Interview: How VCSELs Digitize Thermal Processing

Armand Pruijmboom during the interview with LTJ editor Anna-Lena Idzko (Source: Wiley-VCH)

Armand Pruijmboom during the interview with LTJ editor Anna-Lena Idzko (Source: Wiley-VCH)

Industrial manufacturing equipment for heating of large areas with a high throughput conventionally uses for example gas burners, hot air blowers and electrically or halogen lamp heated belt ovens. Vertical-cavity surface-emitting laser-diodes, VCSELs, offer an innovative solution by heating only when and where it is needed in a short time with cost effective, compact and robust systems.

Laser Technik Journal: You and your team received the first prize for your vertical-cavity surface-emitting lasers. Congratulation! What is special about VCSELs?
Pruijmboom: VCSELs are diode lasers. Typically, the various quantum structures of a diode laser are epitaxially grown. To assemble a resonator the crystal is cleaved and mirrors are vapor-deposited on the facets, formed by the cleavage planes. For VCSELs, also the mirrors are epitaxially grown. For this purpose, layers with alternating refractive indices are grown onto each other to build a distributed Bragg reflector which acts like a mirror for a specific wavelength. The layers consist of AlxGa1-xAs with alternating aluminum content, which varies the refractive index. As a result, the whole laser can be grown epitaxially: first a mirror, then the quantum layers and on top the second mirror. The bottom layer is n-doped and the top layer p-doped.
You get a vertical diode laser by normal semiconductor wafer processes like lithography, etching, dielectric- and metal layer deposition. The entire laser is hence fabricated on wafer-scale, without the need for post processing.
The individual VCSELs have dimensions of a few tens of micrometers. Therefore, the output power is limited – but also the electric power consumption. Traditionally, VCSELs are used for data communication because they can be switched on and off very rapidly. It has been reported that if data transport is increasing at its present rate, in 2020 half of the worldwide energy consumption would be used for data transport. The only way to prevent this is by using optical data transport, instead of transport over copper cables.

LTJ: The optical power of VCSELs is pretty small. How do you reach high output power?
Pruijmboom: Since the lasers are tiny – about 30 µm in diameter – many of them can be placed on a small area. We fabricate chips with an area of 2 × 2 mm2, where 2205 single lasers are densely packed. When every single laser provides 4 mW – the chip with 2205 VCELs provides about 9 W. Of course this is not a lot compared with lasers for cutting steel or shooting rockets from outer space, but compared to on the other side – lamps, halogen lamps, IR emitters – the energy density is ten to a hundred times higher. I would say, we do not compete with lasers but we build better lamps. We are trying to extend the application area of lasers. In the fields where lamps are used and high-power lasers are too expensive we deliver with the VCSELs a new solution. Compared to traditional diode lasers, which use beam-shaping lens systems for the same purpose, VCSELs are much cheaper. And we have just started setting up a business and getting on the learning curve. When we will be able to improve the assembling technology and to produce larger numbers it will be possible to significantly further reduce the price.

Armand Pruijmboom states: “VCSEL-based systems are superior for heating.” (Source: Wiley-VCH)

Armand Pruijmboom states: “VCSEL-based systems are superior for heating.” (Source: Wiley-VCH)

LTJ: What applications the VCSELs are best suited for or are already used?
Pruijmboom: There are a lot of different application areas. Almost everywhere in the industry, heat treatment is used to melt, dry or cure something. And the methods used are numerous: gas burners, hot air blowers, halogen lamps, heated belt ovens or micro waves. A lot of the traditional methods will remain because they are simple and cheap. But I think, that VCSELcan become an essential element. Even if it is only a market fraction – but the market is huge.
Really unexpected for me was our new industrial application in the furniture industry – seamless edge banding of furniture panels. The edge band is melted by a high-power VCSEL heating system before it is welded to the edge of the board, forming an invisible seam with the surface laminates. Another possible application is light weight constructionHere, carbon fiber reinforced plastic needs to be processed, which is not easy at all. We are working on a project with the Fraunhofer IPT. We think that VCSELcould bring advantages, since it is possible to process large areas. Cooperating with the Fraunhofer ISE, we are trying to improve production processes in the photovoltaics industry. 
Another application – which was also surprising for me – is material analysis. VCSELcould be used for a method that is called lock-in thermographyImagine a material and in this material there might be cracks. The material is heated by VCSELand the regions with cracks will cool down in a different way than the regions without cracks. This is very important in the aerospace, where structural integrity is keyOur VCSEL modules are very suitable for this application as they can very homogenously illuminate large areas and switch on and off in a millisecond.

LTJ: Why is it called “digital” thermal processing?
Pruijmboom: From our VCSEL chips we build larger emitters that contain stripes of 4 mm width that deliver each about 200 W. With this standard building block approach, we can make systems of almost arbitrary width, by putting more emitters next to each other. Because of the well-defined illuminating areas, corresponding to these 4 mm-wide stripes, which can switch between 0 and 100 % of power in milliseconds, we can easily create so-called digital heating patterns on the workpiece that are illuminated by our systems.

The interview was conducted by Anna-Lena Idzko. (Source: Laser Technik Journal, Wiley-VCH)

Links: International Laser Technology Congress AKL’16, • Innovation Award Laser Technology • Philips Photonics GmbH Aachen, Germany

Speak Your Mind