experience to Taiwanese researchers to help them better understand the complex system.
"They are now doing research on how to better attach crystals to a particular metal. This can be done by using additional intermediate materials, such as diamond, which has a particularly high thermal conductivity.
And the next question is how to make the laser shine once the thin disk is attached? This is where all sorts of nuances and problems start. The purpose of my visit was to help with laser design: how to evaluate certain parameters and how to solve problems.
As we use the same crystal in our laboratory at FTMC and have been working with similar lasers for a number of years, it was useful for the Taiwanese scientists to hear a bit about it from the experimental side. Because in the articles, in the theory, everything seems too nice and simple.
I think NSYSU will make a huge step towards achieving its goals," shares Mr. Mackonis.
(Dr. Gediminas Račiukaitis, President of the Lithuanian Laser Association, Head of FTMC Department of Laser Technologies, is visiting NSYSU University in Taiwan. Photo: Nsysu.edu.tw)
Lots of work ahead
According to Paulius, thin disk lasers have great potential, but the technology is not easy to master. And the companies that know how to do it, of course, do not share their secrets. For example, the German company Trumpf has been making thin disks for a long time and they are available - but, according to Dr. Mackonis, they are very expensive.
"There are many scientific groups that have tried to do this. Some have done better, others may have just run out of motivation halfway through," he says. - Laser creation is a process that does not have a clear-cut instruction manual on how to do it. You face a lot of problems and potholes.
The Taiwanese are well on their way, they have already done a lot. Of course, like most people, they have problems, but everything is solved.
Moreover, even when you get a laser with the power you want, you have to develop it further. It is always good to develop a "better" version in parallel to a prototype. That's the way it is in laser technology, you can't stand still."
(Dr. Paulius Mackonis. Photo: FTMC)
So the scientific cooperation between Taiwan and Lithuania will continue. In autumn 2021, FTMC signed a Memorandum of Understanding (MoU) with NSYSU Center for Crystal Researches on scientific and technological synergies in the fields of materials science and engineering, solid-state physics and technology, optoelectronics and electrical engineering.
In early 2022, NSYSU and FTMC jointly established the Taiwan and Lithuania Center for Semiconductors and Materials Science, located in the FTMC building. This initiative aims to develop and strengthen next-generation semiconductor materials and laser technologies, not only at the scientific level but also at the industrial level.
Written by Simonas Bendžius