Mingjia Shangguan and Haiyun Xia from the University of Science and Technology of China, part of a research team that developed a new Doppler lidar system for wind measurement. [Image: Quantum Lidar Laboratory/USTC]
The large, destructive hurricanes pounding the Caribbean and southeastern United States have increased concerns about the increasing frequency of damaging storms that’s cited as one outcome of climate change. Researchers in China have now unveiled an improved optical tool for remotely keeping tabs on one especially baleful aspect of those storms—their winds (Opt. Lett., doi: 10.1364/OL.42.003541). The Chinese team’s lidar-based tool could not only improve the stability and usefulness of air- and space-based wind measurement for dramatic events like hurricanes, but could also lead to a better data stream for applications like airport safety and pollution monitoring.
Lidar for wind
Lidar—already eminently familiar to OPN’s readers for its role in everything from autonomous vehicles to airborne terrain mapping to archeological investigation—works by measuring the time between the firing of a laser pulse and the sensing of its reflection from the landscape or target. For the measurement of wind speed, lidar depends on the beam’s interaction not with a solid target, but with molecules such as aerosols in the atmosphere; the movement of these molecules by wind causes a Doppler shift in the received signal that can be detected and converted to wind velocity.
Multiple measurements, at a sufficiently fine spatial and time resolution, allow compilation of a 3-D vector field that permits assessment not only of wind speed but of other parameters like wind shear and turbulence. Getting to a stable lidar system for measuring wind, however, has proved difficult. In part that’s because of the finicky nature of the free-space Fabry-Pérot interferometers (FPIs) used in some setups, which have trouble operating robustly in harsh environments. Another problem has been the use of conventional InGaAs avalanche photodiodes for detection of the signal; these instruments, it turns out, are plagued by low efficiency and high noise, which undermines their use in sensitive data streams such as wispy lidar reflections from moving atmospheric gases.
Improving stability
To get around those problems, the research team, led by Xiankang Dou of the University of Science and Technology of China, started out by focusing on a system that operated at telecom wavelengths (specifically 1.5 microns). That enabled the researchers to use off-the-shelf fiber optic components to dramatically simplify to setup relative to existing wind-speed lidars.
In particular, rather than using a two-channel FPI to retrieve the Doppler shift, the team implemented a much more stable scheme involving a dual-frequency laser and a single-channel, fiber-based FPI. The simplification, according to the researchers, helps to reduce the system’s vulnerability to a host of variables—including angle of incidence, instrument alignment, temperature gradients and pressure fluctuations.
The researchers also improved things at the detector side, by replacing the conventional avalanche photodiode with a superconducting nanowire single-photon detector (SNSPD). The high quantum efficiency, low-noise characteristics and high maximum count rate of the SNSPD boosts the signal to noise, according to the team, and avoids many of the photodiode’s pitfalls in high-speed operation.
Space-based lidar for wind
In ground-based tests of the system, the researchers were able to develop wind profiles to an altitude of 2.7 km, with a time resolution of 10 seconds and a spatial resolution of around 10 meters. And the lidar measurement varied by only around 0.1 meters per second (0.36 kilometers per hour) in speed and a degree in direction relative to measurements by a fixed ultrasonic wind sensor.
The research team is working on improving the instrument to improve its robustness and sensitivity still further, and has formed a company that aims to commercialize the approach as early as next year. Eventually, according to coauthor Haiyun Xia, who leads USTC’s Quantum Lidar Laboratory, the system might find its way not only into aircraft-based wind measurement, but even into satellites. “Space-based Doppler wind lidar,” Xia noted in a press release, “is now regarded as the most promising way to meet the need for global wind data requirements, and to fill gaps in wind data provided by other methods.”