E. A. Serov

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.

Reflectivity of Metals in the Millimeter Wavelength Range at Cryogenic Temperatures

We present the results of theoretical and experimental studies of the reflectivity of metals at cryogenic temperatures in the millimeter wavelength range. High-Q Fabry-Perot resonators operated at a temperature of 4-300 K in the frequency range 150-250 GHz are used for the experimental study. Silver, copper, gold, aluminum, and beryllium reflectors with different structures are examined. It has been shown that the reflection loss at cryogenic temperatures varies considerably (several times) depending on the structure of the sample surface and the presence of impurities. The obtained data make it possible to calculate the thermal noise of cooled reflectors of the antenna systems in millimeter and submillimeter telescopes.

Modern resonator spectroscopy at SubMM wavelengths

The resonance methods of investigations are widely used in precision spectroscopy in all frequency ranges due to their high sensitivity. But the resonance systems have different specific features in each range. In the millimeter (MM) and submillimeter (SubMM) ranges, classical open Fabry-Perot resonators with an empty resonator Q-factor of about 106 (for reasonable sizes) are used for the gaseous and condensed media investigations. The present work based on the previous development is to our knowledge the first advancement of high-sensitivity classical resonator spectroscopy to frequencies of up to 520 GHz. The resonator spectrometer earlier developed at the IAP RAS, methods of measurement, and the results of studying various materials from 40 GHz up to 370 GHz are well described. So in present paper, a detailed description of the resonator excitation system and the frequency phase lock system, which were essentially modified for operation at the 350-520 GHz, is presented. Also the first results obtained using the spectrometer in the extended frequency range are reported.

Atmosphere continuum absorption investigation at MM waves

Investigation of atmospheric gases spectral properties is demanded to develop absorption models. Such models are very important for the global Earths atmosphere monitoring. Most of atmospheric lines and continuum spectral parameters are obtained in laboratory experiments. Recovered atmospheric parameters accuracy of depends directly on accuracy of laboratory measurements. Parameterization of the continuum absorption, in particular, the water vapor continuum is one of the most difficult problems. The problem is related to yet unknown physical origin of the continuum, its weakness in comparison with absorption in regular discrete lines of water molecule, and general difficulties of the water-related measurements. The water-related continuum broadband measurement by the resonator spectrometer [1] revealed strong influence of water adsorbing on resonator elements on results of the continuum parameters measurements. Analysis [2] of the MM/SubMM continuum measurement methods used in the earlier studies leads to the conclusion that water adsorbing onto mirrors was a common factor influencing the water-related continuum parameters obtained in all well-known laboratory experiments. In all these cases gas cells with multiple reflections of radiation off inner mirrors were used as the most appropriate technique for weak continuum absorption investigation, although the problem of separation of water vapor absorption and absorption by water adsorbed on mirrors was not solved in the experiments. To overcome the problem it was proposed to use the cavity length variation method [1], which permits separation of these two types of absorptions and, as a result, the accuracy of continuum absorption measurements increases.