Fiber optic transformation from the "blood vessel" of signal transmission to the "nerve" of monitoring

Now walk into the flower bed in the courtyard of the China Earthquake Administration, crouch down, and you can still see the traces of the excavation that have been turned over. Near the South Gate, look closely and you can see a circle of fibers exposed.

This cheap, well-known household fiber was laid more than 600 meters new in the earthquake bureau. More than 600 meters of ring fiber, connected to the University of Science and Technology of China and the China Earthquake Administration team independently developed DAS (distributed fiber acoustic wave / vibration sensing) and DTS (distributed fiber temperature monitoring) system, equivalent to the arrangement of 1600 vibration and temperature sensors to carry out observation tests.

On October 22, the distributed optical fiber sensing technology application exhibition was held in Beijing. Optical fiber is not only the "blood vessel" that transmits the signal, but also the "nerve" that monitors the signal.

Turn 40 kilometers of fiber optic cable into tens of thousands of seismic sensors

In recent years, optical fibers have been used as sensors to measure physical quantities such as stress and temperature in addition to their applications in the field of communications. However, the core equipment of this new technology, optical signal modulation and demodulation, mainly relies on imports. Foreign equipment is not only expensive, but also the export of high-end products to China is also subject to many restrictions.

In 2021, the University of Science and Technology of China developed a DAS system with independent intellectual property rights, which can use existing communication cables for seismic monitoring and underground space detection.

The technical principle is to use optical time domain reflection technology for positioning. Rayleigh scattering of light is extremely sensitive to changes in the refractive index, length, etc. of optical fibers. When the fiber is disturbed, the intensity and phase of the Rayleigh scattered light changes. Therefore, detecting and demodulating Rayleigh scattered light can obtain external vibration information.

Professor Wang Baoshan of the School of Earth and Space Sciences of the University of Science and Technology of China introduced that the three key points of this set of equipment are the special laser light source, high-sensitivity detection system and high-fidelity demodulation system. DaS devices are capable of turning 40 kilometers of fiber optic cables into tens of thousands of seismic sensors. "Optical fibers are as sensitive as traditional seismometers and can sense a variety of vibrational signals; but ultra-high density observations make fibers have higher monitoring capabilities." In June 2021, DAS equipment was deployed in Hefei Zipeng Mountain for continuous observation, and successfully measured the Xuancheng 2.7 earthquake and the Philippines 6.6 earthquake. After laying optical fibers in the Seismological Bureau, the DAS system recorded the 4.7-magnitude seismic waveform data of Chifeng in Inner Mongolia on October 15, which was well consistent with the traditional short-period seismometer observations.

Wang Baoshan said that this provides a new solution for seismic intensive/ultra-dense array observations in urban areas, which can complement existing monitoring systems.

Continuous temperature measurement, reaching places that cannot be reached by traditional temperature measurement technology

Using another scattering of light, changes in temperature can be sensitively captured. It is Raman scattering, i.e. the frequency of light waves changes after being scattered.

Raman scattering stems from the inherent vibrational interaction of photons with molecules, carrying information about the temperature of molecules, whose intensity is related to temperature. Therefore, the scattering of this light can be analyzed to measure the temperature.

Not long ago, Wang Zhigang, minister of science and technology, issued a "mobilization order" to prepare for the 100-day winter Olympics. Chen Yong, academician of the Chinese Academy of Sciences, geophysicist and director of the Science and Technology Committee of the China Earthquake Administration, told the Science and Technology Daily reporter that the research team actively responded to this "mobilization order", concentrated on key technologies such as laser light sources and core algorithms, and developed a distributed optical fiber temperature sensing (DTS) system that includes laser light source systems, signal demodulation and data processing systems, which has complete intellectual property rights and all devices are domestic.

Temperature has a great influence on snow sports. "Temperature measurement technology is the scientific and technological support of the Ice and Snow Games, and the dynamic temperature change of ice and snow in the sports field is the basic scientific data of the Games." Chen Said.

The traditional temperature measurement method is often a single point measurement, which cannot fully reflect the spatio-temporal changes of temperature in a timely manner. DTS can solve this pain point. By planting a temperature measurement fiber near the ice and snow race venue, the temperature change of the entire track can be monitored every 1 meter or even shorter, and the measurement accuracy can reach 0.1 °C.

Professor Chen Yang of the Hefei National Research Center for Microscale Physical Sciences of the University of Science and Technology of China introduced that distributed optical fiber sensing systems can monitor temperature, strain and vibration, and expand areas that cannot be monitored or difficult to monitor by traditional methods. This kind of monitoring can also detect potential failures in large facilities in a timely manner. It can be used in transmission networks, petrochemicals, large warehouses, and in deep-ground and deep-sea exploration.

Chen Said that in the next step, they will promote the research and development and industrialization of distributed optical fiber sensing software and hardware, promote the construction of distributed optical fiber sensing monitoring network, and expand its application field. (Reporter Zhang Galen)

Source: Science and Technology Daily