Atmospheric boundary layer sensing using ultra-wideband photonic microwave spectrometer

The Atmospheric Boundary Layer (ABL) is the portion of the troposphere that is directly influenced by the Earth’s surface and responds to combined action of mechanical and thermal forcing. Most of the energy exchange with respect to solar heating and evaporation that drive the atmosphere and the ocean occur within the ABL, yet it is one of the most poorly observed and modeled regions of the atmosphere. Conventional passive microwave systems fall well short of being optimized for near surface sensing due to limited number of spectral channels and coarse spectral resolution covering only a small portion of the spectrum of interest for ABL sensing.

 

The so called “window regions” of the microwave spectrum between and on the shoulders of the strong oxygen and water vapor absorption lines carry the information on the near surface thermodynamic structure in the boundary layer. Sampling these regions requires new spectrometers capable of resolving >50 GHz spectral regions at modest spectral resolution (~1GHz). The ultra-wideband photonic spectro-radiometer instrument is funded by NASA ESTO under ACT-20 program to combine low-noise wideband RF technology with a novel photonic integrated circuit (PIC) design for obtaining large bandwidth (>50 GHz) with enhanced channel resolution (<1 GHz). A high-speed, low-loss electro-optic modulator is used to convert radio frequency energy into sidebands on an optical carrier, preserving both amplitude and phase of the radiometric signal. The designed PIC includes an input star-coupler that divides the optical power transmitted from the optical modulator among N waveguides monotonically increasing in length within an arrayed waveguide grating (AWG) that provides chromatic dispersion, an output star-coupler that forms an image of the optical spectrum, and an array of photodiodes that convert the optical power to electrical signals. The ultra-wideband 50 GHz direct acquisition spectrometer capability has been successfully tested and validated on the fabricated PIC. The combination of a low-noise wide-band RF radiometer with an RF Photonics backend system is a key technology development allowing unprecedented ability to spectrally resolve the complete microwave spectrum which is critically needed for the planetary boundary layer sensing. In this paper, we will describe the capabilities of this system for measuring the thermodynamic structure in the lower ~2km of the atmosphere.