The basic working principle of an optical fiber sensor is to send light from a light source through an optical fiber into a modulator, so that the parameter to be measured interacts with the light entering the modulation region, causing changes in the optical properties of the light (such as light intensity, wavelength, frequency, phase, polarization state, etc.), which is called the modulated signal light. The measurement is then completed by utilizing the influence of the measured quantity on the transmission characteristics of the light.
A fiber optic sensor is a sensor that converts the state of a measured object into a measurable optical signal. The working principle of a fiber optic sensor is as follows: a light beam incident from a light source is sent through an optical fiber to a modulator. Within the modulator, the light interacts with the external parameters to be measured, causing changes in the optical properties of the light, such as intensity, wavelength, frequency, phase, and polarization state, resulting in a modulated optical signal. This modulated signal is then sent through an optical fiber to a photoelectric device and, after passing through a demodulator, the measured parameters are obtained. Throughout the process, the light beam is guided through the optical fiber, passes through the modulator, and then exits. The optical fiber's primary function is to transmit the light beam, and secondarily, it acts as an optical modulator.
Optical fiber is a medium that transmits light waves using the principle of perfect internal reflection. It consists of a high-refractive-index core and a cladding. The refractive index of the cladding is lower than that of the core, and its diameter is approximately 0.1 mm to 0.2 mm. When light passes through the end face into the core, it is reflected back to the core layer at the interface with the cladding due to perfect internal reflection. Through continuous reflection, the light propagates forward along the core with only minimal attenuation. Fiber optic sensors guide the light emitted from the transmitter to the detection point using optical fibers, and then guide the detected light signal to the receiver using optical fibers to achieve detection. Depending on the operating method, fiber optic sensors can be divided into various types, such as through-beam and diffuse reflection types. Fiber optic sensors can detect objects over relatively long distances. Due to fiber loss and dispersion, in long-distance fiber optic transmission systems, intermediate amplifiers must be installed at appropriate locations along the line to process and amplify the attenuated and distorted optical pulse signals.
(1) Principle of property-based fiber optic sensors: Property-based fiber optic sensors utilize the sensitivity of optical fibers to environmental changes to transform input physical quantities into modulated optical signals. Their working principle is based on the optical modulation effect of optical fibers, that is, when external environmental factors, such as temperature, pressure, electric field, magnetic field, etc., change, their light transmission characteristics, such as phase and light intensity, will change.
Therefore, if the changes in the phase and intensity of light passing through an optical fiber can be measured, the changes in the measured physical quantity can be known. This type of sensor is also known as a sensitive element type or functional fiber optic sensor. The point source beam of a laser diffuses into a parallel wave, which is then split into two paths by a beam splitter: a reference path and a measurement path. External parameters (temperature, pressure, vibration, etc.) cause changes in the fiber length and the phase of the light, resulting in different numbers of interference fringes. By counting the shifts in their mode directions, temperature or pressure can be measured.
(2) Principle of structural fiber optic sensor: A structural fiber optic sensor is a measurement system composed of a light detection element (sensitive element), a fiber optic transmission loop, and a measurement circuit. The fiber optic cable serves only as the light propagation medium, hence it is also called a light transmission type or non-functional fiber optic sensor.
Point sensors, in which each sensor is discrete and must be backtracked individually, are suitable for deployments over shorter distances. Meanwhile, fiber Bragg grating (FBG) based point sensors can measure parameters at specific locations using FBGs with high resolution and sensitivity.
Quasi-distributed sensors utilize multiple fiber-optic bundled groups (FBGs). By modifying the refractive index of the fiber core, certain wavelengths of light can pass through, while others are reflected back to the light source. Each FBG can reflect a specific wavelength, allowing each FBG to be identified along the fiber path. Quasi-distributed sensing is not sensitive along the entire optical cable, but it is extremely sensitive at specific local points.
Distributed fiber optic sensors (DFOS) are a technology capable of continuous measurement along an entire fiber optic cable. They are characterized by: sensing elements specific to the fiber, high sensitivity, immunity to electromagnetic interference, and a large measurement range. DFOS applications include distributed temperature sensing (DTS), distributed acoustic sensing (DAS), and distributed strain sensing (DSS). DTS uses the fiber itself as a sensing element to measure the temperature distribution along the entire fiber optic cable, representing a cost-effective method for obtaining accurate and high-resolution temperature measurements over long distances. DAS uses fiber optics to detect acoustic vibrations. DSS provides spatially resolved elongation profiles along the fiber optic sensor cable. By combining multiple sensor cables at different locations across the asset's cross-section, DSS is used to calculate the asset's (device under test) elongation (strain), shape (bending radius and bending direction), and torsion.
