A remote sensing instrument based on light detection and ranging (LIDAR) has been recently developed by Researchers. This instrument could provide a robust and simple way to accurately measure wind speed.
The comprehensive, real-time wind measurements could help Scientists gain better insight of how hurricanes form and deliver information that can be used by Meteorologists to identify landfall earlier, thus giving people sufficient time to prepare and evacuate.
As hurricane Harvey approached the U.S., hurricane hunters flew directly into the storm and dropped sensors to measure wind speed. Our Doppler LIDAR instrument can be used from a plane to remotely measure a hurricane’s wind with high spatial and temporal resolutions. In the future, it could even make these measurements from aboard satellites.
Xiankang Dou, Leader of the research team, the University of Science and Technology of China (USTC)
For a number of applications such as the efficient operation of wind turbines, knowledge about the movement of pollution through the air and the determination of safe flying conditions, wind measurements play a critical role. The high-accuracy wind measurement technologies which are already present can be expensive and hard to operate, resulting in gaps in the application of these technologies in conditions where they are most helpful.
“We demonstrated a Doppler wind LIDAR with a simplified optical layout that also substantially enhances the system stability,” said Dou. “Although specialists are typically needed to operate and maintain a sophisticated Doppler LIDAR, we are confident we can develop our approach into a system that will be as easy to use as a smartphone.”
In The Optical Society (OSA) journal Optics Letters, the Researchers showed that the Doppler wind LIDAR system stayed stable throughout a 10 day test period and that it was capable of measuring horizontal wind speed with high accuracy. Compared to previously developed direct detection Doppler wind LIDARs, the Researchers believe that the accuracy and stability of this new system represents a significant improvement.
LIDAR finds its major application in the field of aeronautics, where it can be used from a ground station or on aircrafts to remotely measure air motion. The new system has a vertical spatial resolution of 10 m, and could thus measure small-scale wind phenomena such as wind shear and the wake turbulence caused by an aircraft. In addition to enhancing flight safety, a better understanding of these phenomena could also increase airport capacity by optimizing the separation between aircraft during landing and takeoff.
Using Light to Measure Wind
LIDAR can be defined as a remote sensing technique that has been employed to guide driverless cars, scan the bottom of the ocean floor and create high-resolution maps. In order to measure the wind, a LIDAR system sends out a laser pulse that propagates through the atmosphere where it interacts with aerosols and molecules. A small amount of the light is scattered back towards the LIDAR instrument, and is collected by a telescope. When the air moves because of the wind, it results in a Doppler shift that is capable of being detected by the device.
The Researchers specifically developed a dual frequency direct detection Doppler wind LIDAR that employed a laser emitting 1.5 micron light. As this wavelength is usually used in optical communications networks, it was possible to build the system using fiber-optic components that were commercially available, each integrating a number of light-controlling components into a single device. The all fiber construction of the LiDAR system is thus robust against rough operation handling and vibrations.
Compared to the systems that were developed earlier, the latest simplified design makes it much easier to configure and align each component, reduces the amount of light lost inside the system and increases stability. No calibration is required for this new system after it is initialized and also it does not require any special eye protection.
For LIDAR systems that will be operated full-time in the field, eye safety is an important consideration. Fortunately, the 1.5-micron laser we used exhibits the highest permissible exposure for eye safety in the wavelength range from 0.3 to 10 microns.
Haiyun Xia, Principle Investigator, the Quantum Lidar Laboratory, USTC
The 1.5 micron wavelength is also suitable for atmospheric wind sensing from satellites. This is because, when compared to visible and UV wavelengths, it shows minimal susceptibility to optical contamination and atmospheric disturbance from the sun and other sources. Satellite-based wind measurements are employed for meteorological studies and weather forecasts.
Space-borne Doppler wind LIDAR is now regarded as the most promising way to meet the need for global wind data requirements and to fill gaps in the wind data provided by other methods.
Haiyun Xia, Principle Investigator, the Quantum Lidar Laboratory, USTC
Upgraded Optical Components
The optical setup for the latest Doppler wind LIDAR consists of just one detector, one laser source and a single-channel Fabry-Perot interferometer that transforms the Doppler shift into photon number variations of the backscatter signals. As a Fabry-Perot interferometer made of optical fibers was used instead of the one that contains a large number of individual optical components, the system became stable and robust enough to be used in harsh environments such as aboard satellites or aircraft.
The new system also comprises of a superconducting nanowire single photon detector (SNSPD) which is one of the fastest detectors available for single photon counting. This detector enhanced the LIDAR’s performance compared to the InGaAs avalanche photodiodes usually used to detect 1.5 micron light.
“The high detection efficiency and low dark count rate of the SNSPD means that the weak signal from the backscattered light can be detected with a high signal-to-noise ratio,” said Xia. “Another attractive feature of the SNSPD is its high maximum count rate, which helps avoid detector saturation.”
The Researchers tested their system by primarily examining its stability after calibration. In general, the system’s measurements varied by less than 0.2 m per second over 10 days in the lab. The system was then tested outdoors and the Researchers compared its horizontal wind measurements with measurements from an ultrasonic wind sensor, a non-remote system for measuring wind.
Approximately, the LIDAR measurements were within 1 degree and 0.1 m per second for wind direction and speed, respectively.
Currently, the Researchers are working to improve the spatial resolution of the Doppler wind LIDAR system and wish to make it further practical to use in the field. Additionally, they have established a company to develop the system even more and have planned a commercial version which would be available from next year.
The Optical Society- http://www.osa.org/en-us/about_osa/newsroom/news_releases/2017/light-based_method_improves_practicality_and_quali/