An Atmospheric Limit On Submillimeter Wave Imaging: Observations Of High Visibility Fringes

AbstractMeasurements 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 fluctu­ ations in the complex index of refraction of the near ground atmosphere, as treated theoret­ ically by Armand et al.6 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.IntroductionA few years ago P, W, Kruse and R. L. Hartman et al.1 ^ gained support within the Army for an investigation of the military potential of Submillimeter Waves (SMMW) with arguments that a weapons system operating at these wavelengths could "see" through fogs and aerosols with the capability to image targets using antennas compatible in size with battlefield equipment and environments. This potential payoff to the Army has generated interest in investigating SMMW propagation and imaging through the atmosphere under various weather conditions.At the Gordy Conference in June 1979, we reported observations of rapid and slow fluctu­ ations in transmission of 1,2 mm wavelength radiation in a variety of weather conditions. * As a result of these observations, we were led to the present investigation of phase and amplitude fluctuations in SMMW laser beams propagating thru the near ground atmosphere. Our interpretations of the results presented here involve gradients due to atmospheric turbulence.Turbulence in the atmosphere causes such effects as scintillation fading, fluctuations in the angle of arrival, and gain degradation. It has set limits to resolution and contrast for astronomers for more than a century. Limitations on imaging due to atmospheric turbu­ lence have been studied theoretically.4,5 Kemp has described conditions under which imaging instruments operating in the submillimeter wavelength region are atmospherically limited when angular scintillation exceeds the minimum resolved angle due to diffraction.5 Propagation through a turbulent atmosphere has received considerable attention for the case when turbu­ lence is caused by variations in the real part of the complex index of refraction, under conditions of negligible absorption. In general, good agreement has been found between theory and experiment at optical and radio frequencies where absorption can be neglected. At sub- millimeter wavelengths, rotational and vibrational transitions of water molecules in the at­ mosphere cause absorption that must be taken into account. It is therefore necessary to consider fluctuations in both the real and imaginary parts of the complex index of refraction to describe and predict performance of an imaging radar that operates at wavelengths near 1mm.Propagation of a plane monochromatic wave through a medium in which fluctuations in both real and imaginary parts of the complex index of refraction occur has been treated by Armand, Izyumov, and Sokolov.^ They used the method of smooth perturbations proposed by Rytov? to solve the scalar wave equation.V2^ + K2 N% = 0 (1)where ty and KQ are the wave function and the free space wave number respectively, and N is the complex index of refraction with fluctuating real and imaginary parts. The calculated effects of phase and amplitude fluctuations expected from this treatment were compared to*0n leave to Sandia National Laboratories, Albuquerque, NM