Scintillation

Irradiance fluctuations of an infrared laser beam from a shore-to-ship data link ranging from 5.1 to 17.8 km are compared to lognormal (LN), gamma-gamma (GG) with aperture averaging, and gamma-Laguerre (GL) distributions. From our data analysis, the LN and GG probability density function (PDF) models were generally in good agreement in near-weak to moderate fluctuations. This was also true in moderate to strong fluctuations when the spatial coherence radius was smaller than the detector aperture size, with the exception of the 2.54 cm power-in-bucket (PIB) where the LN PDF model fit best. For moderate to strong fluctuations, the GG PDF model tended to outperform the LN PDF model when the spatial coherence radius was greater than the detector aperture size. Additionally, the GL PDF model had the best or next to best overall fit in all cases with the exception of the 2.54 cm PIB where the scintillation index was highest. The GL PDF model also appears to be robust for off-of-beam cente...

At present, system design usually assumes the Kolmogorov model of refractive index fluctuation spectra in the atmosphere. However, experimental data indicates that in the atmospheric boundary layer and at higher altitudes the turbulence can be different from Kolmogorov's type. In optical communications, analytical models of mean irradiance and scintillation index have been developed for a traditional Kolmogorov spectrum and must be revised for non-Kolmogorov turbulence. The image quality (resolution, MTF, etc.) is essentially dependent on the properties of turbulent media. Turbulence MTF must be generalized to include non-Kolmogorov statistics. The change in fluctuation correlations of the refractive index can lead to a considerable change in both the MTF form and the resolution value. In this work, on the basis of measurements and model calculations, the influence of non-Kolmogorov turbulence on imaging and communications through the atmosphere is estimated for different scenarios of vertical and slant-path propagation. The atmospheric model of an arbitrary (non-Kolmogorov) spectrum is applied to estimate the statistical quantities associated with optical communication links (e.g., scintillation and fading statistics) and imaging system. Implications can be significant for optical communication, imaging through the atmosphere, and remote sensing.

The propagation of satellite signals at frequencies above 10 GHz has been studied to investigate the characteristic features of tropical rain. The propagation effects, namely, attenuation, scintillation and depolarization caused by varying structures of rain have been investigated to indicate their interrelationship. The varying relationship between attenuation and scintillations indicates the evolution of turbulence during different types of rain events. Also, the rain-induced depolarization of the signal is related to rain attenuation in different manner during stratiform events, when the melting layer is present, than during convective events when large rain drops are mainly responsible for depolarization in the absence of melting layer.

This paper shows the impact of atmospheric turbulence-induced fading on the symbol decision position in the on-off keying (OOK) and the binary phase shift keying (BPSK) subcarrier intensity modulated (SIM) laser communication link. Weak turbulence is simulated in the laboratory using a chamber equipped with heating elements and fans. We have shown that in atmospheric turbulence, it is advantageous to employ modulation schemes such as pulse time and subcarrier intensity modulations that do not directly impress data on the optical irradiance as is the case with the OOK. For the OOK-modulated laser communication system, atmospheric turbulence imposes complexity on the symbol decision subsystem and by extension places a limit on the achievable bit error rate (BER) performance.

We are developing high-energy grazing-incidence optics for a balloon-borne hard-x-ray telescope. When completed the instrument, termed HERO for High Energy Replicated Optics, will have 200 cm2 effective collecting area at 40 keV and≤ 30 arcsec ...

ABSTRACT Nd3+-doped Lu2O3 (Nd: Lu2O3) is a candidate for an infrared scintillator because (i) Lu2O3 has a high density of 9.5 g/cm3 and a high atomic number of 67 and (ii) Nd3+-doped materials emit in the infrared range and the emission lines from Nd3+ can be used in medical applications since human body has a transparency window between 600 and 1,100 nm. However, it is extremely difficult to fabricate Lu2O3 single crystals using conventional crystal growth methods because of the high melting point (2,510 ° C). Using solid-state reactions, it is much easier to fabricate Lu2O3 into a ceramic structure. Therefore, Nd: Lu2O3 transparent ceramics were fabricated using a spark plasma sintering method. This technique is comparatively simple and consumes less time than other methods such as vacuum hot pressing. The scintillation properties and transmittance spectra of the as-produced ceramics were studied in both the visible and infrared regions. Radioluminescence spectra were measured in the range 800-1,200 nm. Nd3+ emission lines were observed in the transparency window of human body. Thus, these ceramic materials could be a candidate for medical imaging applications.

The effect of source coherence on the average signal-to-noise ratio  performance of free space optical communication (FSOC) systems operating in weak atmospheric turbulence is investigated with the help of the extended Huygens–Fresnel​ principle. For an FSOC system that uses a partially coherent laser source, first, the received power at the finite-sized receiver aperture is derived. Then, the power scintillation index is evaluated that reflects the aperture averaging. Using these derived optical entities, the variations of  are examined versus parameters such as the degree of source coherence, wavelength, link distance, source size, structure constant of atmosphere and the receiver aperture radius. Obtained results show that a decrease in the degree of source coherence has a positive effect on .

We study the hybrid free-space optical (FSO) and radio-frequency (RF) channel from an information theoretic perspective. Since both links operate at vastly different carrier frequencies, we model the hybrid channel as a pair of parallel channels. Moreover, since the FSO channel signals at a higher rate than the RF channel, we incorporate this key feature in the parallel channel model. Both channels experience fading due to scintillation, which is slow compared to typical signalling rates. Under this framework, we study the fundamental limits of the hybrid channel. In particular, we analyse the outage probability in the large signal-to-noise ratio (SNR) regime, and obtain the outage diversity or SNR exponent of the hybrid system. First we consider the case when only the receiver has perfect channel state information (CSIR case), and obtain the exponents for general scintillation distributions. These exponents relate key system design parameters to the asymptotic outage performance and illustrate the benefits of using hybrid systems with respect to independent FSO or RF links. We next consider the case when perfect CSI is known at both the receiver and transmitter, and derive the optimal power allocation strategy that minimises the outage probability subject to peak and average power constraints. The optimal solution involves non-convex optimisation, which is intractable in practical systems. We therefore propose a suboptimal algorithm that achieves significant power savings (on the order of tens of dBs) over uniform power allocation. We show that the suboptimal algorithm has the same diversity as the optimal power allocation strategy.

ABSTRACT Borogermanate scintillating glasses with nominal composition of (70-x)B2O3-xGeO(2)-29Gd(2)O(3)-1Eu(2)O(3) (20 <= x <= 45) were synthesized by melt- quenching method. Their structure and optical properties were characterized by transmittance spectra, Fourier transform infrared spectra (FTIR), Raman spectra, excitation and emission spectra, and X- ray excited luminescence (XEL) spectra in detail. Transparency of the as- synthesized borogermanate glasses exceeds 80% in 400- 800 nm regions. The cut- off edges of borogermanate glasses are in the vicinity of 308 nm, and they shift towards longer wavelength (red- shifted) with the elevated x values. Both glass density and mean molecular weight increase in a linear relationship with increasing values. With increasing x values, the characteristic emission intensity of both Gd3+ and Eu3+ ions decrease progressively under 276 nm excitation, while these emission intensities seem irregular under 394 nm excitation. The strongest X-ray excited luminescence (XEL) intensity occurs at x = 35 glass sample, which is improved about 35% with comparison to the x = 20 one. Additionally, the information on glass structure is also discussed by FTIR and Raman spectra.

... Am. A, vol. 24, pp. 1994-2002, July 2007. [4] RK Tyson, "Principles of adaptive optics," Academic Press: San Diego, 1991. [5] RH Hudgin, "Wave-front reconstruction for compensated imaging," J. Opt. Soc. Am. A, vol. ... Rev. A, vol. 79, pp. 013810-1-7, 2009. CLEO Pacific Rim 2009

: We characterized a class of modified peptides designed to activate in the extracellular space of tumors in which pH is below 7.0 in cell culture and in tumor bearing mice. These "molecular engines" change shape in a low pH environment, become lipophilic, and embed into the plasma membrane. Our studies suggest that a molecular engine with a pT of 6.8-6.9 will deliver an 8-carboxyfluoroscein ligand to the surface of tumor cells in C57blk-J6 mice bearing subcutaneously transplanted Lewis lung cell-derived tumors following an iv injection of 27 nmoles (0.1 mg) peptide with FITC-labeled ligand. These observations were confirmed by flow cytometric analysis. Proof of principle for their diagnostic utility of this technology was obtained by injection of a 99mTc labeled peptide followed by gamma scintigraphy. This technology has tremendous potential for highly specific medical intervention at the molecular scale.

We present in this paper the new cute-SCIDAR instrument, entirely developed by the Instituto de Astrofísica de Canarias (IAC), delivered recently at the European Southern Observatory (ESO) Paranal Observatory (Chile). This instrument, supported by the European Community (Framework Programme 6, Extremely Large Telescope Design Study), carries out the generalized SCIntillation Detection And Ranging (g-SCIDAR) technique to obtain the temporal evolution of turbulence profiles CN 2 with height. A new design was made in order to fit the VLT Auxiliary Telescopes (ATs) interfaces and control requirements. Also, a new software architecture allows a full remote control, and a data analysis pipeline provides turbulence profiles in real-time, which is the main achievement of this new cute-SCIDAR. Details of its design and results of its excellent performance are included.