Remote sensing of atmospheric temperature has become a central topic in meteorology, in the wind energy industry, and in the atmospheric sciences. Most devices currently employed in atmospheric research are unable to measure temperature in the atmospheric boundary layer comprising the lowest 3 km of the atmosphere with sufficient accuracy and vertical resolution. Rotational Raman lidar, can provide accurate, high vertical resolution measurements of temperature, but the technological approach is hindered by being costly, bulky and high maintenance.
Continuous high-resolution observation of atmospheric thermodynamic variables, such as temperature, wind and water vapor, in the lower troposphere is crucial for improved weather forecasting at the mesoscale. Wind energy turbines are highly sensitive to atmospheric temperature profiles, at heights beyond 200 m where wind shear, wind veer, and turbulence induce stratification. Horizontal variations of temperature also influence the flow characterization on a varying terrain. To obtain the horizontal variations, a network of temperature profilers is required, such as but not limited to lidars. These profilers must be relatively small, low-cost, eye-safe, and reliable enough to run continuously for years without requiring maintenance.
Lidars can provide high spatial and temporal resolution monitoring of thermodynamic variables in the atmosphere; the majority of such lidars benefit from the direct detection principle. In these systems sub-micron wavelengths are employed to take advantage of a stronger Rayleigh backscatter, where β∝λ−4 (β is the molecular volume scattering coefficient and λ is the wavelength). The transceivers in these systems can also be costly and complicated to design and implement. Lidar systems operating between 400 nm and 1400 nm wavelength region have limiting eye-safety requirements. Eye-safety requirements of systems operating above 1400 nm are much more relaxed and allow more compact and lightweight telescopes and transmitters to be employed.
Currently in the art, rotational Raman lidar, which relies on the presence of presumably well-mixed atmospheric nitrogen molecules, is the only reliable active lidar for high resolution remote sensing of atmospheric temperature. The system is hindered by being costly, bulky and requires high maintenance due to the need for powerful transmitters. Another lidar is differential absorption lidar (DIAL) which can employ temperature sensitive absorption lines of oxygen molecules to measure the temperature. This system suffers from a significat bias without correction from the simultaneous presence of Mie and molecular scattering in the backscatter signal.
Another lidar technique to measure atmospheric temperature uses the temperature dependent width of the Rayleigh molecular backscatter spectrum to derive temperature. The technique requires an optical filter, such as a strongly absorbing atomic gas, to remove the spurious Mie backscatter component. This approach is difficult and has never been successfully demonstrated as a viable option.
Another method for remote sensing of atmospheric temperature measures differential absorption of water vapor and includes measurement of a temperature-sensitive absorption line. This approach relies on the presence of water vapor in the lower troposphere. The method may employ three lines with close proximity where one line corresponds to an offline wavelength for the purposes of calibration and the other two correspond to appropriate water vapor absorption lines where the difference between the ground state energies is significant. Overlap of the appropriate lines with absorption lines from any other gas species will cause interference in the measurement and may cause large measurement errors. The absorption lines should be in the optical spectrum where appropriate laser sources are available.
A widely tunable laser source, an optical parametric oscillator (OPO), in combination with a direct detection principle may be used to measure the atmospheric temperature by relying on three appropriate water vapor lines in 1.7 μm portion of the optical spectrum where the presence of interfering gas species is insignificant. In the past, the shortcomings of this approach included, but are not limited to, the inability to fine tune the laser source, slow laser tuning time resulting in the de-correlation of the backscatter signal, and the slow time response of the liquid-nitrogen-cooled InSb photodetectors limiting the range resolution in a range-gated system. Furthermore, the InSb detectors exhibit excess noise and low quantum efficiency. This methodology has been reported to be only capable of path-integrated measurements of temperature, and therefore do not provide the needed high (vertical) range resolutions.
This present application takes a new approach to measuring atmospheric temperature using three appropriate water vapor lines 1.7 μm, by combining low-cost seed lasers, an optimized Thulium-doped fiber amplifier (TDFA), and coherent detection methods. TDFAs have been extensively used in the vicinity of 1900 nm band. By optimizing the TDFA, it is possible to build amplifiers operating around 1770 nm, and provide a reliable high-resolution range-resolved lidar for remote sensing of atmospheric temperature, as well as measure water vapor, and wind simultaneously. The present application provides a way to build a low-cost, low-maintenance temperature profiler which would be suitable for deployment in a network.