Fiber optic sensors can convert the state of a measured object into an observable optical signal, making them a widely used type of sensor. Due to the inherent superior properties of optical fibers, the advantages of fiber optic sensors are also quite obvious. A fiber optic sensor consists of a light source, an input fiber, an output fiber, an optical modulator, a photodetector, and a demodulator. Its basic principle is that light from the light source is sent through the input fiber to the modulation region. Within the modulation region, the light interacts with the external parameters being measured, causing changes in the optical properties of the light (such as intensity, wavelength, frequency, phase, and polarization), thus becoming the modulated signal light. This modulated signal light is then sent through the output fiber to the photodetector and demodulator to obtain the measured parameters.
The advantages of fiber optic sensors compared to traditional sensors are that they use light as the carrier of sensitive information and optical fiber as the medium for transmitting it, possessing the characteristics of fiber optic and optical measurement, and offering many unique advantages. They have excellent electrical insulation, strong resistance to electromagnetic interference, are non-invasive, have high sensitivity, and are very easy to use for long-distance monitoring of the measured signal. They are also corrosion-resistant, explosion-proof, and have a flexible optical path, facilitating connection with computers. Sensors are developing towards greater sensitivity, accuracy, adaptability, compactness, and intelligence. They can act as the eyes and ears of humans in places inaccessible, and can even transcend human physiological limits, receiving external information that is imperceptible to human senses.
Optical fiber can serve as a communication path between a test station and external sensors, a process known as external sensing. However, when the optical fiber itself is used as a fiber optic sensing system, this is called intrinsic fiber optic sensing. The advantage of this type of fiber optic sensing technology is that it eliminates the need for a discrete interface between the optical fiber and the external sensor, thus reducing complexity and cost. For this purpose, external stimuli such as temperature and strain fluctuations need to influence the light source in the cable in a measurable manner to provide useful data.
Rayleigh scattering occurs when photons are randomly scattered after contact with particles in an optical fiber. This principle has proven applicable to various types of fiber testing techniques, such as OTDR fiber testing, because the volume, wavelength, and position of the backscattered light to the detector can determine the amplitude and location of attenuation events in the fiber. Similarly, Raman scattering in the Stokes band produces temperature-induced changes in the photons scattered back to the source. By measuring the difference in intensity between the backscattered light in the Stokes and anti-Stokes bands, the temperature at any given location along the fiber can be precisely determined. Brillouin scattering is a similar phenomenon, where the wavelength of the backscattered light is predictably affected by external temperature and acoustic stimuli. This data, combined with background temperature knowledge at the same point, can be used to accurately determine the strain experienced by the fiber and to analyze which regions (areas) of the fiber are affected.
Fiber optic sensors offer advantages such as extremely high sensitivity and accuracy, inherent good security, resistance to electromagnetic interference, high insulation strength, corrosion resistance, integration of sensing and transmission, and compatibility with digital communication systems. These advantages are summarized below:
(1) High sensitivity;
(2) Lightweight, flexible, and easy to install and embed;
(3) Electrical insulation and chemical stability. Optical fiber itself is a highly insulating and chemically stable material, suitable for harsh environments such as high-voltage isolation and flammable/explosive conditions in power and chemical systems;
(4) Excellent safety. Fiber optic sensors are passive sensing elements, therefore, when used in measurements, there are no safety hazards such as leakage or electric shock;
(5) Electromagnetic interference resistance. Generally, the frequency of light waves is higher than that of electromagnetic radiation, therefore light propagating in optical fibers is not affected by electromagnetic noise;
(6) Distributed measurement capability. A single optical fiber can achieve long-distance continuous measurement and control, accurately measuring information such as strain, damage, vibration, and temperature at any point, thereby forming a monitoring area with a large range and improving the level of environmental monitoring;
(7) Long service life. The main material of optical fiber is quartz glass, with an outer cladding of polymer material, which gives it greater durability compared to metal sensors;
(8) Large transmission capacity. Using optical fiber as the bus, high-capacity optical fiber replaces bulky multi-core underwater cables to collect and store information from various sensing points, and multiplexing technology is used to monitor distributed optical fiber sensors.
