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Silicon photonics solves its "fundamental problem"

20 Jul 2006

The prospect of on-chip optical interconnects comes one step closer as researchers achieve continuous optical amplification in silicon without the need for input electrical power.

Researchers at the University of California, Los Angeles (UCLA) claim to have overcome the key problem associated with building silicon-based photonic devices on a computer chip. While previous silicon amplifiers have required high power levels - and generated large amounts of heat - the UCLA device actually produces small amounts of electrical power.

"We have now demonstrated continuous optical amplification in silicon without electrical power consumption," Bahram Jalali of UCLA Engineering told optics.org. "We now have a technology that is power compatible and heat compatible with silicon chip technology."

The crucial advantage of on-chip silicon photonics is that light waves carry more information, more quickly than copper wires. However, the challenge for chip developers such as Intel is that silicon-based devices continue to be too power hungry.

The technique normally used to induce lasing in silicon is to pump the material with an external laser. However, a high optical intensity is needed to transmit more bits of information and to achieve amplification and lasing in silicon, but some of the incident light is converted into heat through a process known as two-photon absorption (TPA).

"TPA is the fundamental problem in silicon photonics," said Jalali. "What we have shown is that we can recover about two-thirds of the power that is inevitably lost to TPA, and we can convert this to useful electrical power that can be used to power up electronic circuitry on the same chip."

Previously, the only way to overcome the TPA problem was to add an electrical diode, an approach that has been demonstrated by Intel. However, this requires 1 W of power, enough to run about a million of transistors on a chip. What's more, the input power converts to heat: the last thing any chip developer would want.

The UCLA team, which also included Sasan Fathpour and Kevin Tsia, induced lasing by pumping the silicon with an external-cavity 1539 nm laser diode. The stray electrons were then recovered by reversing the voltage bias of the attached electrical diode. "Application of a reverse bias establishes an electric field that pulls the electrons out of the device," said Jalali.

According to the researchers, the result was negative electrical power dissipation - in other words, the device actually produced a few milliwatts of net electrical power as a result of harvesting the free electrons generated by TPA. The findings of the experiment are set to appear in Applied Physics Letters.

According to Jalali, the priority now is to reduce the size of the device. "If we can reduce the size of these devices by about a factor of 10, then you will see them appear in optical interconnects. A silicon electronic chip will then be able to communicate with other chips through optical light paths," added Jalali.

The team is aiming to do this by optimizing the device geometry and investigating ways to enhance the microscopic interaction between light and silicon atoms. But even before size reduction is achieved, the devices could be used as standalone amplifiers or lasers for communications, sensing and military systems.

"Silicon as an optical material has a high crystal quality and high thermal conductivity, which allows it to scale up to high powers - if you can solve the TPA problem," concluded Jalali. "We have shown that this problem can be solved in a way that does not compromise the power efficiency of the device."

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