Atmospheric Transparency Window Research Articles

Aerosol optical depth of the atmosphere over the ocean in the wavelength range 0.37–4 µm

At least two problems, the climatic impact of aerosols and improvement in techniques for space‐borne sensing, require investigation of the spatiotemporal variability of the aerosol optical depth (AOD) over the ocean. The marine atmosphere covers an area 2.5 times greater than the continental atmosphere and has essential differences in the composition and manner of aerosol variability, but the quantity of available data is significantly less. Hence, the importance of measurements of the aerosol optical characteristics in marine expeditions is clear. This paper generalizes the results of investigations of the AOD of the atmosphere carried out in the past decade onboard Russian scientific vessels in different regions of the Atlantic Ocean. Statistical characteristics of the AOD are presented for typical regions of the Northern Atlantic: near midlatitude continents, open ocean, tropical and equatorial zones. We show that aerosol turbidity in the atmosphere of the southern hemisphere decreases with latitude, and the minimum values are observed near Antarctica. The results of ship‐borne measurements of the AOD of the atmosphere are compared with the data from several island stations of the global Aerosol Robotic Network (AERONET). The spectral behaviour of the AOD in the visible and near‐infrared wavelength range (0.37–4 µm) is considered. The Ångström exponent is used for analysis of the wavelength dependence in the shortwave range, and the mean value of the AOD in four ‘atmospheric transparency windows’ from 1.2 µm to 4 µm is used in the longwave range. Special attention is paid to revealing the dependence of the AOD on relative humidity, wind velocity in the near‐water layer and the synoptic factor, the change in air mass.

Infrared (2–12 μm) solid-state laser sources: a review

The infrared domain is very attractive for many applications owing to two unique features: (i) it contains several atmospheric transparency windows, (ii) it corresponds to the ‘molecular fingerprint’ region of the electromagnetic spectrum where various molecules have strong rovibrational absorption lines. In many cases, these applications (e.g. laser surgery, trace gas monitoring, remote sensing, nonlinear spectroscopy, countermeasures, …) require coherent light radiation as the one emitted by a laser source. In this context, the choice of the proper technology is a key issue. Depending on the selected application, it could be required the source to deliver tunable emission, narrow linewidth, nearly diffraction limited beam, pulsed or continuous-wave (CW) radiation, etc. This article briefly reviews the main technologies, restricted to CW and nanosecond pulsed sources emitting in the 2–12 μm range. The technologies considered include rare-earth and transition-metal doped bulk and fiber lasers, semiconductor lasers, and optical parametric sources. Pros and cons of these technologies are then briefly discussed in the context of several selected applications. To cite this article: A. Godard, C. R. Physique 8 (2007).

Promising Material for Infrared Nonlinear Optics: NaI3O8 Salt Containing an Octaoxotriiodate(V) Anion Formed from Condensation of [IO3]− Ions

Off-center: The material NaI3O8 is obtained as millimeter-sized single crystals (see photograph of a single crystal; crystal size: 2×2×5 mm3), and its crystal structure is acentric. It is transparent up to 12.5 μm, thus covering the three atmospheric transparency windows. It generates an intense second-harmonic-generation signal and has a high optical damage threshold.

High-sensitivity 0.13–0.38-THz matrix radiometer based on superconducting bolometers for the BTA telescope

We develop a high-sensitivity matrix radiometer which will be used on the large altazimuth telescope BTA of the Special Astrophysical Observatory (SAO) of the Russian Academy of Sciences (Fig. 1) for performing astronomical observations in atmospheric transparency windows in the poorly explored frequency range from 0.13 to 0.38 THz (wavelengths from 2.3 to 0.8 mm). Mounting of the radiometer at the focal point of the 6-m BTA telescope will enhance the capability of the SAO in the transparency windows of the Earth’s atmosphere from the UV range (wavelengths about 3000 A) up to the ultimate wavelength of the RATAN-600 telescope (about 0.5 m). This will make it possible to perform unique comprehensive multifrequency studies in one observatory. The radiometer is presumed to be used with other radio and optical telescopes, both in the above-mentioned and higher-frequency intervals of the terahertz range.

New materials for infrared non-linear optics. Syntheses, structural characterisations, second harmonic generation and optical transparency of M(IO3)3 metallic iodates

Two new phases α-In(IO3)3 and β-In(IO3)3 have been synthesised by evaporation of concentrated nitric acid solution containing metallic salts and sources of iodate. Single crystal structures reveal an acentric three-dimensional network for α-In(IO3)3 and a centrosymmetric two-dimensional network for β-In(IO3)3. Temperature-dependent X-ray powder diffraction experiments shows the transformation of α-In(IO3)3 to β-In(IO3)3 at 365 °C. The phase transition is not reversible. α-In(IO3)3 is the kinetically favoured phase whereas β-In(IO3)3 is the thermodynamically stable phase. α-In(IO3)3 is isomorphic to Cr(IO3)3, Fe(IO3)3 and Ga(IO3)3 and crystallises in space group P63 (no. 173) with a = 9.541(1) Å and c = 5.266(1) Å. Structural relationships among the anhydrous non-centrosymmetric metallic iodates M(IO3)3 (M = Cr, Fe, In, Ga), M(IO3)2 (M = Mg, Mn, Co, Ni, Zn) and α-LiIO3 are described. Structural analogies between these metallic iodates allow us to prepare solid solutions as In1−xFex(IO3)3 and In1−xCrx(IO3)3. Luminescence of the In1−xCrx(IO3)3 phases has been observed. DSC analyses of β-In(IO3)3 and Ga(IO3)3 show that they decompose at 535 °C and 525 °C respectively. Fe(IO3)3, Ga(IO3)3 and α-In(IO3)3 generate second harmonics and have high non-linear coefficients and high optical damage thresholds in powder form. Furthermore, they do not absorb up to 12 µm. Consequently, they possess a large domain of transparency from visible light to the beginning of the far-infrared, including three atmospheric transparency windows. They are particularly interesting for potential applications in quadratic non-linear optics in atmospheric transparency bands II and III.

Quantum cascade laser-based free space optical communications

The advent of the quantum cascade laser (QCL) with emission wavelengths available in the infrared range from 3 µm through more than 100 µm opens up the possibility of exploiting infrared atmospheric transparency windows for free space optical communications. In an effort to establish the efficacy of using directly modulated QCLs for free space communications we have conducted a series of investigations that demonstrate the potential advantages of this technology. In these experiments we first establish that the QCL has very high modulation bandwidth. We then implement a practical free space communications link that under conditions of atmospheric fog, dust and other obscurants offers significant transparency advantage when compared with near infrared wavelength sources presently used in commercial free space optical communications links.

Experimental testing of correlations to calculate the atmospheric “transparency window” emissivity coefficient

An experimental verification of the correlations existing in literature for the calculation of the coefficient of emissivity inside the atmospheric transparency window, situated in the wavelength band 8–14 μm, has been performed. In addition, the authors propose a new correlation for the calculation of the coefficient ε in, easily applicable since it presupposes the knowledge of the global emissivity of the sky, for which there are a lot of correlations in literature.

Experimental comparison between far- and near-infrared wavelengths in free-space optical systems

We report an experimental comparison of three atmospheric transparency windows (800 nm, 1600 nm, and 10 μm) upon scattering attenuation from haze and fog. Our results prove that a 10-μm window is considerably better than the 1600 and 800 nm ones in dense and thick upslope fog conditions. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 46: 319–323, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20976

Calculation of rovibrational energy states for water vapor using the symmetric top approximation in the Pade form

Considering the water vapor molecule for K a≥ J/2⪢1 as a symmetric top, the simple analytical Pade form for vibrational rotational energy levels was proposed. The rotational spectroscopic parameters for ground and 0 1 0 vibrational states of H 2O molecule in the framework of this model have been obtained with good prediction. The absorption coefficient of water vapor in the spectral range 725–925 cm −1 has been calculated for 1000–6000 K temperatures taking into account the transitions to high-excited states up to J≤35 and J≤30 for the 0 0 0 and 0 1 0 vibrational states, respectively. It is shown that the prominent role of transitions to high-excited states is in 8–12 μm atmospheric transparency window.

Radiation Source in the Ranges 2.1–2.3 and 3.7–4.3 μm for the Spectroscopy of the Atmosphere

To expand the possibilities of spectroscopic systems based on a CO2 laser, we have mixed the frequencies of its radiation with the radiation of erbium lasers. Simultaneous use of Er3+:CaF2 (λ = 2.76 μm) and Er3+:YAG lasers (λ = 2.96 μm) makes it possible for the spectrum of generated sum and difference frequencies to overlap the ranges 2.2–2.3 and 3.6–4.3 μm that coincide with the atmospheric transparency windows. Generation of the difference frequencies of radiation of erbium lasers and of the 10.6-μm radiation of a CO2 laser were observed in CdSe at a phase-matching angle of 72°40′. In a bleached CdSe crystal of length 40 mm the efficiency of transformation has attained 12%. To sum up the frequencies, 4.5-mm ZnGeP2 crystals were used, with the efficiency of transformation being 3% and the phase-matching angle being equal to 52°.

Percolation problem in boron—Implanted mercury cadmium telluride

We used high-resolution x-ray diffraction to measure precisely structural modifications in variously composed Hg1−xCdxTe layers which were fabricated by different growth techniques and subjected to boron implantation to form p-n junctions. Analysis of implantation-induced features in the diffraction profiles allowed us to deduce the interstitials concentration remaining in the sample interior and, thus, to obtain important information on post-implantation defect migration. As a result, a percolation problem in the migration of Cd interstitials was discovered in samples with x<xc (xc=0.265 is the percolation threshold). Due to the percolation problem, the implanted samples having Cd content below and above xc exhibited very different surface recovery, which was visualized by high resolution scanning electron microscopy. It was found that additional annealing at 250–300°C stimulates diffusion of formerly locked Cd interstitials and leads to the change in the conductivity type (n-p) at the expense of remaining non-compensated vacancies. The percolation problem in samples with x < xc seems to be responsible for limited mobility of implanted boron and difficulties in boron activation in Hg1−xCdxTe-based devices for 8–12 µm atmospheric transparency window.

Some capabilities of microwave remote sensing of the atmosphere from drifting vehicles

We study the physical conditions for balloon-borne radiometric sounding of the humidity height profile in the 3-mm atmospheric transparency window, an unusual spectral band for such problems, using a balloon drifting in the tropopause. We solve the inverse problem and discuss the accuracy of reconstruction of the humidity profiles.

Features of limb sounding of minor atmospheric constitutents at various microwave lines

We analyze the characteristics of radio emission from the middle atmosphere during limb sounding of minor gas constituents with rotational transitions of the molecules located in the atmospheric transparency windows 105–118 GHz and 180–210 GHz. Optimal frequencies and height ranges for the sounding of minor gases are found by numerical modeling for typical values of the receiver bandwidth.

Master-oscilltor-amplifier electroionization carbon monoxide laser system and propagation of its radiation through atmosphere

Thus, we have created an EI carbon monoxide laser systems, operating by the scheme “master-oscillator-amplifier”. We have experimentally studied the amplification and absorption of IR (λ∼5−6 μm) by multilevel active medium of EI CO laser. Conditions of saturated amplification of EI CO laser radiation have been found. By using various methods we have performed the formation of the EI CO laser system spectrum in accordance with atmospheric transparency windows. It should be noted that EI CO laser system with selected spectral lines can be used in laser ranging, laser chemistry, and laser isotope separation.

Emission spectrum of a pulsed electron-beam-controlled CO laser with an intracavity water cell

A detailed experimental investigation was made for the first time of the time-resolved spectral and energy characteristics of a pulsed electron-beam-controlled CO laser, as a function of the water vapor content in an intracavity cell. It was found that using such a cell made it possible to considerably increase the number of rotational lines in the emission spectrum and thereby to extend the tuning range of the emission frequency of a pulsed electron-beam-controlled CO laser. The introduction of water vapor into the resonator made it possible to match the emission spectrum of such a laser to the atmospheric transparency windows in the range 5–5.5 μ. The possibility was investigated of utilizing a column of atmospheric air as a selective absorption cell.

Observation of Global 160-min Infrared (Differential) Intensity Variation of the Sun

AbstractThe method developed and the instrument designed for detecting variations of the solar limb darkening at the atmospheric transparency window of the solar opacity minimum region of λ 1.65 µ are described. This differential technique proved to be successful in rejecting undesirable low frequency noises due to the atmosphere and to the instrument. Analysis of observations made in 1977, 1978, and 1981 indicates the persistance of global fluctuations of the IR differential, center-to-limb intensity at the wellknown 160 min period with an average amplitude of about ± 2 x 10-4 in units of the ‘average Sun’ intensity near 1.65 µm.

Observation of global 160-min infrared (differential) intensity variation of the Sun

The method developed and the instrument designed for detecting variations of the solar limb darkening at the atmospheric transparency window of the solar opacity minimum region of λ 1.65 μ are described. This differential technique proved to be successful in rejecting undesirable low frequency noises due to the atmosphere and to the instrument. Analysis of observations made in 1977, 1978, and 1981 indicates the persistance of global fluctuations of the IR differential, center-to-limb intensity at the wellknown 160 min period with an average amplitude of about ± 2 × 10−4 in units of the ‘average Sun’ intensity near 1.65 μm.

Nocturnal and diurnal performances of selective radiators

Radiative cooling of surfaces exposed to a clear sky can be used as an energy source: we have already shown in a previous paper that useful temperatures and powers can be reached using selective surfaces matched to the atmospheric transparency window. Here we present some measurements of the radiative power of a new selective surface and an example of a possible practical application.

Investigation of the absorption of radiowaves in the atmospheric transparency window ?=0.73 mm

Investigation of the absorption of radiowaves in the atmospheric transparency window ?=0.73 mm