Error-Free Three-Dimensional Multimode Crossover Graded-Index Polymer Waveguides for Board-Level Optical Circuitry

In this paper, we experimentally introduce a low-loss 3-dimensional (3D) symmetric crossover multimode optical waveguides realized with parabolic graded-index (GI) profiles using the Mosquito method. This compact crossover multimode waveguide is comprised of two S-bend core groups (6 Ch. × 6 Ch.), where the last six cores cross over the first six cores, maintaining a height gap at the crossover section to avoid the smudging from needle scan. The suitable height gap for this crossover multimode structure is 0.7 mm. The insertion loss of this multimode crossover waveguide is 1.64 dB at 850 nm wavelength. The insertion losses of this multimode crossover waveguide are compared with separately fabricated 3D S-bend waveguides, and it indicates that multimode crossover structures do not exhibit remarkable excess loss. The interchannel crosstalk of this waveguide is less than -45 dB. This crossover multimode waveguide demonstrates sharply open optical and electrical eye diagrams and transmits error-free data at 26 Gb/s.