Academician Dou Xiankang: Use quantum lidar to solve the problem of middle and upper atmosphere detection

The Surging News reporter Liu Hang

Dou Xiankang, academician of the Chinese Academy of Sciences and president of Wuhan University

Wind field detection of the middle and upper atmospheres, which is important for space weather research and forecasting, has long faced a dilemma – it is impossible to detect using air balloons or satellites. However, the quantum lidar innovatively developed by Dou Xiankang, academician of the Chinese Academy of Sciences and president of Wuhan University, provides an effective solution to this.

Recently, Dou Xiankang gave a keynote speech on "Application of Quantum Detection Technology in Atmospheric Detection LiDAR" at the "Yanqi Lake Conference" held in Beijing, and introduced the relevant research in detail.

"Yanqi Lake Conference" is a high-end international academic exchange activity jointly organized by the Chinese Academy of Sciences and Beijing Municipality, which has been held for four sessions so far. With the theme of "Frontiers of Quantum Science and Technology", this year's conference focuses on the frontier key issues in the field of quantum science and technology, and forward-looking to discuss the strategic goals, directions and tasks of the future development of quantum science and technology.

Dou Xiankang has long been engaged in comprehensive research on the theory, observation and experiment of the middle and upper atmosphere, independently developed a series of lidar observation systems, vehicle-mounted wind measurement lidar systems to fill the gap in this field in China, the technical level has reached the international leading level; the first successful development of quantum lidar based on single-photon frequency conversion technology in the world; and based on pioneering work on observation equipment, systematic and innovative results have been achieved in the fields of atmospheric dynamics and photochemistry in the middle aperture region.

In the past, middle and upper atmosphere exploration was a relatively large problem

"The field of space physics research is the space between the surface of the Sun and the surface of the Earth." Dou Xiankang introduced that according to the altitude range, it can be divided into the middle and upper atmosphere, the thermosphere and ionosphere, magnetosphere, and interplanetary space. Among them, the middle and upper atmosphere (more than ten kilometers to one or two hundred kilometers) is the key node in the transition from the earth's neutral atmosphere to the space plasma.

The weather phenomena such as rain and snow that we encounter in daily life occur in the atmosphere below ten kilometers, which is the troposphere; the atmosphere of ten to fifty kilometers is dominated by horizontal flow, which is the stratosphere; the atmosphere of fifty to ninety kilometers is called the middle layer; and above the height of ninety kilometers is the thermal atmosphere. The stratosphere, the mesosphere, and the low-thermosphere atmospheres make up the major regions of the middle and upper atmospheres.

Why study the upper atmosphere? Dou Xiankang gives an example, "The density change of the tropospheric atmosphere is relatively small, although the weather conditions are different on sunny and rainy days, but the atmospheric density change is often only about 1%. In contrast, the middle and upper atmospheres are very thin in density and are susceptible to drastic changes due to disturbances such as solar eruptions. Satellite orbits are affected by changes in density, and satellite orbit prediction can be very difficult if the density of the high atmosphere cannot be accurately observed and predicted. ”

However, due to the lack of observational means, the middle and upper atmosphere has not been adequately studied. Space physics has traditionally focused primarily on above the ionosphere (above one hundred kilometers), while atmospheric science has focused primarily on below the stratosphere (below thirty kilometers). At altitudes of thirty to one hundred kilometres, the middle and upper atmosphere cannot be detected directly by satellites or by balloons.

"For tropospheric altitude atmospheres, we can use weather radar and sounding balloons to detect them. At higher altitudes (thermosphere and ionosphere, magnetosphere, interplanetary, etc.), we can use satellite exploration. However, the middle and upper atmosphere at an altitude of 30 km to 100 km is mainly dominated by atmospheric molecules and has very few scatters, which is a detection blind spot where the air balloon cannot go up and satellite observation cannot come. Dou Xiankang pointed out that although the launch of a sounding rocket carrying a sounding instrument can detect this part of the space, the sounding rocket is a single measurement, and the detection cost is high, and it is impossible to achieve long-term observation. Therefore, the detection of the middle and upper atmosphere requires innovative observation methods.

Lidar is the main means of detection of the middle and upper atmosphere

Lidar is the main means of detection of the middle and upper atmosphere, capable of covering the middle and upper atmosphere from near the ground to a hundred kilometers. Radar uses the scattering process of electromagnetic waves from the target to discover the target and obtain information about its characteristics. The main body of the middle and upper atmosphere at an altitude of thirty kilometers to one hundred kilometers is pure atmospheric molecules, whose scale is comparable to the wavelength of the laser, and the interaction between the laser and the atmospheric molecules can be used to detect the middle and upper atmosphere.

The main parameters of lidar detection of the middle and upper atmosphere include atmospheric density, temperature, and wind field. Among them, the wind field is the most important dynamic parameter of the middle and upper atmosphere, a direct embodiment of the global circulation of the middle and upper atmosphere, and the most difficult parameter of the middle and upper atmosphere. Accurate atmospheric wind field detection is of great significance for numerical weather forecasting, climate model improvement, biochemical gas monitoring, airport wind shear warning, etc.

Dou Xiankang introduced the difficulty of wind field measurement. As mentioned earlier, the middle and upper atmosphere is mainly dominated by atmospheric molecules. Due to the thermal motion of the atmospheric molecules themselves, we emit a laser beam that is scattered by atmospheric molecules, and the scattered laser spectrum will produce a widening. If atmospheric molecules move with the wind field, this spread spectrum will produce a frequency shift. An accurate measurement of this frequency shift can estimate the wind speed of the atmosphere. The main technical difficulties in measuring frequency shift are the wide spread of the scattered laser spectrum, the small amount of frequency shift generated by the wind field, and the weak signal of the scattered laser.

One solution that is being studied internationally and by Dou Xiankang's team is to use "dual edge technology" to detect tiny laser shifts, and convert the shift of weak optical signals into relative changes in signal strength through optical frequency detectors. "This is very useful, it can effectively detect the wind field at high altitude."

Innovative use of quantum detection technology to improve lidar performance

In fact, global wind field measurement also faces the following challenges: under the condition of limited laser power and telescope area, greatly improve the detection signal-to-noise ratio; ensure human eye safety in crowded places such as bases and weather stations; and overcome environmental interference such as strong vibration and large temperature difference on airborne and spaceborne platforms.

"There are two factors in the performance improvement of lidar, one is the telescope aperture that receives scattered photons, and the other is laser energy. Because the high-altitude atmospheric molecular scattering signal is very weak, to improve the performance of lidar, on the one hand, to increase the aperture (area) of the telescope, which leads to a large scale of lidar, the cost is very high, is not conducive to working on the satellite platform; on the other hand, to increase the energy of the laser, which will lead to high-power laser burning out of optical lenses and other issues, which is the technical problem faced by spaceborne lidar. Dou Xiankang pointed out that in addition, lidar also faces another problem: due to the influence of sunlight during the day, the lidar signal in the visible light band is often poor or unable to work.

Therefore, they innovated ideas and cooperated with Academician Pan Jianwei's team Zhang Qiang and others to use optical quantum detection technology to improve the lidar signal-to-noise ratio by improving the quantum efficiency of lidar detection, without increasing laser energy or telescope aperture.

"In the past, we used infrared lasers for atmospheric detection, and the advantage of infrared lasers is that the atmosphere is more penetrating and less affected by sunlight. However, lidar is less efficient for infrared photon detection of atmospheric scattering. Therefore, we worked with Academician Pan Jianwei's team to use single-photon frequency upconversion technology to convert infrared laser photons scattered back from the atmosphere into visible photons of 863 nanometers, and used silicon detectors with higher detection efficiency for detection. In this way, the efficiency and performance of lidar detection can be effectively improved. ”

In recent years, they have built a single-photon frequency conversion quantum wind measurement lidar for the first time in the world by tackling a series of key technologies such as quantum (single-photon) frequency upconversion and all-fiber lidar integration, which breaks through the quantum efficiency limit of detecting infrared single photons at room temperature, and the detection signal-to-noise ratio is better than that of traditional lidar by 3 orders of magnitude, laying the foundation for high-precision, high-space-time resolution of the middle and upper atmosphere detection.

Dou Xiankang's team also realized wind measurement lidar based on superconducting nanowire single-photon detector for the first time in the world, obtaining the highest precision wind field detection with a spatial resolution of 10 meters and a time resolution of 10 seconds.

Editor-in-Charge: Li Yuequn

Proofreader: Liu Wei