Making ultrasmall integrated optics

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by Yvonne Carts-Powell

Fast, low-voltage, active components that integrate optics with electronic circuitry would be a boon to the WDM industry and researchers at Columbia University (NY, NY) have demonstrated one step toward making such devices. Rokan Ahmad and colleagues in Richard Osgood's group fabricated tiny mirrors and splitters in a silicon layer deposited on an insulating substrate.1

Optical devices in a 1-mm or thinner silicon guiding layer atop an insulating substrate could be integrated with CMOS circuitry, providing the potential for low-voltage devices with fast switching times. Such silicon-on-insulator (SOI) integrated optics confine light by taking advantage of the large difference in refractive indices between air and silicon, and silicon and silicon dioxide.

Most waveguiding devices use large radii bends and shallow angle branching to control loss, which results in larger devices. Because the SOI integrated optics have tight confinement of the optical field in the vertical, as well as in the lateral direction, however, the bends and branches can be made unusually small without generating excessive loss.

Last year, the Columbia group suggested that abrupt turns could be made using a structure that combines high index contrast, a phase retarder, and a corner reflector (see figure). They have now built such a device using Unibond SOI, and report, "the corner mirrors fabricated are the smallest to date in any material system." The group also created ultrasmall 1 x 2 power splitters in T branches, rather than the traditional shallow Y branching structures found in conventional integrated optics.

Model and fab
The researchers modeled the structures using two-dimensional finite-difference time-domain calculations, rather than the beam-propagation method (BPM) modeling often used for integrated optics. This is because BPM, in general, in not well-suited to modeling strongly reflective structures or those with abrupt and large index contrasts. The structures that were fabricated agreed well with calculations.

The group optimized the design for TE-polarization transmission at 1.55 μm, and fabricated the components in a 0.34-μm-thick silicon guiding layer atop a 1-μm-thick buried oxide layer. The mirror area was necessarily larger than the cross section of the waveguide, and varied from 1.49 to 2.96 mm2. The depth of the mirror is important because it acts as a phase retarder, and therefore the mirror thickness varied from 0.4 to 0.75 mm. The highest measured transmission was 73%, possibly because the mirror was as far as 14° from vertical, or because of mirror roughness.

For more information contact: Richard Osgood at osgood@columbia.edu.

REFERENCE
1. R. U. Ahmad, et al., Photon. Tech. Lett. 14(1), 65 (January 2002).

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