A basic fiber optic sensor detects changes in light intensity through a phenomenon known as "total internal reflection." Here's a simple explanation of how it works:
Optical Fiber: The sensor consists of an optical fiber, which is a thin, flexible, and transparent filament made of high-quality glass or plastic. The fiber has a core through which light travels and is surrounded by a cladding that has a lower refractive index than the core.
Light Source: At one end of the optical fiber, there is a light source that emits light (typically in the form of a laser or LED).
Total Internal Reflection: When light travels within the fiber core and encounters the boundary between the core and cladding, it can undergo a phenomenon called "total internal reflection." Total internal reflection occurs when the angle of incidence of light is larger than a critical angle determined by the refractive indices of the core and cladding. Instead of passing through the boundary, the light reflects back into the core.
Sensing Area: In the middle of the fiber, there is a section called the sensing area. This section is usually treated or coated with a substance that interacts with the external environment or parameter you want to measure. For example, the coating may be sensitive to temperature, pressure, strain, chemical changes, or any other physical property you wish to monitor.
Light Modulation: Changes in the external environment at the sensing area cause alterations in the light's intensity. For instance, if the sensing area experiences a change in temperature, pressure, or strain, the refractive index of the coating may change. This, in turn, affects the angle of incidence of light hitting the core-cladding boundary.
Detection: At the other end of the optical fiber, there is a light detector (such as a photodiode) that measures the intensity of light that reaches it. The detector converts the light intensity into an electrical signal, which can be further processed and interpreted by electronic devices or systems.
Signal Analysis: The change in light intensity detected by the photodetector is analyzed to determine the corresponding environmental change or parameter being measured. The relationship between the light intensity and the parameter under consideration is calibrated beforehand to ensure accurate readings.
By monitoring the changes in light intensity, the fiber optic sensor can provide valuable information about the external environment or the physical property it is designed to measure. Fiber optic sensors are widely used in various fields, including telecommunications, industrial process control, environmental monitoring, and medical applications. They offer several advantages, such as immunity to electromagnetic interference, small size, lightweight, and the ability to operate in harsh environments.