The principle and characteristics of fiber optic grating sensors

Fiber optic temperature sensor, Intelligent monitoring system, Distributed fiber optic manufacturer in China

In recent years, with the rapid development of fiber optic technology, a large number of new types of fibers and fiber optic devices have emerged. At the same time, with the use of various new light sources and fiber optic detectors, fiber optic sensing technology has made breakthrough progress in technological development and practical applications.

Fiber optic sensors, as an important application of optical fibers, are receiving increasing attention due to their small size, light weight, and high sensitivity. They have unparalleled advantages in preventing magnetic field interference, corrosion, waterproofing, and high temperature and pressure resistance compared to traditional electronic sensors.
Fiber optic grating sensing technology is an important branch of the field of fiber optic sensing technology. Its sensing principle is that the center wavelength of the fiber optic grating shifts due to changes in external physical parameters. By monitoring the size of the center wavelength of the fiber optic grating, specific information of the physical parameters is obtained, thereby achieving sensing. Fiber optic grating sensor is a wavelength modulated fiber optic sensor that, in addition to the characteristics of ordinary fiber optic sensors, has higher measurement accuracy, better resolution, and is more suitable for distributed measurement.

Principle and Application Characteristics of Fiber Bragg Grating Sensors

Due to the light induced effect, a grating effect can be generated in germanium doped fibers, resulting in the successful production of the world’s first fiber grating. Since then, the basic principles, fabrication methods, and practical applications of fiber Bragg gratings have been extensively studied by a large number of researchers.
The main material of optical fibers is quartz, which is composed of a core layer and a cladding layer, and the outer layer is protected by a coating layer. By doping the core layer, the refractive index (n1) of the core layer is greater than that of the cladding layer (n2), forming an optical waveguide where light can propagate within the core layer. When the optical fiber is modulated by external factors, the refractive index of the core layer undergoes periodic changes, forming a fiber grating. The phase mask method is currently the most convenient and effective method for producing fiber Bragg gratings, simplifying the production process and reducing production costs. There are various types of fiber Bragg gratings, among which the typical one is fiber Bragg grating (FBG), which is also one of the earliest developed and most widely used fiber Bragg gratings.

Basic Sensing Principles of Fiber Bragg Grating

By forming a spatial phase grating on the fiber optic core in a certain way, when light passes through the fiber optic grating, mode coupling occurs between the forward and backward transmission of the fiber optic core modes, causing the forward transmission of the fiber optic core mode to transfer to the backward transmission of the fiber optic core mode. That is, the incident light with a wavelength reflected at the center of the fiber optic grating (λ B) is reflected, and the center reflection wavelength of the fiber optic grating is affected by strain and temperature. By monitoring the magnitude of the center reflection wavelength of the fiber optic grating, corresponding strain and temperature sensing can be achieved. Other physical parameters such as pressure, displacement, etc. can be converted into factors such as temperature or pressure that directly affect the fiber optic grating
By doing so, fiber Bragg gratings can be used to achieve sensing of various physical parameters.

Application characteristics of fiber optic gratings

Based on the material and structural characteristics of fiber Bragg gratings, as well as their sensing principles, they have many significant advantages.

(1) Small size, light weight, simple structure, and variable appearance.

Usually, fiber Bragg gratings are only 3-5 cm long and have a lightweight fiber. The fiber Bragg grating sensing head is a small, flexible, and portable device that can be packaged according to the actual use of fiber Bragg grating sensors to meet different sensing needs.

(2) Good chemical stability.

The main component material of fiber optic grating is silicon dioxide, which has good chemical stability. Therefore, fiber optic grating sensors have strong corrosion resistance and are suitable for harsh environments with chemical corrosion, as well as various biological environments.

(3) Good physical stability.

The main component material of fiber optic gratings is silicon dioxide, so fiber optic gratings are electrically insulating, waterproof, and resistant to high temperature and high pressure.

(4) Not affected by electromagnetic interference.

Due to the working principle of fiber optic gratings, they have the characteristic of being immune to electromagnetic interference and are particularly suitable for harsh environments with high radiation intensity, such as medical detection in special environments such as nuclear magnetic testing.

(5) High sensing sensitivity.

Fiber Bragg Grating achieves sensing by monitoring changes in the reflected wavelength of the center, resulting in high measurement sensitivity. Due to the extremely small changes in many medical detections, some ordinary sensors cannot measure the changes. Therefore, this characteristic of fiber Bragg grating ensures its applicability in the field of medical device sensing.

(6) High measurement accuracy.

Based on the sensing principle of fiber optic grating, it is not affected by other factors such as light sources, and its measurement accuracy is high. However, medical sensors have high requirements for measurement accuracy, which makes the characteristic of fiber optic grating better meet the measurement accuracy requirements of medical sensors.

(7) Distributed real-time measurement.

Multiple multiplexing techniques can be used to connect multiple fiber Bragg gratings in series, forming a distributed sensing network to measure the physical quantities of multiple points. The measurement data is transmitted in real-time to the detector through optical fibers, ultimately achieving real-time detection of multiple physical quantities. This feature can meet the requirements of multi parameter measurement for medical sensors.

(8) Wide measurement range.

By designing the structure and packaging of fiber Bragg gratings, various physical parameters such as strain, temperature, pressure, rotational speed, pH value, and humidity can be measured.