Technical Insight
Research Review: VCSELs retain speeds at high temperatures
A GERMAN team claims to have broken the record for data transmission from an oxideconfined 980 nm VCSEL operating at 85 °C. Their device, which is capable of 25 Gbit/s operation at that elevated temperature, is an ideal source for very short optical links in high performance computers, according to the researchers from the Technical University of Berlin and VI Systems.
“Since temperatures inside computers are as high as 85 °C, or even higher, good temperature stability is indispensable for robust, inexpensive optical links,” says Dieter Bimberg, head of the research team at the Technical University of Berlin.
Today 850 nm is the standard wavelength for short-reach optical links and local and storage-area networks, but Bimberg believes there is a strong case for 980 nm sources in all these applications.
“980 nm has the crucial advantage of transparency of the GaAs substrate, so one can easily realize bottom-emitting devices, increasing and simplifying packaging density. This is very important, for example, in the case of a large number of VCSELs for parallel optical links.”
VCSELs are fabricated via MOCVD growth of an epistructure containing 24 pairs of Al0.12Ga0.88As and Al0.90Ga0.10As layers for the bottom mirror, and 37 pairs for the top mirror. Sandwiched between these mirrors is an active region with five compressively strained In0.21Ga0.79As quantum wells that are 4.2 nm thick, which are interlaced with 6 nm thick GaAs0.88P0.12 tensile strained barriers.
Selective wet etching forms two 30 nm-thick Al0.98Ga0.02As oxide apertures positioned just above the microcavity, in the field intensity nodes in the first two periods of the upper mirror.
Output from this 10 μm-diameter oxide aperture VCSEL is 4.3 mW at 20 °C, falling to 2.6 mW at 85 °C. This relatively small reduction in power stems from an intentional red-shift detuning of 15 nm between the quantum well gain peak and the cavity resonance.
Future targets for the team are to speed the 980 nm VCSELs to 40 Gbit/s and maintain this rate at 100 °C. “We will use an optimized active region to improve the temperature stability even further, and an optimized cavity design to increase the speed beyond 25 Gbit/s,” reveals Bimberg.
The 980 nm VCSELs produced by researchers at the Technical University of Berlin and VI Systems have bit error rates at 25 Gbit/s of less than 10-12.
A. Mutig et al. Appl. Phys Lett. 97 151101(2010)
