OH Detection Using Gas Correlation Radiometry

The OH Gas Detector combines the properties of Gas Correlation Radiometry (GCR) with Broad Band Cavity Enhanced Absorption Spectroscopy (BBCEAS) to measure the absorption of OH remotely. GCR utilizes a precise filter with a high spectral resolution that is matched to the absorption features of a specific gas. BBCEAS measures light intensity through a stable optical cavity. OH gas absorbs strongly at specific wavelengths in ultraviolet light. Using the sun as a light source, modulated sunlight is split into two, independent gas cells. One gas cell contains a vacuum and the other gas cell contains OH gas. The OH Gas Detector uses the gas cell containing OH gas as a precise bandpass filter for GCR. Due to the short lifespan and reactivity of OH gas, OH gas must be actively generated to use as a reference and only a limited amount can be generated. BBCEAS is used to increase the path length within the gas cell for a measurement to be possible using the small amount of OH gas present within the gas cell. The OH Gas Detector measures the absorption rate of each gas cell, which is ratioed to determine the remote level of OH absorption.

This technique takes advantage of Ozone's strong absorption at specific wavelengths in the UltraViolet (UV). To take measurements, UV light is projected through a cell bounded by highly reflective mirrors. These mirrors increase the path length of the cell by inducing the light to bounce back and forth within the cell, increasing the likelihood of Ozone absorption within the cell. Finally, an absorption measurement is taken via a detector outside the cell and opposite of the UV source. This innovation produces an ozone detection precision of 0.12 parts per billion (ppb) in 0.1 second integration. This corresponds to 0.012 ppb in ten seconds integration.

This NASA invention is a highly compact and sensitive 530-600 GHz, 1x8 receiver array employing a multi-pixel approach to enhance simultaneous detection capabilities. The receiver has a conversion loss of 1 THz and the center frequency can be tuned by adjusting design parameters. While NASA originally developed this receiver to enable miniaturized, low power consumption, high sensitivity heterodyne-based submillimeter wave spectrometers for small satellite-based planetary atmospheric sensing, potential applications of the novel receiver are broad. The multi-pixel, wideband receiver can be used in spectrometer and radar systems for applications including astronomy, plasma fusion, military, and emerging communication technologies such as 5G and 6G. The invention is available for patent licensing.

The novel mixer offers wideband and sub-harmonic conversion capabilities for enhanced signal processing across a broad frequency range. The mixer operates at 470-600 GHz and includes a LO waveguide to allow 265-300 GHz input signal and a radio frequency (RF) waveguide for the 470-600 GHz operation. The LO and RF signal multiply and down-convert the RF signal to an IF signal to a much lower frequencies for further digitization. The mixer is designed on a gold and quartz substrate for a lower dielectric constant. The filter design uses a triangular patch resonator-based low-pass filter to reduce the size of the mixer as well as isolates the LO signal and the wide IF signal. Additionally, an IF filter, RF filter, Schottky diode, LO, and RF probes are integrated into a single chip to further reduce the dimensions of the mixer. The invention also leverages an antiparallel diode orientation, where the LO frequency is half of the RF input. This LO signal is amplified and multiplied up to 265-300 GHz to provide an input power of 3-5 mW to pump the antiparallel mixer. The technology offers significant advantages in remote sensing and high-speed communications, enabling simultaneous detection of multiple molecular species and enhancing the efficiency of submillimeter-wave heterodyne spectrometers. The wideband functionality achieves high data rates required in emerging 6G networks and offers exceptional sensitivity, with prototype tests showing a conversion loss below 12 dB and noise temperatures under 4000 K at 470 GHz. The integration of components such as filters and diodes into a single chip reduces system size and complexity, contrasting with traditional multi-chip setups. The design is scalable across frequencies from 1 GHz-1 THz with minimal modifications, with the system's form factor inversely scaling with frequency. These features make the technology versatile for applications in environmental monitoring, planetary exploration, radar systems, and advanced communication systems.

Conventional ambient-temperature oxygen sensors are limited in various ways: optically based sensors can be expensive and challenging to manufacture; electrochemical cells with liquid electrolytes can have limited lifetimes and become leak sources; and both types of sensors are difficult to miniaturize. These problems are addressed with Glenn's novel ambient temperature oxygen microsensor, which is based on a Nafiontm polymer electrolyte, microfabricated using thin-film technologies. In the past, one drawback of Nafiontm film has been that it can lose conductivity when the moisture content in the film is too low, potentially affecting sensor operation. Glenn researchers devised a method to use certain salts to hold water molecules in the Nafiontm film structure at room temperature. The presence of these salts provides extra sites in the film to promote proton (H+) mobility, thus improving film conductivity and overall sensor performance, particularly in arid and high-temperature environments. The innovative use of metal/metal oxide as the reference electrode enables miniaturization by eliminating the reference gas and sealing the reference electrode. The combination of interdigitized electrodes with the unique metal/metal oxide reference electrode permits sensor operation in either potentiometric or amperometric mode, as appropriate. In potentiometric mode, which measures voltage differences between working and reference electrodes in different gases, the voltage differences can be monitored with a voltmeter; however, the sensor itself does not need a power source. In room-temperature testing, the sensor achieved repeatable responses to 21 percent oxygen in nitrogen (using nitrogen as a baseline gas), and also detected oxygen from 7 to 21 percent, making Glenn's breakthrough technology usable for personal health monitoring as well as fire detection, fuel-leak detection, and environmental monitoring.