共查询到20条相似文献,搜索用时 15 毫秒
1.
The room-temperature cross sections for the Nd3+4F_{3/2} levels to the4I_{11/2} and4I_{9/2} manifolds (lower laser state and ground state, respectively) in NdP5 O14 are measured by two spectroscopic methods. A value for the largest cross section ofsigma(R_{1} - Y_{2}) = 1.7 times 10^{-19} cm2is found. The highest effective cross section, resulting from superposition of two lines at 1.051 μm, gives a laser gain per Nd ion which is about 2/3 of the maximum gain in YAG:Nd. The relative branching ratio into the4I_{11/2} and4I_{9/2} manifolds is 0.65:0.35. Comparison of the integrated cross sections with the measured lifetime for 1-percent Nd in LaP5 O14 indicates a combined efficiency <0.1 for the remaining transitions, namely radiative decay into the4I_{13/2} and4I_{15/2} manifolds and multiphonon quenching. A measurement of temperature dependence of fluorescence lifetime supports this last result. 相似文献
2.
Depolarization properties of offset reflector antennas 总被引:2,自引:0,他引:2
The cross polarized radiation for linearly polarized excitation and the beam displacement for circularly polarized excitation have been investigated for offset reflector antennas. Numerical calculations are given to illustrate the dependence upon the angletheta_{0} between the feed axis and the reflector axis as well as upon the half-angletheta_{c} subtended at the focus by the reflector. In the casetheta_{0} = theta_{c} = 45deg , measured results have been obtained for both linearly and circularly polarized excitations with a dual mode feed illuminating an offset paraboloid. The cross polarized radiation of horn reflector and open Cassegrainian antennas rises sharply to rather high values off the beam axis; however, in general, the maximum cross polarized radiation of offset reflector antennas can be made small by using a small angle between the feed and reflector axes. The cross polarization caused by offset is compared with that caused by an unbalanced feed pattern. The effect of the longitudinal current distribution and of departure of the surface from a paraboloid on cross polarization are also examined. The clarification of these cross polarization properties is found to be valuable in the design of reflector antennas. 相似文献
3.
Choon Lee Shung-Wu Lee 《Antennas and Propagation, IEEE Transactions on》1987,35(4):391-398
The radar cross section (RCS) of a circular waveguide terminated by a perfect electric conductor is calculated by the geometrical theory of diffraction (GTD) for the rim diffraction and by a physical optics approximation for the interior irradiation. The interior irradiation is generally more than 10 dB higher than the rim diffraction fora/lambda geq 1 (a is the waveguide radius,lambda is the free-space wavelength). At low frequencies (a/lambda sim 1 ), the interior irradiation can be significantly reduced over a broad range of incident angle if the interior waveguide wall is coated with a thin layer (1 percent of the radius) of lossy magnetic material. Our theoretical prediction is confirmed by measurements. At higher frequencies (a/lambda sim 3 ), a thin layer of coating is effective for the case of near axial incidence, provided that a good transition of theTE_{11} mode near the waveguide opening to theHE_{11} mode inside the waveguide is made. A thicker layer of coating is required for the RCS reduction over wider incident angle. 相似文献
4.
《IEEE transactions on information theory / Professional Technical Group on Information Theory》1985,31(4):543-545
A "slowly" fluctuating target is assumed to keep its radar cross section constant for the duration of several(M) dwells on target. To resolve multiple range and/or Doppler ambiguities, the received signal, which is presumably coherently processed (i.e., predetection integrated or matched filtered) over each dwell, must often be tested against a threshold, {em independently} of those on other dwells. Such a procedure is referred to as {em multiple detection}. A technique for the evaluation of a tight lower bound on the multiple-detection probabilityP_{M} , under Swerling case I statistics for the cross section, is presented in term of an infinite series and worked out in detail forP_{2} andP_{3} . Estimates on the computation error due to the truncation of the series are derived. Numerical results indicate thatP_{3} comes much closer toP_{1} than top_{1}^{3} or even toP_{1}P_{2} ; at an expected signal-to-noise ratio of13 dB and atP_{1} = 0.51 , it obtains thatP_{3} geq 0.40 , whereasP_{1}P_{2} = 0.23 andp_{1}^{3} = 0.17 . 相似文献
5.
On the polarization and the wavelength dependence of sea echo 总被引:1,自引:0,他引:1
Data on radar cross section per unit areasigmadeg of the sea at 6.3 Gc/s and 35.0 Gc/s and for grazing angles between1.5deg and4.0deg are reviewed. The data were obtained by simultaneously receiving both horizontally and vertically polarized echo components for transmitted polarizations which were sequentially changed between horizontal and vertical. Results indicate that sea echo is primarily caused by two scattering mechanisms: 1) a wind-dependent fine structure of the sea (presumably ripples) that partly depolarizes and has a scattering cross section which depends on wavelength in accordance withlambda^{-1} , and 2) a gross structure of the sea (presumably smooth facets between the ripples) that does not depolarize and has a scattering cross section which is independent of wavelength. The cross sections for transmitting and receiving vertical polarization (sigmadeg VV ) and for transmitting and receiving horizontal polarization (sigmadeg HH ) are caused by the sum of the contributions from the two mechanisms. Therefore, although dependent on sea state and polarization, it seems thatsigmadeg VV andsigmadeg HH will tend to be independent of wavelength at the lower frequency end of the microwave spectrum and will tend to depend on wavelength in accordance withlambda^{-1} at the higher end of the spectrum. A comparison of data at 6.3 Gc/s and 35.0 Gc/s and for grazing angles between1.5deg and4.0deg indicates that the wavelength dependences ofsigmadeg VV andsigmadeg HH are functions of sea state but are greater thanlambdadeg and are considerably less thanlambda^{-1} . 相似文献
6.
《Microwave and Wireless Components Letters, IEEE》2008,18(8):509-511
7.
The cross polarization from dual mode horn antennas supporting the TE11 and TM11 -modes has been analyzed both theoretically and experimentally. The cross polarization has been shown to be strongly dependent on the length and flare angle of the horn. When the flare angle for a given horn length becomes larger than a certain threshold angle, the cross-polar sidelobes increase rapidly. Simple formulas for this threshold angle are presented. This cross-polar degradation will not occur if a plane phase front is provided over the horn aperture, either by profiling the horn or by applying a lens in the aperture. 相似文献
8.
Youn Choung 《Antennas and Propagation, IEEE Transactions on》1984,32(12):1288-1291
The gain and the radiation patterns of a conical horn excited by a circular waveguide operating in theTE_{mn} andTM_{mn} modes are predicted. The expressions are rigorously derived by using the accurate calculation of the associated Legendre function with the real order and the integer degree. 相似文献
9.
Tunable alexandrite lasers 总被引:6,自引:0,他引:6
Walling J. Peterson O. Jenssen H. Morris R. O'Dell E. 《Quantum Electronics, IEEE Journal of》1980,16(12):1302-1315
Wavelength tunable laser operation has been obtained from the solid-state crystal alexandrite (BeAl2 O4 :Cr3+) over the continuous range from 701 to 818 nm. The tunable emission was observed at room temperature and above in a homogeneously broadened, vibronic, four-level mode of laser action. In this mode the laser gain cross section increases from7 times 10^{-21} cm2at 300K to2 times 10^{-20} cm2at 475K, which results in improved laser performance at elevated temperatures. Efficient 2.5 percent, low-threshold (10 J) operation has been obtained with xenon-flashlamp excitation of the 6 mm diameter × 76 mm length laser rods. Output pulses of greater than 5 J and average power outputs of 35 W have been demonstrated, limited by the available power supply. The emission is strongly polarizedEparallelb , with a gain that is 10 times that in the alternate polarization. The 262 μs, room-temperature fluorescence lifetime permits effective energy storage andQ -switched operation. TunableQ -switched pulses as large as 500 mJ have been obtained with pulsewidths ranging between 33 and 200 ns depending on the laser gain. Laser action has also been demonstrated on the high-gain (3 times 10^{-19} cm2emission cross section)R line at 680.4 nm and is also polarizedEparallelb . This three-level mode is analogous to the lasing in ruby except that the stimulated emission cross section in alexandrite is ten times larger than for ruby. 相似文献
10.
11.
《Lightwave Technology, Journal of》2009,27(13):2282-2288
12.
《Quantum Electronics, IEEE Journal of》2009,45(6):654-660
13.
Evidence of the combined influence of disaligning and velocity-changing collisions on laser light-induced saturation peaks or holes in the velocity distribution of Ne2p_{4} atoms was obtained by transmitting 1.15-μ light of a short single-mode He-Ne laser through the discharge tube of a long single-mode Michelson interferometer-type He-Ne laser oscillating at 0.63 μ, and by measuring the frequency-dependent single pass gain of the IR light in the discharge of the latter laser. Atoms that have undergone a transition in the linearly polarized red-laser light field are partially aligned. The width of the detected signal depends on the He gas pressure and also on the mutual polarization of the two laser fields. This indicates a smaller velocity broadening for aligned atoms than for collisionally disaligned ones. An evaluation of the data provides information on the correlation of the above-mentioned collision processes and on phase-disturbing and phase-nondisturbing contributions to the line broadening. Estimate values of the integral elastic Ne2p_{4} -He scattering cross section and a cross section for phase shifting collisions are given. 相似文献
14.
A method is presented for the solution of second-order electromagnetic fields in a bounded plasma which is excited by two high-frequency electromagnetic waves. Due to nonlinearities inherent in any plasma, incoming waves are mixed, creating sum and difference frequencies. The signal from the difference frequencyomega_{1}-omega_{2} , whereomega_{1} andomega_{2} are source frequencies, can be of considerable consequence whenomega_{1}-omega_{2} is sufficiently low to cause a significant reaction in a plasma as resonant conditions develop. In particular, the problem considered in this paper is the optimization of second-order fields in a uniform plasma confined to a circular waveguide including reference to diagnostic applications. 相似文献
15.
An analytical approach to the problem of scattering by composite random surfaces is presented. The surface is assumed to be Gaussian so that the surface height can be split (in the mean-square sense) into large (zeta_{l} ) and small (zeta_{s} ) scale components relative to the electromagnetic wavelength. A first-order perturbation approach developed by Burrows is used wherein the scattering solution for the large-scale structure is perturbed by the small-scale diffraction effects. The scattering from the large-scale structure (the zeroth-order perturbation solution) is treated via geometrical optics since4k_{0}^{2}bar{zeta_{l}^{2}} gg 1 . The first-order perturbation result comprises a convolution in wavenumber space of the height spectrum, the shadowing function, a polarization dependent factor, the joint density function for the large-scale slopes, and a truncation function which restricts the convolution to the domain corresponding to the small-scale height spectrum. The only "free" parameter is the surface wavenumber separating the large and small height contributions. For a given surface height spectrum, this wavenumber can be determined by a combination of mathematical and physical arguments. 相似文献
16.
A new type of dual-frequency antenna using a hollow-fin trough waveguide is described. The hollow fin is used as aTE_{10} mode waveguide with sidewall radiating elements. These Sidewall radiating elements constitute the high-frequency section of the antenna. The low-frequency section consists of a thick hollow-fin trough waveguide antenna. The attenuation due to radiation and waveguide wavelength of the trough waveguide section are analyzed. This analysis shows good agreement with experimental measurements. Existing data may be used as a guide in the design of the high-frequency section of this antenna, and the data presented may be used as a guide in the design of the low-frequency section. 相似文献
17.
In attempting to estimate the radar cross section of airborne vehicles, it is often only necessary to consider average values of the radar returns. A method of providing a quick estimate of the average bistatic radar cross section of the vehicle components would be useful. Ray optics provides a method of predicting the radar cross section of electrically large, perfectly conducting, simply curved, convex bodies such as spheres, ogives, ellipsoids, etc. This paper extends the method of ray optics to the case of an arbitrary body, which may be concave and/or convex, on which doable reflection and depolarization can occur. The incident radiation on the scattering body is represented by a large number(10^{5}-10^{6}) of rays. The rays reflected in a given direction with a given polarization are collected at infinity and combined by phasor addition. For the bodies investigated, this method yields results withinpm2 dB of measured results except in small angular regions where trapped waves contribute significantly to the radar return. 相似文献
18.
The existence of excited-state absorption of 1.06-μ radiation in transitions from the upper laser level upward (4F_{3/2} -4G_{9/2} ) is suggested by the ground-state absorption spectrum of Nd3+in soda-lime glass. The strength of this absorption was measured as follows. In the unpumped material the upper laser level was populated at high temperatures (600 and 783°K) by saturating the laser transition with an intense 1.06-μ probe laser beam. The residual absorption was attributed to4F_{3/2} -4G_{9/2} transitions. The steady-state saturation behavior was calculated on the assumption of rapid spectral cross relaxation, using spectroscopic data appropriate to the elevated temperature. Comparison of theoretical and experimental saturation behavior yielded an effective excited-state absorption cross section equal to one-third the gain cross section of the laser transition. This sizable value may be expected to have a significant adverse effect on laser threshold and efficiency. Also, it provides a mechanism for internal fracture such that the fracture threshold decreases with increasing neodymium concentration and with increasing temperature. 相似文献
19.
The "spherical-harmonics" problem is investigated for a cone of arbitrary cross section. The analysis shows that two basic singularities must be considered: 1) the electric singularity, in whichbar{e} becomes infinite likeR^{nu-1} near the tip of the cone, 2) the magnetic singularity, in whichbar{h} becomes infinite likeR^{tau-1} . Numerical results, in particular concerningnu andtau , are given for: 1) the elliptic cone and its limiting case the sector, 2) the pyramidal corner. 相似文献
20.
A general formula is derived for the scattering cross section of a passiven -element linear array consisting of isotropic radiators. When all the reactances are tuned out and scattering in the mirror direction is investigated, it is found thatA_{sr} , the relative scattering cross section is equal to the square of the maximum gain the array can produce. As a consequence, for forward scattering in the limiting case of zero spacing between the elements,A_{sr} = n^{4} . 相似文献