Analysis of Etch Depth for Polarization-free Directional Couplers

We numerically analyze the etch depth as a design parameter for the realization of integrated polarization independent directional couplers. Through finite-element-method simulations, the coupling coefficients of transverse-electric (TE) and transverse-magnetic (TM) polarizations are investigated according to different etch depth levels in the coupling section of the studied device. By optimizing etching depth, the proposed coupler can perform similarly for both polarizations in broadband. According to simulation results, the minimum difference between the TE and TM coupling coefficients can be kept less than 1% in 100 nm of the wavelength range.