Seasat over-land scatterometer data. I. Global overview of theKu-band backscatterer coefficients

Statistics on the backscatter coefficient σ⁰ from the Ku-band Seasat-A Satellite Scatterometer (SASS) collected over the world's land surfaces are presented. This spaceborne scatterometer provided data on σ⁰ between latitude 80° S and 80° N at incidence angles up to 70°. The global statistics of vertical (V) and horizontal (H) polarization backscatter coefficients for 10° bands in latitude are presented for incidence angles between 20° and 70° and compared with the Skylab and ground spectrometer results. Global images of the time-averaged V polarization σ⁰ at a 45° incidence angle and its dependence on the incidence angle are presented and compared to a generalized map of the terrain type. Global images of the differences between the V an H polarization backscatter coefficients are presented and discussed. The most inhomogeneous region, which contains the deserts of North Africa and the Arabian Peninsula, is studied in greater detail and compared with the terrain type     

A numerical procedure for recovering scattering coefficients frommeasurements with wide-beam antennas

A numerical procedure for estimating true scattering coefficients, σ⁰, from measurements made using wide-beam antennas is discussed. The use of wide-beam antennas results in an inaccurate estimate of σ⁰. To reduce this error, the authors propose a correction procedure that estimates the error resulting from the use of narrow-beam approximation and utilizes the error to obtain a more accurate estimate of σ⁰. An exponential model is assumed to take into account the variation of σ⁰ with incidence angles, and the model parameters are estimated from measured data. Based on the model and knowledge of the antenna pattern, the procedure calculates the error due to the narrow-beam approximation. The procedure is shown to provide a significant improvement in the estimation of σ⁰ obtained in wide-beam antennas. The proposed procedure is also shown to be insensitive to the assumed σ⁰ model     

95-GHz scattering by terrain at near-grazing incidence

This study, consisting of three complimentary topics, examines the millimeter-wave backscattering behavior of terrain at incidence angles extending between 70 and 90°, corresponding to grazing angles of 20° to 0°. The first topic addresses the character of the statistical variability of the radar backscattering cross section per unit area σ_A. Based on an evaluation of an extensive data set acquired at 95 GHz, it was determined that the Rayleigh fading model (which predicts that σ_A is exponentially distributed) provides an excellent fit to the measured data for various types of terrain covers, including bare surfaces, grasses, trees, dry snow, and wet snow. The second topic relates to the angular variability and dynamic range of the backscattering coefficient σ⁰, particularly near grazing incidence. We provide a summary of data reported to date for each of several types of terrain covers. The last topic focuses on bare surfaces. A semi-empirical model for σ⁰ is presented for vertical (VV), horizontal (HH), and cross (HV) polarizations. The model parameters include the incidence angle &thetas;, the surface relative dielectric constant ϵ, and the surface roughness ks, where k=2π/λ and s is the surface root mean square (RMS) height     

Emission cross sections and spectroscopy of Ho3+ laserchannels in KGd(WO4)2 single crystal

The spectroscopic properties of Ho³⁺ laser channels in KGd(WO₄)₂ crystals have been investigated using optical absorption, photoluminescence, and lifetime measurements. The radiative lifetimes of Ho³⁺ have been calculated through a Judd-Ofelt (JO) formalism using 300-K optical absorption results. The JO parameters obtained were Ω₂=15.35×10^-20 cm², Ω ₄=3.79×10^-20 cm², Ω₆ =1.69×10^-20 cm². The 7-300-K lifetimes obtained in diluted (8·10¹⁸ cm^-3) KGW:0.1% Ho samples are: τ(⁵F₃)≈0.9 μs, τ( ⁵S₂)=19-3.6 μs, and τ(⁵F₅ )≈1.1 μs. For Ho concentrations below 1.5×10²⁰ cm^-3, multiphonon emission is the main source of non radiative losses, and the temperature independent multiphonon probability in KGW is found to follow the energy gap law τ_ph ^-1(0)=βexp(-αΔE), where β=1.4×10^-7 s^-1, and α=1.4×10³ cm. Above this holmium concentration, energy transfer between Ho impurities also contributes to the losses. The spectral distributions of the Ho³⁺ emission cross section σ_EM for several laser channels are calculated in σ- and π-polarized configurations. The peak a σ_EM values achieved for transitions to the ⁵I₈ level are ≈2×10^-20 cm² in the σ-polarized configuration, and three main lasing peaks at 2.02, 2.05, and 2.07 μm are envisaged inside the ⁵I₇→⁵I₈ channel     

LSE- and LSM-mode sheet impedances of thin conductors

The sheet resistance of very thin conductors is commonly taken as R=1/σt. We show that the sheet impedance, defined as the ratio of the tangential electric field at the surface of the conductor to the conduction current per unit length in the conductor, depends on the field distribution. The LSE (TE-to-y) and LSM (TM-to-y) modes used in the spectral domain immittance approach have sheet impedances which are distinct for vanishingly small or large values of the wavenumber γ in the medium surrounding a thin conductor. In the limit γ→0 and t/δ≪1, Z_sh^LSE approaches R=1/σt while Z_sh^LSM→2/σt. In the limit γ→∞ and t/δ≪1, Z_sh^LSE approaches R=2/σt and Z_sh^LSM approaches R=1/σt. When t/δ≫1, the sheet impedance approaches the surface impedance Z_s=(1+j)/σδ and is independent of the field distribution     

Kernel estimation of the instantaneous frequency

Considers kernel estimators of the instantaneous frequency of a slowly evolving sinusoid in white noise. The expected estimation error consists of two terms. The systematic bias error grows as the kernel halfwidth increases while the random error decreases. For a nonmodulated signal g(t), the kernel halfwidth that minimizes the expected error is proportional to h~[(σ²)/(N|∂_t² g|2)]^1/5 where σ² is the noise variance and N is the number of measurements per unit time. The author shows that estimating the instantaneous frequency corresponds to estimating the first derivative of a modulated signal, A(t)exp(iφ(t)). For instantaneous frequency estimation, the halfwidth which minimizes the expected error is larger: h_1,3~[(σ²)/(A²N|∂_t ³(e^iφ¯(t/))|sup 2/)]¹/$ u7. Since the optimal halfwidths depend on derivatives of the unknown function, the authors initially estimate these derivatives prior to estimating the actual signal     

Theoretical and experimental investigation of rhodamine B-dopedpolymer optical fiber amplifiers

High-power (620 W) and high-gain (28 dB) amplification was achieved by a rhodamine B (RB)-doped GI polymer-optical fiber amplifier (POFA) with a l-m length. A theoretical analysis of the amplification in the RE-doped GI POFA was carried out using the rate equation for the fast three level system. The calculated gain behaviour reasonably fit the experimental data. The possibility of higher power (more than 620 W) and higher gain (more than 28 dB) amplification covering a wide range in the visible region is shown from the calculation. Absorption and emission cross sections of RE in poly(methyl methacrylate) bulk were determined and used in the calculation. The values of the cross sections (σ_{a max}=3.4·10^-20m² ,σ_{e max}=3.4·10^-20 m ²) were approximately 10,000 times larger than those of Er ³⁺ in GeO₂-SiO₂ glass     

Impulse response modeling of indoor radio propagation channels

If indoor radio propagation channels are modeled as linear filters, they can be characterized by reporting the parameters of their equivalent impulse response functions. The measurement and modeling of estimates for such functions in two different office buildings are reported. The resulting data base consists of 12000 impulse response estimates of the channel that were obtained by inverse Fourier transforming of the channel's transfer functions. It is shown that the number of multipath components in each impulse response estimate is a normally-distributed random variable with a mean value that increases with increasing antenna separations; a modified Poisson distribution shows a good fit to the arrival time of the multipath components; amplitudes are lognormally distributed over both local and global areas, with a log-mean value that decreases almost linearly with increasing excess delay; for small displacements of the receiving antenna, the amplitude of the multipath components are correlated; the amplitudes of adjacent multipath components of the same impulse response function show negligible correlations; and the RMS delay spread over large areas is normally distributed with mean values that increase with increasing antenna separation     

Petrophysics of magnetic dipole fields in an anisotropic earth

Measurement-while-drilling (MWD) resistivity log data are often acquired in highly deviated or horizontal holes. The loop sensors are located on the drill collar and are approximated as magnetic dipoles. The conductivity of the earth in the vertical direction σ_v and horizontal direction σ_h are almost always different. When an MWD resistivity tool enters a new bed, the response is compared with the precomputed logs to aid in the determination of the location of the drill bit. The MWD tool response, however, is sensitive to resistivity anisotropy. An alternative method is used to derive analytical expressions for the Sommerfeld-type integrals. Numerical results give typical MWD tool response as a function of the inclination angle &thetas; the tool makes with respect to the axes of anisotropy and also as a function of the anisotropy index κ=(σ_h/σ_v)^1/2     

Relationship between and measurement of differential scatteringcoefficient (σ0) and bidirectional reflectancedistribution function (BRDF)

The microwave and optical scattering formulations are reviewed, a relationship between them at the same wavelength is presented, and the measurement conditions for obtaining range-independent and equipment-independent values are discussed. It is shown that the physical mechanism of scattering of electromagnetic beams from a surface can be quantified by either a differential scattering coefficient (σ⁰) or a bidirectional reflectance distribution function (BRDF). For a diffuse surface, σ⁰=4π BRDF cos &thetas;_i cos &thetas;_r, where &thetas;_i and &thetas;_r are the incident and reflected angles, respectively, from the surface normal. The geometrical constraints inferred in the measurement of coherent electromagnetic wave scattering from relatively small objects are easily satisfied to obtain range- and equipment-independent values, but care in experimental design is required when dealing with large surfaces. More care is also required when dealing with surfaces entailing specular features in addition to diffuse features     

Generation-recombination noise in GaN/AlGaN heterostructure fieldeffect transistors

Local levels with a large activation energy E_a~0.8-1.0 eV have been observed in low-frequency noise measurements of GaN/AlGaN heterostructure field effect transistors (HFETs and MOS-HFETs) grown on 4N-SiC substrates. The noise might come from the thin (30 nm) AlGaN barrier layer. The estimates of the level parameters based on this assumption resulted in reasonable values of capture cross section σ_n≈(10^-12-10^-13) cm² and trap concentration N_t≈5-10¹⁶ cm^-3     

Analysis of competing mechanisms in transitions between excitedstates in Er-doped integrated waveguides

A new procedure for determining the prevalence of either excited-state absorption (ESA) or nonradiative cooperative energy transfer between erbium ions in transitions between excited states in Er-doped integrated waveguides is presented. These transitions are currently very attractive for the development of upconversion lasers. The procedure is based on the analysis of the dependence on the transition-originating mechanisms of the modulation transfer from the pump to the excited levels' population. The accuracy and validity range of the method are studied numerically using ordinary integrated structures. By using this procedure, the ratio of the contributions of the two competing mechanisms to the ⁴I_13/2 ⇒ ⁴I_9/2 transition is determined from fluorescence measurements on a Er,Ti:LiNbO₃ sample excited by a 1480-nm pump. Moreover, new values of the excited-state pump-absorption cross section from level ⁴I_13/2, σ₂₄ (≈1480 nm) = 0.8 × 10^-26 m², and of the parameter associated with nonradiative cooperative energy transfer between Er³⁺ ions, C₂₂ = 3 × 10^-24 m³ s·^-1, are reported     

A semi-empirical representation of antenna diversity gain atcellular and PCS base stations

Predictions of uplink space-diversity gain in the cellular and personal communications systems (PCS) bands (near 850 MHz and 1.9 GHz, respectively), suffer from incomplete modeling of multipath angular spread σ. Using previously published measurements to reduce the gap, we show that σ~[distance]^-1/2 and that diversity gains are about 2 dB higher for PCS than for cellular     

Maximum likelihood DOA estimation and asymptotic Cramer-Rao boundsfor additive unknown colored noise

This paper is devoted to the maximum likelihood estimation of multiple sources in the presence of unknown noise. With the spatial noise covariance modeled as a function of certain unknown parameters, e.g., an autoregressive (AR) model, a direct and systematic way is developed to find the exact maximum likelihood (ML) estimates of all parameters associated with the direction finding problem, including the direction-of-arrival (DOA) angles Θ, the noise parameters α, the signal covariance Φ_s, and the noise power σ². We show that the estimates of the linear part of the parameter set Φ_s and σ² can be separated from the nonlinear parts Θ and α. Thus, the estimates of Φ_s and σ² become explicit functions of Θ and α. This results in a significant reduction in the dimensionality of the nonlinear optimization problem. Asymptotic analysis is performed on the estimates of Θ and α, and compact formulas are obtained for the Cramer-Rao bounds (CRB's). Finally, a Newton-type algorithm is designed to solve the nonlinear optimization problem, and simulations show that the asymptotic CRB agrees well with the results from Monte Carlo trials, even for small numbers of snapshots     

Measuring the quantum-limited linewidth of a laser by using theZeeman effect

We explore Zeeman operation of a laser as a way to measure its quantum phase noise. We measure the differential phase diffusion of a σ⁺ and a σ^- mode; the degeneracy of these modes is lifted by a longitudinal magnetic field. Experiments are performed on a high-gain HeXe gas laser, comparing quantum linewidths measured using this technique with measurements on the same laser using the well-established self-heterodyne technique. The two methods are found to be equivalent when the magnetic field used in the Zeeman technique is sufficiently large. The advantages of the Zeeman technique as compared to the self-heterodyne technique are its extreme simplicity and very modest laser-power requirement     

Fabrication of planar optical waveguides in LiB3O5 by 2 MeV He+ ion implantation

The fabrication of planar optical waveguides in LiB₃O ₅ is discussed. Using 2-MeV ⁴He⁺ implantation with a dose of 1.5×10¹⁶ ions/cm² at 300 K, the refractive indexes of a 0.2-μm-thick layer 5.1 μm below the crystal surface are reduced to form optical barrier guides. For this ion dose the maximum change from the bulk values of refractive index at a wavelength of 0.488 μm are 1.5%, 5.25%, and 4% for n_x, n_y, and n_z, respectively. The refractive indexes of the guiding region change by less than 0.02% from the bulk values. The dose dependence of the optical barrier height has been measured. A threshold ion dose of about 0.75×10¹⁶ ions/cm² is required to form a refractive index barrier and ion doses higher than about 2.5×10¹⁶ ions/cm². saturate the refractive index decrease. Waveguide propagation losses for annealed single energy implants of dose 1.5×10¹⁶ ions/cm² are dominated by tunneling and are estimated to be ~8.9 dB/cm for the z-cut waveguides used. Multiple energy implants broaden the optical barrier, and losses of <4 dB/cm have been observed     

Comparison of GTD propagation model wide-band path loss simulationwith measurements

The geometrical theory of diffraction (GTD) wedge diffraction has been used successfully in the GTD propagation model to predict narrowband continuous-wave (CW) radiowave propagation characteristics. The GTD propagation model uses a two-dimensional terrain profile approximated as piecewise-linear and computes reflection and diffraction effects with model output representing a complex approximation to the narrowband channel transfer function. Using the narrowband GTD model as a starting point, a wideband terrain-sensitive model has been developed which is capable of predicting wide-bandwidth propagation characteristics. The complex wideband channel transfer function calculated by the GTD model is transformed to the time domain by a fast Fourier transform (FFT). The results are then used to predict time-domain radio transmission loss in the form of a bandlimited approximation to the channel impulse response. Important channel parameters such as delay spread, and wideband received signal level can then be calculated. The GTD predicted results are put in a suitable format and compared with measurements obtained by SRI International     

A high-speed and highly uniform submicrometer-gate BPLDD GaAsMESFET for GaAs LSIs

The authors present formation conditions for ion-implanted regions of a GaAs buried p-layer lightly doped drain (BPLDD) MESFET that can improve short-channel effect, V_th uniformity, and FET operating speed, simultaneously. For 0.7-μm gates, a Mg⁺ dose of 2×10¹² cm^-2 at 300 keV and a Si⁺ dose of 2×10¹² cm^-2 at 50 keV are suitable for the p layer and n' layer, respectively. A σV _th of 7 mV is realized. Gate-edge capacitance of the 0.7-μm-gate BPLDD that consists of both overlap capacitance and fringing capacitance is successfully reduced to 0.5 fF/μm, which is about 50% of that of a non-LDD buried p-layer (BP) FET. Another parasitic capacitance due to the p-layer was found to have less effect on the speed than the gate-edge one. Consequently, the gate propagation delay time of the BPLDD can be reduced to 15 ps at power dissipation of 1 mW/gate, which is about 65% of that of a BP. Applying the 0.7-μm-gate BPLDD to 16-kb SRAMs, the authors have obtained a maximum access time of less than 5 ns with a galloping test pattern     

New-user identification in a CDMA system

We present three detector/estimators (DEs) which allow multiuser detection and parameter estimation without a side channel in a dynamic asynchronous code-division multiple-access (CDMA) system in which users are entering and leaving the system. These DEs optimally detect a new user given only the chip rate and the spreading factor of the new user. Two of these DEs, the maximum-likelihood detector/estimator (MLDE) and the generalized maximum-likelihood detector/estimator (GMLDE), produce maximum-likelihood estimates of the new user's signature sequence, delay, and amplitude, which are then incorporated into a multiuser detector. The third DE, the cyclic detector/estimator (CDE), is the most computationally efficient of the three processors. This DE detects the new user by testing for cyclostationarity and then uses suboptimal schemes to estimate the new user's signature sequence, delay, and amplitude. Simulations indicate that all three DEs reliably detect a new user for an E_s/σ² (symbol-energy-to-noise ratio) of 5 dB. The MLDE and GMLDE produce signature sequence and delay estimates with probability of error less than 0.07 for an E_s/σ² of 10 dB, and the CDE produces signature sequence and delay estimates with probability of error less than 0.13 for an E_s/σ² of 15 dB     

Carrier-dependent nonlinearities and modulation in an InGaAs SQWwaveguide

The authors report measurements of optically induced carrier-dependent refractive index changes and their saturation in an InGaAs single quantum well centered within a linear multiple quantum well guided-wave Fabry-Perot resonator using diode laser sources. A low-excitation nonlinear refractive cross-section, σ_n=-1×10^-19 cm³, was deduced for probe wavelengths near the TM (transverse magnetic) absorption edge, falling only to σ_n=-3.1×10^-20 cm³, at over 0.16 μm from the band edge. For an incident irradiance of 18 kW/cm ², refractive index changes in the InGaAs quantum well as large as -0.16 were deduced near the absorption edge, while the index change at a wavelength 0.16 μm from the absorption edge was -0.055. This large off-resonant index change is attributed to an enhanced free-carrier contribution within a 2D system