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At least 235 records · Page 13

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NASA NTRS1980

Sensitivity studies and laboratory measurements for the laser heterodyne spectrometer experiment

Several experiments involving spectral scanning interferometers and gas filter correlation radiometers (ref. 2) using limb scanning solar occultation techniques under development for measurements of stratospheric trace gases from Spacelab and satellite platforms are described. An experiment to measure stratospheric trace constituents by Laser Heterodyne Spectroscopy, a summary of sensitivity analyses, and supporting laboratory measurements are presented for O3, ClO, and H2O2 in which the instrument transfer function is modeled using a detailed optical receiver design.

Allario, F.↗

NASA NTRS1980

Heterodyne detection of the 752.033-GHz H2O rotational absorption line

A tunable high resolution two stage heterodyne radiometer was developed for the purpose of investigating the intensity and lineshape of the 752.033 GHz rotational transition of water vapor. Single-sideband system noise temperatures of approximately 45,000 K were obtained using a sensitive GaAs Schottky diode as the first stage mixer. First local oscillator power was supplied by a CO2 laser pumped formic acid laser (761.61 GHz), generating an X-band IF signal with theoretical line center at 9.5744 GHz. Second local oscillator power was provided by means of a 3 GHz waveguide cavity filter with only 9 dB insertion loss. In absorption measurements of the H2O taken from a laboratory simulation of a high altitude rocket plume, the center frequency of the 752 GHz line was determined to within 1 MHz of the reported value. A rotational temperature 75 K, a linewidth 5 MHz and a Doppler shift 3 MHz were measured with the line-of-sight intersecting the simulated-plume axis at a distance downstream of 30 nozzle diameters. These absorption data were obtained against continuum background radiation sources at temperatures of 1175 and 300 K.

Dionne, G. F.↗

NASA NTRS1980

Heterodyne Systems and Technology, part 2

Various aspects of optical heterodyning are covered including active and passive systems, local oscillators, tunable diode lasers, detectors and related systems.

Source record↗

NASA NTRS1980

Conceptual design and applications of HgCdTe infrared photodiodes for heterodyne systems

The significance of HgCdTe photodiodes are discussed relative to their existance in heterodyne detection systems operating in the 9 to 11 micrometer CO2 laser wavelength region. Their successful fabrication as well as the physical properties of the materials are described. The implementation of controlled industrial processes are reported with emphasis on the yield of predictable and repeatable detector characteristics to the discriminating systems, demands for high cutoff frequencies, quantum efficiency, and reliability. The most salient production steps and diode characteristics are presented. Measured results from production units are also given.

Sirieix, M. B.↗

NASA NTRS1980

Infrared heterodyne receivers with IF responses approaching 5 GHz

Specialized coherent 10.6 micrometer infrared receivers were developed with infrared frequency responses approaching 5 GHz. Carbon dioxide lasers were employed for these applications because of their availability, stability, and high average power levels. Heterodyne receivers which employ extrinsic photoconductive Ge:Cu(Sb) mixers were developed for measurements of carbon dioxide laser scattering to determine: (1) the drive lower hybrid wave density fluctuations; and (2) the plasma ion temperature of the dense Tokamak plasmas.

Wolczok, J. M.↗

NASA NTRS1980

RF spectrometers for heterodyne receivers

Several types of spectrometers developed for radio astronomy receivers which utilize RF filters, multiple oscillators and mixers, digital autocorrelators and acoustic/optic devices are considered. The RF spectrometer developed at GSFC to provide wide bandwidths (greater than 1 GHz) as well as high resolution (5MHz) is described. The 128 channel filter bank is divided into high and low resolution sections. The high resolution section is tunable by providing a second mixer ahead of the filter bank. This is necessary because infrared receivers which use gas lasers as local oscillators are only tunable to specific laser frequencies. To compensate for astronomical Doppler shifts and molecule frequency differences a second local oscillator and mixer is needed. A diagram of the RF section of the filter bank is shown. The RF spectrometer is shown to be the best means of achieving ultra-wide bandwidths for infrared heterodyne receivers. For high resolution with a large number of channels, the acousto/optical spectrometer is the principle instrument, particularly for balloon or space flight applications.

Buhl, D.↗

NASA NTRS1980

Submillimeter local oscillators for heterodyne spectroscopy

The major technological innovations in continuous wave (CW) submillimeter sources which are specifically suitable for application as local oscillators in heterodyne systems are reviewed. A description of the various sources is given which underscores the general principles and operating features for each type of device. Particular emphasis is placed on CW optically pumped lasers, which have had a dramatic impact as widely available sources of narrow linewidth coherent radiation. The state-of-the-art is summarized for these lasers and performance data are presented for several local oscillator packages. Optically pumped lasers are then compared and contrasted with other competing sources such as backward wave oscillators, IMPATT diodes, and Josephson junctions. By comparing their advantages and limitations for use as local oscillators, the potential applications of these different sources are projected. The prospects for increased tunability, reliability, and scalability are briefly considered, and several novel techniques for generating partially tunable radiation using Schottky diode mixers or CW Raman lasers are highlighted.

Danielewicz, E. J., Jr.↗

NASA NTRS1980

Mixing efficiency in heterodyne systems

How the spatial distributions of the signal and local oscillator fields affect the heterodyne signal to noise ratio is examined for both a single detector and an array of detectors. For an array, the distribution of gain among the intermediate frequency (i.f.) amplifiers is included. The emphasis is on understanding why the distributions are important and what is the key to maximizing the signal to noise ratio in any system. It is shown that for a single detector, the highest signal to noise ratio is obtained with a local oscillator (LO) distribution the same as that of the signal. For an array, the product of the i.f. gain times the LO field at each detector has the same distribution as that of the signal field.

Fink, D.↗

NASA NTRS1980

DIAL with heterodyne detection including speckle noise: Aircraft/shuttle measurements of O3, H2O, and NH3 with pulsed tunable CO2lasers

A parametric analysis of DIAL sensitivity with heterodyne detection is presented and comparisons with direct detection are discussed. Examples are given for monitoring vertical distributions of O3, H2O, and NH3 using a ground-, aircraft-, or shuttle-based pulsed tunable CO2 laser DIAL system. Results indicate that maximum sensitivity at minimum laser energy per measurement requires multiple pulse operation with the energy per pulse selected so that the measured photon rate is approximately equal to the detector IF bandwidth. Measurement sensitivities can be maximized and interference effects minimized by fine adjustment of measurement frequencies using the tunability of high pressure lasers. The use of rare isotope lasers minimizes loss due to CO2 atmospheric absorption.

Brockman, P.↗

NASA NTRS1980

Heterodyne signal-to-noise ratios in acoustic mode scattering experiments

The relation between the signal to noise ratio (SNR) obtained in heterodyne detection of radiation scattered from acoustic modes in crystalline solids and the scattered spectral density function is studied. It is shown that in addition to the information provided by the measured frequency shifts and line widths, measurement of the SNR provides a determination of the absolute elasto-optical (Pockel's) constants. Examples are given for cubic crystals, and acceptable SNR values are obtained for scattering from thermally excited phonons at 10.6 microns, with no external perturbation of the sample necessary. The results indicate the special advantages of the method for the study of semiconductors.

Cochran, W. R.↗

NASA NTRS1980

DIAL with heterodyne detection including speckle noise: Aircraft/shuttle measurements of O3, H2O, and NH3 with pulsed tunable CO2 lasers

Atmospheric trace constituent measurements with higher vertical resolution than attainable with passive radiometers are discussed. Infrared differential absorption lidar (DIAL), which depends on Mie scattering from aerosols, has special advantages for tropospheric and lower stratospheric applications and has great potential importance for measurements from shuttle and aircraft. Differential absorption lidar data reduction involves comparing large amplitude signals which have small differences. The accuracy of the trace constituent concentration inferred from DIAL measurements depends strongly on the errors in determining the amplitude of the signals. Thus, the commonly used SNR expression (signal divided by noise in the absence of signal) is not adequate to describe DIAL measurement accuracy and must be replaced by an expression which includes the random coherent (speckle) noise within the signal. A comprehensive DIAL computer algorithm is modified to include heterodyne detection and speckle noise. Examples for monitoring vertical distributions of O3, H2O, and NH3 using a ground-, aircraft-, or shuttle-based pulsed tunable CO2 laser DIAL system are given.

Brockman, P.↗

NASA NTRS1980

Remote infrared heterodyne radiometer measurements of atmospheric ammonia profiles

An infrared heterodyne radiometer has been used to study the seasonal variation of the vertical distribution of atmospheric ammonia. The ammonia profiles are inferred from high-resolution atmospheric solar transmittance measurements around the ammonia absorption feature at 927.32323 kaysers. The transmittance data are obtained from six IF channels with spectral resolutions ranging from .0067 to .033 kaysers. Results from this study show a marked decrease in the tropospheric ammonia level with a decrease at ground level from approximately 10 ppb in March 1979 to 1.3 ppb in August 1979.

Hoell, J. M.↗

NASA NTRS1981

Vibration characteristics of a laser heterodyne spectrometer tunable diode laser system using a mechanical cooler platform

The Langley Research Center redesigned the cooler test bed hardware for the refrigerator for the purpose of isolating the tunable diode laser (TDL) from the cold tip in a laser heterodyne spectrometer system. Deflection in the lateral and vertical directions were managed on the cold tip and on the TDL. Measurements were analyzed over the frequency range of 0.100 Hz. The results show that the TDL responds approximately one order of magnitude less than that of the cold tip. The redesign of the system provided for adequately isolating the TDL for future operation.

Catherines, J. J.↗

NASA NTRS1981

Design, development, and field testing of Infrared Heterodyne Radiometer (IHR) for remote profiling of tropospheric and stratospheric species

The performance of a dual-channel infrared heterodyne radiometer, designed to remotely monitor the concentration and vertical distribution of selected atmospheric species, is described. Ground based solar viewing measurement using the IHR were performed at selected laser transitions for ammonia (NH3 and ozone O3). Flight tests were conducted aboard the Galileo II, NASA Ames CV-990, on the Latitude Survey Mission. Ozone was the selected atmospheric species for the airborne flight measurements because of the scientific interest in this atmospheric species, the availability of in situ monitors, the coordinated ozone measurements, and the availability of ground truth data. The IHS was operated in the solar viewing mode to determine ozone distributions in the stratosphere and in the nadir viewing mode to determine the ozone distribution in the troposphere. Airborne atmospheric propagation measurements also were carried out at selected CO2 laser transitions.

Lange, R.↗

NASA NTRS1981

Balloon-borne laser heterodyne radiometer for measurements of stratospheric trace species

A laser heterodyne radiometer has been flown to measure chlorine monoxide in the stratosphere, which has an important role in the catalytic destruction of ozone. The measured values in the 35-38 km region are found to be significantly higher than model predictions, and the drop-off at lower altitudes occurs at a higher rate, even after adjusting the profile to include the solar zenith angle effects.

Menzies, R. T.↗

NASA NTRS1980

A 10 micron heterodyne receiver for ultra high resolution astronomical spectroscopy

An improved CO2 laser heterodyne spectrometer is examined. The present system uses reflective optics to eliminate refocusing at different wavelengths, and the local oscillator is a line-center-stabilized isotopic CO2 laser. A tunable diffraction grating makes possible easy and rapid selection of over 50 transitions per isotope of CO2. The IF (0 to 1.6 GHz) from the HgCdTe photomizer is analyzed by a 128-channel filter bank, consisting of 64 tunable 5-MHz filters and 64 fixed 25-MHz RF filters. These filters provide resolving powers of about 1,000,000 to 10,000,000 and velocity resolution of 50 to 250 m/sec; their output is synchronously detected, integrated, multiplexed and stored in a buffer memory for the desired integration period. Kitt Peak observations show the wide spectral coverage, wide mixer and electronics bandwidth, and high sensitivity of the system.

Buhl, D.↗

NASA NTRS1982

Spectroscopic measurements of Mars and Venus with heterodyne techniques

Measurements were made of the exact shape of CO2 absorption lines in Mars and Venus to determine the vertical pressure temperature structure using high resolution heterodyne spectroscopy. Accurate measurements of absolute wind velocities in both the mesosphere and stratosphere of Venus were made from Doppler shifts of narrow CO2 lines, and searches were made for minor molecular species of interest in modeling the stratospheric photochemistry for Mars and Venus.

Betz, A.↗

NASA NTRS1981

Development of a diode laser heterodyne spectrometer and observations of silicon monoxide in sunspots

A state of the art, tunable diode laser infrared heterodyne spectrometer was designed and constructed for ground based observations throughout the 8 to 12 micron atmospheric window. The instrument was optimized for use with presently available tunable diode lasers, and was designed as a flexible field system for use with large reflecting telescopes. The instrument was aligned and calibrated using laboratory and astronomical sources. Observations of SiO fundamental (v = 1-0) and hot band (v = 2-1) absorption features were made in sunspots near 8 microns using the spectrometer. The data permit an unambiguous determination of the temperature pressure relation in the upper layers of the umbral atmosphere, and support the sunspot model suggested by Stellmacher and Wiehr.

Glenar, D. A.↗