A basic fiber optic sensor detects changes in light intensity through the principle of total internal reflection within an optical fiber. The sensor consists of an optical fiber, which is a thin, flexible, and transparent filament made of glass or plastic. It has a core surrounded by a cladding layer with a lower refractive index.
Here's a step-by-step explanation of how it works:
Light Source: The sensor requires a light source, typically an LED (Light Emitting Diode), to emit light into one end of the optical fiber.
Total Internal Reflection: When light enters the core of the fiber, it undergoes total internal reflection. Total internal reflection occurs when light travels from a medium with a higher refractive index (the core) to a medium with a lower refractive index (the cladding) at an angle greater than the critical angle. This causes the light to be trapped within the core and keeps it traveling through the fiber by bouncing off the core-cladding interface.
Sensing Region: Within the fiber, there is a sensing region where the light interacts with the external environment. The sensing region can be exposed to certain physical parameters like temperature, pressure, strain, or chemical substances, depending on the specific application of the sensor.
Light Intensity Modulation: Changes in the external environment cause changes in the properties of the sensing region. For example, if the fiber optic sensor is designed to detect temperature, the temperature changes would lead to changes in the refractive index or length of the sensing region.
Light Propagation: The changes in the sensing region result in a variation in the light propagation characteristics within the fiber. This can cause light to be scattered, absorbed, or altered in other ways.
Output Detection: At the other end of the fiber, there is a light detector, such as a photodiode or a photodetector, which receives the light that has traveled through the sensing region and is now exiting the fiber.
Signal Processing: The light detector converts the received light into an electrical signal, and this signal is processed by electronic circuits or a data acquisition system. The intensity of the detected light corresponds to the changes in the external parameter being measured.
Analysis and Measurement: The electrical signal can be further analyzed and calibrated to determine the magnitude of the detected parameter, such as temperature or pressure.
By monitoring the changes in light intensity as it travels through the fiber, the fiber optic sensor can effectively detect variations in the external environment and convert them into measurable signals, making it a valuable tool for various applications in industries ranging from telecommunications to environmental monitoring and structural health monitoring.