What is a common use of directional couplers in waveguides?

Directional couplers are specifically designed to sample signals in waveguides. They allow a small portion of the input signal to be extracted and sent to a different port, enabling monitoring or measurement of the signal without significantly affecting the main transmission path. This sampling capability is particularly useful in applications where the signal's characteristics, such as power levels or phase, need to be analyzed without interrupting the flow of the primary signal.

In contrast, the other options represent functions that are not typical for directional couplers. For instance, amplifying signals generally requires active components like amplifiers, while reflecting signals is associated with devices like reflectors or mirrors, which redirect signals rather than sample them. Modulating signals involves changing signal characteristics, often using modulators or mixers, rather than simply taking a portion of the signal for analysis.