The optical transmitter is an electronic-optical assembly that converts electrical signals into light signals and conditions them for transmission via optical fibers.
An optical transmitter consists of an E/O converter, which generates the light modulation, and a downstream optical amplifier, which performs the adaptation to the optical waveguide. E/O converters are light-emitting diodes or laser diodes.
The coupling of the light energy into the optical fiber can occur directly at the photodiode or via an optical extension and depends on the light power, the emitting surface from the photodiode or laser, the emission angle and the coupling angle, as well as the scattering losses and refexion losses. The ratio of the emitting surface area to the size of the fiber face plays a crucial role in light coupling, since only the light energy that hits the core glass at the correct coupling angle can be coupled in. This light power is reflected in the numerical aperture( NA). The smaller the ratio between emitting surface and core glass surface, the better the coupling efficiency.