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Dual-resonator VCSEL delivers higher bandwidths

12 Jun 2007

A laser configuration known as a composite-resonator vertical-cavity laser (CRVCL) could offer the key to high-speed optoelectronics.

A team from the University of Illinois at Urbana-Champaign has fabricated a single-mode CRVCL that could achieve higher speeds than that of traditional vertical-cavity surface-emitting lasers (VCSELs). The device currently achieves a small-signal modulation bandwidth of 12.5 Ghz at -3dB, but the researchers claim that refinements to the device design could improve this to 40 GHz (described in CLEO Postdeadline Paper CPDA11).

"Optimizing the design to remove parasitic effects like series resistance, capacitance between the contact pads and junction capacitance of the top cavity, should allow us to achieve a higher modulation bandwidth than that of conventional VCSELs," researcher Chen Chen told optics.org. "CRVCLs demonstrate great potential to achieve higher speed and lower chirp compared to VCSELs, which will lead to applications in high-speed optical interconnects."

A CRVCL is essentially a VCSEL with two electrically independent but optically coupled regions. The key feature of the CRVCL is the ability to vary the laser characteristics by increasing or decreasing the gain in one cavity while the current injected into the other is fixed. This unique property allows novel modulation behavior, according to the Illinois team.

To date the highest reported small-scale modulation bandwidth is 21.5 GHz for a hybrid VCSEL and 15 GHz for a single-mode implant confined photonic crystal VCSEL. But theory predicts that it will be difficult to go beyond 30 GHz with a VCSEL under conventional direct modulation, while the Illinois team say that CRVCLs could ultimately achieve 40 GHz.

The composite design also allows a higher level of device integration than in a conventional VCSEL. The improved flexibility and additional functionalities of CRVCLs have allowed them to be utilized successfully in many applications such as picosecond pulse generation, high-contrast optical switching, high single-mode power, wavelength division multiplexing and polarization switching.

"The next step is to flatten the modulation response curve and reduce the peak height," said Chen. "One possible method is to modulate both optical cavities simultaneously. This will enable us to make use of the wide bandwidth that the CRVCL provides."

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