A basic fiber optic sensor detects changes in light intensity using the principle of total internal reflection. Here's how it works:
Principle of Total Internal Reflection: Fiber optic cables consist of a core made of a material with a higher refractive index surrounded by a cladding with a lower refractive index. When light enters the core at an angle greater than the critical angle, it undergoes total internal reflection, effectively bouncing off the core-cladding interface and propagating along the core.
Sensor Setup: In a basic fiber optic sensor for detecting changes in light intensity, the core of the fiber is exposed to the environment where the light changes need to be measured. This exposed part of the fiber is usually coated with a material that interacts with the parameter being measured (e.g., temperature, pressure, strain, etc.).
Light Source: At one end of the fiber, a light source (typically an LED) sends light into the core of the fiber. This light propagates through the core via total internal reflection.
Detection End: At the other end of the fiber, there is a photodetector that monitors the light intensity. The photodetector converts the incoming light into an electrical signal.
Interacting with the Environment: The exposed section of the fiber interacts with the environment being monitored. For example, if the fiber is being used as a temperature sensor, changes in temperature cause the fiber's core dimensions to change, which alters the critical angle for total internal reflection. This, in turn, affects the amount of light that reaches the photodetector.
Detecting Changes: As the environment changes and affects the core's properties, the light that reaches the photodetector changes in intensity. An increase or decrease in the parameter being monitored will cause the core's properties to change, leading to a change in the critical angle and, subsequently, a change in the amount of light coupled out of the fiber core. The photodetector measures this change in light intensity.
Signal Processing: The electrical signal generated by the photodetector is then processed by electronic circuits to quantify the change in light intensity. This information can be calibrated to correspond to the specific environmental parameter being measured, such as temperature, pressure, strain, etc.
In summary, a basic fiber optic sensor for detecting changes in light intensity relies on the variation of total internal reflection within the fiber's core due to environmental changes. The light intensity changes are converted into electrical signals and processed to provide information about the monitored parameter.