Atmospheric Molecular Absorption Research Articles

Atmospheric transmission calculation of diode‐pumped alkali laser energy transfer

AbstractWireless power transfer has become a research hotspot in the field of energy transmission, among which laser energy transfer is one of the best approaches for long‐distance wireless transmission. Based on good atmospheric transmittance, especially the wavelengths consistent with the response range of mainstream Si and GaAs photovoltaic cells with relatively high photoelectric efficiency, diode‐pumped alkali laser (DPAL) energy transfer will have huge prospects. To solve the problem of long‐distance laser energy transfer of DPALs, laser atmospheric molecular absorption was calculated by analyzing the atmospheric transmission absorption effects of laser beams with specific wavelengths. The atmospheric transmission of three typical alkalis at different atmospheric modes and altitudes was calculated and compared, and practical suggestions were given.

Rapid calculation of atmospheric molecular absorption characteristics in the terahertz band based on wideband K-distribution method

This paper focuses on rapidly calculating atmospheric molecular absorption properties in the 0.3–30 THz frequency range of terahertz waves. It conducts research related to spectral radiation models and fast computation model databases. Based on the latest version of the High-resolution transmission molecular absorption database-2020, in conjunction with the line-by-line (LBL) spectral radiative calculation method, the spectral absorption characteristics of nine atmospheric molecules (H2O, CO2, O3, NO, N2O, NO2, NH3, SO2, CH4) are analyzed. The relative contributions of the eight other gas molecules absorption abilities in the terahertz range are examined, with H2O molecule serving as the reference. Taking into account the non-gray absorption properties of atmospheric molecules and the need for fast computation, this study divides the frequency range of 0.3–30 THz into 24 calculation bands based on the spectral absorption distribution characteristics of atmospheric molecules. Utilizing LBL calculations with a resolution of 0.01 cm−1, a non-correlated wideband K-distribution model (WBK) is developed, which achieves nearly the same computational accuracy as LBL. Furthermore, the WBK radiation calculation model is validated under both uniform and non-uniform atmospheric conditions based on earth atmospheric environmental parameters. An exponential kernel fitting coefficient table for the rapid calculation of atmospheric molecular absorption coefficients is established based on the WBK radiation calculation model to improve computational efficiency further. This database enables fast calculation of 24 broadband absorption coefficients (corresponding to 7 Gaussian integration points) within 0–100 km altitude in earth atmosphere.

Analytical study on the atmospheric absorption properties of fiber lasers in a 1 µm band.

Atmospheric absorption is one of the significant factors influencing the atmospheric propagation efficiency of high-power fiber lasers. Based on typical atmospheric environment parameters, the atmospheric absorption (aerosol and atmospheric molecular absorption) of fiber lasers with different linewidths and center wavelengths in a near 1µm band is numerically calculated. The results show that the atmospheric absorption of common (several nanometer scales) and narrow linewidth (<1n m scale) lasers have distinctly different external characteristics, but the intrinsic mechanisms are interconnected. Due to the high wavelength selectivity of atmospheric molecular absorption, this work focuses on the factors influencing water vapor (main absorbing gas) absorption of different linewidth lasers and the corresponding low absorption region. Based on the fine atmospheric absorption spectra of different types of fiber lasers, the output spectra of fiber lasers can be artificially designed to avoid strong absorption during atmospheric propagation and achieve improved high-energy laser propagation efficiency. The above method provides a partial reference for designing and optimizing the light source parameters of high-power fiber lasers for atmospheric propagation.

Energy and Spectral Efficiencies of Cell-Free Millimeter-Wave Massive MIMO Systems Under Rain Attenuation Based on Ray Tracing Simulations

Future wireless communication systems depend on the network’s ultra-densification, on the application of massive multiple-input multiple-output (mMIMO) techniques and on the use of higher frequency bands to satisfy the ever-increasing demands for capacity. The operation of cell-free (CF) networks in the millimeter wave (mmWave) spectrum combines those principles, because they are composed of multiple access points (APs) distributed over a geographic region which serve a small number of users. Despite the extensive available bandwidth, the mmWave spectrum imposes high path losses and significant atmospheric molecular absorption to the links. In addition, in this frequency range, rain attenuation can notably degrade communications. Therefore, this article presents a study of the impact of rain attenuation on CF networks operating in the mmWave spectrum, this is in the 26 GHz, 38 GHz and 73 GHz frequency bands, based on site-specific ray tracing simulations. The propagation simulation is characterized under the effects of reflection, diffraction, diffuse scattering, atmospheric molecular absorption, vegetation losses and rain attenuation. The channel model is characterized using a hybrid approach, with the large-scale parameters determined by ray tracing in an environment subject to random Rician small-scale fading. The system performance is measured by the sum spectral efficiency (SSE) and energy efficiency (EE). According to the results, it was observed that AP densification protects the network against the effect of rain attenuation. Furthermore, even in sparse networks, the CF system has low sensitivity with respect to the precipitation rate, resulting in relatively small reductions in the average SSE and EE.

Calibration of NOMAD on ESA's ExoMars Trace Gas Orbiter: Part 2 – The Limb, Nadir and Occultation (LNO) channel

The Nadir and Occultation for MArs Discovery (NOMAD) instrument is a 3-channel spectrometer suite on the ESA ExoMars Trace Gas Orbiter. Since April 2018, when the nominal science mission began, it has been measuring the constituents of the Martian atmosphere. NOMAD contains three separate spectrometers, two of which operate in the infrared: the Solar Occultation (SO) channel makes only solar occultation observations, and therefore has the best resolving power (∼20,000) and a wider spectral region covering 2.2–4.3 ​μm. The Limb, Nadir and Occultation (LNO) channel covers the 2.2–3.8 ​μm spectral region and can operate in limb, nadir or solar occultation pointing modes. The Ultraviolet–VISible (UVIS) channel operates in the UV–visible region, from 200 to 650 ​nm, and can measure in limb, nadir or solar occultation modes like LNO. The LNO channel has a lower resolving power (∼10,000) than the SO channel, but is still typically an order of magnitude better than previous instruments orbiting Mars. The channel primarily operates in nadir-viewing mode, pointing directly down to the surface to measure the narrow atmospheric molecular absorption lines, clouds and surface features in the reflected sunlight. From the depth and position of the observed atmospheric absorption lines, the constituents of the Martian atmosphere and their column densities can be derived, leading to new insights into the processes that govern their distribution and transport. Surface properties can also be derived from nadir observations by observing the shape of the spectral continuum. Many calibration measurements were made prior to launch, on the voyage to Mars, and continue to be made in-flight during the science phase of the mission. This work, part 2, addresses the aspects of the LNO channel calibration that are not covered elsewhere, namely: the LNO ground calibration setup, the LNO occultation and nadir boresight pointing vectors, LNO detector characterisation and nadir/limb illumination pattern, instrument temperature effects, and finally the radiometric calibration of the LNO channel. An accompanying paper, part 1 (Thomas et al., 2021, this issue), addresses similar aspects for SO, the other infrared channel in NOMAD. A further accompanying paper (Cruz-Mermy et al., 2021, this issue) investigated the LNO radiometric calibration in more detail, approaching the work from a theoretical perspective. The two calibrations agree with each other to within 3%, validating each calibration method. • The LNO channel is 1 of 3 spectrometers in NOMAD on ExoMars Trace Gas Orbiter. • The LNO channel observes Mars in limb, nadir and solar occultation modes. • This article describes the operational modes and calibration of the LNO channel. • The boresight, detector illumination, slant, bad pixels and saturation were found. • The procedure for radiometrically calibrating nadir observations is described.

Evaluation of Cell-Free Millimeter-Wave Massive MIMO Systems Based on Site-Specific Ray Tracing Simulations

Ultra densification of the number of antennas combined with the use of large bandwidths in the millimeter wave (mmWave) spectrum is considered one of the main methodologies to achieve the quality of service requirements for future generations of wireless communications. Massive multiple-input multiple-output (mMIMO) cell-free (CF) systems have a large number of access points (APs) distributed in the coverage area, serving simultaneously a smaller number of users consuming the same time-frequency resources. In order to support realistic CF networks designs, this work proposes a performance analysis based on ray tracing simulations. The propagation modeling considers reflection, diffraction, diffuse scattering, atmospheric molecular absorption and foliage and rainfall losses. CF networks with APs equipped with multiple antennas operating in the 26 GHz, 38 GHz and 73 GHz bands are evaluated. From the simulation results, the communication channel is characterized and parameterized. Different performance analysis of CF networks are performed, based on downlink spectral efficiency. In addition, the performance of CF networks in rainy environments is evaluated, and it has been observed that this architecture promotes resistance to the effect of rain attenuation.

Impact of atmospheric molecular absorption on the temporal and spatial evolution of ultra-short optical pulses.

We present a rigorous study on the impact of atmospheric molecular absorption on the linear propagation of ultrashort pulses in the mid-infrared wavelength region. An ultrafast thulium-based fiber laser was employed to experimentally investigate ultrashort-pulse propagation through the atmosphere in a spectral region containing several strong molecular absorption lines. The atmospheric absorption profile causes a significant degradation of the pulse quality in the time domain as well as a distortion of the transverse beam profile in the spatial domain. Numerical simulations carried out in the small signal limit accurately reproduce the experimental observations in the time domain and reveal that the relative loss in peak power after propagation can be more than twice as high as the relative amount of absorbed average power. Although their nature is purely linear (i.e. the intensities considered are sufficiently low) the discussed effects represent significant challenges to performance-scaling of mid-infrared ultrafast lasers operating in spectral regions with molecular absorption bands. Guidelines for an efficient mitigation of the pulse quality degradation and the beam profile distortion are discussed.

Design of a High Efficiency Acoustic Phased Array Antenna for an Acoustic Wind Profiler

Improvements to the performance of acoustic wind profilers necessitate designing efficient acoustic antennas for the profilers, so that weak echoes can be received to enhance data availability. An efficient acoustic antenna needs to optimize electrical to acoustic and acoustic to electrical conversion efficiency as measured at the antenna axis. It also needs to provide a good directional response, and be able to handle increased electrical power. Phased array antennas, designed using piezo-electric transducers, should provide better characteristics in this regard. In the present study several different transducer elements were characterized with respect to their relative efficiencies. On the basis of this characterization, measurement of acoustic background noise at the expected site of antenna location and comparative study of atmospheric molecular absorption at the element peak efficiency frequencies, it is concluded that Motorola KSN 1025 A transducer is the best speaker element for the array antennas for profilers up to around 500 meters. The present array antenna has been designed using 104 KSN 1025 A piezo-electric transducer elements. The corner elements were removed from a 16 × 8 element speaker matrix, yielding an irregular octagonal pattern. Before installing the elements in the array, each element was individually characterized for its transmit and receive conversion efficiency, and variations in the product of these efficiencies (conversion gain) amongst the different elements, has been usefully employed to taper the array gain from centre to periphery. The antenna was systematically characterized, in an acoustic anechoic chamber, with respect to its axial transmit and receive conversion efficiencies and directional response.

Review of empirical methods for the calculation of the diurnal NO 2 photolysis rate coefficient

The rate coefficient for the photolysis of nitrogen dioxide ( J NO 2 ) is used in ambient photochemical air quality models as this photodissociation leads to the formation of tropospheric ozone. This review examines the effectiveness and efficiency of four methods for the calculation of J NO 2 . The nature of J NO 2 is reviewed briefly with regard to temperature, pressure, zenith angle, altitude, albedo, clouds, atmospheric turbidity and atmospheric molecular absorption. Comparison of the four methods against a radiation transfer model, for three clear-sky days at different latitudes in Australia, shows that any of the methods may be used to approximate a clear-sky diurnal profile of J NO 2 , as the maximum variation between all models at noon was found to be 15%. However, more care needs to be taken when calculating other photolysis rate coefficients, such as J O 3 , as these are more susceptible to other variables such as the ozone column concentration. A simple procedure for calculating cloudy-day J NO 2 is also illustrated.

Finite amplitude distortion of spherically diverging intense sound waves in air

Waveform distortion in a free field of intense sinusoidal pressure waves propagating in air was measured along the axis of a 122-mm-diam plane piston source for four fundamental frequencies, 14.7, 24.6, 35.7, and 63.6 kHz. The acoustic pressure of the fundamental and harmonic components was measured versus source amplitude at distances from 0.15 to 2.00 m from the source at amplitudes up to 160 dB re 20 μPa. It was observed (1) that the pressure waveform reaches the stable form of an iterated shock at a smaller distance from the source for larger source pressure amplitudes, (2) that waveform distortion increases with frequency at a given source amplitude and a given distance, and (3) that a limiting level exists for the fundamental amplitude at a given point in a spherically diverging sound field with increasing source level. An empirical propagation relation due to Allen was further developed to include frequency dependence and air absorption. Complete shock formation was inhibited for the highest frequency sound field, presumably because of atmospheric molecular absorption.

Molecular absorption of infrared laser radiation in the natural atmosphere

This paper discusses the information available concerning atmospheric molecular absorption of radiation in the 1 to 25μm range. Particular attention is given to the high spectral resolution requirements for predicting laser transmittance. The following subjects are reviewed: the content and variability of the atmosphere, calculations of transmittance and the data on which they are based, and laser measurements of molecular absorption.