Showing papers by "Norman S. Kopeika published in 1983"

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

Abstract: 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.

Wavelength tuning of GaAs LED's through surface effects

Abstract: A novel wavelength tuning technique, applicable to shallow junction surface-emitting LED's, is shown to exhibit wavelength changes on the order of 50 nm, depending on current and gas environment, the tuning technique is external and is continuous, repeatable, reversible, controllable, and apparently nondestructive. Thermal and pressure changes in E g do not appear to be dominant mechanisms. The dominant mechanism is suspected to involve surface effects, such as desorption of adsorbed gases, which depend upon self-heating in vacuo. The one-to-one relationship between diode voltage and peak emission wavelength suggests photon-assisted tunneling as the tuning mechanism although surface band-bending changes may play some role. Surface effects alone, such as generated by UV illumination, are shown to yield noticeable LED wavelength shift, while bulk heating with red light of much higher average irradiance does not. This technique is, in principle a general technique independent of semiconductor material. The results described here should also have implications for optical-fiber communication, gas detection and identification, and space applications.

Observation of Cooper minima in excited- s -state photoionization cross sections in neon and argon

Abstract: Observations of Cooper minima in excited-state photoionization cross sections for the Ne $3s$ and Ar $4s$ levels are reported. Prebreakdown Ne and Ar discharges under irradiation by light in the 200-400-nm wavelength range yield spectral dependences of photoionization cross sections with minima at the ionization threshold of these states in Ne and slightly distant from them in Ar. They are in close agreement with certain theoretical predictions. Autoionization structure which exists between the two ionization limits $^{2}p_{\frac{3}{2}}$ and $^{2}p_{\frac{1}{2}}$ was not observed in any of the Ne or Ar photoionization spectra due to both low spectral resolution and the high density of levels in this spectral region. Application of these measurements to ultraviolet-radiation detection is examined. High detection sensitivity of ultraviolet light is indicated.

Spectral tuning and linewidth narrowing of shallow-junction surface emitting GaAs LED́s through γ-ray irradiation

Abstract: Alterations of device characteristics as a result of γ-ray irradiation of shallow-junction surface emitting devices are different from those of deep-junction devices with respect to LED emission intensity reduction, I-V curve, line shape, and spectral shift. In particular, much larger spectral shifts in the opposite direction-toward longer wavelengths-are reported here than those found in the literature for deep-junction devices. A qualitative model based upon photochemical doping and changes in surface band bending is proposed to explain these phenomena. Changes in surface emitting shallow-junction optical radiation source device characteristics brought about by γ-ray irradiation are desirable ones for utilization in most optical fiber communication systems. These changes include linewidth narrowing, decreased time response, and decreased material dispersion because of the emission wavelength change.

Effects Of Aerosols On Imaging Through The Atmosphere: Spatial Frequency And Wavelength Dependent Effects

Abstract: Experiments carried out over diagonal lines of sight through the entire atmosphere support the concept of spatial coherence degradation through forward scattering as described by an aerosol transfer function which strongly affects the wavelength dependence of imaging through the atmosphere. Airborne-particulate size and concentration are affected strongly by wind strength and soil moisture. Changes in weather that result in changes in average particulate size of airborne soil-derived particulates also strongly change the wavelength dependence of resolution through the atmosphere as a result of changes in the wavelength dependence of the scattering coefficient. Knowledge of such effects can therefore permit prediction of spectral regions most suitable for imaging through the atmosphere.© (1983) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.