Abstract
Silicon photonics holds great promise for low-cost large-scale photonic integration. In its future development, integration density will play an ever-increasing role in a way similar to that witnessed in integrated circuits. Waveguides are perhaps the most ubiquitous component in silicon photonics. As such, the density of waveguide elements is expected to have a crucial influence on the integration density of a silicon photonic chip. A solution to high-density waveguide integration with minimal impact on other performance metrics such as crosstalk remains a vital issue in many applications. Here, we propose a waveguide superlattice and demonstrate advanced superlattice design concepts such as interlacing-recombination that enable high-density waveguide integration at a half-wavelength pitch with low crosstalk. Such waveguide superlattices can potentially lead to significant reduction in on-chip estate for waveguide elements and salient enhancement of performance for important applications, opening up possibilities for half-wavelength-pitch optical-phased arrays and ultra-dense space-division multiplexing.
Highlights
Silicon photonics holds great promise for low-cost large-scale photonic integration
Waveguide arrays are widely used in emerging applications such as optical-phased arrays[18,19,20,21], space-division multiplexing[22] and chip-scale optical interconnects[23,24], and conventional applications such as wavelength-division multiplexers[25,26]
A waveguide array or a waveguide lattice can be viewed[27] as fully analogous to a periodic chain of atoms, which lends itself to a broad spectrum of fascinating scientific possibilities ranging from Anderson localization of light[28,29] to parity-time symmetric effects[30]
Summary
Low crosstalk can be achieved for such a pair at small spacing on the basis of the asymmetric directional coupler theory In such a coupler, it is well known that the normalized power coupling (that is, crosstalk) from one waqveffigffiffiuffiffiffiiffidffiffiffieffiffiffiffitffioffiffiffiffiffiaffiffinffiffioffiffither is given by ref. Crosstalk in a large array of waveguides is fundamentally different because light in one waveguide of the array can be transported to the second-, third-nearest waveguide-neighbours and beyond (at sub-l pitches, the mode overlap with second-, third-nearest waveguide-neighbours is not negligible and crosstalk will rise). At a sufficiently narrow w2, crosstalk starts to increase (for a given pitch) Considering this limit and noting that narrow waveguides tend to have higher loss (see loss information in Methods), the narrowest waveguide used in the superlattices is set to be B330 nm wide. It can be shown that the amplitude oX f nthec0nmðzoÞDdeA, mcnn0þ, caX nn be io@b@ctz0nainþedbnfroBmmn 1⁄4 0; ð1Þ where bn is the propagation constant of the original mode of the n-th waveguide, Bmn is related to the overlap integral between modes m and n and DAmn is the perturbation potential matrix element (see detailed derivation of equation (10) in the
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