A basic fiber optic sensor detects changes in light intensity using the principle of total internal reflection. Fiber optic sensors are composed of an optical fiber made of a highly transparent material, usually glass or plastic. The fiber is capable of transmitting light through its core due to the phenomenon of total internal reflection, which occurs when light traveling through the core of the fiber is reflected back into the core instead of escaping through the cladding (outer layer).
Here's how the process works:
Light Source: A light source, such as an LED (Light Emitting Diode) or a laser, emits light into one end of the optical fiber.
Light Propagation: The emitted light travels through the core of the optical fiber, which has a high refractive index compared to the cladding. This difference in refractive indices ensures that the light is confined within the core through total internal reflection.
Sensing Region: In the sensing region of the fiber optic sensor, the core is exposed to the external environment. This sensing region is designed to interact with the parameter being measured (e.g., temperature, pressure, strain, etc.).
Changes in Light Intensity: When there is a change in the external parameter being measured, it can affect the behavior of light within the sensing region. For instance, if the temperature changes, the refractive index of the medium around the fiber might also change.
Interaction with Sensing Region: The changes in the external parameter cause a change in the refractive index of the material surrounding the fiber core. This, in turn, affects the total internal reflection phenomenon.
Light Loss: As a result of the changes in the refractive index, some of the light may escape from the core, resulting in light loss within the fiber.
Light Detection: At the other end of the fiber, there is a light detector, typically a photodiode or a photodetector, that measures the intensity of light that reaches it. The detector converts the light signal into an electrical signal that can be analyzed and processed.
Signal Analysis: The electrical signal from the detector is analyzed to determine the changes in light intensity. The magnitude of the change corresponds to the variation in the external parameter being measured. This information is then used to infer the value of the measured parameter.
In this way, a basic fiber optic sensor can detect changes in light intensity due to external influences and use this information to make measurements and monitor various physical quantities. Fiber optic sensors are widely used in various industries for their numerous advantages, including their immunity to electromagnetic interference, small size, and ability to be deployed in harsh environments.