A basic fiber optic sensor detects changes in light intensity using the principle of total internal reflection within an optical fiber. Here's how it works:
Optical Fiber: An optical fiber is a thin, flexible, and transparent strand of glass or plastic that can transmit light along its length. The core of the fiber is made of a material with a higher refractive index, surrounded by a cladding with a lower refractive index.
Principle of Total Internal Reflection: When light travels from a material with a higher refractive index to a material with a lower refractive index at an angle greater than the critical angle, it undergoes total internal reflection. This means that the light is completely reflected within the core of the optical fiber, allowing it to travel over long distances without significant loss of signal.
Sensing Mechanism: In a basic fiber optic sensor for light intensity detection, a portion of the optical fiber's cladding is usually removed or modified. This modified region is exposed to the external environment or the parameter being measured (such as changes in light intensity).
Input Light Source: An external light source, typically a light-emitting diode (LED) or a laser diode, is used to launch light into one end of the optical fiber. This light travels through the core of the fiber through multiple internal reflections.
Sensing Interaction: When the modified cladding region of the fiber is exposed to changes in light intensity, the intensity of light reaching this region affects the boundary conditions for total internal reflection. If the intensity changes, the angle at which the light strikes the core-cladding interface changes as well.
Output Detection: At the other end of the optical fiber, a photosensitive detector, such as a photodiode, is placed to detect the transmitted light. The photodiode converts the received light into an electrical signal. The changes in the intensity and angle of the incident light due to external factors cause variations in the amount of light that reaches the detector.
Signal Processing: The electrical signal from the photodiode is then processed by electronic circuitry. By analyzing the variations in the electrical signal, the sensor can determine changes in light intensity. These changes could be due to external factors such as the presence of an object, a change in the ambient light, or any other parameter that affects the interaction between the light and the modified cladding region.
In summary, a basic fiber optic sensor for light intensity detection relies on changes in the angle of incident light due to variations in the external environment. These changes affect the amount of light that reaches the detector, and the resulting electrical signal is processed to determine the corresponding changes in light intensity.