William B. Cook

Multiplex Fabry–Perot interferometer: I. Theory

The Multiplex Fabry-Perot Interferometer (MFPI) is a unique instrument, incorporating the wide spectral-bandwidth capability of the Michelson interferometer with the small size and high resolution of the Fabry-Perot interferometer. The MFPI is, structurally, a standard Fabry-Perot in which the scanning distance is allowed to be very large, of the order of centimeters. The signal recorded through this distance is Fourier transformed as would be the interferogram produced by a Michelson interferometer. The result is a spectrum containing very high-resolution information over a moderately large optical bandwidth. The MFPI is much smaller than a Michelson producing the same resolution and covers a much broader bandwidth than a Fabry-Perot used in the usual fashion. We present a basic description of the operating theory for the MFPI in terms familiar to the Michelson spectroscopist.

Multiplex Fabry-Perot interferometer: II. Laboratory prototype.

The Multiplex Fabry-Perot interferometer (MFPI) consists of a Fabry-Perot interferometer in which the étalon plate separation is changed over a large optical distance. Fourier transformation of the resultant interferogram allows one to treat the multiple reflections within the étalon cavity in a manner analogous to an array of Michelson-type interferometers. However, the scan distance required by the MFPI is much less than for a comparable Michelson. The design and construction of the MFPI are described. Solar absorption spectra measured with this instrument are compared with results from the FASCODE atmospheric model.

Fabry-Perot interferometer for geostationary-based observations of tropospheric ozone

Monitoring tropospheric chemistry from space is the next frontier for advancing present-day remote sensing capabilities to meet future high-priority atmospheric science measurement needs. Paramount to these measurement requirements is that for tropospheric ozone, one of the most important gas-phase trace constituents in the lower atmosphere. Such space-based observations of tropospheric trace species are challenged by the need for sufficient horizontal resolution to identify constituent spatial distribution inhomogeneities (that result from non-uniform sources/sinks and atmospheric transport) and the need for adequate temporal resolution to resolve daytime and diurnal variations. Both of these requirements can be fulfilled from a geostationary Earth orbit (GEO) measurement configuration. An advanced atmospheric remote sensing concept for the measurement of tropospheric ozone from a GEO-based platform is presented. The concept is centered about an imaging Fabry-Perot interferometer (FPI) observing a narrow spectral interval within the strong 9.6 micron ozone infrared band with a spectral resolution approximately 0.07 cm-1. This concept could also simplify other atmospheric chemistry sensor designs (which typically require spectral resolutions in the range of 0.01 - 0.1 cm-1), since such an FPI approach could be implemented for those spectral bands requiring the highest spectral resolution and thus simplify overall design complexity.

Tropospheric ozone sensing Fabry-Perot interferometer: II. Recent laboratory advancements

Space-based observation of tropospheric pollution has been identified as an important measurement to be included in Earth science missions of the 21st century. This presentation will summarize on-going efforts focused on enabling such a new capability, a high-priority atmospheric science mission for the measurement of tropospheric ozone from a space-based platform, through the implementation of Fabry-Perot interferometry. The measurement technique involves a double-etalon series configuration FPI along with an ultra-narrow bandpass filter to achieve single-order operation with an overall spectral resolution of approximately .068 cm-1, sampling a narrow spectral region within the strong 9.6 micrometers ozone infrared band form a nadir-viewing satellite configuration. Current research efforts are focusing on technology development and demonstration activities to address technology drivers associated with this measurement concept. To this end we have developed a small-scale, modular, double-etalon prototype FPI for laboratory characterization and testing, modelled the instrument optical configuration, and performed R and D associated with an etalon optical control scheme. This presentation will cover advancements pertaining to all aspects of this effort, however, emphasis will be placed on integration and testing activities associated with the laboratory prototype FPI. This will include multichannel operation considerations pertaining to different configurations for spectral tuning. In addition, implications associated with extrapolation toward a full- scale flight instrument design will also be addressed.

New design of a Multi-Order Etalon Sounder for measuring stratospheric temperature profiles

Sounding the stratosphere using the 4.3 micrometers CO2 lines requires observations where radiance contributions from the very thin upper atmosphere form a significant part of the measured signal. In order to properly account for the influence of these extremely narrow emission features in the temperature retrievals, extremely high spectral resolution is required. The multi-order etalon sounder (MOES) is an instrument which capitalizes on the extremely high spectral resolution achievable with a standard Fabry-Perot etalon and on the regular spacing of the lines in portions of the vibration-rotation spectrum of gases such as CO2. The MOES instrument simultaneously measures several identically shaped CO2 lines when its etalon spacing is set to produce a free spectral range equal to the uniform line intervals in the spectrum. By combining the signals from a large number of lines a MOES sensor greatly improves the signal-to-noise ratio without reducing its inherent spectral resolution. A detailed design for the most recent MOES concept is presented along with its expected/simulated performance parameters.

Prototype broadband/high-resolution spectrometer designed for the study of planetary atmospheres

Optical remote sensing plays an important role in the study of planetary atmospheres, especially in determining trace gas abundance, temperature profiles and dynamics/winds. Instruments to be flown aboard interplanetary platforms must be small, have low mass and consume little power. Fabry- Perot interferometers (FPI) satisfy these physical constraints and are capable of acquiring spectra suitable for analysis of the atmospheric parameters. Two new applications of FPI technology have recently been developed at UM/SPRL: the multiplex Fabry-Perot interferometer (MFPI) and the multi-order etalon spectrometer (MOES). The MFPI produces a broad bandwidth high resolution spectrum via Fourier transformed interferograms produced by scanning the etalon over large distances. The MOES simultaneously measures several similar lines in a regular spectrum by matching its free spectral range to the line spacing. Thus MFPI provides a means for broadening the usable bandwidth and MOES can record improved signal-to-noise spectra at extremely high resolution. This paper reports recent progress in the design, construction and testing of a prototype instrument incorporating both the MFPI and the MOES concepts using a single set of etalon plates.

Retrieval of stratospheric temperature profiles in the presence of non-LTE radiance contributions

A sounder using a high-finesse Fabry-Perot etalon offers substantial potential to extract temperature profiles from the stratosphere atmosphere. The multi-order etalon sounder (MOES) is such an instrument. Its extremely high spectral resolution makes it possible to selectively observe emission at the very line centers and near shoulders of individual CO2 lines. However, the radiances at the centers of these lines can contain large non-LTE contributions originating at much higher altitudes. At high altitudes, the non-equilibrium absorption and re-radiation of solar illumination enhances the vibrational temperature compared to the kinetic temperature. The kinetic temperature profile can only be estimated from the complex line radiance spectrum by relating the observed vibrational temperature to the probable kinetic temperature. The effective separation of layer contributions requires that the instrument be designed so that (1) its dynamic range preserves the much larger range of expected radiances and (2) its spectral response function is very well known so that the line wing radiances can be accurately determined in the presence of large line center radiances. This paper discusses the retrieval of stratospheric temperatures in terms of typical measured radiance covariances, the required solar illumination model and a sensor suitable for routine temperature profile retrieval.

Atmospheric spectra from a multiplex Fabry-Perot interferometer

Remote sensing of major and minor constituents in the earths atmosphere is of great importance to the study of climate and global change. Because much of remote sensing involves placing instrumentation in environments that are not easily accessible, such as balloons, spacecraft, or remote field stations, it is usually necessary that the instrumentation be compact, lightweight, and rugged. This paper describes the development of a new type of remote sensing instrument we have chosen to call the multiplex Fabry-Perot interferometer (MFPI). We present atmospheric spectra obtained with our working prototype instrument. The MFPI is a Fabry-Perot interferometer for which the etalon plate separation is changed over a large optical distance during a measurement. When the resulting interferogram is Fourier transformed the multiple reflections within the etalon cavity produce a spectrum analogous to that which would be produced by an array of Michelson interferometers. However, for high spectral resolution measurements the scan distance required by the MFPI is much less than for the comparable Michelson. The MFPI will be ideal for remote sensing applications where weight, size, and mechanical reliability are primary considerations.