Manufacturing Customized Semiconductor Saturable Absorber Mirrors

In this interview, Antonio Castelo, EPIC's Technology Manager for Bio-Medical and Lasers, talks to Eero Koivusalo, COO at RefleKron, a Finnish manufacturer of customized semiconductor saturable absorber mirrors (SESAMs) for ultrafast lasers.

What's the background to your appointment as COO RefleKron?

After doing an MSc in Engineering Physics at Tampere University of Technology, I studied for a PhD focusing on Molecular Beam Epitaxy (MBE) and basic research on gallium arsenide-based nanowires. It was in-depth MBE fundamentals, but a really good introduction to the basic physics underlying semiconductor growth. In 2019, towards the end of my PhD, I became Product Manager of Reflekron, a spin-off from the Optoelectronics Research Centre at Tampere University of Technology. The company had originally been set up in 2004 during the rise of ultrafast fiber laser industry and its mission was to produce semiconductor saturable absorber mirrors (SESAMs) - key components for manufacturing ultrafast lasers.

When I started at RefleKron, the company was just beginning to develop our current volume product lines, so I had the opportunity to participate in their development from the start and acquire a lot of SESAM knowhow particularly in relation to the processing and customisation of components. I started by taking the lead of all the new development projects, and in 2020, I was appointed COO.

How has the company developed?

In 2004, as the first commercial mode-locked fiber lasers started to come on to the market, RefleKron was established in a close partnership with Fianium, the first EU company introducing SESAM-based ultrafast fibre laser at 1 µm. While for most of RefleKron's initial existence SESAMs were exclusively supplied to Fianium we also identified niche applications where our expertise could provide competitive advantages, for example, custom SESAMs for Q-switched microchip lasers, which required a completely different set of characteristics compared with mode-locked fiber lasers. Just in the recent years the market began to mature diversifying into many types of lasers, and currently we are providing our expertise globally with a specific focus on addressing the qualification process for large volume products.

Until recently, we were based at Tampere university, but in April 2023, we moved to our new factory in Tampere where we have an MBE-cleanroom, and sufficient space and equipment to further scale up SESAM production.

Although we’re still a relatively small company, Reflekron is very proud of its key role in the ecosystem as enabler of volume scaling capability for ultrafast laser market. We offer a unique combination of semiconductor technology expertise, in-house epitaxy, SESAM design and characterization, and extensive knowledge in laser physics. Our technology allows to develop SESAMs for both mode-locking and Q-switching applications, covering a wavelength range from 0.6 µm to 3 µm. As each type of laser has to be paired with a specific SESAM all our products are customised according to the needs of the customer and then volume production is secured for each product line.

What have been your personal challenges?

In the last couple of years, we’ve spent a lot of time and effort on our new factory. Before the move, we were based at Tampere university. While our birthplace environment is excellent enabling us to enter the market at very early stage, we are very happy that we are now having own premises with volume production capability for MBE, the core technology supporting our production and differentiation. We were fortunate to get financial support from Business Finland and EU for developments of new product lines, and also to find a partner who had experience of building similar facilities. The building design was tailored for us and is also big enough to accommodate two other spin-offs from the Optoelectronics Research Centre at Tampere University. Becoming independent has been really crucial as we can now guarantee a stable SESAM supply for volume production of pulsed laser systems, which is key for our customers and future growth.

On a personal level, I have been continuously gaining new expertise outside technology area, for example related to financing and HR. The learning path continues as we are increasing the operation for new types of products and larger volumes, which requires the development of new expertise related to qualification process and scaling up the production.

How do you see the future?

Our SESAM product line is increasing nicely, and everything looks good for the future. The volumes of existing customers are growing and we’re finding new customers at a steady pace. Furthermore, the customer base is quite versatile, so we are not dependent on any single customer or even any single market.

We expect that more of the current development projects will see large volume, something that is still relatively small compared to other type of laser and semiconductor technologies. Again, availability of reliable and SESAMs with repeatable performance is essential for the volume development of practical ultrafast lasers. Our approach of customizing SESAMs for each laser allows operation in an optimal working regime, which is the key to long lifetime. Secondly, as a result of our comprehensive know-how of process optimization and experience in MBE-technology, our products have a high wafer-to-wafer repeatability suitable for industrial volume production. In this context, we have repeatedly qualified SESAM product lines with lifetimes exceeding 10 000 hours and excellent wafer-to-wafer repeatability ensuring volume production.

We believe that RefleKron will continue to impact positively on the ultra-fast laser community because our products will play a crucial role in enabling new technologies and allowing customers to scale up their production lines. The new MBE production capability allows us to expand us to new product lines requiring specialized III-V wafers.

What's your advice for next generation of entrepreneurs?

For us, joining EPIC has been very positive. From personal perspective, us Finns tend to be silent types, it's not easy to go out and talk to others about what we do. For this reason, joining EPIC has been great because it's given us the opportunity to network, find good connections and attend meetings that have really pushed us into discussions with other companies. In this way, we’ve not only been able to highlight our capabilities but also to learn. It's also done a lot to increase our community feeling and made us see even more clearly that we are an important part of the European photonics and laser technology ecosystem.

Written by Antonio Castelo, EPIC's Technology Manager for Bio-Medical and Lasers.