VCSEL drives higher-bandwidth LANs

June 10, 2004 Fort Collins, CO -- With today's corporate LANs required to support an increasingly sophisticated and data-intensive application mix, component developers are under constant pressure to come up with active devices that deliver that much needed bandwidth "headroom" at a compelling price-performance point, reports Tami Freeman.

June 10, 2004 Fort Collins, CO -- With today's corporate LANs required to support an increasingly sophisticated and data-intensive application mix, component developers are under constant pressure to come up with active devices that deliver that much needed bandwidth "headroom" at a compelling price-performance point, reports Tami Freeman.

With this in mind, researchers at Colorado State University have designed an 850-nm vertical-cavity surface-emitting laser (VCSEL) that offers -3 dB modulation bandwidths of up to 17 GHz. The big plus of their design: the fabrication process is relatively simple and cost-effective.

Two key factors that influence the modulation bandwidth of a VCSEL are its series resistance and parasitic capacitances--both of which must be minimized in order to increase the bandwidth. The researchers reduced the resistance by fabricating the VCSEL from an AlGaAs structure on a p-type substrate (whereas most VCSELs are grown on n-substrates).

"VCSEL resistance was also minimized by using MOCVD [metallo-organic chemical vapour deposition] to grow carbon-doped mirrors with engineered grading profiles," explains Kevin Lear, Rockwell Anderson associate professor of electrical and computer engineering at Colorado State University. "Careful fabrication also helped achieve low resistances--for example, contact regions were etched before metal deposition."

Lear says that the team reduced the capacitance by employing moderately sized VCSEL pads placed on thick polyimide layers and by keeping the mesa size relatively small. "In the past, various researchers have used methods such as growth on semi-insulating substrates or ion-implantation to reduce capacitance, but neither of these methods needed to be used here, which simplifies the [fabrication] process," adds Lear.

He continues: "We think that our current laser devices can certainly be used for 20-Gbit/sec modulation and are close to being able to be used for 30-Gbit/sec modulation. Our goal is to make VCSELs that can be directly modulated at 40 Gbits/sec within the next few years. To reach that, we will have to continue to address a variety of limiting mechanisms, including parasitic circuit, thermal and intrinsic laser-response issues."

The research is part of a US Defense Advanced Research Projects Agency program investigating the interconnection of integrated circuits on a board with light instead of electrical signals.


--Tami Freeman, deputy editor, FibreSystems Europe in association with LIGHTWAVE Europe.

• This article originally appeared in FibreSystems Europe in association with LIGHTWAVE Europe May 2004 p7.

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