Physicist Zheng-Ping Li and his colleagues at the China University of Science and Technology in Shanghai have developed a system for photographing at distances of tens of kilometers, even in urban environments. The urban environment is very unfavorable for such work - there are many parasitic sources of light, evaporation, etc. The presence of moving objects and environmental pollution also leave their mark - but it is precisely in cities that a large-scale monitoring system is needed at long distances.
Li's setup is based on an innovative photonic lidar, a detector capable of capturing a single photon. For example, as part of an impulse reflected from a target to measure the distance based on the return time. The device is sensitive enough to see objects at a distance of 10 km, but Lee's team went further and assembled a device based on the telescope that sends and receives photon beams at a distance of 20, and at the limit of possibilities, 45 km.
The system uses an infrared laser with a wavelength of 1550 nanometers, a frequency of 100 kilohertz and a power of only 120 milliwatts. It is safe for the eyes and allows you to experiment in urban areas. The main merit of the Chinese scientists is in the development of the gating algorithm: they have learned to calculate the "time window" of the return of photons from the target and, based on this data, filter out all other parasitic photons. Including sunlight, which poses a major problem in such observations. As a result, where conventional optics show solid noise, the features of the object begin to appear.
The accuracy of the camera is amazing - when filmed from the roof of a 29-story skyscraper on Chongming Island, Shanghai, testers accurately counted the number of windows in the airport building in Pudong, about 45 km away. And, in addition, we improved the resolution of the system from the previous minimum of 1 m to 60 cm. And all this with the help of equipment that fits into a container the size of a shoebox! Now Lee and his colleagues intend to overcome the bar of 100 km, as well as improve the algorithms - if you correctly vary the strobing, you can display the measurement results immediately in the form of three-dimensional models.