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Cascaded VCSELs beat bandwidth record

29 Nov 2007

A direct modulation bandwidth of 66 GHz has been achieved from two daisy-chained VCSELs, and the principle could be scaled up to reach over 100 GHz.

An international research team has shown that a novel configuration of two cascaded vertical-cavity surface-emitting lasers (VCSELs) can achieve a modulation bandwidth of 66 GHz at 1.55 µm, claimed to be a record high. The team, led by researchers at the University of California, Berkeley (UCB), say that such high bandwidth will be essential for increasing the speed of transmitters used in optical communications networks (Optics Express 15 14810).

"This technique using cascaded optical injection locking (COIL) is a totally new approach that has never been attempted before," Xiaoxue Zhao of UCB explained to optics.org. "Although the modulation speed of VCSELs is limited by device parasitics to about 20 GHz, the dynamic performance can be drastically improved by applying strong optical injection locking."

Optical injection locking is a method for improving the performance of a direct-modulated laser by using a second source, called the master laser, to inject photons at a similar wavelength into the transmitter or slave laser. Coherent nonlinear interaction effectively locks the wavelength and phase of the slave laser to that of the master.

The Berkeley team extended this approach to include two slave lasers. The first is injection-locked and directly modulated, while the second is kept under CW operation and injection locked to the output of the first slave laser. The result is that the VCSEL cavity essentially works as a red-shifted high-Q optical amplifier, providing strong single-sideband amplification of the modulation signal up to a frequency range close to one order of magnitude higher than that of free-running VCSELs.

Wideband lasers and modulators are being developed for use in high-speed optical communications networks, but lasers based on multiple quantum wells and distributed Bragg reflectors have so far been limited in bandwidth to 30 GHz at the preferred wavelength of 1.55 µm. Earlier this year a UCB team reported a bandwidth of 52 GHz from two COIL VCSELs working at 1.55 µm (see previous coverage on optics.org). Now they have managed to go further and achieved 66 GHz.

"The strong injection-locking regime requires the power ratio between master and slave laser to be 5 dB or higher," commented Zhao. "This imposes a restriction on which master laser can be used."

In a multiple stage structure this injection ratio needs to remain high for the cascaded stage as well, which Zhao confirmed can be difficult due to coupling loss. "Under strong injection the wavelength difference between master and slave can be as large as 3 nm, so fine tuning of the slave wavelength can be necessary to tailor the RF response."

Having now cascaded two VCSELs, the team are planning for the next logical step: cascading more slave lasers in a daisy-chain structure. As long as the master laser has enough power to lock the slave with the largest detuning value the system could eventually achieve ultrahigh bandwidth modulation greater than 100 GHz.

"It's too early to tell if there is a limit for the maximum bandwidth," said Zhao. "Theoretically there should be no fundamental limit to the resonance frequency enhancement possible by optical injection locking. We haven't seen any practical limitations so far."

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