Polarization converter and design method

The invention provides a polarization converter, which comprises a silicon dioxide substrate, top silicon and an air cladding, and is characterized in that the top silicon comprises an input waveguide, a polarization conversion area, a mode conversion area and an output waveguide which are connected in sequence, and the polarization conversion area is divided into a plurality of first rectangular units; the mode conversion area is divided into a plurality of first rectangular units, the state of each first rectangular unit is calculated through a direct binary search algorithm to form a first punching array enabling the first output function to reach the maximum value, the mode conversion area is divided into a plurality of second rectangular units, and the state of each second rectangular unit is calculated through the direct binary search algorithm to form a second punching array enabling the second output function to reach the maximum value; forming a second punching array which enables a preset second out