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Silicon photonics makes giant strides

18 Feb 2005

It's been a week to remember in the world of silicon photonics with two groups releasing news of further advances.

Hot on the heels of its announcement of an all-silicon laser chip last month, a team from Intel in the US and Israel has unveiled a continuous wave Raman silicon laser. (Nature 433 725)

Researchers from the University of California at Los Angeles, who announced the first silicon Raman laser in October 2004, have now demonstrated a directly modulated version emitting peak powers of around 3Watts at 1698 nm. (Optics Express 13 796)

Intel's laser is based on a low-loss s-shaped silicon-on-insulator rib waveguide. The difference is that Intel has found a way to overcome the main drawback of its original design where the laser could only operate for about 100 nanoseconds.

"The biggest challenge was identifying that two-photon- absorption induced free carrier absorption was stopping the gain," Mario Paniccia, Director of Intel's Photonics Technology Lab told Optics.org. "We now integrate a reversed-biased p-i-n diode into the waveguide. When a voltage is applied to the p-i-n, it acts like a vacuum and removes the excess electronics from the path of the light allowing continuous lasing from silicon."

Optically pumped by an amplified external cavity diode laser emitting 3W at 1550 nm, the team says that the lasing threshold reduces with increasing reverse-bias voltage. Intel reports thresholds of 280 mW and 180 mW with biases of 5V and 25V respectively.

The Intel laser emits at 1686 nm and a lensed fiber is used to couple the emission out of the waveguide. According to Paniccia, the laser has operated at room temperature throughout testing without any significant degradation.

The research team from the University of California has also addressed criticisms voiced after its announcement in October 2004. "The criticism had to do with the fact that Raman lasers are purely optical devices, compared with electrically powered lasers that can be electronically switched," researcher Bahram Jalali explained.

"We have answered this criticism," Jalali continued. "We show that the silicon Raman laser does have an electronic interface and can be modulated directly to carry data. It has the same modulation properties as diodes used in telecom equipment."

Jalali's laser uses a silicon chip placed in a fiber loop cavity. The chip contains a 2 cm long waveguide plus a p-n junction diode. A modelocked laser emitting 30 ps pulses at 1560 nm pumps the ring cavity. A tap coupler extracts 5% of the power as the output with the remainder going back into the ring cavity.

An intracavity-switching scheme allows Jalali to modulate the laser's output. "The linear dependence of free-carrier density on diode forward current provides direct electronic modulation of the intra-cavity gain," said Jalali. "The laser will be turned off when the loss induced by the diode current exceeds the gain per round trip in the cavity."

According to Jalali, data rates of 10Gbit/second are possible. "We have shown 30 dB (1:1000) modulation depth," he told Optics.org. "There is no physics reason why SNR measurements should not be as high as conventional lasers."

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