Design Optimization and Comparative Analysis of Silicon-Nanowire-Based Couplers

Three kinds of highly compact 2
$\times$
2 couplers based on silicon nanowire are designed and optimized for the array waveguide grating (AWG) demodulation integration microsystem in this paper. These couplers are directional (X) coupler, cross gap coupler (CGC), and multimode interface (MMI) coupler. The couplers are simulated using the beam propagation method. The distance between the input/output waveguides is set to 10
$\mu\hbox{m}$
considering the test of a single device in the following work. The total footprint of X coupler is
$10 \ \mu\hbox{m} \times 300 \ \mu\hbox{m}$
. The length of parallel film waveguide is 1
$\mu\hbox{m}$
. After optimization, the minimum excess loss is 0.73 dB. CGC has a footprint of
$10 \ \mu\hbox{m} \times 300 \ \mu\hbox{m}$
, a coupling region length of 24
$\mu\hbox{m}$
, and a minimum excess loss of 0.6 dB. Taper waveguides are used as input/output waveguides for MMI coupler. The footprint of MMI region is only
$6 \ \mu\hbox{m} \times 57 \ \mu\hbox{m}$
. The excess loss is 0.46 dB after optimization. Uniformity is 0.06 dB with transverse electric polarization when the center wavelength is 1.55
$\mu\hbox{m}$
. The maximum excess loss is 1.55 dB in the range of 1.49
$\mu\hbox{m}$
to 1.59
$\mu\hbox{m}$
. The simulation results show that a small 2
$\times$
2 MMI coupler exhibits lower excess loss, wider bandwidth, and better uniformity than X coupler and CGC. MMI coupler is suitable for the requirements of optoelectronic integration.