Polar cap mesosphere wind observations: comparisons of simultaneous measurements with a Fabry-Perot interferometer and a field-widened Michelson interferometer.

Polar cap mesospheric winds observed with a Fabry-Perot interferometer with a circle-to-line interferometer optical (FPI/CLIO) system have been compared with measurements from a field-widened Michelson interferometer optimized for E-region winds (ERWIN). Both instruments observed the Meinel OH emission emanating from the mesopause region (approximately 86 km) at Resolute Bay, Canada (74.9 degrees N, 94.9 degrees W). This is the first time, to our knowledge, that winds measured simultaneously from a ground-based Fabry-Perot interferometer and a ground-based Michelson interferometer have been compared at the same location. The FPI/CLIO and ERWIN instruments both have a capability for high temporal resolution (less than 10 min for a full scan in the four cardinal directions and the zenith). Statistical comparisons of hourly mean winds for both instruments by scatterplots show excellent agreement, indicating that the two optical techniques provide equivalent observations of mesopause winds. Small deviations in the measured wind can be ascribed to the different zenith angles used by the two instruments. The combined measurements illustrate the dominance of the 12-h wave in the mesopause winds at Resolute Bay, with additional evidence for strong gravity wave activity with much shorter periods (tens of minutes). Future operations of the two instruments will focus on observation of complementary emissions, providing a unique passive optical capability for the determination of neutral winds in the geomagnetic polar cap at various altitudes near the mesopause.

Global tropospheric and total ozone monitoring with a double-etalon fabry-perot interferometer. I. Instrument concept.

The global distribution of tropospheric ozone (O(3)) can be observed from a satellite-based instrument by spectrally isolating the pressure-broadened wings of strong O(3) lines. The Fabry-Perot interferometer (FPI) provides high spectral resolution and high-throughput capabilities that are essential for performing such a measurement. Through proper selection of channel spectral regions, the FPI optimized for tropospheric O(3) measurements can simultaneously observe a stratospheric component and thus the total O(3) column abundance. We present a conceptual instrument design that involves a double-etalon fixed-gap series configuration FPI along with an ultranarrow bandpass filter to achieve single-order operation with an overall spectral resolution of approximately 0.068 cm(-1), sampling the narrow 1054.2-1055.2 cm(-1) spectral region within the strong 9.6-mum ozone infrared band from a nadir-viewing satellite configuration.

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.

Performance of a circle-to-line optical system for a Fabry-Perot interferometer: a laboratory study.

We describe the characterization and laboratory study of a new optical device, a circle-to-line interferometer optical (CLIO) system, for a Fabry-Perot interferometer. The CLIO system converts a circular Fabry-Perot interferometer fringe pattern into a linear fringe pattern that can be analyzed by a linear array detector or a charge-coupled device. One can achieve the circle-to-line conversion by the use of a mirrored kaleidoscope and a 90-deg segment of a 45-deg half-angle internally reflecting cone. Our laboratory results are in good agreement with theoretical predictions. The aberrations associated with this optical system are minor for large F-number optical systems, especially for the cone segment. The finesse degradation caused by this CLIO system is relatively small, which agrees with the results of a ray-trace study.

Image plane detector spectrophotometer: construction and nightglow observations.

Following the theoretical simulation of an image plane detector spectrophotometer (IPDS) presented in a previous paper, a ground-based IPDS for airglow measurements is constructed. This low resolution spectrometer can measure a molecular band spectrum at 12 wavelength positions simultaneously without mechanical scan. The band brightness and rotational temperature of the emitter can be obtained from the measurements. We describe the optical design and calibration of the instrument and present some results of mesospheric nightglow observations. The evaluation and analysis on the performances of the new instrument are also given. The result shows that, although the constructed instrument does not represent the optimized IPDS, the instrument performed reasonably well, and with some improvements the IPDS has great potential of being used to study small-scale oscillations in airglow layers.

Observations of winds with an incoherent lidar detector.

We have developed a Fabry-Perot interferometer and image-plane detector system to be used as a receiver for a Doppler lidar. This system incorporates the latest technology in multichannel detectors, and it is an important step toward the development of operational wind profiler systems for the atmosphere (troposphere, stratosphere, and lower mesosphere). The instrumentation includes a stable highresolution optically contacted plane talon and a multiring anode detector to scan the image plane of the Fabry-Perot interferometer spatially. The high wavelength resolution provided by the interferometer permits the aerosol and molecular components of the backscattered signal to be distinguished, and the Doppler shift of either component can then be used to determine the wind altitude profile. The receiverperformance has been tested by measuring the wind profile in the boundary layer. The Fabry-Perot interferometer and image-plane detector characteristics are described and sample measurements are presented. The potential of the system as a wind profiler in the troposphere, the stratosphere, and the mesosphere is also considered.

Image plane detector for Fabry-Perot interferometers: physical model and improvement with anticoincidence detection.

Modern Fabry-Perot interferometers use image plane array detection to obtain the multiplex advantage. The imaging quality of these image plane detectors (IPD's) limits their ultimate resolution and usefulness. The influence of pulse spreading on photon-counting IPD's is investigated, first theoretically and then experimentally on real devices. The model developed in our study is in good agreement with laboratory measurements and suggests a more reliable technique to measure the blurring of the Fabry-Perot fringe caused by the IPD. Based on the model, an anticoincidence detection circuit is designed; the circuit is simple, greatly reduces the blurring, and increases the resolution of the IPD.

Multiplex Fabry-Perot interferometer.

The Fabry-Perot interferometer (FPI) traditionally has been used to examine either small spectral ranges or relatively imple spectra. Recently, however, studies have shown that the FPI can be competitive with the Michelson interferometer ver extended spectral ranges. This paper describes a relatively new FPI technique in which one of the étalon plates is moved over a large optical distance while the other remains fixed, thus exploiting the multiplex advantage of the instrument. we have chosen to call this instrument the multiplex Fabry-Perot interferometer (MFPI). It is shown that this technique could be useful for the remote sensing of minor atmospheric species because the MFPI, like a Michelson interferometer, has the ability to examine large spectral regions at high resolution but retains the small size of the FPI.

Circle to line interferometer optical system.

This study describes a technique and apparatus for converting the circular fringes of a Fabry-Perot interferometer into a linear pattern which can be detected with a conventional linear detector. The conversion from a circular ring pattern at the focus of the objective lens of the interferometer is accomplished using a segment of an internally reflecting cone to direct the light from a sector of the ring pattern onto the linear detector. Light is concentrated into a single sector by the use of a kaleidoscopic mirror system. The aberrations associated with the transformation are shown to be small for high resolution devices which have large F numbers. A laboratory demonstration of a linearly scanned Fabry-Perot interferometer with a 0.7 cm. spacing illustrates a spectrum taken over ten orders with very small aperture defects. These results imply that a high resolution interferometer using five orders and a CCD detector would have a sensitivity 87.5 x greater than is presently achieved with those used to observe the winds in the upper atmosphere.

Photoacoustic technique for the measurement of absorption line profiles.

A spectrophone utilizing a resonant cylindrical cavity and operated by driving the first azimuthal mode of the cavity has been developed for the study of weak absorption lines of gases at pressures from 100 to 1300 Torr. Presented are the acoustic resonant amplification factor as a function of pressure, and a description of the noise sources inherent in this spectrophone. An example is given of the optical frequency resolution resulting when this spectrophone is used in conjunction with a tunable ring dye laser as a high resolution spectrometer.

Absorption and emission line shapes in the O(2) atmospheric bands: Theoretical model and limb viewing simulations.

A multiple scattering radiative transfer model has been developed to carry out a line by line calculation of the absorption and emission limb measurements that will be made by the High Resolution Doppler Imager to be flown on the Upper Atmosphere Research Satellite. The multiple scattering model uses the doubling and adding methods to solve the radiative transfer equation, modified to take into account a spherical inhomogeneous atmosphere. Representative absorption and emission line shapes in the O(2)((1)Sigma(+)(g)-(3)Sigma(-)(g)) atmospheric bands (A, B, and gamma) and their variation with altitude are presented. The effects of solar zenith angle, aerosol loading, surface albedo, and cloud height on the line shapes are also discussed.

Image plane detector spectrophotometer: application to O(2)atmospheric band nightglow.

A new variety of low resolution spectrometer is described. This device, an image plane detector spectrophotometer, has high sensitivity and modest resolution sufficient to determine the rotational temperature and brightness of molecular band emissions. It uses an interference filter as a dispersive element and a multichannel image plane detector as the photon collecting device. The data analysis technique used to recover the temperature of the emitter and the emission brightness is presented. The atmospheric band of molecular oxygen is used to illustrate the use of the device.

Use of gamma ray radiation to parallel the plates of a Fabry-Perot interferometer.

A technique is described by which radiation is used to shrink Zerodur posts and parallel the plates in anairspaced Fabry-Perot etalon.

Optimization of a triple etalon interferometer.

The High Resolution Doppler Imager (HRDI) is a triple etalon Fabry-Perot interferometer designed to measure Doppler shifts of rotational lines in the O(2) atmospheric system from the Upper Atmosphere Research Satellite. These shifts are used to determine wind vectors in the stratosphere and mesosphere. This paper presents the techniques used to determine the gap thicknesses and reflectivities of the three etalons of the HRDI instrument. The spacings are found to be 1.000, 0.186, and 0.025 cm. These spacings are independent of the reflectivity of the etalons. The reflectivities of the three etalons should be nearly equal to minimize the errors in the wind measurement caused by mistuning of the etalons. The choice of the reflectivity does not strongly influence the statistical error in the wind error when the values are less than ~0.90.

Image plane detector for the Dynamics Explorer Fabry-Perot interferometer.

A compact twelve-channel photon-counting device based on existing Generation II imaging technology has been developed for use as the image-plane detector of the Dynamics Explorer Fabry-Perot interferometer. The device has an S-20 photocathode, three-microchannel plate electron multiplication stages, and an equal-area concentric-ring segmented anode whose geometry mimics that of the interference ring pattern produced by a plane etalon. The twelve channels sample equal and contiguous intervals in the spectrum. The purpose of the development has been to utilize the signal multiplex advantage of a multichannel detector in the measurement of Doppler shifts and line-broadening effects for naturally occurring atmospheric emission features of low intensity. The design, testing, calibration, and flight performance of the novel detector system are presented. In addition, measured emission line profiles at high resolution from the satellite instrument are presented to illustrate the operation of the device.