High-Efficiency Two-Dimensional Perfectly Vertical Grating Coupler With Ultra-Low Polarization Dependent Loss and Large Fibre Misalignment Tolerance

A two-dimensional grating coupler (2D GC) with four access waveguides is proposed and demonstrated for polarization independent and perfectly vertical coupling. Benefitting from the perfectly vertical coupling and
$1\times 4$
power splitting/combing scheme, broadband polarization independent operation can be achieved. Through grating apodization with etched square holes, a simulated coupling efficiency (CE) of 65% (−1.87 dB) is obtained with a minimum feature size of 180nm which is compatible with the 193-nm deep UV lithography technology. To characterize the device performance, a back-to-back configuration of this 2D GC was fabricated, where phase-delay lines with similar patterns are introduced to ensure the phase equality between different waveguide arms. A CE of 49% (−3.1 dB) was experimentally measured, with a 1-dB bandwidth of 42 nm and PDL below 0.2 dB across the wavelength range from
$1.5~\mu \text{m}$
to
$1.6~\mu \text{m}$
. The fibre misalignment tolerance of this device was also investigated by measurement. The excess coupling loss and PDL caused by
$\pm 2~\mu \text{m}$
fibre misalignment are lower than 1 dB and 0.8 dB respectively, and the maximum excess coupling loss and PDL caused by ±2° fibre angle error is 1 dB and 0.7 dB respectively. Finally, optical modulation using this device is preliminarily demonstrated by the embedded thermal-optic functions.