Direct modulation is a technique in optical communication where the drive current of a laser diode is directly varied to encode information onto the optical carrier. When discussing optical transceiver parameters, modulation schemes are a key consideration, and the transmitter modulation method is specified in the datasheet of some optical modules, as shown in the figures below: • The transmitter laser modulation mode is marked as EML in the Moduletek 25G ER. In the introduction of product parameters of optical modules, we often mention the modulation mode as a key indicator, DML (Directly Modulation Laser) and EML (External Modulation Laser) are two major modulation technologies for optical modules. Its primary function is to achieve optoelectronic conversion by converting electrical signals into optical signals and vice versa.