  A novel mode-selective thermo-optic phase shifter (MS-TOPS) enabled by
subwavelength grating (SWG) structures is proposed and experimentally
demonstrated on a 220 nm waveguide thick silicon photonics chip for the first
two quasi-transverse electric modes (TE0, TE1). Mode-selective relative phase
manipulation of modes unlocks several processing tasks in mode division
multiplexing systems. This integrated solution provides a direct phase
manipulation of modes without converting them to their fundamental modes. A
Mach-Zehnder interferometer is deployed as a test structure incorporating the
proposed MS-TOPS in one arm and a mode-insensitive thermo-optic phase shifter
(MI-TOPS) in another. The effect of the SWG duty cycle ratio is investigated by
both numerical simulations and experimental measurements. A mode-selectivity of
1.44 is experimentally demonstrated. In other words, the thermo-optic
coefficient of TE0 is 44% larger than the one for TE1. The phase shifter's
insertion loss is at most 2.5 dB and a worst-case crosstalk of -13.1 dB over a
40 nm wavelength range from 1520 to 1560 nm. A cascaded configuration of the
proposed MS-TOPS and an MI-TOPS provides sufficient degrees of freedom to
manipulate the relative phase of each mode independently. Potential numerous
applications of such devices include optical switching, multimode quantum
optical processors, and scaling-up conventional optical processors with a mode
selective building block.
