Ultrabroadband Power Coupling and Mode-Order Conversion Based on Trapezoidal Subwavelength Gratings

We propose and experimentally demonstrate the broadband power coupling and mode-order conversion based on the trapezoidal subwavelength grating (SWG) structures. The slowly varying trapezoidal SWGs alter the modal field distribution in two closely spaced waveguides and result in an enhanced and adiabatically changing coupling strength between them. As a result, high coupling efficiencies over broad bandwidths are obtained for the coupled-waveguide devices with small sizes. Simulations show that the complete power transfer and the mode-order conversion can be accomplished using the coupling structure. The power coupler has a 1-dB bandwidth of 708 nm spanning from 1353 nm to 2061 nm, while the mode-order converter has a conversion loss less than 1 dB and crosstalk below -12.5 dB over a 324-nm bandwidth from 1388 nm to 1712 nm. Both devices were fabricated and experimentally characterized. The power coupler shows a measured insertion loss of <; 0.25 dB and the mode-order converter exhibits a measured conversion loss of <; 1.2 dB and a crosstalk value of <; -10 dB, over a ~120-nm bandwidth from 1507.2 nm to 1627.8 nm that covers the whole C- and L-band. The coupling length and the total length of our devices are 12.5 μm and 62.5 μm, respectively, which are very compact for adiabatic coupling structures.