Y-branch is an important optical passive component. It can be used as power splitter and mode conversion. In this paper, a variable optical attenuator was designed using an asymmetric Y-branch. The device was simulated by FDBPM (Finite Difference Beam Propagation Method). It has the characteristics of compact size, large scale dynamic attenuation and low power consumption.
First, in order to get the low-loss and small-size 2-arc S-shaped bend, the lateral offset between waveguides with different curvature was optimized to reduce the mode mismatch. To get the smoother S-shaped bend, which also has the overall bending loss as small as possible, another S-shaped bend based on functional approximation theory of B-spline was put forward. By this means, the transition loss at the junctions can be completely eliminated and also make the pure bending loss as small as possible. A comparison among sine-generated, cosine-generated, 2-arc and numerical calculated S-shaped bends was also been conducted. Using Beam Propagation Method (BPM) and analytical method, the result was verified and good agreement was got.