Fiber optic sensors are commonly used in temperature monitoring applications due to their unique advantages, such as immunity to electromagnetic interference, small size, and the ability to cover long distances. The principle behind fiber optic temperature sensors is based on the modulation of light properties as a function of temperature. There are several types of fiber optic temperature sensors, but the most common ones are based on the principles of intensity, wavelength, or phase modulation. Let's briefly explore each type:
Intensity-based fiber optic sensors: These sensors rely on the change in the intensity of light propagating through an optical fiber as a result of temperature variations. One common type is the "fiber Bragg grating" (FBG) sensor. An FBG is a periodic refractive index variation written into the core of an optical fiber. When light passes through the FBG, certain wavelengths are reflected back, while others continue to propagate. The reflected wavelength, known as the "Bragg wavelength," is affected by temperature changes, leading to intensity variations that can be measured and correlated to temperature.
Wavelength-based fiber optic sensors: In these sensors, the temperature-induced changes affect the wavelength of light, rather than its intensity. An example is the "Fabry-Perot interferometer" sensor. It consists of a cavity between two reflective surfaces. As the temperature changes, the length of the cavity changes, leading to a shift in the interference pattern of the reflected light. By measuring the wavelength shift, the temperature can be determined.
Phase-based fiber optic sensors: These sensors exploit the change in the phase of light due to temperature variations. One example is the "Mach-Zehnder interferometer" sensor. It consists of two arms with different refractive indices. As temperature changes, the refractive index of each arm changes, leading to a phase shift between the two arms. The phase shift can be measured to determine the temperature.
In all these fiber optic temperature sensors, the changes in light properties are converted into electrical signals using photodetectors, and the resulting signals are processed to obtain temperature readings. The advantage of fiber optic sensors lies in their ability to be multiplexed, meaning multiple sensors can be connected to the same fiber, allowing for distributed temperature sensing over long distances.
Fiber optic temperature sensors find numerous applications in industries such as aerospace, oil and gas, healthcare, and structural monitoring, where accurate and reliable temperature measurements are crucial.