A basic fiber optic sensor detects changes in light intensity through the principle of total internal reflection. Here's a step-by-step explanation of how it works:
Fiber Optic Cable: The sensor consists of a fiber optic cable, which is a thin, flexible, and transparent strand of glass or plastic that can transmit light along its length.
Light Source: At one end of the fiber optic cable, there's a light source (usually an LED) that emits a beam of light into the cable.
Total Internal Reflection: The core of the fiber optic cable has a higher refractive index than the surrounding cladding. This means that when light enters the core at a certain angle, it can undergo total internal reflection, bouncing off the core-cladding interface and staying within the core instead of escaping through the cladding.
Detection End: At the other end of the fiber optic cable, there's a detection end. This could simply be a portion of the fiber exposed to the environment you're monitoring.
Intensity Measurement: In a basic setup, there is no active modulation of the light signal. Instead, the intensity of the light that reaches the detection end is measured. The light intensity decreases as the light travels along the length of the fiber due to various factors such as absorption and scattering.
Sensing Mechanism: When the detection end of the fiber is exposed to the external environment, any change in the environment that affects the intensity of the light can be detected. For example, if the detection end is in contact with a liquid or gas, changes in the refractive index of that medium due to changes in temperature, pressure, or the presence of specific analytes will affect the amount of light that gets transmitted through the fiber.
Photodetector: A photodetector placed at the detection end measures the intensity of light that reaches it. The photodetector converts the light signal into an electrical signal, which can be easily measured and processed.
Signal Processing: The electrical signal from the photodetector is then sent to signal processing electronics. These electronics can analyze the changes in light intensity and interpret them as changes in the external environment that the sensor is monitoring.
In summary, the basic fiber optic sensor operates by transmitting light through a fiber optic cable and measuring the changes in light intensity at the detection end. These changes are caused by alterations in the external environment that affect the transmission characteristics of the fiber, such as changes in refractive index due to temperature, pressure, or the presence of specific substances. This makes fiber optic sensors useful for a wide range of applications, including temperature, pressure, strain, and chemical sensing.