Advanced directly modulated VCSELs
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The development of VCSELs supporting reliably data
rates of 40 Gbit/s under direct modulation represents the greatest
challenge. The modulation bandwidth of a VCSEL is, like any other
semiconductor laser, limited by damping, parasitic and thermal
effects. With the VCSEL being a very small volume laser, parasitic and
thermal effects related to the relatively high differential resistance
are commonly limiting the bandwidth. To achieve high bit rates
conventional directly modulated devices require high power densities
in the active region. This in turn causes degradation problems while
also the beam and spectral quality of the device is reduced.
Therefore, a proper impedance, differential gain and thermal
management are required to reach high bandwidths. For high speed
operation at high temperatures, attention also has to be paid to the
design of the active region using e.g. strain to improve differential
In addition to generating open eyes and providing error free transmission under ideal conditions, there are requirements on the maximum spectral width under modulation, on the extinction ratio, on compatibility with drive circuitry, etc. that also have to be met. There should also be margins to account for environmental variations etc. With the techniques used in the project largely advancing the high speed VCSEL technology beyond the state-of-the-art should be possible.
There is an intermediate bit rate where directly-modulated (DM) devices can be used. DM GaAs-based VCSELs, emitting at 850 nm and capable of direct modulation at 16-32 Gbit/s up to 85°C, will be developed as a bridging component meeting the demand of the 16 G and 32 G FC standard and supporting fiber ribbon multiplexed 100 G Ethernet systems. SuchVCSELs, with a design allowing low cost manufacturing and providing reliability does not exist today. In a second phase, GaAs-based VCSELs, emitting at 850 or 980 nm aiming reliable direct modulation at 40 Gbit/s will be developed.
Strained QWs will be optimized to achieve high differential gain and gain compression to enable high speed operation at elevated temperatures. Various designs for low capacitance (insulating substrate, advanced oxide layers, thick layers of dielectric material, etc.) will be developed. Self-heating will be minimized using an epitaxial structure precisely designed for low differential resistance, low free carrier absorption, and a large oxide aperture. Heat transport will be improved using integrated heat spreaders. Mode filters will be applied for limiting the spectral width under direct modulation.