Jun 9 2010
Ridges of silicon can trap and guide light on dimensions as small as just a few hundred nanometers. But the way light behaves in these structures is dependent on its polarization. This effect limits the potential of the technology for a number of applications. A polarization rotator that can help to correct this has now been integrated into a waveguide, which is no more than 400 nanometers across, by Jing Zhang and co-workers at the A*STAR Institute of Microelectronics in Singapore.
Silicon-based photonics research opens up the possibility of tiny optical circuits similar to those that have revolutionized the electronics industry. But at such small dimensions, the difference between the height and the width of the waveguides means that an electromagnetic wave with an electric field oriented along the horizontal direction—known as the transverse-electric (TE) mode—travels at a different speed to that with vertical polarization—the transverse-magnetic (TM) mode. “This ‘structural birefringence’ will affect devices used in high-data-rate applications,” explains Zhang.
An effective solution to this problem is to use a ‘polarization diversity scheme’, in which the optical signal is split into two separate beams: one with vertical polarization and one with horizontal polarization. A special optical component rotates one of the beams by 90°, which orients the beams in the same direction. The beams then travel along identical silicon waveguide circuits before the reverse sequence of events reconstructs the optical beam at the far end.
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