Bidirectional mode slicing and re-combining for mode conversion in planar waveguides

Abstract

Photonics integrated circuits and planar lightwave circuits are crucial in future high capacity optical communication networks. Such circuits can perform complicated and sophisticated signal processing functions' on-chip level. However, they should have high bandwidth in order to handle ultrafast data flow and thus achieve high-capacity transmission. One way to achieve such high bandwidth is using few-mode waveguides. Therefore, it becomes mandatory to find a way to convert from current conventional single-mode planar waveguides into few-mode waveguides, and moreover to selectively excite the desired modes within the converted multimode waveguides. In this paper, a novel silica-glass planar waveguide converter is numerically demonstrated for the first time. This device can convert in a bidirectional fashion between single-mode and two, three, or four modes planar waveguides, in addition, to selectively excite the desired mode. The device principle of operation mainly depends on spatial modes slicing and re-combining to achieve the desired higher or lower order modes. That could be accomplished by cascading stages of V-shape and M-shape graded-index planar waveguides. The graded-indices allow for flexible and simple geometrical designs that can split or combine fundamental as well as higher order modes. The finite-difference time-domain numerical method is utilized to verify the device operation and evaluate its performance for each excited mode in bidirectional directions. The device shows good performance over the entire C-band wavelength range with the worst-case magnitude of insertion-loss ≅ 3 dB, polarization-dependent loss ≅ 0.6 dB, and return-loss ≅ 21 dB. It also shows a worst-case cross-talk of ≅-24.1 dB among the excited high-order modes in the forward direction, and a worst-case mode-rejection ratio of ≅-23 dB for non-excited modes in the reverse direction. Moreover, the device shows reasonable performance tolerance to variations in V-shape and M-shape waveguide design parameters.