Abstract
Horizontal multiple slot waveguides of polysilicon and silicon nanocrystalline oxide were grating coupled to a surface normal fiber array. Measurements yielded a coupling efficiency of 60% per grating. The fabrication-tolerant, four-stage grating design was genetically evolved from a random seed without starting from first-principle design. Theoretical coupling efficiency was 68% and was re-designed to 63% after accommodating all sources of fabrication error. To our knowledge, this is the first implementation of a purely polysilicon and silicon nanocrystalline oxide slot waveguide platform.
Highlights
Slot waveguides have become a platform of interest for integrated optics due to their ability to confine a guided mode into areas smaller than the diffraction limit [1]
Efficient grating coupling from a normally incident fiber array to multiple horizontal slot waveguides have been experimentally demonstrated with a 60.1% coupling efficiency per grating
A traditional coupler designed solely from first principles yielded a 27% theoretical efficiency and plateaued at 40% when augmented with the genetic algorithm
Summary
Slot waveguides have become a platform of interest for integrated optics due to their ability to confine a guided mode into areas smaller than the diffraction limit [1]. Surface normal grating couplers exhibit potential advantages over angularly detuned gratings, previous surface normal couplers on silicon waveguides have required extreme fabrication complexity [18], an extensive bottom reflector [19], extra high resolution fabrication steps [20], or significant expansion of the device's footprint by simultaneously coupling to two counter-propagating waveguides [21] Such elaborate workarounds are due to grating couplers often being designed solely from first principles with a single period and fill factor, effectively eliminating two of the most significant degrees of freedom available to a designer. Partially-etched, dual horizontal nanocrystal slot input/output grating coupler pairs were evolved via a genetic algorithm [23] (which is introduced ) with no initial design, fabricated, re-evolved to match fabrication errors, and simultaneously coupled to and from a single mode fiber array at perfectly normal incidence
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