Stacked VCSEL Integration in PPG Sensor for Enhanced Optical Signal Collection

This study investigated the impact of the spatial arrangement between the Vertical-Cavity Surface-Emitting Laser (VCSEL) and Photodiode (PD) within the PPG (Photoplethysmography) sensor package structure on the signal collection efficiency of sensors. Compared to traditional LED-based sensors, VCSELs exhibit a significantly narrower emission angle and higher optical precision, enabling closer placement to the sensing chip and reducing noisy scattered light. In contrast, the wider emission angle and larger size of LEDs make them less suitable for compact integration due to adding light blocking structure to avoid signal interference. Optical simulations were conducted using LightTools to comprehensively evaluate overall sensing efficiency in the PPG waveform. Signal collection efficiency increased significantly (4 times) when light source-PD distance reduced less than 1 mm. Moreover, setting VCSEL above the top of PD 0.5 mm showed enhancement of signal collection as well. A stacking prototype design was developed to verify simulation results.