| CPC G02B 6/12004 (2013.01) [G02B 6/12002 (2013.01); G02B 2006/12061 (2013.01); G02B 6/136 (2013.01)] | 20 Claims |

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1. A structure comprising:
a first waveguide core including a section; and
a back-end-of-line stack including a first dielectric layer, a second dielectric layer on the first dielectric layer, an opening in the second dielectric layer, a second waveguide core including a section that overlaps with the section of the first waveguide core, and a plurality of third waveguide cores arranged in a vertical direction between the first waveguide core and the second waveguide core, the plurality of third waveguide cores positioned inside the opening in the second dielectric layer, the first dielectric layer comprising a first material with a first refractive index, and the second dielectric layer comprising a second material with a second refractive index different from the first refractive index.
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