PROCEEDINGS ARTICLE | September 10, 2013
Proc. SPIE. 8844, Optical System Alignment, Tolerancing, and Verification VII
KEYWORDS: Packaging, Semiconductors, Mid-IR, Finite-difference time-domain method, Waveguides, Copper, Quantum cascade lasers, Single mode fibers, Integrated optics, Waveguide modes
We present Finite-Difference Time-Domain (FDTD) simulations to explore feasibility of chip-to-chip waveguide
coupling via Optical Quilt Packaging (OQP). OQP is a newly proposed scheme for wide-bandwidth, highly-efficient
waveguide coupling and is suitable for direct optical interconnect between semiconductor optical sources, optical
waveguides, and detectors via waveguides. This approach leverages advances in quilt packaging (QP), an electronic
packaging technique wherein contacts formed along the vertical faces are joined to form electrically-conductive and
mechanically-stable chip-to-chip contacts. In OQP, waveguides of separate substrates are aligned with sub-micron
accuracy by protruding lithographically-defined copper nodules on the side of a chip. With OQP, high efficiency chip-to-chip
optical coupling can be achieved by aligning waveguides of separate chips with sub-micron accuracy and reducing
chip-to-chip distance. We used MEEP (MIT Electromagnetic Equation Propagation) to investigate the feasibility of OQP
by calculating the optical coupling loss between butt coupled waveguides. Transmission between a typical QCL ridge
waveguide and a single-mode Ge-on-Si waveguide was calculated to exceed 65% when an interchip gap of 0.5 μm and
to be no worse than 20% for a gap of less than 4 μm. These results compare favorably to conventional off-chip coupling.
To further increase the coupling efficiency and reduce sensitivity to alignment, we used a horn-shaped Ge-on-Si
waveguide and found a 13% increase in coupling efficiency when the horn is 1.5 times wider than the wavelength and 2
times longer than the wavelength. Also when the horizontal misalignment increases, coupling loss of the horn-shaped
waveguide increases at a slower rate than a ridge waveguide.
