V.V. Parshin

The absorption investigation in CVD-diamond plates and windows at 50–200 GHz

Abstract Absorption investigations into a number of CVD-diamond plates, as well as absorption investigations into the output windows of megawatt gyrotrons in the range of frequencies 50–200 GHz have been carried out. Significant additional surface absorption was found and its influence was defined. The theory for measurement of this additional absorption was developed. The possible mechanism of microwave absorption in CVD-diamond is briefly discussed.

Resonator spectrometer for precise broadband investigations of atmospheric absorption in discrete lines and water vapor related continuum in millimeter wave range.

The instrument and methods for measuring spectral parameters of discrete atmospheric lines and water-related continuum absorption in the millimeter wave range are described. The instrument is based on measurements of the Fabry-Pérot resonance response width using fast phase continuous scanning of the frequency-synthesized radiation. The instrument allows measurement of gas absorptions at the cavity eigenfrequencies ranging from 45 to 370 GHz with the highest to date absorption variation sensitivity of 4x10(-9) cm(-1). The use of a module of two rigidly bounded maximum identical resonators differing in length by exactly a factor of two allows accurate separation of the studied gas absorption and spectrometer baseline, in particular, the absorption by water adsorbed on the resonator elements. The module is placed in a chamber with temperature controlled between -30 and +60 degrees C, which permits investigation of temperature dependence of absorption. It is shown that systematic measurement error of discrete atmospheric line parameters does not exceed the statistical one and the achieved accuracy satisfies modern demands for the atmospheric remote sensing data retrieval. Potential systematic error arising from the neglect of the effect of water adsorption on mirror surfaces is discussed. Examples of studies of water and oxygen spectral line parameters as well as continuum absorption in wet nitrogen are given.

Modern Resonator Spectroscopy at Submillimeter Wavelengths

Classical resonator spectroscopy methods have been realized for the first time in the submillimeter wave range. A resonator spectrometer was developed earlier at the IAP RAS on the basis of an open Fabry-Perot resonator excited by the radiation of a backward wave oscillator whose frequency is stabilized by the phase lock loop system. This spectrometer is successfully used for high-precision measurements of the dielectric properties of solid, liquid, and gaseous dielectrics as well as for the metal and coating reflection measurements in the 36-370 GHz range. In this paper, we report an extension of the upper limit of the spectrometer operation frequency to 520 GHz. Features of operation of the main spectrometer systems in the extended frequency range are analyzed. The broadband measurements of absorption in modern MPCVD diamonds are presented. A continuous record of the absorption spectrum of the laboratory atmosphere in the 350-500 GHz range obtained for the first time by the high-sensitivity microwave method is demonstrated. Further prospects for extension of the spectrometer range to terahertz frequencies are discussed.

Uncertainties in the temperature dependence of the line-coupling parameters of the microwave oxygen band: impact study

A sensitivity study was performed with the commonly used millimeter-wave propagation model (MPM) to assess the impact of prescribed uncertainty in the temperature dependence of the line-coupling factors used to model the oxygen line shape. In some cases, apparent nonphysical behaviors were noticed and their impact on microwave channels was found to be significant and more importantly, nonremovable by simple calibration or bias removal. The Advanced Microwave Sounding Unit configuration in particular was tested because of its prime importance in current data assimilation. This impact, which reached a maximum of a few Kelvin in some channels, is modulated nonlinearly by the amount of water vapor present in the atmosphere, with maximum impact in dry air situations, which makes it dependent on the brightness temperature itself. These errors directly impact the error budget of any physically based geophysical retrieval. The line shape of these O/sub 2/ lines has impacts across the microwave spectrum, through the wing effects. This includes channels near 23, 36, and 85 GHz, commonly used in operational radiometry.

Methods for investigating thin dielectric films in the millimeter range

An original method based on determining the characteristics of open Fabry-Perot resonator modes with different polarizations is proposed for measuring parameters of dielectric plates and films with a thickness smaller than λ/2 in the millimeter and submillimeter wavelength ranges. This method is used for determining the refractive index and tanδ, as well as the thickness of films made of isotropic materials. For anisotropic films of known thickness, the method makes it possible to measure the permittivity tensor components. Popular film materials such as Teflon (polytetrafluoroethylene, PTFE), lavsan (Mylar, polyethyleneterephthalate, PETP), and polyamide with a minimal thickness of ∼5 μm are investigated. Appreciable anisotropy of roll film materials and the dependence of the dielectric properties on the thickness, which is associated with manufacturing technology, are revealed. The dependence of the refractive index and tanδ on the air humidity is investigated

Precision Resonator Microwave Spectroscopy in Millimeter and Submillimeter Range

By the use of Fabry–Perot resonator with quality factor ≃ 600 000 and fast precision (down to one Hertz) frequency control of coherent millimeterwave radiation source the 20 Hz accuracy in measurements of the width of resonance curve is obtained. This accuracy corresponds to the detection of 1.8 × 10−3 dB/km absorption in the sample filling the resonator and exceeds the before known sensitivity more than by an order of magnitude. The example of precise measurement of 60 GHz oxygen absorption band in the real atmosphere is presented. The new possibilities of application of precision resonator microwave spectroscopy to the atmospheric problems as well as to the ultra-low absorptions measurements in dielectrics and metal surfaces up to Terahertz frequencies are discussed.

Reply to the comment on "Uncertainties in the temperature dependence of the line-coupling parameters of the microwave oxygen band: impact study"

For original paper see Boukabara et al., ibid., vol. 43, no.5, p.1109 (2005) and for comment see Rosenkranz., p.2160 (2005).The impact study described by Boukabar is, in essence, a theoretical sensitivity analysis of the temperature dependence of the line coupling coefficients in the microwave oxygen absorption band as accounted for in the millimeter-wave propagation model (MPM). The study was driven by emerging highly demanding requirements in terms of accuracy of the brightness temperatures measurements and therefore equally demanding requirements for models that simulate them. The main conclusions of the study are twofold: 1) the need to extend experimental measurements to the lower range of the atmospheric temperatures and 2) that errors due to uncertainties in the temperature dependence, are not necessarily entirely removable by a bias correction. The spectral region, where most of the impact was found, is located at the edge of the absorption band, especially in dry conditions. Rosenkranz commented on the work by Boukabara et al. and they now reply to these comments.

Material aspects of high power microwave windows

Summary form only given. The candidate materials for high power mm-wave window materials like sapphire, boron nitride, high-resistivity silicon, CVD-diamond and silicon carbide will be presented and the production technology briefly described. The material parameters will be discussed which are most relevant for high power operation. Experimental data of absorption values in selected grades of window materials will be reported resulting from measurements at frequencies from 40 GHz up to 380 GHz and in the temperature interval of 40-900 K. The main absorption mechanisms for ultra-low loss materials will be described and prospects will be given for advancing these materials in the THz region. It will be shown that CVD diamond is the only material currently available with a transmission capability of several Megawatt in CW operation and that CVD Diamond is the primary candidate material for high-power THz applications.

The reflectivity investigation of satellite and ground based antennas AT 100-380 GHZ and 80 K

The satellite and ground based antenna reflectivity investigations have been made. It was revealed that the reflective coverings of some composite carbon fibers antennas have inadmissibly more reflective losses which lead to the essential noise temperature increasing of high sensitive cooled receivers. The significant anisotropy of carbon fibres antenna covering have been found. Unexpected reflectivity behaviour of satellite antenna reflectors at liquid nitrogen temperatures was found.

Polycrystalline CVD diamond-a new dielectric material for microwave electronics

Properties of polycrystalline diamond wafers produced by chemical vapor deposition (CVD) are described which show their importance for applications in microwave electronics. Today CVD diamonds become an engineering material whose unique thermal and electrophysical parameters make it very promising for a new generation of microwave devices.