Active Anomalous Transmission and Its Application in Compensating Waveform Distortions

In theory, engineered anomalous transmission in passive materials and waveguide devices can be used to compensate for waveform distortions. However, they suffer from inherent dissipation. Recently, active non-Foster elements with imaginary immittance monotonically decreasing with frequency have shown important potentials in broadening bandwidths of electromagnetic devices. So far, they are implemented based on negative impedance convertors(NICs) loaded with Foster devices. This makes them intrinsically one-port elements and thus cannot be used to compensate for distortions of signals. We construct a two-port network with a non-Foster transmission coefficient based on an unconventional use of NICs. Simulation and experiments show that it can compensate for extremely distorted signals. The proposed method can be used to broaden existing applications in different areas such as antennas, circuits and systems, and physical-layer signal processing.     

Wide-angle emission lasers

Conclusion Analysis of the data obtained showed that the diameters of the nonaxial beam rings did not satisfy the condition for the appearance of rings of equal slope for the Fabry-Perot interferometer. Radiation of this type can be attributed to the appearance in the distorted resonator of vibrational modes propagating at some angles to the crystal axis. Among the causes of this phenomenon may be the presence of inhomogeneities in the active material, the quality of the rod surfaces, and the geometry of the rods. The low excitation threshold was due to the longer path length of the light between the mirrors as compared with that in the case of ordinary axial radiation. This fact indicates that in certain experiments on stimulated radiation (particularly in the study of lasers with optically inhomogeneous active media and of continuous lasers employing long crystals of small diameter) the appearance of nonaxial radiation may actually prove useful. In the case of long small-diameter lasers, distortions of the resonator can arise through deformation of the crystal as a result of heating by the power from the excitation source.     

Formation of laser beams with a minimum divergence in the UV spectral region

The results of investigations of the divergence of an amplified diffracted beam of diameter from 1.5 to 150 mm produced by a XeCl laser are presented. It has been shown that if the intensity distribution of the amplified radiation is spatially nonuniform, the wave front in the active medium changes substantially due to the diffraction of the radiation at a small-diameter diaphragm. The active medium of the amplifier excited by an electron beam makes it possible to amplify without visible distortions a laser beam with a divergence of ≈10 μrad. Institute of High-Current Electronics, Siberian Division of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 64–68, April, 2000.     

Study of thermo-optical distortions in a plate active element with zigzag beam path

Phase distortions introduced into the laser beam propagating in a plate under diode pumping of the active element (AE) and for intentionally caused temperature gradients in the AE are studied. The temperature distribution along the optical axis of the active element is measured during its end diode pumping. Phase distortions of the test beam propagating through the nonuniformly heated active element are studied. Interference patterns are obtained and optical distortions of the test beam are measured at various temperature gradients in the crystal.     

On the possibility of compensating for small-scale wave-front distortions using a Zernicke cell and an optically controlled spatial light modulator

The possibility of compensating for small-scale wave-front distortions using a spatial light modulator which is optically controlled by the output beam of a Zernicke interferometer was studied theoretically. The optimal parameters of the system, requirements of its elements, the range of the compensated distortions, and the compensation quality are determined for the case of compensation for sinusoidal distortions. It is shown that this technique allows for the compensation of small-scale sinusoidal phase distortions with a depth of up to 3λ and a 20-fold gain in the Strehl number can be achieved.     

Repetitively pulsed CO2 laser driven by an electron accelerator with a gas-discharge plasma cathode

A repetitively pulsed electron-beam-controlled CO₂ laser driven by an electron accelerator with a plasma electron emitter based on low-pressure glow discharge with hollow anode and cathode is presented. The application of the proposed emitter makes it possible to sharply increase the current and to control the electron-beam duration and, hence, the energy and time characteristics of the laser. It is demonstrated that the radiation pulse duration ranges from 300 to 1200 μs, whereas the radiation energy amounts to 200 J at an efficiency of 18%. In the course of lasing, the focal spot is not affected by the heterogeneous heating of the active medium. A significant heterogeneity in the gas flow upon an incomplete renewal of the gas in the zone filled with radiation leads to an increase in the focal spot owing to the wavefront distortions. Original Text ? Astro, Ltd., 2006.     

The possibility of compensating small-scale wavefront distortions with a Zernike cell and a spatial light modulator in an optical feedback scheme

The possibility of compensating small-scale wavefront distortions in an optical feedback scheme by using a spatial light modulator optically addressed by the output beam of a Zernike cell is theoretically considered. The optimal parameters of the system for compensating sinusoidal distortions and distortions characteristic of phasing of subapertures, the requirements for the elements of the system, the range of compensated distortions, and the quality of compensation are determined. It is shown that the use of this method makes it possible to compensate small-scale phase distortions with a peak-to-peak amplitude of up to 3λ; in this case, the gain in the Strehl number can be as high as 80.     

Lineshape distortions in fir cyclotron resonance of MOS structures

Cyclotron resonance lineshape distortions which include appreciable shifts in the peak position have been observed in far infrared laser transmission measurements of MOS structures. Experimental studies and a complete theoretical treatment show that these distortions arise from multiple interference effects in plane parallel sample substrates and from partial circular polarization of long wavelength coherent radiation owing to the light pipe optics.     

Thermal aberrations and high power frequency conversion in a barium nitrate Raman laser

Raman frequency conversion of high average power pulsed Nd:YAG laser radiation into the near IR spectral region in a barium nitrate Raman laser was studied with the emphasis on thermal effects inside the Raman-active medium. The probe-beam technique together with numerical reconstruction, done by integrating the transient heat conduction and paraxial wave equations, revealed dynamics of the induced distortions featuring high-order optical aberrations. By utilizing the Zernike expansion of the reconstructed phase profile and implementing a special focusing geometry of the pump beam, partial compensation of the distortions was realized in a stable configuration of the Raman cavity. Generation of the first-, second-, and third-order Stokes radiation with output power of 17, 9.5, and 5.5 W corresponding to a quantum conversion efficiency of 32, 21, and 13% is reported.     

Single-photon emission computed tomography in a proportional scattering medium

The reconstruction of the spatial distribution of radiation sources in a proportional scattering medium has been considered. An exact solution of the inverse tomographic problem has been obtained by analytically solving the radiation transport equation for arbitrary distributions of radiation sources and extinction coefficient and exact boundary conditions. The type and scale of distortions associated with radiation scattering in tomographic reconstructions have been demonstrated.     

Compensation of dynamic phase distortions in the phase conjugation system based on the film of azocontaining liquid-crystalline polymer

The compensation dynamics of phase distortions in optical phase conjugation (OPC) using the film of azocontaining liquid-crystalline polymer was experimentally investigated. The quality of compensation of distortions of the wave front was determined, and the effect of experimental conditions on the OPC dynamics was studied. The coefficient of reflection in OPC reached 35% for a total incident light intensity of ～17 mW/cm² at a wavelength of 532 nm. The response time of the OPC system was ～1 s at a polymer temperature of 69°C. The high-quality compensation of the wave front distortions of the laser radiation was obtained. The residual error of compensation decreased by a factor of ～100 during the transient OPC process. It is demonstrated that the spatially modulated light beam can be retrieved during repeated passages through a phase plate with strong small-scale aberrations.     

Combined passive and active shimming for in vivo MR spectroscopy at high magnetic fields

The use of high magnetic fields increases the sensitivity and spectral dispersion in magnetic resonance spectroscopy (MRS) of brain metabolites. Practical limitations arise, however, from susceptibility-induced field distortions, which are increased at higher magnetic field strengths. Solutions to this problem include optimized shimming, provided that active, i.e., electronic, shimming can operate over a sufficient range. To meet our shim requirements, which were an order of magnitude greater than the active shim capacity of our 7T MR system, we developed a combined passive and active shim approach. Simple geometries of ferromagnetic shim elements were derived and numerically optimized to generate a complete set of second-order spherical harmonic shim functions in a modular manner. The major goals of the shim design were maximization of shim field accuracy and ease of practical implementation. The theoretically optimized ferro-shim geometries were mounted on a cylindrical surface and placed inside the magnet bore, surrounding the subject's head and the RF coil. Passive shimming generated very strong shim fields and eliminated the worst of the field distortions, after which the field was further optimized by flexible and highly accurate active shimming. Here, the passive-shimming procedure was first evaluated theoretically, then applied in phantom studies and subsequently validated for in vivo 1H MRS in the macaque visual cortex. No artifacts due to the passive shim setup were observed; adjustments were reproducible between sessions. The modularity and the reduction to two pieces per shim term in this study is an important simplification that makes the method applicable also for passive shimming within single sessions. The feasibility of very strong, flexible and high-quality shimming via a combined approach of passive and active shimming is of great practical relevance for MR imaging and spectroscopy at high field strengths where shim power is limited or where shimming of specific anatomical regions inherently requires strong shim fields.     

Collimation of the initial wavefront of a partially coherent light beam by the backscattered signal

The possibility of compensating distortions of the initial wavefront of a partially coherent beam by the backscattered radiation power is analyzed. A new algorithm for controlling the flexible mirror by the signal of the volume atmospheric scattering is proposed and its efficiency is studied. It is shown that the developed algorithm makes it possible to considerably reduce the compensation time of distortions compared to the gradient method.     

Diagnostics of the acoustic properties of unconsolidated media under natural conditions

Results of experimental studies carried out at the Bezvodnoe test area in 2005 and 2006 are presented. The goal of the studies is to investigate the potentialities of vibration diagnostics of the surface layers of soil under natural conditions. The most attention is given to the problem of measuring the shear modulus of soil and determining the power of seismic acoustic radiation with the help of an impedance method. The results obtained testify to the high sensitivity of the impedance method soil diagnostics and to the possibility of monitoring the variations of soil properties. Sources of nonlinear distortions and the problem of stability of radiation characteristics are also considered. A comparison with theoretical estimates is performed. It confirmed a special role of the contact region as the major source of radiation signal distortions. The results of the study are important for seismoacoustic investigation of the structure of natural media under natural conditions.     

Coexistence of two different Peierls distortions within an atomic scale wire: Si(553)-Au

The ground state property of a Au-induced atomic wire array on a stepped Si(553) surface with interesting 1D metallic bands was investigated. Electron diffraction and scanning tunneling microscopy reveal an intriguing coexistence of triple- and double-period lattice distortions at low temperature. Angle-resolved photoemission observes both the nearly 1/3- and 1/2-filled bands to gradually open energy gaps upon cooling. We explain these unusual findings as due to the occurrence of Peierls distortions of triple and double periods on the two different atomic-scale chain elements, respectively, within a single unit wire. The two Peierls distortions are suggested to have different transition temperatures and little lateral correlation between each other.     

A study of possibilities of development of GaAs detectors for high-power nanosecond X-ray pulses

It is proposed to use chromium-compensated semi-insulating GaAs detectors for detecting high-power nanosecond X-ray pulses. An X-ray facility based on a small direct-acting electron accelerator “Sinus-150” developed at the Institute of High-Current Electronics of the Siberian Branch of the Russian Academy of Sciences, Tomsk was used as a source of pulsed X-ray radiation. The detectors are shown to detect 5 ns pulses under exposure to 30 mR radiation during one pulse without distortions. In so doing, one-ampere currents flow through the active detector area 0.2 cm². It is found that the physical processes limiting the working capacity of GaAs detectors are due to capture of nonequilibrium electrons and holes by deep centers. At the nonequilibrium charge-carrier concentrations higher than 5·10¹³ cm³, nonuniform distributions of the electric field are formed in the active region and these can result in distortion of the output detector signal shape. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 14–18, September, 2008.     

The microscopic mechanism behind the fission barrier asymmetry

The instability at the second saddle point of actinide elements towards asymmetric distortions is explained by a decrease in energy of the neutron orbitals [40 Λω] (orbitals at the waistline of the nucleus) for asymmetric distortions. These orbitals are situated at the Fermi surface and couple strongly to [51 Λω] levels slightly above the Fermi surface.     

Analysis of Operating Regimes of EHF Hybrid-Integrated Autodynes Based on the Gunn Micropower Mesaplanar Diodes

The results of investigations into the operating regimes of EHF hybrid-integrated autodyne generators are given in the present paper. Multipurpose crystals with three mesaplanar structures, one of which serves as an active element and the two others serve as control elements, are used as active elements. The segment of a coplanar strip transmission line are used as a resonant system of the autodyne. The cw regime of autodyne operation and regimes with various types of radiation modulation, which allow the class of problems solved by short-range radar systems to be extended, are examined.     

Coherence and wavefront characterization of Si‐111 monochromators using double‐grating interferometry

A study of the coherence and wavefront properties of a pseudo‐channel‐cut monochromator in comparison with a double‐crystal monochromator is presented. Using a double‐grating interferometer designed for the hard X‐ray regime, the complex coherence factor was measured and the wavefront distortions at the sample position were analyzed. A transverse coherence length was found in the vertical direction that was a factor of two larger for the channel‐cut monochromator owing to its higher mechanical stability. The wavefront distortions after different optical elements in the beam, such as monochromators and mirrors, were also quantified. This work is particularly relevant for coherent diffraction imaging experiments with synchrotron sources.     

Formation of Two-Pulse Lasing with a Single Photoelectrical Switching of the Electro-Optical Shutter in Lasers with Anisotropic Active Elements

We have developed an optical scheme of a laser with anisotropic active elements providing, upon a single switching of the electro-optical shutter and with the use of a bleaching filter in the cavity, the generation of double giant pulses the variation range of the time interval between which is practically unlimited. The laser provides the generation of two giant pulses whose radiation polarization planes are mutually orthogonal. An experimental study of a ruby laser of such a type with an orientation of the optical axis of the active elements close to 60° has been made.