A basic fiber optic sensor detects changes in light intensity using the principle of total internal reflection. Fiber optic sensors are based on the use of an optical fiber, which is a thin, flexible, and transparent strand of glass or plastic that can transmit light signals along its length.
The basic components of a fiber optic sensor include a light source (such as an LED or laser), the optical fiber itself, and a light detector (usually a photodiode or phototransistor). Here's how the sensor works:
Light transmission: The light source emits light, which is guided into one end of the optical fiber. The fiber is designed to have a core surrounded by a cladding layer with a lower refractive index than the core. This difference in refractive index allows the light to be guided along the fiber through a process called total internal reflection.
Propagation through the fiber: Once the light enters the core of the fiber, it bounces off the boundary between the core and cladding at shallow angles, preventing it from escaping the fiber. As a result, the light is transmitted through the length of the fiber with minimal loss.
Interaction with the sensing element: At some point along the length of the fiber, there is a region called the sensing element or sensing tip. This part of the fiber is designed to interact with the external environment. For example, it could be coated with a substance that reacts to changes in temperature, pressure, strain, or other parameters.
Light interaction with the sensing element: When the external parameter being measured changes, it affects the properties of the sensing element. This, in turn, changes the refractive index of the sensing element or alters the conditions for total internal reflection at the boundary between the core and the sensing element.
Changes in light intensity: As the light interacts with the sensing element, it may scatter or reflect differently based on the external parameter. This leads to changes in the amount of light that exits the sensing element and returns back to the fiber core.
Light detection: The altered light that emerges from the sensing element is guided back through the fiber and reaches the other end of the fiber where the light detector is located. The light detector converts the received light into an electrical signal, which can be measured and processed.
Signal processing: The electrical signal from the light detector is processed by suitable electronic circuitry to interpret the changes in light intensity and convert them into meaningful measurements of the external parameter being sensed.
In summary, a basic fiber optic sensor relies on changes in light intensity caused by interactions between the light and the sensing element to detect various physical quantities. This allows fiber optic sensors to be used in a wide range of applications, such as temperature sensing, pressure sensing, strain sensing, and more, due to their immunity to electromagnetic interference and ability to cover long distances.