Why are laser diodes considered most effective when coupled to a single-mode fiber?

Laser diodes are considered most effective when coupled to a single-mode fiber primarily due to their high coupled power and directionality. Single-mode fiber has a small core diameter, typically around 8 to 10 microns, which is designed to carry light signals with minimal modal dispersion. This alignment demands a light source, like a laser diode, that can offer precise control over the emitted light.

The nature of laser diodes allows for a highly directional output, meaning that the light they emit can be coupled efficiently into the single-mode fiber with minimal loss. The focused beam of light produced by the laser matches well with the small core of the single-mode fiber, maximizing the amount of power transmitted and ensuring a cleaner signal over long distances.

In contrast, other alternatives, such as multimode fibers, may present challenges with light dispersion and modal noise when using laser diodes, as the broader core size is more suited to light sources that emit in a less focused manner, like LEDs. Thus, single-mode fibers combined with laser diodes yield superior performance characteristics, especially for applications requiring high bandwidth and long transmission distances.