The key components that perform electro-optical conversion in optical modules are called optical sub-assemblies (OSA). OSAs generally fall into three main categories: TOSA, ROSA, and BOSA. • TOSA TOSA: Transmitting Optical Sub-Assembly Used in dual-fiber bidirectional or transmit-only optical. TOSA, ROSA, and BOSA are critical components in optical transceivers. Many engineers and buyers ask: what optical devices are mainly composed of optical modules? What are TOSA and. In the era of 5G, AI, and high-speed data centers, optical modules serve as the core bridge for converting electrical signals to optical signals (and vice versa), enabling fast, reliable data transmission across networks. SFP modules are compact, hot-swappable.