Silicon grating couplers for low loss coupling between optical fiber and silicon nanowires

Justin C Wirth, Purdue University

Abstract

The promise of silicon nanophotonic devices is constrained by the large inherent size difference between comparatively large optical fibers and much smaller photonic waveguides, which causes an unacceptable amount of loss without a mode size conversion solution. One such solution is the vertical grating coupler, which allows light to be efficiently coupled in from the top of a device. However, for standard 250nm crystalline silicon top layers of silicon-on-insulator wafers, no such published designs existed. The initial focus of this research was to design and test a grating coupler for operation at 1550nm in the near infrared which could be used for coupling to photonic devices on these wafers. Coupling quasi-TE mode polarized light at less than 10dB fiber-to-fiber loss with a 3dB bandwidth across the C-band was required. Grating layouts were designed and simulated, and a maximally efficient solution was found. This design was then fabricated in both straight grating and curved grating varieties. Testing showed a fiber-to-fiber loss as low as 9.5dB, with 43nm of 1dB bandwidth and 76nm of 3dB bandwidth. Therefore, coupler performance exceeded the required efficiency and far surpassed the bandwidth target. Further expanding the design to other silicon structure types, amorphous versions of the same couplers were also fabricated. Performance was slightly less but comparable to crystalline couplers. Both material types were incorporated into devices and demonstrated as effective coupling solutions. Future work will focus on increasing efficiency, utilization of the couplers' Fabry-Perot properties, and developing amorphous couplers for use on flexible substrates.

Degree

M.S.E.C.E.

Advisors

Qi, Purdue University.

Subject Area

Electrical engineering|Nanotechnology|Optics

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