The mode transition part is composed of two asymmetrical inverted cones with a gradient grating embedded in the middle of which the pitch width and gap width are gradually changed, and the longitudinal lengths are L1 and L2, respectively.
The mode transition section is composed of two asymmetric inverted vertebrae with a gradient grating embedded in the middle that gradually changes the pitch width and gap width, and longitudinal lengths of L1 and L2, respectively,
The mode transition part is composed of two asymmetric inverted cones with longitudinal lengths L1 and L2, in which a gradient grating with gradually varying pitch width and gap width is embedded in the middle.