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Updated: July 8th, 2008 05:26 PM CDT

Photonic Crystals Enable Smaller Silicon Modulators

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Semiconductor International
via NewsEdge Corporation

Engineers at the University of Texas at Austin have, for the first time, demonstrated an ultra-compact silicon electro-optic modulator based on silicon photonic crystal waveguides. "We were able to get our new silicon modulator to control the transmission of laser light, while using 10 less power than normally needed for silicon modulators," said Ray Chen, a professor of electrical engineering.

Chen, with graduate students Wei Jiang, YongQiang Jiang and Lanlan Gu, initially published findings on the silicon modulator in the Nov. 28, 2005, issue of the journal Applied Physics Letters. That article described how less than 3 mW of power was needed for light modulation. Because of the way the researchers modified the silicon ? by using line defects to create large regions of regularly spaced, nano-size holes ? the length the light needed to travel before being modifiable was 80 m. That is about 10 shorter than the best conventional silicon optical modulators.

Using silicon for such photonic functions has two main advantages. First, it is compatible with conventional CMOS processing, so monolithic integration of silicon photonic devices with advanced electronics on a single silicon substrate is possible. Second, silicon is surprisingly useful as a photonic material in that it is transparent in the range of optical telecommunication wavelengths, 1.3 and 1.55 m. It also has a high refractive index that allows for the fabrication of high-index-contrast nanophotonic structures.

For light to encode meaningful information, its intensity or other characteristics need to be modulated. For broadband optical intensity modulators, a structure known as the Mach-Zehnder interferometer (MZI) is widely used to convert phase modulation into an intensity modulation. In 2004, scientists from Intel Corp. made news using such a device to encode data onto a light beam. Intel researchers split a beam of light into two separate beams as it passed through silicon, then induced a phase shift using an MZI-type device. When the two beams of light are recombined, the phase shift induced between the two arms makes the light exiting the chip go on and off resulting in a frequency rate >1 GHz, which was 50 faster than what had been previously produced on silicon (see "Transistor-Like Modulator Helps 'Siliconize' Photonics," March 2004, p. 28). The problem, according to the UT

Longhorn researchers, is the size: Conventional silicon MZI modulators are based on rib waveguides, which usually need one-half to several millimeters to achieve the required phase shift. To get around that, the researchers employed photonic crystal waveguides with nanopores, which help disperse the light and maximize phase modulation efficiency (efficiency is a function of phase change, which is related to the change of propagation constant and waveguide length). This means the same phase change can be produced by a photonic crystal waveguide that is 100 shorter than a conventional waveguide.

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