Arrays of VCSELs provide configurable heat at high efficiency for
industrial thermal processes and are scalable to multikilowatt levels.
ARMAND PRUIJMBOOM
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Many industrial
manufacturing processes use some kind of thermal process for melting, drying,
curing, annealing, or diffusing. The types of heaters used range from
hot-air blowers, gas burners, and convection ovens to halogen lamps, induction
heaters, and microwave ovens. All these approaches have their specific merits,
but for a significant fraction of industrial heating needs, vertical-cavity
surface-emitting laser (VCSEL)-based photonic heating has advantages.
and can be switched on and off in milliseconds. Furthermore, they allow accurate
dosing control for either very homogeneous heating of large areas or the
imposition of heat patterns with local variations in the deposited energy
density. In addition, solid-state light sources like VCSELs are very robust and
have lifetimes exceeding multiple 10k hours of continuous operation.
VCSEL arrays are
distinctly different from the world of conventional high-power laser diodes. The
latter is dominated by lasers for cutting or welding steel, where it is all
about what laser system has the highest brightness and power to cut even faster
through even thicker steel. VCSELs are more benign lasers that can also be
scaled to very high power by massive parallel operation of millions of
microlasers. Because of this multibeam concept, their effective brightness is
lower, and while they are not suited for metal machining, they are an excellent
solution for industrial heating.1
VCSEL geometry
The cavity of a VCSEL is
formed perpendicularly to the surface of a gallium arsenide (GaAs) wafer by epitaxial growth of layers with alternating
refractive index, forming so-called distributed Bragg reflectors (DBRs), called
"DBR mirrors" (see Fig. 1). Usually, n-type doped DBR mirrors are formed first,
followed by quantum wells forming the active layer and p-type doped DBR mirrors
forming the emission facet. The current flows through a ring-shaped contact on
the upper mirrors to the substrate.
 |
FIGURE 1. A cross-section through the optical cavity of a VCSEL shows |
A current confinement aperture, usually formed by selective oxidation of an
aluminum-rich layer in the middle of the cavity, is introduced to improve the
optical-electrical conversion efficiency by only electrically pumping the area
that overlaps with the optical modes. The emission wavelength can range between
about 800 and 1100 nm, depending on the design of the quantum well and the
cavity.
Depending on the aperture size, which can range between 4 and 20 μm, one
VCSEL can emit 1 to 10 mW of power. The VCSEL cavities (often referred to as
"mesas" because of their shape) have a diameter of 30 to 40 μm; when hexagonally
arranged, more than 500 of them can be placed on one square millimeter of a
chip, leading to power densities of several watts per square millimeter.
An advantage VCSELs have over edge-emitting laser diodes is that the laser
arrays are fully completed by wafer processing; functional testing and chip
selection can be done by wafer probing. The chip assembly can be done with
standard automatic semiconductor processes and equipment for pick-and-place and
wire-bonding. As a result, costs are comparable to those for LEDs.
Philips Photonics has
scaled VCSEL technology to allow fabrication of very large arrays. For example,
an array chip of 1.9 × 2.0 mm2, containing more than 2000 VCSELs, is
assembled on a board to form an illumination module for night-vision cameras
(see Figs.
2a-c). Depending on the heat-sink temperature, it emits more than 8 W of
infrared power. A larger 2D array can then be formed by assembling the same
chips next to each other (see Fig.
2d). By series connection of the chips, the current stays low (the same as
that for a single chip) and the voltage high, allowing cost-effective driver
design.
 |
FIGURE 2. A VCSEL-array chip assembled on a board forms an |
Figure
2e shows a 400 W emitter, including the cooling structure. The emitting area
is 8.5 × 40 mm2 and consists of 56 chips arranged in four rows of 14
chips, two of which are connected in series to form emitting zones of 4.2 × 40
mm2. Each zone can be independently adjusted between 0 and 200 W
output power.
Scalable by stacking
These VCSEL modules are
easily scalable: Larger arrays can be created by stacking these emitters next to
each other. Figure
2f shows a 9.6 kW module with an emitting area of 40 × 200 mm2.
The power density, which is above 1 W/mm2, can be enhanced to above
10 W/mm2 by higher output-power densities of the VCSEL chip and
further by the use of microlenses and secondary optics.2,3 The
performance characteristics of an 808 nm emitter show that an output power above
400 W is achieved at 40% power-conversion efficiency (see Fig.
3). An even higher power is possible at lower efficiency.
High performance of densely packed large-area arrays is much more challenging
to achieve than for a single chip assembled on a heat sink. In the latter case,
lateral heat spreading leads to 3D heat conduction, whereas for a densely packed
emitter the heat conduction is essentially one-dimensional. Despite this
challenging thermal situation, performance of the large-area arrays is on par
with benchmark results.
An electronic driver
unit has been developed that can drive up to 12 zones of a maximum of six
emitters (see Fig.
4). The response time is 5 ms, allowing fast switching of the laser power
between 0 and 100% of its maximum power. An EtherCAT industrial bus interface
enables software control of the power. The calibration data of an emitter is
stored on an SD memory card, which allows the user to set the true output power.
Safety switches are included which, for example, switch off the power when the
cooling water is interrupted.
Together with the
scalable modules shown in Figure
2f, the electronic driver forms a "heating system" that allows control of
the heating dose spatially as well as temporally. At a 30 mm distance from the
emitter surface, the mixing of radiation from the individual chips evens out the
gaps between the chips, leading to a ripple in the received optical intensity of
less than 5%.
Local control of heating
With these heating systems, large areas can be heated homogeneously. The
system's zoning allows for alteration of the power locally to adjust for
conditions such as locally higher heat conduction at the edges of a workpiece.
Between 800 and 1100 nm, the wavelength can be chosen for maximum absorption of
the material. A major advantage compared to halogen lamps is that, when dimming
the output, the wavelength—and hence the material absorption-does not
change.
In conveyor-belt ovens, the entire oven needs to be heated up to heat
materials within it, which requires a long time and hence long ovens for high
throughput. In contrast, VCSEL-based photonic heating systems are much more
compact due to their much higher power density, as well as the fact that the
infrared (IR) radiation only has to heat the workpiece rather than the entire
oven. The fast switching allows the system to switch off when there are gaps
between the materials on the belt.
Heat patterns can be generated on linearly moving materials by switching the
output power spatially as well as temporally. This can be used for
belt-oven-like processes such as sintering of screen-printed thick films for
passive electronic components and solar cell manufacturing. This approach can
also be used in roll-to-roll processes, which have been recently pursued for
cheap and efficient manufacturing of printed electronics, organic LEDs (OLEDs),
and solar cells.
VCSEL heating systems are compact and easy to integrate as compared to other
approaches. To date, VCSEL arrays are successfully used for melting and welding
of plastics, as well as fast firing for solar-cell manufacturing. Tape laying of
carbon-fiber-reinforced thermoplastic composites, which is a special case of
plastic welding, is used for weight savings in the aerospace industry, and more
recently also in the automotive industry.
In the automotive
industry, high-strength steel is used to save weight. Local softening of this
steel facilitates cold-forming of automotive parts. It has been demonstrated
that VCSELs enable treating larger areas without the use of scanning
systems.4 On a 1.5-mm-thick steel sample, an 8.5 × 40 mm2
large area was heated to 900°C by a VCSEL module. The Vickers hardness was
measured at different locations as a function of the distance from the area that
remained cold (see Fig.
5). A more than 50% reduction in Vickers hardness is observed in the treated
area. Material analysis confirms that the initial martensitic structure observed
in the untreated area is converted to the softer ferrite and cementite structure
in the heated area.
Currently, traditional laser diodes or even fiber and disk lasers are used in
conjunction with complicated, bulky, and expensive optics to generate a
homogeneous illumination for heating. In contrast, VCSEL illumination systems
are a compact, efficient, and cost-effective solution for demanding
heat-treatment applications requiring outstanding homogeneity.
REFERENCES
1. H. Moench and G. Derra, "High power VCSEL systems," Laser Technol.
J., Wiley, no. 2 (2014).
2. H. Moench et al., "Modular VCSEL solution for uniform line illumination in
the kW range," Proc. SPIE 8241, 824110 (2012).
3. H. Moench et al., "VCSEL arrays with integrated optics," Proc. SPIE
8639, 86390M (2013).
4. S. Wollgarten, Fraunhofer ILT, unpublished.
Armand Pruijmboom
is general manager of Philips GmbH, Photonics Aachen (Aachen, Germany), a branch
of Philips Photonics; e-mail: systems.photonics@philips.com;
www.philips.com/photonics.
