Optimization of vertical coupling induced by 45° micro-mirrors in double-layer polymer optical waveguides with CO₂ laser smoothing

The vertical coupling performance induced by 45° micro-mirrors in double-layer polymer optical waveguides is enhanced through CO₂ laser smoothing. The optimal double- layer waveguides were designed via ray-tracing simulation. The results show that the minimum vertical coupling loss is 0.28 dB for the upper-layer waveguide and 0.40 dB for the lower-layer waveguide with a core size of 42 µm × 42 µm. A vertical coupling model considering the roughness of 45° micro-mirrors indicates that the surface roughness should be less than 100 nm to ensure a vertical coupling loss of less than 1 dB. The 45° micro-mirrors in double-layer polymer optical waveguides were fabricated using a combination of ultraviolet lithography and stepped laser-ablation method. CO₂ laser smoothing at 0.6 W for 40 s reduces the average roughness of the micro-mirrors by 16.67 nm. The vertical coupling loss induced by the micro- mirrors treated with CO₂ laser smoothing was measured as 2.89 dB at 850 nm and 1.29 dB at 1310 nm using a 50-µm core multi-mode fiber receiver. This is significantly lower than the average vertical coupling loss of 5.47 dB at 850 nm and 2.50 dB at 1310 nm induced by untreated 45° micro-mirrors. This straightforward and effective method for fabricating and optimizing 45° micro-mirrors holds significant potential for application in vertical coupling within optical printed circuit board (OPCB) interconnection technologies.