Which type of optical detection involves a photodiode that converts the light waves to an electrical signal that varies in proportion to intensity changes?

Direct detection is the process in which a photodiode responds to incoming light waves by converting them into an electrical signal. This signal's intensity varies based on the intensity of the light it receives. In this method, the relationship between the light and the electrical output is straightforward; as the light intensity increases or decreases, the corresponding electrical signal also changes in direct proportion. This makes direct detection an efficient and widely used technique in optical communications, where accurate real-time conversion of optical signals into electronic signals is crucial.

The other detection types, such as indirect detection, often involve more complex processes where the light is not converted directly, and additional signal processing might be required. Photodetection is a broader term that encompasses various methods of detecting light, including both direct and indirect approaches, and mixed detection combines elements of both direct and indirect methods. However, direct detection specifically refers to the immediate and proportional conversion of light into an electrical signal, making it the most accurate answer for this question.