High-extinction directional coupler switches by compensation and elimination methods

A conventional guided-wave optical directional coupler switch always presents a low-extinction switching property less than 30 dB. One main reason is that coupling occurs between input and output lead regions of the coupler, causing low extinction in the through switching state despite the coupling strength being very weak. Another reason may be due to the asymmetric structural designs of couplers in optical integrating circuits, making low extinction result in the cross switching state. In this paper, we propose two methods which can solve this crosstalk problem easily. One is by the compensation technique in which we design an optimum structural construction for the reversal Δβ-directional coupler switch, and for which theoretical prediction shows that infinity extinction can be achieved where the fabricated switch element gave above 36 dB extinction ratio for both the through and cross switching states. In another method, coupling effect between lead waveguides are eliminated by means of refractive index control of the waveguides. Theoretical prediction shows that if the change of refractive index δn is greater than 3×10^-3 , over 60 dB extinction can result, where the fabricated switch element also gave more than 38 dB extinction in both the through and cross switching states. Fabrication error, existence of propagation loss, and stray lights due to scattering may be the causes of difference between predicted and experimental values. However, both proposed methods show evidence that it is possible to obtain an extinction ratio as high as nearly 40 dB