Abstract. Laboratory calibrations of the Cloud Droplet Probe (CDP) sample area and droplet sizing are performed using water droplets of known size, generated at a known rate. Although calibrations with PSL and glass beads were consistent with theoretical instrument response, liquid water droplet calibrations were not, and necessitated a 2 μm shift in the manufacturer's calibration. We show that much of this response shift may be attributable to a misalignment of the optics relative to the axis of the laser beam. Comparison with an independent measure of liquid water content (LWC) during in-flight operation suggests much greater biases in the droplet size and/or droplet concentration measured by the CDP than would be expected based on the laboratory calibrations. Since the bias in CDP-LWC is strongly concentration dependent, we hypothesize that this discrepancy is a result of coincidence, when two or more droplets pass through the CDP laser beam within a very short time. The coincidence error, most frequently resulting from the passage of one droplet outside and one inside the instrument sample area at the same time, is evaluated in terms of an "extended sample area" (SAE), the area in which individual droplets can affect the sizing detector without necessarily registering on the qualifier. SAE is calibrated with standardized water droplets, and used in a Monte-Carlo simulation to estimate the effect of coincidence on the measured droplet size distributions. The simulations show that extended coincidence errors are important for the CDP at droplet concentrations even as low as 200 cm−3, and these errors are necessary to explain the trend between calculated and measured LWC observed in liquid and mixed-phase clouds during the Aerosol, Radiation and Cloud Processes Affecting Arctic Climate (ARCPAC) study. We estimate from the simulations that 60% oversizing error and 50% undercounting error can occur at droplet concentrations exceeding 400 cm−3. Modification of the optical design of the CDP is currently being explored in an effort to reduce this coincidence bias.

Water Droplet Calibration of the DMT Cloud Droplet Probe (CDP) and In-Flight Performance in Liquid, Ice and Mixed-Phase Clouds during ARCPAC

Terahertz (THz)-band communications are a key enabler for future-generation wireless communication systems that promise to integrate a wide range of data-demanding applications. Recent advancements in photonic, electronic, and plasmonic technologies are closing the gap in THz transceiver design. Consequently, prospect THz signal generation, modulation, and radiation methods are converging, and the corresponding channel model, noise, and hardware-impairment notions are emerging. Such progress paves the way for well-grounded research into THz-specific signal processing techniques for wireless communications. This tutorial overviews these techniques with an emphasis on ultra-massive multiple-input multiple-output (UM-MIMO) systems and reconfigurable intelligent surfaces, which are vital to overcoming the distance problem at very high frequencies. We focus on the classical problems of waveform design and modulation, beamforming and precoding, index modulation, channel estimation, channel coding, and data detection. We also motivate signal processing techniques for THz sensing and localization.

An Overview of Signal Processing Techniques for Terahertz Communications

Terahertz (THz)-band communications are a key enabler for future-generation wireless communication systems that promise to integrate a wide range of data-demanding applications. Recent advances in photonic, electronic, and plasmonic technologies are closing the gap in THz transceiver design. Consequently, prospect THz signal generation, modulation, and radiation methods are converging, and corresponding channel model, noise, and hardware-impairment notions are emerging. Such progress establishes a foundation for well-grounded research into THz-specific signal processing techniques for wireless communications. This tutorial overviews these techniques, emphasizing ultramassive multiple-input–multiple-output (UM-MIMO) systems and reconfigurable intelligent surfaces, vital for overcoming the distance problem at very high frequencies. We focus on the classical problems of waveform design and modulation, beamforming and precoding, index modulation, channel estimation, channel coding, and data detection. We also motivate signal processing techniques for THz sensing and localization.

Time-of-flight (ToF) sensors using light pulses or continuous waves allow accurate distance measurements. Three-dimensional imagers can be based on an array of timing or time-gated integration pixels. Single-photon avalanche diodes (SPADs) have been increasingly chosen as the pixel’s photodetector device to develop fast, long-range ToF sensors. Solid-state ToF cameras are replacing other alternatives, showing attractive characteristics and bringing up new potential applications. This paper presents the technical evolution of SPAD ToF 3D imaging sensors, and provides insight into their development over the last few decades. Starting with the first SPAD imagers reported in the early 2000’s, various direct and indirect arrays up to present day state-of-the-art prototypes are referenced. The existing methods, options and possible implementations are described, addressing their advantages and drawbacks, and showing the progress yet to be made. The performance of the different presented approaches are given and compared.

A Review of Single-Photon Avalanche Diode Time-of-Flight Imaging Sensor Arrays

Transparent wood (TW) is an interesting polymer biocomposite with potential for buildings and photonics applications. TW materials need to be eco-friendly and readily processed with few defects, for high optical transmittance and low transmission scattering at wide angles (haze). Two wood templates with different lignin-content are impregnated with a new thiol-ene thermoset system. The more eco-friendly bleached wood template results in transparent wood with high optical transmission and much reduced transmission haze, due to strong reduction of interfacial air gaps. Characterization includes template composition, thiol-ene distribution, and polymerization in wood cell wall by EDX and confocal Raman microscopy, also NMR and DSC, tensile testing and FE-SEM fractography for morphology and wood/thiol-ene interface adhesion assessment. The wood template is a true nanocomposite with thiol-ene polymer located inside the nanoporous wood cell wall. Advanced TW applications require not only appropriate wood template modification and careful polymer matrix selection but also tailoring of the process to impregnation and polymerization mechanisms, in order to reduce optical defects.

Transparent Wood Biocomposites by Fast UV-Curing for Reduced Light-Scattering through Wood/Thiol-ene Interface Design.

An instrument for simultaneous measurement of absorption and reduced scattering coefficients of optically turbid materials is presented. Determination of the coefficient is based on scattering profile of a laser beam illuminating the sample at oblique incidence angle. The back reflected profile is recorded by imaging the scattered laser spot on material surface, and the coefficients calculated by determining the asymmetry and the slope of the reflectance profile. Several calibrated phantoms having different combinations of absorption and reduced scattering coefficients were measured. Measurement ranges were from 0.0047 to 0.118 mm-1 for absorption and from 0.41 to 16.24 mm-1 Results given by our instrument were compared together with manufacturer's values against calibrated reference results. Acquired absorption and scattering properties were in most of the cases smaller than reference values and variations of the relative errors were small. Instrument was not observed to be sensitive to small spatial deviations of sample from ideal measurement geometry. 95% of the determined parameters were within 10% of the average values in repeatability experiments.

Instrument for measurement of optical parameters of turbid media by using diffuse reflectance of laser with oblique incidence angle

Results at mm wave frequencies have shown that it is important to evaluate co-channel interference between directional beams that cross each other. Terahertz frequency bands, especially around 300 GHz, has been considered for future wireless communications. As far as we know, no work has considered the rain induced interference at THz frequencies. In this work, we evaluate the rain induced interference at 300 GHz. Results are calculated also at 60 GHz in order to make a comparison. We combined bistatic radar equation, first order multiple scattering approximation and full Mie scattering calculations to existing drop size distribution models to estimate interference due to rain. Considered effective path lengths between transmitter and receiver are 100 m and 500 m and the effect of selected drop size distribution is studied. Overall interference levels was observed to be approximately 20 dBm smaller at 300 GHz than at 60 GHz and angular dependency of interfering power was much more forward oriented for 300 GHz frequency. The results show that rain induces interference has significantly different behaviour at THz as compared to lower frequencies.

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Rain Induced Co-Channel Interference at 60 GHz and 300 GHz Frequencies

Computational fluid dynamics and particle tracing simulations are presented for a cloud droplet sensor Airspeeds and streamlines around the sensor are calculated at several wind speeds and their effect on the droplet sampling are examined Particle tracing is used to study the effect of different wind speeds and droplet sizes on the sampling of the cloud droplets Simulated droplet concentrations are confirmed by comparing them with measured wind tunnel data Results demonstrate clear sampling effects that are functions of both wind speed and droplet size Optimal compromise between maximal measurement volume and sampling effects is found and a simple approximation for sensor’s sampling bias is presented The results show that CFD simulations can give valuable information about the sampling of droplets in an ideal environment with known droplet concentrations Even in a wind tunnel, the true test conditions are often impossible to accurately determine Thus by simulating the sampling effects in different conditions, the sensor can be calibrated for a wide range of naturally occurring cloud conditions

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Study of the Aerodynamic Sampling Effects of a Holographic Cloud Droplet Instrument

Scattering and attenuation properties of rain using various drop models are calculated using Mie theory, the T-matrix method and numerical technique respectively. Ellipsoidal and oblate drops use the axis ratio of raindrops reported in literature. Scattering are presented both for small volumes containing a single drop and for large volumes with multiple drops in which the statistics of drop size distribution need to be taken into account. The angular dependence of scattering, absorption and scattering cross sections and polarization ratios are investigated. For single scattering the angular dependence and cross sections of nonspherical drops differed from those of a spherical drop. Differences between ellipsoid and oblate drops were minimal. Drop shape affected the polarization mostly at 40–140 degree detection angles. The averaged linear attenuation and rain induced cross channel signal were studied. Attenuation was close to the ITU-R.838 model and in the same order of magnitude as the attenuation due to atmospheric moisture. The cross channel signal was calculated as a function of rain rate and transmitter/receiver angle. Vertical polarization was observed to produce a higher cross channel signal than horizontal polarization. It was concluded that drop shape is not an important factor at 300 GHz due to small drops dominating the scattering signal. Overall, the results showed that in single scattering, the ellipsoid is a good approximation for raindrops and that for large volumes, a spherical drop approximation and a Mie solution may be sufficient at frequencies of 300 GHz and higher.

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The Effect of Drop Shape, Sensing Volume and Raindrop Size Statistics to the Scattered Field on 300 GHz

Immersion liquid method is used to determine refractive indices of wood material An integrating sphere setup is used to measure optical transmission of liquid containing small wood particles Sweeping the refractive index of the liquid affects its transmission properties through light scattering caused by refractive index mismatch between solid wood particles and the liquid The refractive index of the solid material is obtained by finding the refractive index value of the liquid which provides the lowest scattering value (maximum transmission) The refractive indices of softwood samples were determined within a wavelength range from 500 nm to 800 nm The refractive index of sapwood and heartwood were found to be 156 and 157 at sodium D-line (5893 nm), respectively Measured refractive index values will be used when building models of wood for optical simulations

Harri Juttula

Papers

Instrument for measurement of optical parameters of turbid media by using diffuse reflectance of laser with oblique incidence angle

Rain Induced Co-Channel Interference at 60 GHz and 300 GHz Frequencies

Study of the Aerodynamic Sampling Effects of a Holographic Cloud Droplet Instrument

The Effect of Drop Shape, Sensing Volume and Raindrop Size Statistics to the Scattered Field on 300 GHz

Determination of refractive index of softwood using immersion liquid method