Design optimization of a low-loss and wide-band sharp 120° waveguide bend in 2D photonic crystals

For two dimensional photonic crystals containing finite cylinders on triangle lattice, a 120° waveguide bend with low-loss and wide-band is obtained in this paper. The optimal process can be divided into two steps: firstly, a conventional waveguide bend can be introduced by maximizing the photonic bandgap; then further optimization involves shifting the position and modifying the radius of only one air hole near the bend. An optimization problem at a given frequency or over a frequency range needs to be solved. It depends on both the field solutions obtained by using the finite element method and the optimization of photonic bandgap obtained by using the plane wave expansion method. With the proposed optimal technique, the result of our optimized design for sharp 120° waveguide bends shows that an obvious low-loss transmission at wavelength 1550 nm can be observed and the maximum value of objective function is able to be rapidly obtained.