Design of a Silicon Fresnel Lens for Optimized Light Coupling in a Transmitter Optical Subassembly

The silicon microlens is a key element in advanced optical packaging. However, the difficulty of its fabrication process and low yield have hindered the development of an silicon lens based optical device module. A silicon Fresnel lens was studied using ray tracing optical simulation, because its small sagittal height allows easy fabrication and high yield. The Fresnel lens was constructed from reference spherical lens with a controlled zone depth and partition number. The optimum zone depth was determined by target wavelength and the refractive index of the lens. The partition number was controlled for the highest coupling efficiency. The maximum coupling efficiency varied with the radius of curvature (ROC) of the lens and the coupling efficiency of the Fresnel lens was always higher than that of a reference lens. When the ROC was 700 mu m, the Fresnel lens showed a coupling efficiency of -0.331 dB while the reference lens showed -0.674 dB. It was noted that the high coupling efficiency of the Fresnel lens could be obtained for a wide range of ROCs, which is huge advantage in the design of an optical module. The superior performance of the Fresnel lens was attributed to the smaller beam waist, as analyzed by the beam synthesis propagation method. The alignment tolerance of the Fresnel lens was similar to the reference lens in lateral offset, but it exhibited tighter tolerance with longitudinal offset.