N. S. Kopeika

Heterodyne detection through rain, snow, and turbid media: effective receiver size at optical through millimeter wavelengths.

Both scattering and turbulence can effect the spatial coherence of short wavelength signals propagating through the open atmosphere. In this paper, the influence of forward scattering on heterodyne receiver performance is investigated, taking into account turbulence. It is shown that the effect of forward scattering is to reduce the effective heterodyne receiver area through spatial coherence degradation. A common approach to scattering as an attenuation phenomenon is not always valid. Generally, this approach underestimates the SNR. The accuracy of the attenuation approach depends on the ratio R of the actual receiver diameter to the scattering particle diameter. If R >100, scattering is essentially large angle and the typical treatment of scattering as an attenuation effect is indeed justified. However, for small R, forward scattering is primarily small angle, field coherence is noticeably affected by forward scattering, and the attenuation approach is not valid. Further, it is shown that the SNR is improved when the ratio of the scattering particulate size to turbulence coherence diameter decreases. From the practical point of view, the most important result of this study is that small receivers use their area more effectively than large receivers. Thus, an array of several small receivers may perform better than one large receiver with the same total area. The treatment here is particularly relevant for coherent detection through clouds, fog, precipitation, and turbid media in general, including liquid media.

Laser satellite communication network-vibration effect and possible solutions

A number of serious consortiums develop satellite communication networks. The objective of these communication projects is to service personal communication users almost everywhere on Earth. The intersatellite links in those projects use microwave radiation as the carrier. Free-space optical communication between satellites networked together can make possible high-speed communication between different places on Earth. Some advantages of an optical communication system over a microwave communication system in free space are: (1) smaller size and weight, (2) less transmitter power, (3) larger bandwidth, and (4) higher immunity to interference. The pointing from one satellite to another is a complicated problem due to the large distance between the satellite, the narrow beam divergence angle, and vibration of the pointing system. Such vibration of the transmitted beam in the receiver plane decreases the average received signal, which increases the bit error rate. We review: (1) the present status of satellite networks, (2) developing efforts of optical satellite communication around the world, (3) performance results of vibration effects on different kinds of optical communication satellite networks, and (4) seven approaches to overcome the problems caused by transmitter pointing vibration.

Inexpensive detector for terahertz imaging

Glow discharge plasma, derived from direct-current gas breakdown, is investigated in order to realize an inexpensive terahertz (THz) room-temperature detector. Preliminary results for THz radiation show that glow discharge indicator lamps as room-temperature detectors yield good responsivity and noise-equivalent power. Development of a focal plane array (FPA) using such devices as detectors is advantageous since the cost of a glow discharge detector is approximately

Terahertz detection mechanism of inexpensive sensitive glow discharge detectors

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Inexpensive THz Focal Plane Array Imaging Using Miniature Neon Indicator Lamps as Detectors

0.5 per lamp, and the FPA images will be diffraction limited. The detection mechanism of the glow discharge detector is found to be the enhanced diffusion current, which causes the glow discharge detector bias current to decrease when exposed to THz radiation.

THz Polarization Effects on Detection Responsivity of Glow Discharge Detectors (GDDs)

The detection mechanism of glow discharge plasma, which is derived from direct current gas breakdown, in neon indicator lamps was investigated in the terahertz and microwave regimes. Such devices exhibit high sensitivity to terahertz radiation. Experimental setups at 10, 100, and 250GHz were carried out and analyzed. The analysis of the experimental results shows that the dominant mechanism of the glow discharge detector (GDD) in these regimes is enhanced cascade ionization. Furthermore, the responsivity at 10GHz decreases with the increase in the dc bias current between the electrodes, while the responsivity at 100 and 250GHz increases with the dc current. This is attributed to electron-neutral atom collision frequency (ν) of the GDD being tens of gigahertz and its increasing with dc bias current according to dc field increase.

Design of inexpensive diffraction limited focal plane arrays for millimeter wavelength and terahertz radiation using glow discharge detector pixels

Development of focal plane arrays (FPAs) for mm wavelength and THz radiation is presented in this paper. The FPA is based upon inexpensive neon indicator lamp Glow Discharge Detectors (GDDs) that serve as pixels in the FPA. It was shown in previous investigations that inexpensive neon indicator lamp GDDs are quite sensitive to mm wavelength and THz radiation. The diameters of GDD lamps are typically 3-6 mm and thus the FPA can be diffraction limited. Development of an FPA using such devices as detectors is advantageous since the costs of such a lamp is around 30-50 cents per lamp, and it is a room temperature detector sufficiently fast for video frame rates. Recently, a new 8 × 8 GDD FPA VLSI control board was designed, constructed, and experimentally tested. First, THz images using this GDD FPA are given in this paper. By moving around the 8 × 8 pixel board appropriately in the image plane, 32 × 32 pixel images are also obtained and shown here, with much improved image quality because of much reduced pixelization distortion.

Effect of particulates on performance of optical communication in space and an adaptive method to minimize such effects

The detection mechanism of glow discharge plasma in neon indicator lamps has been investigated in the terahertz and microwave spectral regions. The influence of the THz radiation polarization on the responsivity of glow discharge detectors (GDD) is considered here. There are two detection mechanisms in the GDD that exist simultaneously, each at the expense of the other. Experiments with GDD neon indicator lamps in the THz regime prove that the dominant detection mechanism of the GDD is enhanced cascade ionization rather than enhanced diffusion current. Our polarization experiments in the THz regime show that when the electric field of the THz radiation is in the same direction as the DC bias electric field of the lamp, the responsivity is about 50% higher than when the THz electric field is orthogonal to the DC field. This supports the concept that the dominant detection mechanism of the GDD in the THz regime is enhanced cascade ionization.

Heterodyne Detection by Miniature Neon Indicator Lamp Glow Discharge Detectors

Development of focal plane arrays (FPAs) for millimeter wavelength and terahertz radiation is presented in this paper. FPA is based on an inexpensive glow discharge detector (GDD) that serves as a pixel in the FPA. It was shown in previous investigations [A. Abramovich et al., Appl. Opt. 46, 7207 (2007)] that those inexpensive neon indicator lamp GDDs are quite sensitive to millimeter wavelength and terahertz radiation. The diameter of the GDD lamp is 6 mm and thus the FPA can be diffraction limited. Development of a FPA using such devices as detectors is advantageous since the cost of such lamps is around

W-Band Chirp Radar Mock-Up Using a Glow Discharge Detector

0.2–0.5 per lamp, and it also serves as a room temperature detector. Experimental results at 100 GHz show that the responsivity of the terahertz FPA 4×4 GDD pixel is three times better than in previous measurements of A. Abramovich et al. [Appl. Opt. 46, 7207 (2007)].The addition of a parabolic reflector improves the accuracy of the noise equivalent power measurement which was found to be 6×10−9 W/√Hz ...