A basic fiber optic sensor detects changes in light intensity through a phenomenon known as total internal reflection (TIR). Here's a simplified explanation of how it works:
Fiber Optic Cable: The sensor consists of a fiber optic cable, which is a thin, flexible strand made of high-quality glass or plastic. The cable is designed to transmit light signals through its core.
Light Source: At one end of the fiber optic cable, there is a light source, typically an LED (Light Emitting Diode) or a laser diode. This light source emits a continuous beam of light into the fiber.
Total Internal Reflection: The core of the fiber optic cable has a higher refractive index than its cladding (outer layer). When light travels through the core and reaches the interface with the cladding at a certain angle (called the critical angle), total internal reflection occurs.
Sensing Element: The sensing element is the part of the fiber optic cable where the light interacts with the external environment. This element is often coated with a substance or material that is sensitive to the property being measured, such as temperature, pressure, strain, or chemical changes.
External Influence: When an external parameter (e.g., temperature, pressure) changes, it affects the properties of the sensing element. This alteration, in turn, affects the light propagation in the fiber.
Light Modulation: The changes in the sensing element cause a variation in the refractive index of the core in the vicinity of the sensing element. As a result, the angle at which light hits the core-cladding interface changes. This affects the amount of light that can escape through the cladding, causing modulation of the light intensity in the fiber.
Detection: At the other end of the fiber optic cable, there is a photodetector. The photodetector detects the modulated light intensity and converts it into an electrical signal. The electrical signal is then processed and interpreted by the sensor's electronics to determine the magnitude of the external influence.
In summary, the basic fiber optic sensor relies on total internal reflection and the modulation of light intensity within the fiber to detect changes in the external environment. These sensors are widely used in various applications, including temperature sensing, pressure sensing, strain measurement, and chemical analysis, due to their immunity to electromagnetic interference and ability to transmit signals over long distances with minimal loss.