原创 Lasers for area heating, really?

We tend to picture lasers, regardless of power rating, as highly focused, coherent light sources. After all, one of the virtues of the laser is that its beam doesn't spread, so it can be used for targeted illumination (such as scanner or rangefinder), or high-intensity localized heating (cutting or welding metals), to cite just a few of their thousands of applications. (Historical side note: when the laser was first demonstrated, one pundit wagged "it was a solution looking for a problem to solve," and we know how that quip turned out!)

But using lasers for area heating seems to be a contrary to their virtues. After all you can heat with IR lamps, microwaves, heated air, electric coils, induction coils, or gas-fired burners, to cite just a few possibilities. Why would you go to the complications of using lasers unless you had no alternative?

That's why I was surprised when I saw the story on the benefits of "photonic" heating in Laser Focus World, "High-power VCSEL arrays make ideal industrial heating systems." By setting up an array of hundreds of vertical-cavity surface-emitting lasers (shown below), you not only obviously get a different source of heat, but you attain some other unique operating advantages that are non-obvious and beneficial. Yes, the author's company (Photonics Aachen, part of Philips Photonics) makes this system and so he is somewhat biased, but nonetheless, it's worth seeing what he has to say.

A one or two-dimensional array of VCSELs can be used as tightly spaced, easily modulated high-intensity heat source.
(Source: Philips Photonics)

 

In contrast to conventional edge-emitting laser diodes, the collection of vertical-emitting laser diodes (each with a diameter of 30-40 μmeter) can be fabricated in one pass of wafer processing, including test, with about 500 VCSELs per mm² of a die. Since each laser emits 1 to 10 mW, a 2 × 2 mm chip array holding 2,000 VCSELs can emit over 20 W of infrared power — that's impressive power density for this technology. These arrays can be connected in series, so arrays of hundreds of watts and even kW have been built. While heat sinking of the die is an issue, it’s a manageable one, the author claims.

All this is impressive, but why bother when you can use standard halogen lamps, for example, to get the IR heating? First, the VCSEL IR brightness is 100 to 1,000 greater than halogens, with a lifetime of over than 10,000 hours, the author says (and I'll have to accept those numbers for now).

But the advantages of VCSEL-based heating go beyond just density and lifetime. The VCSEL array can be switched on and off in milliseconds for precise dosing control, since it does not have the thermal lag of a halogen emitter or similar sources. Also, the VCSEL array is well suited to highly targeted, localized zones, where the material to be heated may not be homogenous, with some areas needing more or less heat or specialized heat-treating patterns are preferred (think of PC boards to be wave soldered and loaded with different-size/mass components). Even if this level of control is not a requirement, the output power and thus heat is tightly focused, so that the entire oven does not have to be heated; only the part of the material that needs treatment. Further, unlike bulbs, the VCSEL's output wavelength does not change as it is dimmed, which means the target material's thermal absorption characteristics are unchanged, a factor in precision situation. Dimming control is relatively easy, as the VCSELs are driven by a controllable DC current.

I don’t have a need to heat anything with VCSELs, of course, nor do I fully understand what downsides of this heating approach. Still, reading about this application reminded me that what we often consider a key attribute of a technology can sometimes be less relevant, while its downside can become a virtue. While we prize lasers for their ability to deliver highly focused beams of photonic power, and use ever-bigger single-source lasers to deliver increasingly powerful punches, some out-of-the-box thinking shows that an aggregation of many small lasers as heat sources can be used to advantage for some applications.

Have you seen other cases where contrary thinking has solved a power problem or turned a thermal weakness into an advantage? Have you ever done this?