Elena M. Georgieva

Development of a Fabry?Perot interferometer for ultra-precise measurements of column CO2

Progress on a passive Fabry?Perot-based instrument for detecting column CO2 through absorption measurements at 1.58 ?m is described. In this design, solar flux reaches the instrument platform and is directed through two channels. In the first channel, transmittance fringes from a Fabry?Perot interferometer are aligned with CO2 absorption lines so that absorption due to CO2 is primarily detected. The second channel encompasses the same frequency region as the first, but is comparatively more sensitive to changes in the solar flux than absorption due to CO2. The ratio of these channels is sensitive to changes in the total CO2 column, but not to changes in solar flux. This inexpensive instrument will offer high precision measurements (error <1%) in a compact package. Design of this instrument and preliminary ground-based measurements of column CO2 are presented here as well as strategies for deployment on aircraft and satellite platforms.

Total column oxygen detection using a Fabry-Perot interferometer

A passive instrument based on a Fabry-Perot interferometer was designed and used for oxygen atmospheric column absorption measurements. The instrument operates in the oxygen A-band spectral region from 759 to 771 nm. Surface solar irradiation reflected off the Earth is detected using two channels at two wavelengths—one for pressure sensing and the other for temperature sensing. Each channel of the O2 column measurement system consists of two subchannels—Fabry-Perot and reference. Solid Fabry-Perot etalons were designed and used to confine the response to the O2 absorption bands. The etalons have free spectral ranges of 0.575, 0.802, and 2.212 nm. Two narrow bandpass filters (760 to 764 and 767 to 771 nm) were also used. The instrument is sensitive to changes in oxygen column as small as 0.88 mbar for ground-based experiments and 5 mbar for airborne measurements. The major advantages of the optical setup are its compactness, high sensitivity, high signal-to-noise ratio, and stability for both ground and airborne experiments.

Differential Radiometers Using Fabry–Perot Interferometric Technique for Remote Sensing of Greenhouse Gases

A new type of remote-sensing radiometer based upon the Fabry-Perot (FP) interferometric technique has been developed at NASAs Goddard Space Flight Center and tested from both ground and aircraft platforms. The sensor uses direct or reflected sunlight and has channels for measuring the column concentration of carbon dioxide at 1570 nm, oxygen lines sensitive to pressure and temperature at 762 and 768 nm, and water vapor (940 nm). A solid FP etalon is used as a tunable narrow bandpass filter to restrict the measurement to the gas of interests absorption bands. By adjusting the temperature of the etalon, which changes the index of refraction of its material, the transmission fringes can be brought into nearly exact correspondence with the absorption lines of the particular species. With this alignment between absorption lines and fringes, changes in the amount of a species in the atmosphere strongly affect the amount of light transmitted by the etalon and can be related to gas concentration. The technique is applicable to different chemical species. We have performed simulations and instrument design studies for CH4, 13CO2 isotope, and CO detection.

PRECISION MEASUREMENT OF ATMOSPHERIC TRACE CONSTITUENTS USING A COMPACT FABRY-PEROT RADIOMETER

To address the problem of sources and sinks of atmospheric CO2, measurements are needed on a global scale. Satellite instruments show promise, but typically measure the total column. Since sources and sinks at the surface represent a small perturbation to the total column, a precision of better than 1% is required. No species has ever been measured from space at this level. Over the last three years, we have developed a small instrument based upon a Fabry-Perot interferometer that is highly sensitive to atmospheric CO2. We have tested this instrument in a ground based configuration and from aircraft platforms simulating operation from a satellite. The instrument is characterized by high signal to noise ratio, fast response and great specificity. We have performed simulations and instrument designs for systems to detect, H2O, CO, 13CO2, CH4, CH2O, NH3, SO2, N2O, NO2, and O3. The high resolution and throughput, and small size of this instrument make it adaptable to many other atmospheric species. We present results and discuss ways this instrument can be used for ground, aircraft or space based surveillance and the detection of pollutants, toxics and industrial effluents in a variety of scenarios including battlefields, industrial monitoring, or pollution transport.

Experimental data on CO2 detection using Fabry-Perot-based optical setup for atmospheric observations

The experimental data on CO2 detection in atmosphere using Fabry-Perot technique are presented. The optical setup consists of two channels. Channel one measure the total reflected light, whereas channel two uses a solid substrate Fabry-Perot etalon to restrict measurement to light in CO2 absorption bands. The free spectral range of the etalon is calculated to be equal to the almost regular spacing between the CO2 spectral bands located near 1.58 μm, where CO2 absorption is significant. The ratio of the intensities detected by the two channels is then sensitive to CO2 change. We are exploring the temperature dependence of the index of refraction of the optical media to align the pass bands of the Fabry-Perot etalon to the appropriate CO2 absorption lines. The experimental data presented show excellent agreement with our theoretical expectations. They are recorded at different gas pressures and temperatures. Some of the major advantages of the optical setup are its compactness, high sensitivity, high signal-to-noise ratio, and stability.

A hollow-waveguide gas correlation radiometer for ultra-precise column measurements of formaldehyde on Mars

We present preliminary results in the development of a miniaturized gas correlation radiometer that implements a hollow-core optical fiber (hollow-waveguide) gas correlation cell. The substantial reduction in mass and volume of the gas correlation cell makes this technology appropriate for an orbital mission?capable of pinpointing sources of trace gases in the Martian atmosphere. Here, we demonstrate a formaldehyde (H2CO) sensor and report a detection limit equivalent to ~30 ppb in the Martian atmosphere. The relative simplicity of the technique allows it to be expanded to measure a range of atmospheric trace gases of interest on Mars such as methane (CH4), water vapor (H2O), deuterated water vapor (HDO), and methanol (CH3OH). Performance of a formaldehyde instrument in a Mars orbit has been simulated assuming a 3 m long, 1000 ?m inner diameter hollow-core fiber gas correlation cell, a 92.8? sun-synchronous orbit from 400 km with a horizontal sampling scale of 10 km ? 10 km. Initial results indicate that for 1 s of averaging, a detection limit of 1 ppb is possible.

Experimental data on O 2 absorption using Fabry-Perot-based optical setup for remote sensing atmospheric observations

The experimental data on O2 absorption using reflected sunlight and a passive Fabry-Perot technique are presented. The atmospheres irradiance measurements are an important tool for the remote sensing study. In this work we focus on the O2 A band (759-771 nm) composed of about 300 absorption lines, which vary in strength and width according to pressure and temperature. We performed measurements using solid Fabry-Perot etalons with different FSR and two different pre-filters. The first pre-filter selects a spectral range around 763 nm which is between the P and R branches, where the absorption coefficient is insensitive to temperature, but is sensitive to pressure changes and therefore to the variations in the O2 column. The second pre-filter is selecting several absorption bands between 765 and 770 nm, which are more sensitive to temperature changes. The optical setup consists of two channels. Channel one measure the total reflected light, whereas channel two uses a solid substrate Fabry-Perot etalon to restrict measurement to light in the O2 absorption bands. The ratio of the intensities detected by the two channels is sensitive to O2 pressure change or temperature change depending of the spectral region and is a function of the air mass, solar zenith angle and altitude. The experimental data presented shows excellent agreement with our theoretical expectations. They were recorded at different gas pressures and temperatures, and also at various weather conditions. The goal of the experiment is to demonstrate that variations of the column density of the O2 can be detected using a solid Fabry-Perot etalon. Results can be used for normalization of the other trace gases column densities, (to measure CO2 column density) because the Oxygen is well mixed throughout most of the atmosphere (to an altitude of about 100 km) and can help to interpret the influence of scattering from aerosol and clouds, polarization of the reflected light, and the reflection properties of the surface. Some of the major advantages of our optical setup are its compactness, high sensitivity, high signal-to-noise ratio and stability.

Ultra-precise measurement of CO2 from space

The experimental data on CO2 and O2 detection in atmosphere using Fabry-Perot technique are presented. The atmospheres irradiance measurements are an important tool for the remote sensing study. We show results from lab, ground and flight testing of a new instrument called FPICC (Fabry-Perot Interferometer for Column CO2) which is intended for a very precise measurements of atmospheric carbon dioxide and oxygen. The optical setup consists of three channels. The first channel is built to measure the carbon dioxide. This channel operates using the reflected sunlight off the ground and solid Fabry-Perot etalon to restrict the measurement to light in CO2 absorption bands. The free spectral range of the etalon is calculated to be equal to the almost regular spacing between the CO2 spectral bands located near 1,571 μm, R band, where CO2 absorption is significant. The precise alignment of the transmission peaks of the Fabry-Perot etalon to the CO2 absorption lines is achieved through altering the refractive index of the material (fused silica) using its temperature dependence. The second and third channels foucs on the O2 A band (759 - 771 nm) composed of about 300 absorption lines, which vary in strength and width according to pressure and temperature. We performed measurements using solid Fabry-Perot etalons with different FSR and two different pre-filters. The first pre-filter selects a spectral range around 762 nm which is between the P and R branches, where the absorption coefficient is insensitive to temperature, but is sensitive to pressure changes and therefore to the variations in the O2 column. The second pre-filter is selecting several absorption bands between 765 and 770 nm, which are more sensitive to temperature changes. The experimental data presented show excellent agreement with our theoretical expectations. They are recorded at different gas pressures, temperatures and different weather conditions. Some of the major advantages of the optical setup are its compactness, high sensitivity, high signal-to-noise ratio, and stability.

Robust IR Remote sensing technique of the total column of trace gases including carbon dioxide and methane

Progress on the development of a differential radiometer based upon the Fabry-Perot interferometer (FPI) for methane (CH4) and carbon dioxide (CO2) detection in the atmosphere is presented. Carbon dioxide and methane measurements are becoming increasingly important as a component of NASAs programs to understand the global carbon cycle and quantify the threat of global warming. Methane is the third most important greenhouse gas in the Earths radiation budget (after water vapor and carbon dioxide) and the second most important anthropogenic contributor to global warming. The importance of global warming and air quality to society caused the National Research Council to recommend that NASA develop the following three missions1,2 ASCENDS (Active Sensing of CO2 Emissions over Nights, Days, and Seasons), GEOCAPE (Geostationary Coastal and Air Pollution Events), and GACM (Global Atmosphere Composition Mission). The importance of CO2 and methane study is reflected in the successful operation of the Japanese Greenhouse gases Observing Satellite (GOSAT) monitoring those trace gases globally from orbit.3

New broadband lidar for greenhouse carbon dioxide gas sensing in the Earth's atmosphere

We present airborne measurements of a novel eye-safe spectrally broadband LIDAR capable of dealing with the atmospherically-induced variations in CO2 absorption using a Fabry-Perot based detector. The Fabry-Perot solid etalon in the receiver part is tuned to match the wavelength of several CO2 absorption lines simultaneously. The broadband technique tremendously reduces the requirement for source wavelength stability, instead putting this responsibility on the Fabry- Perot based receiver. The instrument technology we are developing has a clear pathway to space and realistic potential to become a robust, low risk space measurement system.