ROSA is the receiving core of the optical module. It receives optical signals from the fiber, converts them into weak electrical signals through photodetectors, and amplifies them through TIA/APD-TIA circuits to restore stable digital electrical signals. TOSA is the. TOSA: Transmitting Optical Sub-Assembly, used in dual-fiber bi-directional or single-emission optical modules, converts electrical signals into optical signals, and then couples the light in the optical path to the optical fiber through optical parts.