Low threshold 8x8 VCSEL arrays with high uniformity for optical
interconnects
H.-P. Gauggel*, S. Eitel, K. Gulden
Centre Suisse d’Electronique et de Microtechnique SA
Badenerstrasse 569, CH-8048 Zurich, Switzerland
Vertical cavity surface emitting lasers (VCSELs) offer the great advantage of simple
fabrication in large 2-dimensional (2D) arrays. In combination with recently achieved
performance progresses, including record low threshold values, high power
conversion efficiencies and modulation bandwidths, they become more and more
attractive for massively parallel high speed optical interconnects. However, besides
the performance characteristics of the individual devices, high uniformity in electrical
and optical properties across the arrays and high reliability is required.
We present results of very uniform, individually addressable 8 x 8 top-emitting
VCSEL arrays, designed and fabricated as a component of a free-space optoelectronic
interconnect within the SPOEC project. The epitaxial layer structure was designed for
top-emission at 960 nm and was grown by metallorganic vapor phase epitaxy
(MOVPE). The VCSELs were processed as air-post devices in an 8 x 8 array
configuration, including selective oxidation for better electrical and optical
confinement. The pitch between the individual devices is 250 μm, resulting in an area
for the whole array of 2.8 x 2.8 mm2.
A systematic variation of the oxide aperture diameter between 2 μm and 8 μm enabled
the optimization of various laser parameters. Threshold currents down to 0.24 mA and
threshold voltages down to 1.41 V could be achieved. The threshold and output power
characteristics as well as the emission wavelength of the individual lasers are
remarkably uniform across the arrays. Apart from the VCSEL array with the smallest
oxide apertures (2 μm in diameter) the threshold current within all arrays was found to
vary by less than ±4%, the corresponding uniformity in threshold voltage is better
than ±1%. Typical wavelength variations across the arrays of less than 1 nm (±0.05%)
have been achieved.
Eye-diagram measurements show that rf-packaged devices are capable of operation at
a data rate of 1.5 Gbit/s non-return-to-zero (NRZ) even without biasing, so that a
potential aggregate array capacity of 96 Gbit/s (NRZ) results. When the lasers are
biased, we measured fully opened eyes at 3 Gbit/s (NRZ), which is the limit of our
measurement set-up. For the application in the SPOEC demonstrator system, a
VCSEL array was packaged onto a driver printed circuit board (PCB) and
successfully operated at 500 Mbit/s NRZ.
In order to study the reliability characteristics a VCSEL array was mounted into a
lifetime test station and operated at 8 mA and 70°C. Up to now it has been operated
for more than 2000 hours and even under this harsh conditions the device shows only
a small output power decrease of around 3.5%.
The achieved homogeneity of the 8x8 arrays in combination with the high frequency
modulation characteristics and the device reliability makes these arrays very suitable
for applications in free space optical interconnects.