Process development and application of grayscale lithography for efficient three-dimensionally profiled fiber-to-waveguide couplers

While recent advances in optical integrated circuits and photonic crystal devices have been impressive, there presently exists an unsatisfied need for an efficient means of coupling into these systems from the outside world. To this end, we have developed writing techniques for continuous-tone grayscale masks in high-energy beam sensitive (HEBS) glass, which we subsequently employ in the fabrication of tapered coupling devices. These devices demonstrate efficient coupling of free-space and fiber signals into waveguides fabricated on silicon-on-insulator substrates. This approach significantly reduces losses as compared to standard butt-coupling and end-fire coupling methods, in addition to being inherently broadband. In this paper, we discuss grayscale mask process development, fabrication techniques for the coupling devices, and characterization of device performance.

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