The functional sensor in the optical fiber sensor type uses the optical fiber itself as a sensitive element, and is measured to modulate the light transmitted in the optical fiber, so that the intensity, phase, frequency or polarization state of the transmitted light are changed. The measured signal is obtained by demodulating the modulated signal. The optical fiber is not only a light-guiding medium, but also a sensitive element. The light is modulated by the measurement in the optical fiber, and multi-mode optical fiber is mostly used.
Non-functional fiber optic sensors use other sensitive components to sense the changes to be measured. Optical fibers are only used as information transmission media, and single-mode fibers are often used. The optical fiber only plays the role of guiding light, and the light on the optical fiber-type sensitive element is modulated by the measurement. Advantages: Optical fibers can be used for both electrical isolation and data transmission, and the signals transmitted by optical fibers are not affected by electromagnetic interference. Most of the practical ones are non-functional fiber optic sensors.
Fiber Bragg Grating Sensor (FBS) is a fiber-optic sensor with the highest frequency and widest range. This sensor can change the wavelength of the reflected light wave according to the change of ambient temperature or strain. Fiber Bragg gratings use holographic interferometry or phase masking to expose a small section of light-sensitive fiber to a light wave with a periodic distribution of light intensity. In this way, the optical refractive index of the fiber is permanently changed according to the intensity of the light wave to which it is irradiated. The periodic changes in the refractive index of light caused by this method are called fiber Bragg gratings.
When a broad-spectrum light beam is propagated to the fiber Bragg grating, each small section of the fiber after the refractive index of the light is changed will only reflect a light wave of a specific wavelength, which is called the Bragg wavelength. Fiber Bragg gratings reflect only one specific wavelength of light, while all other wavelengths are propagated.
The light-transmitting fiber-optic sensor in the fiber-optic sensor type is to input the optical signal modulated by the measured object into the optical fiber, and then measure the optical signal at the output end. This type of sensor has an additional photosensitive element that is sensitive to the physical quantity to be measured, the optical fiber is only used as a light-transmitting element, and a sensitive element that can modulate the light transmitted by the optical fiber must be attached to form a sensing element. Fiber optic sensors can also be divided into three types: point fiber optic sensors, integral fiber optic sensors, and distributed fiber optic sensors according to their measurement ranges. Among them, distributed optical fiber sensors are used to detect the strain distribution of large structures, which can quickly and non-destructively measure important parameters such as displacement, internal or surface stress of structures. Optical fiber sensors have become a rising star in the sensor family by virtue of their many advantages, and play their own unique roles in various measurements, becoming an indispensable member of the sensor family.