Abstract Optical mode switch is the key component in mode-division multiplexing systems, where mode coupler plays a critical role by controlling both mode exchange and phase relations. It must split an injected 1st mode-light into two fundamental modes with a π phase difference and regenerate the 1st mode-light from two fundamental modes with the same phase condition. Although such functions have been demonstrated using Y-junction couplers, the device length is typically several hundred micromet