Fabrication and Loss Characteristics Analysis of Multimode Curved Polymer Waveguide for Compact Board-Level Optical Interconnection
Objective The physical architecture realization of electronic devices constantly imposes higher requirements on the integration density and complexity of board-level interconnection systems. The optical power attenuation of curved waveguides should be minimized by routing design to meet high-density interconnection requirements. In the integrated system of board-level optical interconnections,it is necessary to change the direction of the optical path and the transmission optical axis for achieving high-density transmission. The inherent strong anti-interference characteristics of the optical waveguide enable the formation of complex links such as crosses and bends,providing extremely high wiring flexibility and integration ability. This allows for bending and turning of the optical path to meet the high-density interconnection requirements of board-level optical interconnection systems. The curved waveguide can alter beam propagation direction and realize redirection of non-collinear optical paths. However,bending will inevitably introduce radiation losses which are affected by factors such as material refractive index,waveguide size,bending radius,and radian. In designing board optical paths,the bending angle and radian are critical structural parameters. A smaller bending radius results in a more compact structure and greater insertion loss. Conversely,a larger bending radius occupies more space and is not conducive to high-density integration. Therefore,it is essential to analyze light transmission characteristics in curved waveguide structures to provide a theoretical basis for designing integrated optical waveguide devices. For multimode waveguides with numerous transmission modes and large cross-section sizes,simulation modeling poses a significant challenge that usually requires substantial computing power support. The mode transmission characteristics of single-mode curved waveguides with specific parameters and simple structures are widely studied. However,most relevant references only give theoretical simulation models and simulation results,with the lack of corresponding experimental support. Meanwhile,few studies have been reported on the loss characteristics of multimode curved waveguides for board-level inter-connect applications,and the design and fabrication of large-size curved waveguides are still lacking in corresponding experimental and theoretical guidance. Without considering the scattering loss introduced by the defects of the waveguide structure,the S-shaped waveguide link loss mainly includes coupling loss,propagation loss,and bending loss. The bending loss can be obtained by subtracting the linear waveguide insertion loss of the same length from the sample insertion loss,while the port coupling loss can be systematically subtracted. Waveguide bending loss contains radiation loss and mode conversion loss. For curved waveguides with a cross-section size of tens of μm and a bending radius of mm level,the mode conversion loss caused by curvature change is the main factor affecting link power attenuation. Although the radiation loss can be calculated by numerical simulation,the mode conversion loss of curved multimode waveguides is difficult to calculate directly by simulation. It is proven to be an effective method to predict the mode conversion loss caused by bending by subtracting the bending radiation loss from the bending waveguide link insertion loss and the calibrated straight waveguide insertion loss of the corresponding length.Methods To reveal the optical transmission characteristics of curved waveguides designed by different optical paths with specific optical axis lateral migration,we apply the geometric ray tracing method combined with numerical simulation to investigate the transmission characteristics. Meanwhile,this method is employed to numerically simulate the propagation path,local numerical aperture,and radiation loss in the curved structure of multimode waveguides,and reveal the loss characteristics of S-shaped waveguides with tangent arc transitions of different bending radii. The relationship between different bending radii and radiation loss of the bending waveguide is clarified. Additionally,the numerical simulation results show that the local numerical aperture increases sharply with the increase in bending radii,while the radiation loss decreases significantly. Furthermore,based on the practical application requirements of board-level optical waveguide transmission,S-shaped curved polymer waveguides with different geometric topologies are designed and fabricated,and the corresponding link loss performances of these waveguide samples are also evaluated. What's more,combined with theoretical numerical simulation results and experimental insertion loss results,the compositions of insertion loss for the S-shaped curved waveguide with different bending radii are also clarified.Results and Discussions The results show that the link insertion loss of the straight waveguide with a length of 140 mm is 2.11 dB,while the link insertion loss of the S-shaped waveguide with a bending radius of 2‒14 mm presents oscillation. Due to the mode coupling effect between the whispering gallery rays and tunnel rays,the loss of the curved waveguide link with a bending radius of 5 mm reaches the maximum value of 21.22 dB. When the bending radius reaches 12 mm,the insertion loss drops to a lower level and tends to be stable. This is mainly due to the suppression of radiation loss and mode conversion loss (including mode coupling loss). By combining the numerical simulation and experimental test results,we further predict and analyze the loss composition of S-shaped waveguides with different bending radii. The results show that when the bending radius of the S-shaped waveguide is 5 mm,the mode conversion loss caused by bending reaches the maximum,and the insertion loss is also the maximum. When the bending radius is greater than 10 mm,the radiation loss and mode conversion loss of the waveguide are the lowest.Conclusions Our study not only provides a method for studying the loss characteristics of curved waveguides but also lays a theoretical and practical foundation for route design of complex optical waveguides for board-level optical interconnections.
waveguide devicegeometric ray tracing methodcurved waveguideoptical interconnectioninsertion loss
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