One is a light-transmitting (non-functional) sensor, and the other is a sensing (functional) sensor. In fiber optic sensors, the optical fiber is only used as a light transmission medium; the sensing of the measured signal is accomplished through other sensing elements. The output and input optical fibers in the sensor are discontinuous; the modulator between them is a spectral change sensing element or other sensing elements. What are the working principles and characteristics of fiber optic sensors?
1. Working principle of fiber optic sensors
Optical fiber is a medium for transmitting light waves based on the principle of complete internal reflection. It consists of a high-refractive-index core and a cladding. The cladding has a lower refractive index than the core and a diameter of approximately 0.1 mm to 0.2 mm. When light passes through the core and reaches the interface with the cladding, it is reflected back to the core due to complete internal reflection. Through continuous reflection, the light can propagate forward along the core with only slight attenuation. An optical fiber sensor consists of a light source, an incident fiber, an output fiber, an optical modulator, a photodetector, and a modulation device. Its basic principle is to transmit light from the light source to the modulation region through the ejected fiber. The light interacts with external measurement parameters within the modulation region, changing its optical properties (such as intensity, wavelength, frequency, phase, and polarization), becoming modulated signal light. This modulated signal light is then transmitted through the ejected fiber to the photodetector and demodulator to obtain the measurement parameters. Optical fiber sensors can be used for remote object detection. Due to fiber loss and dispersion, long-distance optical fiber transmission systems must be equipped with intermediate amplifiers at appropriate locations along the line to process and amplify attenuated and distorted optical pulse signals.
2. Characteristics of fiber optic sensors
(1) High sensitivity
Since light is an electromagnetic wave with a very short wavelength, its optical length is obtained through its phase. Take a fiber optic interferometer as an example. Because the diameter of the optical fiber used is very small, even a small change in external mechanical force or temperature will cause a change in optical length, resulting in a large phase change. If a 10m fiber is used, a 1℃ change will result in a minimum detectable phase change of 0.01 ard (1000 ard). Therefore, the minimum measurable temperature change is 10℃, indicating its high sensitivity.
(2) Electromagnetic interference resistance, electrical insulation, corrosion resistance, intrinsic safety
Since fiber optic sensors use light waves to transmit information, and optical fiber is an electrically insulating and corrosion-resistant transmission medium, it is safe and reliable and can be conveniently and effectively applied in various large-scale electromechanical, petrochemical, mining and other harsh environments with strong electromagnetic interference and flammable and explosive conditions.
(3) Fast measurement speed
Light has the fastest transmission speed and can transmit two-dimensional information, making it suitable for high-speed measurements, radar, and other signals.
The analysis requires high detection rates, which are difficult to achieve using electronic methods. This can be addressed by performing high-speed spectral analysis on optical diffraction phenomena.
(4) Large information capacity
The signal being measured uses light waves as its carrier, has an extremely high optical frequency, and encompasses a wide bandwidth. Multiple signals can be transmitted through the same optical fiber.
(5) Suitable for harsh environments
What are the working principles and characteristics of fiber optic sensors? Optical fiber is a medium resistant to high voltage, corrosion, and electromagnetic interference, allowing it to be used in harsh environments where other sensors cannot operate. Furthermore, fiber optic sensors are lightweight, small in size, easy to wind, can measure a wide range of objects, are highly reusable, and are low in cost. Light has the fastest transmission speed, enabling the transmission of two-dimensional information and facilitating high-speed measurements. Radar and other signal analysis require high detection rates, which are difficult to achieve electronically. This can be addressed through high-speed spectral analysis of optical diffraction phenomena.
