Optical engine
Beginner
A tiny chip set that turns electrical signals into light and back, small enough to sit beside a processor.
Novice
A compact transmitter-receiver: a photonic chip with modulators and photodetectors, plus an electronic chip with drivers and amplifiers. In CPO, several engines surround the switch chip.
Expert
PIC (modulators, photodiodes, waveguides, fiber coupling) stacked or bonded with an EIC (drivers, TIAs, control). Lasers are usually external. Power, bandwidth per mm of package edge and yield of known-good engines are its key metrics.