High-efficiency dual-layer grating coupler for vertical fiber-chip coupling in two polarizations.

Efficient coupling between optical fibers and high-index-contrast silicon waveguides is essential for the development of integrated nanophotonics. Herein, a high-efficiency dual-layer grating coupler is demonstrated for vertical polarization-diversity fiber-chip coupling. The two waveguide layers are orthogonally distributed and designed for y- and x-polarized L P 01 fiber modes, respectively. Each layer consists of two 1D stacked gratings, allowing for both perfectly vertical coupling and high coupling directionality. The gratings are optimized using the particle swarm algorithm with a preset varying trend of parameters to thin out the optimization variables. The interlayer thickness is determined to ensure efficient coupling of both polarizations. The optimized results exhibit record highs of 92% (-0.38d B) and 85% (-0.72d B) 3D finite-difference time-domain simulation efficiencies for y and x polarizations, respectively. The polarization-dependent loss (PDL) is below 2 dB in a 160 nm spectral bandwidth with cross talk between the two polarizations less than -24d B. Fabrication imperfections are also investigated. Dimensional offsets of ±10n m in etching width and ±8 nm in lateral shift are tolerated for a 1 dB loss penalty. The proposed structure offers an ultimate solution for polarization diversity coupling schemes in silicon photonics with high directionality, low PDL, and a possibility to vertically couple.

Ultra-broadband Bragg concave diffraction grating designs on 220-nm SOI for wavelength demultiplexing.

The appropriate broadband design of a de/multiplexer can significantly increase the channel number and consequently the transmission capacity of a wavelength division multiplexing system. Herein, we present the first ultra-broadband Bragg concave diffraction grating (CDG) on a 220-nm silicon-on-insulator, covering most of the E, S, C, L, and U telecommunication wavebands spanning from 1.425 to 1.675 µm. A wide-band-gap Bragg mirror is employed to facilitate broadband reflection, with a low diffraction order of grating for a sufficient free spectral range. Numerical simulations show that the proposed approaching blazed concave diffraction grating (AB-CDG) for the two-material case exhibits a high integration, simple fabrication process, and promising spectral performance. We fabricate the grating for design verification with a low transmission loss of -0.6 dB and a crosstalk below -33.7 dB for the eight measured wavelength channels covering the spectral range from 1.5 to 1.61 µm that is limited by the bandwidth of the grating coupler. This design can be used for broadband wavelength demultiplexing, frontier astronomical observation, and spectroscopic imaging.