Some embodiments of the present disclosure provide an optical sensor. The optical sensor includes a semiconductive block having a front side and a back side, a wave guide region, and a light sensing region. The wave guide region is positioned over the back side of the semiconductive block and having a core layer. The wave guide region is configured to guide an incident light. The light sensing region is positioned in the semiconductive block, having a multi-junction photodiode. The light sensing region is configured to sense emission lights from the wave guide region.