Precision Heterodyne Oxygen-Calibration Spectrometry: Development and Initial Deployment Tests

We describe the development and initial tests of a new laser heterodyne radiometry (LHR) technique: Precision Heterodyne Oxygen-Calibration Spectrometry, or PHOCS. Atmospheric gas analysis using solar occultation with heterodyne detection was pioneered at the Jet Propulsion Laboratory (JPL) beginning in the 1970s. Incoming light is combined with light from a narrow-band laser source (the local oscillator or LO) on a photodetector. The detector output will contain AC electronic signals at the (optical) difference frequencies. PHOCS heterodyne signals are proportional to the solar spectrum at the LO wavelength. When the LO coincides with an optical absorbance, the heterodyne signal intensity will drop by an amount proportional to the absorbance.

PHOCS builds on earlier work by using continuously tunable, near-infrared diode lasers as heterodyne local oscillators, conventional fiber optics, and low-noise room temperature detectors. In addition, PHOCS includes recently developed rf power sensors that have -70 dBm noise floors and that are USB-powered and readout directly through a USB interface. The prototype instrument is equipped with two active laser channels for oxygen (measured in the a ¹Δ_g band near 1.27 mm) and carbon dioxide (a portion of the 30012 ← 00001 vibrational transition near 1.57 mm) determinations. The latter may be substituted by a heterodyne receiver module equipped with a laser to monitor atmospheric methane near 1.65 mm. Oxygen measurements provide dry gas corrections and - more importantly - determine accurate temperature profiles that improve the precision of the CO₂ column retrievals. PHOCS complements results from the Orbiting Carbon Observatory (OCO-2), Active Sensing of CO₂ Emissions over Nights, Days, and Seasons (ASCENDS), and ground-based Fourier transform spectrometers.

The presentation will describe the development of the instrument by Mesa Photonics and will present the results of initial field tests in New Mexico and in the vicinity of Washington, DC during early 2018. [Note: this abstract was drawn largely from material in the Final Report of NASA Contract NNZX14CP06C.]