Measuring the world with "light", they increase the highest accuracy by a factor of 10

Original title: Measuring the world with "light", they increase the highest accuracy by 10 times

China Science News reporter Li Qingbo

In the near future, the deformation of space stations and wings, the temperature of polar ice shelves, the cracks of dams, bridges and tunnels, the carriers of semiconductor devices, the sound and temperature of mines, and even the cancerous transformation of human organs and organs can all be measured by laying a layer of sensory "nerves" - optical fibers, and all of this stems from the continuous breakthrough of original technology of distributed optical fiber sensing.

"We can sense where the fiber is laid for a few cents a meter. The larger the space and the longer the distance, the higher the accuracy requirements and the wider the range of perception data. Zhang Mingjiang, a professor at the School of Physics of Taiyuan University of Technology, said that the distributed fiber sensor based on chaotic laser is like a combination of a meter ruler and a tape measure, with length and accuracy, helping us to detect the world.

Recently, the Key Laboratory of New Sensors and Intelligent Control of the Ministry of Education and the School of Physics of Taiyuan University of Technology have developed a new Raman distributed optical fiber temperature sensing technology, which can achieve centimeter-level spatial resolution at a sensing distance of kilometers, and the best spatial resolution is more than 10 times higher than the world's most advanced level. The results were published in Light: Science and Applications.

Ten years of "pieces"

Two months ago, two PhDs from China's 40th Antarctic expedition came to Zhang's lab to borrow a distributed fiber-optic sensor to monitor the runway temperature at China's northern airport and the Antarctic ice sheet. The shape of the sensor resembles a home computer host, including a chaotic laser transmitter, receiving card and other self-developed precision instruments, and a peripheral optical fiber interface. As long as the optical fiber is laid on the surface of the object to be observed, the sensor can emit and recover the laser light, and through a series of complex calculations, the optical signal is accurately converted into physical quantities such as temperature, stress, vibration, etc.

"The research and development of this new sensor is based on our continuous research on its light source, the chaotic laser, for many years, and there are only three or four domestic units that can develop this laser instrument. We were the first to combine chaotic lasers with fiber optic sensors, and the equipment developed was easy to carry and easy to operate. Since the equipment is not available for sale, we lend it to our counterparts who need it to further promote our team's latest research results. Zhang Mingjiang said. In his lab, the fiber-optic sensor, which had just finished working at the bottom of the coal mine, had not yet had time to wipe the dust off the casing.

The two corresponding authors are also mentors and apprentices, the teacher Zhang Mingjiang on the left and the student Li Jian on the right. Photo courtesy of the interviewee

This chaos is not the other chaos. Modern chaos theory began with the "butterfly effect" discovered by meteorologist Eduard Lorenz in his weather prediction research in the 60s of the 20th century. After 60 years of research, chaos theory has made great progress in mathematics, physics, astronomy, chemistry, sociology and other fields, especially combined with advanced optics and optoelectronics, and has given birth to a series of transformative optical technologies in the application fields of secure optical communication, lidar, and sensing.

"The concept of chaotic laser was born in the 90s of the 20th century, and it was not until the beginning of the 21st century that scientists discovered that chaotic laser has an irreplaceable role in secure communication, random number generation, and lidar. Since 2003, Zhang Mingjiang's team has explored the preparation of chaotic lasers from scratch, and now it is one of the few teams in China that can independently develop the laser.

"No country will openly sell Chaos lasers. Zhang Mingjiang, who took 10 years to stand on this starting line, has a "diamond diamond" and can decide "which porcelain job" to take.

Win in chaos

Zhang Mingjiang decided to open up a field that no one has set foot on - try to introduce chaotic laser as a light source on the fiber sensor with the traditional pulsed laser as the light source, and move to a new frontier that is difficult to reach with the original technology.

Choosing a new direction means that you need to continue to invest more energy, manpower, and may even face the embarrassment of tearing down the east wall and replacing the west wall. "It took us 11 years to go from the idea and concept in our heads to the mass production of the product we have today. Zhang Mingjiang's investment in chaotic laser-based fiber optic sensors is no less than a big gamble.

"The risk comes from the Chaos Laser, which is not favored. Zhang Mingjiang said, "For fiber optic sensors, changing the light source and changing the pulsed laser into a chaotic laser is a earth-shaking change, and based on completely different scientific principles, compared with traditional instruments, they are completely two types of instruments." ”

The timing waveform of pulsed laser fluctuates regularly, and the timing waveform of chaotic laser presents noise-like characteristics. Conventional pulsed laser is limited by its pulse width in fiber optic sensors, and the narrower the pulse width, the better its spatial resolution. However, the associated signal-to-noise ratio increases, similar to the "impurities" and "noise" in the signal, which contradict each other and are difficult to achieve.

The new fiber optic sensor is characterized by a wide spectrum of pulses, irregular pulse timing, and extremely large amplitudes, which theoretically makes it difficult to improve spatial resolution.

"After realizing the problem, we started with the calculation on paper, pondered the theoretical design repeatedly, and spent more than a year in the simulation to solve the problem that the spatial resolution of the traditional system is limited by pulse width in principle. Another corresponding author, Li Jian, an associate professor at the School of Electronic Information and Optical Engineering of Taiyuan University of Technology, said.

The successful combination of chaotic laser and fiber optic sensors can push the performance boundaries of traditional fiber optic sensors. On traditional fiber optic sensors, 1 kilometer of fiber can only locate meter-level data, which will generate about 1,000 pieces of data. The new sensor can capture data per centimeter of fiber, generating 100,000 pieces of data, an increase of two orders of magnitude.

It's hard to ask questions than to overcome them

Optical fibers are nerves, lasers are the means of perception, and the interrogator is the "brain" that processes information.

The chaotic laser incident enters the optical fiber, and through the backscattered light generated during light propagation, the optical timing, optical frequency, and optical phase along the optical fiber can be obtained, and then the information is collected back to the sensor acquisition card by the original way. It takes a very short time to walk 1 kilometer, but at least 200,000 pieces of data can be generated, and complex curves and messy signals mean that decoding is difficult and the amount of data processed is huge. Zhang Mingjiang's team exclusively developed a spatiotemporal compression modem, using the latest computer technologies such as edge computing to achieve massive data collation and strong decoding.

Distributed fiber optic sensors are divided into three directions: Rayleigh scattering, Brillouin scattering, and Raman scattering. "Each direction is a mountain to climb, and different sensors need to be made in these three directions. Zhang Mingjiang explained.

The more you know, the more you do. From the idea proposed in 2012 to the publication of the paper in 2023, Zhang Mingjiang's team has completed theoretical innovation and breakthroughs in three research directions, and realized the maturity and availability of equipment research and development, miniaturization and mass production.

In fact, as early as the initial stage, Zhang Mingjiang was under great pressure. Some experts have pointed out that if the research on the direction of Raman scattering is not completely and thoroughly improved, it will basically reach a dead end and have no future. Therefore, in order to prepare for this new track, Zhang Mingjiang arranged different tasks for the students in the research group, the first of which was to read literature.

In 2015, Li Jian became a master's student of Zhang Mingjiang, and has been following up since the initial stage of the new project. Zhang Mingjiang's requirement for him is not only to read it, but also to read out the defects of this article, so as to exercise his academic thinking ability.

"The biggest difficulty is the moment when the tutor first asks a 'scientific question', because not all questions can be a 'scientific question'. Li Jian said.

In the next three years, they undertook a key research and development program of the Ministry of Science and Technology to develop a fiber frequency domain reflectometry that can be applied in the fields of life sciences, aerospace sciences and geological sciences.

Zhang Mingjiang said: "This is a chaotic laser as the light source of high-precision scientific measuring instruments, our goal is to be more sensitive than the current most advanced fiber sensor 10 times, the laying length is increased by 10 times, in addition to the temperature field, strain field, and then add vibration field, magnetic field, evanescent field 3 physical fields, large to the state of submarine optical cables, mining area collapse changes, small to the power grid magnetic field, tumor markers in vivo can be observed." ”

Zhang Mingjiang said that an excellent "ruler" is not only a measuring tool, but also an indispensable helper for large scientific research, large projects and large equipment.

Source: China Science News