Fiber optic transmitters convert electrical signals into optical signals and then inject these optical signals into light- conducting cable. They use light emitting diodes (LED) or laser diodes as their optical source, and are designed for use with either single-mode or multi-mode. Optical transmitters are a crucial component in modern telecommunications, enabling the transmission of data as light signals through optical fibers. Nowadays, the applications of optical fibers mainly involve in telecommunication systems and also in the Internet & LAN (local area networks) to attain. The OPA621 is a low-noise, wide-band op amp in classical configuration, which functions as an amplifier in the I/V conversion section behind the photodiode and as an I/V converter behind the AGC multiplier. Essentially, they act as the starting point for a fiber optic.