It diverges, like a gaussian beam travelling in free-space. If the photodiode is placed far away then you can use Fraunhofer diffraction. A fiber optic sensor is a measurement device that uses light traveling through a glass or plastic filament to determine a physical quantity such as temperature, pressure, or strain. These sensors replace traditional electronic sensors by using light waves instead of electrical signals. It is often defined by numerical aperture, which simply means the sine of the half angle. Heating the material enables the trapped states to interact with phonons and decay into lower-energy. The Fotonic Sensor™ is a non-contact instrument, which uses the fiber optics lever principle to perform displacement measurement, vibration analysis and surface-condition measurements. The Fotonic Sensor transmits a beam of light through a flexible fiber-optic probe, receives light reflected from a. A fiber-optic sensor is a sensor that uses optical fiber either as the sensing element ("intrinsic sensors"), or as a means of relaying signals from a remote sensor to the electronics that process the signals ("extrinsic sensors").
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