These sensors mainly include fiber Bragg grating strain sensors, temperature sensors, acceleration sensors, displacement sensors, pressure sensors, flow sensors, liquid level sensors, etc.

Strain

This type of sensor is the most widely used and technologically mature optical fiber sensor in the engineering field. Strain directly affects the wavelength drift of fiber Bragg gratings. In cases where the working environment is good or the structure to be tested requires a small sensor, people directly attach bare fiber Bragg gratings as strain sensors to the surface of the structure to be tested or embed them inside the structure. Because fiber Bragg gratings are relatively fragile and very easy to be damaged in harsh working environments, they need to be encapsulated before they can be used. At present, the commonly used packaging methods mainly include substrate type, tube type and tube-based two-end clamping type.

Temperature

Temperature is one of the fundamental physical quantities provided by the International System of Units. It is a major parameter that needs to be frequently measured and controlled in industrial and agricultural production as well as scientific experiments, and it is also an important physical quantity closely related to People's Daily lives. At present, the commonly used electrical temperature sensors mainly include thermocouple temperature sensors and thermistor temperature sensors. Optical fiber temperature sensing has many advantages over traditional sensors, such as high sensitivity, small size, corrosion resistance, electromagnetic radiation resistance, flexible optical path, and ease of remote measurement. The temperature sensor based on fiber Bragg grating technology adopts wavelength coding technology, eliminating the influence of light source power fluctuations and system losses, and is suitable for long-term monitoring. Moreover, the temperature sensing system composed of multiple fiber Bragg gratings, using a single optical cable, can achieve quasi-distributed measurement.

Temperature is also a factor that directly affects the wavelength variation of fiber Bragg gratings. People often directly apply bare fiber Bragg gratings as temperature sensors. Just like fiber Bragg grating strain sensors, fiber Bragg grating temperature sensors also need to be packaged. The main function of packaging technology is to protect and sensitize. People hope that fiber Bragg gratings can have strong mechanical strength and a long service life. At the same time, they also hope that the response sensitivity of fiber Bragg gratings to temperature can be improved through appropriate packaging technology in fiber sensing. The temperature sensitivity of a common fiber Bragg grating is only about 0.010 nm/℃. Therefore, for a fiber Bragg grating with a working wavelength of 1550nm, the wavelength change when measuring a temperature range of 100℃ is only 1 nm. Demodulation with a decoder with a resolution of lpm can achieve a very high temperature resolution. However, if a spectrometer with a resolution of 0.06nm is used for measurement due to equipment limitations, the resolution is only 6 degrees, which is far from meeting the needs of actual measurement. The commonly used packaging methods at present include substrate type, tube type and polymer packaging method, etc.

Displacement

Researchers have carried out studies on displacement measurement using fiber Bragg gratings. Currently, these studies all measure the strain on the surface of the cantilever beam and then calculate the vertical deformation of the cantilever beam, that is, the vertical displacement at the end of the cantilever beam. This kind of "displacement sensor" is not a true displacement sensor in the real sense. Currently, this type of sensor has been applied in actual engineering, and there are also commercial products available in China.

Accelerometer

In 1996, Berkoff et al. from the United States designed a fiber Bragg grating vibration accelerometer by utilizing the pressure effect of fiber Bragg gratings. The converter is composed of a mass plate, a base plate and composite materials. Both the mass plate and the base plate are 6mm thick aluminum plates. The base plate serves as a rigid plate for support, and in the middle is an 8mm thick composite material sandwica between the two aluminum plates to act as a spring. Under the action of the inertial force of the mass block, the fiber grating buried in the composite material is subjected to the lateral force and undergoes strain, thereby causing the Bragg wavelength of the fiber grating to change. The relationship between the strain and acceleration of the fiber Bragg grating was demodulated by using a non-equilibrium M-Z interferometer. In 1998, Todd designed a grating accelerometer using a double flexible beam as the converter. The acceleration sensor consists of two rectangular beams and a mass block. The mass block is welded between the two parallel beams through point contact, and the fiber Bragg grating is attached to the lower surface of the second rectangular beam. When the sensor is subjected to vibration, under the action of inertial force, the mass block drives the two rectangular beams to vibrate, causing strain in them, which is transmitted to the fiber Bragg grating to cause wavelength shift. This kind of sensor has also been commercialized in China.

Pressure

The monitoring of tensile or compressive forces is also an important part of the monitoring process, such as the overall cable force of bridge structures, ice pressure on high-latitude offshore platforms, as well as soil pressure and water pressure on roads. Ou Jinping and others from Harbin Institute of Technology have successively developed fiber Bragg grating cable pressure rings and fiber Bragg grating ice pressure sensors. The British Naval Research Centre has developed fiber Bragg grating soil pressure sensors to monitor the load conditions inside highways. And various countries have successively begun the research work on fiber grating pressure sensors for oil and gas Wells.

In addition to the fiber Bragg grating sensors introduced above, fiber Bragg grating researchers and sensor designers have also designed fiber Bragg grating extensometers, fiber Bragg grating curvature meters, fiber Bragg grating hygrometers, as well as fiber Bragg grating inclinometers and fiber Bragg grating communicating tubes based on the sensing principle of fiber Bragg gratings. In addition, people have also made transport gauges for measuring road transportation conditions and strain gauges for measuring the strain of asphalt during road construction through fiber Bragg grating strain sensors.