Michael S. Shumate

Laser absorption spectrometer: remote measurement of tropospheric ozone.

The laser absorption spectrometer (LAS) is an airborne instrument able to perform remote measurement of trace atmospheric gases. The instrument uses a pair of carbon dioxide laser-heterodyne receiver systems that are directed downward from an airplane and determines gas concentration by the differential absorption method. Configured to measure tropospheric ozone, the LAS has been used in conjunction with another instrumented aircraft to test the accuracy of this method of measurement. A complete description of the instrument is presented, and the results of extensive flight testing are summarized.

Interferometric Measurement of Large Indices of Refraction

A method for determination of refractive indices which may be applied to thin flat plates of optical materials is considered. It is particularly suited for use with materials whose refractive indices are large (>1.8), but is not limited in the range of refractive index it can determine. The method uses an interferometer to measure the optical pathlength through a sample, and is shown to have moderate accuracy: +/-2 x 10(-4) in refractive index for a sample 0.5 mm thick. The effect of a nonideal sample is considered, and is shown to have only a small effect on the accuracy. The method has been applied to singlecrystal barium titanate in the visible spectrum, and tabulated results are given.

Optoacoustic detection using Stark modulation.

Stark modulation of the absorbed laser radiation in an optoacoustic detector (or spectrophone) is reported. Measurements were made over a range of total pressure between 760 Torr and 50 Torr. Greatly enhanced molecular discrimination is suggested due to the tuning ability of the Stark-shifted absorption. The background signal obtained by operating in this mode is more than 500 times smaller than that obtained by operating the same optoacoustic detector in the conventional chopped radiation mode. The responsivity of the optoacoustic detector and the absorption coefficient of C(2)H(4) are presented as a function of total pressure.

Analysis of errors caused by optical interference effects in wavelength-diverse CO2 laser long-path systems.

The measurement errors in a CO2 laser differential absorption system caused by optical interference effects are analyzed. Major sources of errors are identified and design suggestions to improve the system accuracy are made. The effects of temperature changes, short measurement paths, and spectral features of the measured species are taken into account.

Stark cell optoacoustic detection of constituent gases in sample

An optoacoustic detector for gas analysis is implemented with Stark-effect cell modulation for switching a beam in and out of coincidence with a spectral line of a constituent gas in order to eliminate the heating effect of laser energy in the cell as a source of background noise. By using a multiline laser, and linearly sweeping the DC bias voltage while exciting the cell with a multiline laser, it is possible to obtain a spectrum from which to determine the combination of excited constituents and determine their concentrations in parts per million.

Simulation of sampled-data systems using analog-to-digital converters

Until recently, systems simulation problems could generally be split into two classes: problems requiring only an analog computer for solution and problems requiring only a digital computer for solution. Any general problem has a number of characteristics which adapt it to either one or the other method of solution.

Underwater measurements of off-axis radiance compared with various analytical treatments of the radiative transfer equation: a comment

A recent paper [ R. Anderson L. Stotts , J. Opt. Soc. Am.72, 738– 746 ( 1982)] gives plots of undersea radiance measurements that show anomalous brightness at unlikely angles. Difficulties with a critical alignment could have produced these results.

ON THE POSSIBILITY OF MEASURING ATMOSPHERIC WATER VAPOR WITH AN AIRBORNE LASER ABSORPTION SPECTROMETER

The Laser Absorption Spectrometer (LAS) is an airborne instrument that is capable of remotely measuring several important atmospheric constituents. The instrument design concept is also feasible for operation from earth orbiting altitudes, as part of a shuttle pay load. The LAS utilizes two carbon dioxide lasers and two heterodyne receivers tuned to a selected wavelength pair, and operates in a differential absorption mode. One laser is tuned to an absorption line of a molecular specie of interest, and the other laser is tuned to a nearby wavelength that is not affected by variable atmospheric constituents. The two laser transmitter/receiver combinations are oriented in a nadir viewing configuration, and the differential transmission is determined. The instrument is designed to be installed in a twin-engine Beechcraft and operates through an open port in the belly of the aircraft. The LAS has been used for the last 3 years to measure atmospheric ozone. An extensive series of test flights over the Los Angeles Basin and over Southeastern Virginia, some in conjunction with other instrument aircraft, have demonstrated that the LAS performs accurate remote measurement of tropospheric ozone burdens. By choosing the proper set of wavelengths, this