Design of a Broadband Integrated Mode Converter Based on Non-Uniform Binary Long-Period Grating by Direct Multiparameter Optimization

We present a novel approach for designing non-uniform binary long-period gratings with predefined transmission characteristics by directly optimizing the coupling strengths and lengths of the constituent waveguides in all grating segments. The optimal grating parameters were determined by minimizing the deviation of the transmission characteristics calculated using the transfer matrix method from the predefined target functions using the multistart optimization procedure. We demonstrate the effectiveness of the proposed method by optimizing the performance of grating-based TE0/TE1 mode converters in a few-mode buried waveguide composed of medium-contrast materials (TiO2:SiO2/SiO2). Two converters with coupling caused by the lateral shift and step-like thickness change of adjacent waveguides were numerically optimized for the 1.3 and 1.55 mu m bands. Each of the optimized devices has a total length of approximately 250 mu m. For both types of converters, a spectral range of 150 nm with crosstalk lower than -20 dB was obtained for the two communication windows using at most nine grating segments, with twice lower excess loss for the grating with a step-like thickness change. Moreover, for such grating composed of only 9 segments it is possible to obtain the conversion band of 400 nm, covering both communication windows with an excess loss lower than 0.45 dB and crosstalk below -20 dB. We also numerically analyzed the impact of the fabrication tolerances on the converter characteristics.