research activities in 2007 - CSEM

Compact Illumination Modules Based on High-Power VCSEL Arrays
C. Gimkiewicz, M. Columbus, S. Schneiter
Multi-mode VCSEL (vertical-cavity surface-emitting laser) arrays are candidates for compact illumination modules with applications as flash light
illumination for the detailed imaging of fast moving objects (> 100 m/s) or for time-of-flight cameras with modulation frequency in the > 10 MHz
domain. CSEM has realized an illumination unit with modulation frequencies of up to 80 MHz and an optical peak output power of 1.3 Watt.
VCSELs are prominent light sources in the area of in sensing
applications and communication, allowing data rates up to
10 GBit/s. Here, they are proposed for illumination
applications, offering a circular beam profile, low fabrication
cost and long life-time. Especially for the imaging of fast
moving objects they are of interest, since rise and fall times as
short as a few nanoseconds are feasible. The optical output
power per VCSEL diode is in the order of 10 mW; for a
23 VSCEL array it is in the order of 100 mW and for a
complete module it can be in the order of one Watt at a
wavelength of 850 nm. An illumination unit with modulation
frequencies of up to 80 MHz and an optical output peak power
of 1.3 Watt has been realized (Figure 1).
(a) (b)
Figure 1: Photography of (a) a VCSEL array with 23 diodes (Avalon
Photonics), (b) the test board.
One may note that for thermal and eye-safety reasons and for
the shadow free illumination of the field of view it is of
advantage to distribute the laser diode arrays. The developed
test board on a thermally conductive substrate has a size of
45 mm x 45 mm and can support at maximum 16 arrays with
23 devices per array (Figure 1). A high power transistor
driving circuit provides driving currents up to 1 Ampere.
However, in this current range the self-heating of the VCSELs
decreases the optical power. In cw mode for a single array
with 23 devices, this thermal roll-over occurs at around
380 mA at room temperature. In pulsed mode, the self-heating
effect is reduced and peak powers above 130 mW per array
can be achieved (Figure 2).
Figure 2: Optical DC power output of a VCSEL array with 23 diodes
(Avalon Photonics).
In a set-up with 10 VCSEL arrays of this kind, an optical DC
power of 565 mW has been demonstrated. Since the signal
has a sinusoidal shape, the peak power output is 1.33 W at
80 MHz modulation frequency (Figure 3). It has been
achieved with fan cooling, only.
Figure 3: Ten VCSEL arrays are modulated at 80 MHz modulation
frequency. The red line corresponds to the optical power curve, violet
line to the driving current, green line to the voltage and blue line to
the fast Fourier transform signal generated over the sampled interval.
A draw-back of multi-mode VCSELs is their varying far-field
intensity, which is not only dependent on the radiation angle
but also on the driving current: marginal areas of an
illuminated area suffer from a lower, non-linear illumination
intensity at low driving currents, since the higher order modes
appear only at higher driving currents (Figure 4a). With the
help of a diffusing element, the current dependency of the farfield
pattern can be reduced (Figure 4b). The power variation
at a far-field angle of 10° is only +/- 10 % compared to over
+/- 50 % for a laser diode array without diffuser.
(a) (b)
Figure 4: Angular intensity distribution in the far-field of a multi-mode
VCSEL array for different driving currents; (a) with a glass plate in
front of the array only; (b) with a diffusing element at 5.5 mm.
CSEM would like to thank Michael Moser from Avalon
Photonics. This work has been partly financed by the
European Commission under contract number IST-34107.
CSEM thanks them for their support.
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