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Silicon achieves optical success

12 Feb 2004

A high-speed optical modulator made from silicon threatens the role of optoelectronics made from compound semiconductors.

Silicon’s success in making electronics may be about to repeat itself in the world of optoelectronics. In this week’s issue of Nature, scientists at the microelectronics giant Intel report the first silicon-based optical modulator that has a bandwidth that exceeds 1 GHz -- 50 times faster than previous efforts. (Nature 427 615)

Although the prototype device’s performance is still far below that of commercial products based on materials such as lithium niobate (LiNbO3) and III-V semiconductors, the news is important. It suggests that silicon, the basis of cheap yet powerful computer chips, could also make high-speed optical switches. In future, the hope is that both optical and electronic functions could be combined on cost-effective silicon superchips made at high-volume microelectronics factories.

“Breakthrough developments such as this fast silicon modulator suggest that a low-cost silicon optical superchip could soon be a reality,” commented Graham Reed, a semiconductor scientist from the University of Surrey, UK, in an accompanying review of the breakthrough in Nature. “It [silicon] is already the world’s favorite electronic material and could yet come to dominate the photonics industry as well.”

The silicon-waveguide modulator is based on a so-called Mach-Zehnder interferometer design where a silicon waveguide is split into two arms which recombine a short distance later. Applying a voltage to one of the arms causes a small change in the refractive index of the silicon and, through optical interference, controls the amount of light leaving the device.

Intel’s prototype operates at the all-important telecoms wavelength of 1.5 microns and requires a drive voltage of about 8 V to achieve an extinction ratio of around 16 dB. However, it currently suffers from a large insertion loss of 15.3 dB. In contrast, commercial LiNbO3 modulators have an insertion loss that is about ten times smaller (5 dB) and can operate at least ten times faster (>10 GHz). However the Intel team is confident that it can significantly improve device performance by tweaking the design.

Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.

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