The different types of coherent millimeter and submillimeter detectors are reviewed. Emphasis is placed on heterodyne detection using GaAs Schottky diodes as quasi-optical mixers. A sufficiently sensitive radiometer has been developed to make the first ground-based submillimeter astronomical experiment using a laser local oscillator. Results for a millimeter wave monolithic mixer and a solid state local oscillator are also discussed.
The Indium Phosphide (InP) Gunn device is fast becoming a key component in the millimeter wave region. The device has superior performance as wideband low noise amplifier, low noise local oscillator, self-oscillating mixer, and low to medium power source. Its development and advances are primarily fueled by military systems requirements and developments covering missile and projectile guidance, high resolution radar systems, intercept secure communications and Eli and ECM functions. In this paper an overview of the current status of indium phosphide Gunn device development and future trends is presented. Covered will be in particular key parameters leading to indium phosphide's advantage at millimeter-wave frequencies, recent material and device developments, oscillators, amplifiers and systems applications with emphasis on current capabilities. Other devices such as InP FET and InP IMPATT devices are not considered in this paper because of their current limitations to microwave frequencies.
Optical pumping of a submillimeter wave (SNI4W) laser with a relatively compact CTS CO2 waveguide laser is reported. The increased frequency tunability of the waveguide pump laser has resulted in new low threshold SMMW emissions by pumping into absorption lines which are beyond the tuning range of a conventional CO2 laser.
A formalism is developed for determining the possibility of operation of a gyrotron at both the fundamental, w = wc in a TEmpq mode and at the second harmonic, w' = 2wc, in a TEm'p'q' mode. This formalism is then applied to two physical problems. First, the conditions are derived for conversion of a gyrotron from operation at w = wc to operation at w' = 2wc. Such a conversion is shown to be most practical when the mode at w = wc is a higher order mode (m or p large) and/or the beam position is at a higher radial maximum. Several specific examples are considered. Second, the conditions are derived for oscillation of a parasitic mode at w = wc in a gyrotron designed for operation at w' = 2wc. Again, this problem is more severe for higher order modes and beams at larder radii. For a whispering gallery mode TEm'11 oscillating at w' = 2wc, if m' is odd, there is a resonant para-sitic whispering gallery mode TEmll at w = wc, with m = 0.5 (m' - 1). This would be expected to spoil the 2wc operation of all TEmy 11 modes, with m' odd.
In this paper we describe the US Army Metrology and Calibration Center's (USAMCC's) efforts to obtain about a five-fold improvement in the accuracy of power and energy standards for the Far Infrared (FIR) spectral region, 0.1-1.2 mm. Total uncertainties for most FIR calibrations are placed at ± 25% or worse, with no traceability to National Standards. Our goal is to establish, by the end of 1981, FIR reference standards with assigned uncertainties approaching ± 5%, directly traceable to infrared standards maintained by the US National Bureau of Standards (NBS). At that time, we also expect to initiate measurement and calibration services for the Army and other Department of Defense (DOD) agencies. To date, our program has resulted in the National Physical Laboratory (NPL), the national standards laboratory of England, delivering to us a British power meter calibrated with an overall uncertainty of ± 11%. This recent NPL effort represents the latest state-of-the-art for FIR calibrations.
Results of thermal switching of the electrical and optical properties of thin films of Ag2S are presented. The procedures for the preparation of the thin film are described. Film characteristics including crystal structure, composition for various thicknesses, and spectral transmittance/reflectance versus electrical conductivity are given. In addition, we have demonstrated electrical control (not due to Joule heating) of the spectral transmittance over the Far-IR to millimeter wavelength region. The application of the latter effect to infrared-millimeter devices such as modulators, variable attenuators, switchable polarizers, and tunable filters is stressed.
This paper summarizes the recent state-of-the-art results in silicon IMIPATT sources beyond 100 GHz. A bridge type double-quartz-standoff diode package has been developed and successfully used for frequency up to 255 GHz. Power combining techniques have been demonstrated to incorporate several diodes in a circuit that combines the power efficiently at 140 GHz and 217 GHz. Finally, a phase-locked source has been developed to achieve frequency stabilization at 217 GHz.
Near millimeter wave (NMMW) propagation problems are divided into three classes: propagation through homogeneous, turbid, and turbulent atmospheres. These classical forms include anomalous water vapor absorption in a homogeneous atmosphere as well as scintillation phenomena associated with propagation through severe weather and "dirty battlefield" environments. Examples of the existing, inadequate, scintillation data base are given and the lack of supporting meteorological data noted. Carefully designed NMMW scintillation experiments with equally carefully designed micro-meteorological support are needed.
Atmospheric models of fog, clouds and rain are described. These models are typical for mid-latitude temperate regions of the globe. A computer code for incorporating the models into a new efficient computer algorithm of the AFGL HITRAN series named FASCOD1 has been completed. The computer models presented allow calculation of atmospheric transmission or attenuation for millimeter and submillimeter waves (1-34 cm-1 or 1-1000 GHz). Four (4) fog models, eight (8) cloud types and rainfall rates from 1-150 mm hr available. All models consider hydrometeors as having temperatures between 0°C and 40°C and permit arbitrary input of atmospheric parameters and geometry (slant range).
Gasses tend to have low absorption coefficients in the millimeter wavelength region; absorption cells with path lengths of hundreds of meters are needed for millimeter wave gas-phase spectroscopy. Three types of long-path cell are discussed here: tuned cavities, untuned cavities, and optical multiple-pass cells. The operating principles of each type are described, along with the advantages and limitations of each type when used in the millimeter wavelength region. Several examples of each type of cell are given. An optical analysis of a three-mirror optical multiple-pass cell is performed, for the purpose of optimizing this cell for millimeter wave spectroscopy, with the result that a cell with mirrors one meter in diameter can give a path length of 500 meters while conserving the power from a presently available black body source.
Unique techniques are being utilized to develop self-contained imaging radiometers operating at single and multiple frequencies near 35, 95 and 183 GHz. These techniques include medium to large antennas for high spatial resolution, low-loss open structures for RF confinement and calibration, wide bandwidths for good sensitivity, plus total automation of the unit operation and data collection. Applications include: detection of severe storms, imaging of motor vehicles, and the remote sensing of changes in material properties. The radiometric imaging system of principal interest in this report makes use of both 35 and 95 GHz receivers, both vertical and horizontal polarizations, an elevation-over-azimuth antenna positioner, highly automated scanning and data acquisition routines, real-time TV display of scene being scanned, and immediate color display of recorded radiometric images. The RF sections make use of Rexolite lenses for low loss beam confinement, and use low loss reflective metallic surfaces for both Dicke chopping and calibration beam selection.
Water in the atmosphere is the principal natural impediment to propagation of both infra-red and near-millimeter wave electromagnetic energy. Attenuation by water in the vapor phase is greater near one millimeter than near 10 μm. When fog is present, extinction by the water or ice particles depends upon the wavelength of the propagating energy, the complex index of refraction of the particles for that wavelength, and the particle-size distribution. This paper summarizes a thorough survey of the literature on fog drop-size distributions throughout the world. A representative sample of data is selected for use in computations of the extinction of 10.6, 870, and 1250 μm by numerous fogs. Attenuation by fog drops is much greater near 10 μm than near one millimeter. Computations made with German data show that attenuation of liquid plus vapor is smallest for 1250 μm in fogs of different densities in all seasons. In very dense German fogs, 10.6 μm has the largest attenuation in all seasons. In spring, summer, and autumn attenuations of 10.6 μm and 870 μm in moderate fogs are the same order of magnitude.
Potential explanations of the anomalous continuous absorption coefficient in water vapors (ACA) are examined theoretically. We show how the single line shape is determined by the time dependence of the dipole-dipole autocorrelation function(DDA) whose short time behavior dominates the frequency dependence in the far wing region. Based on general theoretical argument, it is shown that the DDA must be a Gaussian function for short times and an expotential function for long times. Empirical formula are obtained to fit the experimental data of CO2, whose absorbtion coefficient diminishes much faster than the Lorentz line shape. Since it is difficult to understand how CO2 or H2O can be different in the far wing region, we have to conclude that the ACA cannot be explained by modified local Lorentzian lines of H2O in the far wing region. We point out that both H2O and HCl which have permanent dipole moments have ACA. The possibility of the dipole-dipole interaction that may be important for ACA is examined.
Battlefield obscurants including smoke, dust, and inclement weather limit the ability of our Armed Forces to recognize targets. Infrared imaging systems have proven extremely useful for day/night operation, but are of limited use on the "dirty battlefield". To penetrate obscurants, yet meet the spatial resolution required for target recognition, a near-millimeter wave imaging system is needed.
Performance parameters for a passive and active FLIR-type imager will be presented. Refrigerated photon detectors or power detectors (bolometers) have nearly background-limited performance in the NMM spectral region. An imager equipped with such a detector will therefore have high sensitivity which is further enhanced by the fact that the bandwidth can be extremely broad and, in fact, can cover more than one atmospheric window. System performance of the NMM FLIR will be derived from the detector performance figures, i.e., detector sensitivity, system bandwidth, receiver aperture, field of view and dwell time. Target recognition capability will also be given in terms of target albedo, target-to-background contrast, range and atmospheric conditions. It will be shown that techniques developed for FLIR's also can enhance the performance of NMM wave imagers. Conversion to an active mode of operation using, for example, a Far IR laser and the resulting performance will also be presented.
The Electronically Scanned Phased Array concept is considered as a technique for obtaining near millimeter (NIVIM) wave imaging. Consideration is given to the various factors of importance for obtaining near millimeter images in a dynamic tactical environment.
Synthetic aperture (SA) imaging techniques are considered for identifying targets at ranges of a few kilometers using near-millimeter waves. A generalized concept of SA imaging is presented. A hybrid SA array technique is proposed that would significantly diminish the motion required for the SA data generation. A scanning SA technique is presented and analyzed in detail. Images are shown that were generated using the scanning SA technique with a near-millimeter wave homodyne system operating at 1.22 mm.
Measurements have been made on the Redstone Arsenal Submillimeter Wave range of weather related changes in the coherence within a transmitted laser beam whose wavelength is 1.2 mm and divergence is less than five milliradians. Our experimental techniques, variations of a Michelson interferometer, are described in brief. Our preliminary observations of rapid fluctuations superposed on high visibility fringes are discussed in terms of local fluctuations in the complex index of refraction of the near ground atmosphere, as treated theoretically by Armand et al. For our beam geometry, a limit to the observed fringe visibility appears to be set by local fluctuations in the real part of the complex index of refraction.
An all-weather, kilometer-range, high resolution battelfield imaging system, capable of detecting and identifying enemy targets, is of considerable importance. In that view, this paper seeks to describe a state-of-the-art scanning mirror imaging system operating in the 1.3mm atmospheric window that provides simulated data on resolution and expected target signatures. Both passive and active imaging is performed using scale model targets to simulate reasonable tactical scenerios.
A method for the evaluation of passive millimeter wavelength sensors for the detection of point targets is described. The basis of the performance evaluation is the determination of signal-to-noise ratio where noise includes both thermal noise and background clutter. Because passive millimeter wavelength sensors depend on target contrasts against background, similar to detection of point targets by visible and infrared wavelength sensors, the method is an adaptation from that used in the evaluation of visible and infrared wavelength sensors. Background clutter as well as thermal noise are described by power spectral densities. Because of the limited amount of data on power spectral densities of backgrounds at millimeter wavelengths, a power spectral density was derived. Methods used in visible and infrared sensors for signal-to-noise ratio enhancement are discussed for adaptation to passive millimeter wavelength sensors.
Systems technology and applications of millimeter optics in the general frequency renion of 100 to 1000 GHz are discussed. Potential applications include tactical target detection, target acquisition, secure communications, remote sensing of earth environment and industrial process monitoring and control. Applications of millimeter optics to date have been very limited due to lack of a mature component technology as well as insufficient data on atmospheric propagation and materials reflectance.
A submillimeter radar modeling facility has been developed for the acquisition of data from scale models of tactical targets to determine the scattering of millimeter waves from the corresponding full size vehicles. Advances in submillimeter optically pumped laser technology and minicomputer control and display have been utilized to achieve a versatile radar modeling system in a laboratory. High resolution imaging and radar cross section (RCS) measurements have been made and the necessary calibration techniques have been developed. Evolving submillimeter technology should permit the modeling of frequency-agile radar systems.
Despite years of investigation, the solid, quasi-permanent component of comets, the nucleus, remains largely a mystery. Its composition and thermal properties determine the evolution of the more familiar and often spectacular cometary features, the coma and the tail. Under some circumstances, the (≈200 K) nucleus may be obscured by a dust cloud of much higher temperature. It appears that the most appropriate technology for the investigation of the surface and subsurface layers of the nucleus is millimeter-wave sensing from an interplanetary spacecraft. Simple radiative transfer models, adapted from methods used for the interpretation of remote-sensing data on terrestrial ice and snow fields, are used to predict the millimeter-wave spectra of representative model nuclei. The spectra guide the choice of the minimum set of observing frequencies that is required. An instrument configuration driven by these requirements and guided by available technology and the constraints of a proposed NASA spacecraft, is then derived.
This paper discusses three brassboard millimeter wave systems which are being built for the Army by Georgia Tech. These systems are: (1) a 94 GHz coherent transmitter/receiver, (2) a 140 GHz incoherent transmitter/receiver, and (3) a 220 GHz radar. The first two systems are to be used primarily for evaluation of millimeter guidance techniques, and the third will be used to evaluate performance of a 220 GHz radar system in a tank-mounted, target acquisition application. All of the systems use extended interaction oscillator transmitters and Schottky-barrier mixer receivers, and the radar employs a unique quasi-optical duplexer, and an all solid-state receiver.
A theoretical analysis of the spectral content of a frequency modulated (FM) and amplitude modulated (AM) electromagnetic plane wave is presented. An unmodulated transmitted millimeter wave is modulated when reflected from a sinusoidally oscillating structure. The FM results from the doppler effect, while AM results from the optical properties (antenna beamwidths, radar cross sections (RCS)) of millimeter waves. The purpose of the analysis is to investigate the effect of the AM on the spectral lines resulting from FM. It is anticipated that AM is largely due to the RCS characteristics of the vibrating structure. For FM only, the amplitude of the first side band (spectral line) is maximized when the transmitted wavelength is approximately seven times the structure oscillation displacement amplitude. When the waveform also has AM, the spectral line amplitudes are increased, and the maximization criterion no longer applies.
Millimeter wave radio development has been limited by the very high cost of millimeter wave components. Millimeter wave receivers almost exclusively use the superheterodyne approach and coherent detection. An alternate approach with low cost potential would utilize direct incoherent thermal detection of millimeter wave energy. Some of the problems and prospects of such an optoelectronic receiver are explored in this paper.