Iterative method for serpentine mode converter design

Iterative method based on Eric's work for serpentine mode converter design is established. It makes use of steepest ascent technique to search the local optimum, and jump mechanism can be learned to find the global optimum if necessary. Step size is given and works well during the procedure. Structure control methods can be used to realize some structural characteristics. Three kinds of mode converters are designed on the basis of this method and verified by commercial software CST.