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Intel boffins build first continuous beam silicon laser

Two-photon absorption problem solved

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Intel scientists have developed what they claim is the world's first continuous-wave laser constructed from silicon in a single chip.

To date, a number of silicon-based lasers have been developed but they have all been capable only of emitting pulses of optical energy. Getting a silicon laser to operate continuously is a key step in the development of optical interconnects between microprocessors and, ultimately, chips that operate using optical rather than electronic switches.

Intel's chip uses the Raman Effect, a process whereby one laser beam - called the 'pump' - is used to amplify a second, low-power, data-carrying beam of a different wavelength. The Raman Effect has predominantly been used to transmit data across very long, multi-kilometre glass fibre optical links, but Intel's goal is to utilise the technique in silicon.

Intel's chip laser consists of a silicon waveguide sandwiched between two semi-transparent mirrors, with the pump laser input at one end and the signal output at the other. Building a waveguide in silicon to channel the beams is relatively easy, because silicon is transparent to infra-red light. It's desirable because silicon chip fabrication is cheap and a relatively straightforward process to apply. But the trick has been getting the pump strength up to a point where the energy of the amplified data beam is greater than the energy lost as imperfections in the waveguide impede the beam.

Then there's the so-called two-photon absorption problem, which limits how far you can turn up the pump strength. Beyond a certain point, pairs of photons - particles of light - collide simultaneously with an atom of silicon providing sufficient energy to create a free electron in the waveguide. These electrons, in turn, absorb energy from the pump and amplified data beams, reducing the strength of the output signal, potentially far enough to counteract the Raman Effect. It's this two-photon absorption problem that has so far limited silicon lasers to pulse-only operation.

Intel's breakthrough is to use a transistor structure to surround the waveguide and pull the free electrons away from the beam, allowing the pump strength to rise to a point where a good, continuous output beam is generated.

"We have demonstrated stable single mode laser output with side-mode suppression of over 55dB and line width of less than 80MHz," the company said in an article published today in the science journal Nature.

The technique is simply an R&D project today, but Intel said it reckons the process could be commercialised by the end of the decade. ®

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