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We present fiber-nonlinearity-tolerant transmission of polarization-division multiplexed binary phase-shift keying (PDM-BPSK) through a nonlinear noise squeezing (NLNS) effect that is achieved by optimized digital electronic dispersion precompensation. With the improved nonlinear tolerance, a 406.6-Gb/s superchannel consisting of eight 37.5-GHz-spaced 32-Gbaud Nyquist-filtered PDM-BPSK signals is transmitted over a 12800-km (160 × 80-km) EDFA-only amplified dispersion-unmanaged nonzero-dispersion-shifted fiber link. We establish a connection between the beneficial NLNS effect and the recently reported phase-conjugated twin wave (PCTW) concept and further generalize the PCTW concept to vector twin waves that are traveling through a fiber link along orthogonal dimensions such as time and space. Moreover, we apply the PCTW concept to wavelength-division-multiplexed (WDM) superchannel transmission by treating the entire WDM superchannel as a twin wave to further mitigate interchannel nonlinear effects. Through numerical simulations, we show that the generalized PCTW technique can effectively mitigate interchannel nonlinear impairments, in addition to mitigating intrachannel nonlinear impairments. [ABSTRACT FROM PUBLISHER]

Highlights: [•] Synthesis of thin film by thermal evaporation technique. [•] Degenerate four-wave mixing technique is employed to exploit the optical nonlinear properties. [•] Comparison of results in amorphous thin films. [Copyright &y& Elsevier]

We consider an opto-mechanical effect that is related to the appearance of additional optical phase due to motion of an ultra-dispersive medium, the enhanced Fizeau effect. We have theoretically shown that the enhanced Fizeau effect on a single atom in a cavity can be observed. It has been shown that the change of phase is sensitive to the mechanical motion and can be applied to measurement of possible displacements as small as cm Hz. One of application can be the detection of weak forces exerted on a single atom. [ABSTRACT FROM AUTHOR]

Transmission characteristics of polarization shift keying (PolSK) signal is experimentally demonstrated in this work. Performance degradation due to transmission is improved using optical phase conjugation (OPC) technique. The wavelength converter based on the proposed state of polarization (SOP) transparent scheme for PolSK signal is effective to use it as mid-link optical phase conjugator and which is difficult to achieve using conventional wavelength conversion scheme. It is found that for fiber transmission, the performance of both of the orthogonal SOPs of the PolSK signal is different for their respective chirping characteristics which can be optimized to be similar using proposed OPC technique. [ABSTRACT FROM AUTHOR]

We have analyzed the impact of digital and optical signal processing algorithms, that is, Volterra equalization (VE), digital backpropagation (BP), and optical phase conjugation with nonlinearity module (OPC-NM), in next generation 10 Gbit/s (also referred to as XG) DP-QPSK long haul WDM (fixed-grid) passive optical network (PON) without midspan repeaters over 120 km standard single mode fiber (SMF) link for downstream signals. Due to the compensation of optical Kerr effects, the sensitivity penalty is improved by 2 dB by implementing BP algorithm, 1.5 dB by VE algorithm, and 2.69 dB by OPC-NM.Moreover, with the implementation of NL equalization technique, we are able to get the transmission distance of 126.6 km SMF for the 1 : 1024 split ratio at 5GHz channel spacing in the nonlinear region. [ABSTRACT FROM AUTHOR]

Focusing light inside highly scattering media is a challenging task in biomedical optical imaging, manipulation, and therapy. A recent invention has overcome this challenge by time reversing ultrasonically encoded diffuse light to an ultrasound-modulated volume inside a turbid medium. In this technique, a photorefractive (PR) crystal or polymer can be used as the phase conjugate mirror for optical time reversal. Accordingly, a relatively long ultrasound burst, whose duration matches the PR response time of the PR material, is usually used to encode the diffuse light. This long burst results in poor focusing resolution along the acoustic axis. In this work, we propose to use two intersecting ultrasound beams, emitted from two ultrasonic transducers at different frequencies, to modulate the diffuse light at the beat frequency within the intersection volume. We show that the time reversal of the light encoded at the beat frequency can converge back to the intersection volume. Experimentally, an acoustic axial resolution of ~1.1 mm was demonstrated inside turbid media, agreeing with theoretical estimation. [ABSTRACT FROM AUTHOR]

Controlling light through dynamically varying heterogeneous media is a sought-after goal with important applications ranging from free-space communication to nanosurgery. The underlying challenge is to control a large number of degrees of freedom of the optical wavefront, at timescales shorter than the medium dynamics. Many advances have been reported recently following the demonstration of focusing through turbid samples by wavefront shaping, where spatial light modulators with more than 1,000 degrees of freedom were used. Unfortunately, spatial light modulator-based wavefront shaping requires feedback from a detector or camera and is currently limited to slowly varying samples. Here, we demonstrate a novel approach for wavefront shaping utilizing all-optical feedback. We show that the complex wavefront required to focus light scattered by turbid samples (including thin biological tissues) can be generated at submicrosecond timescales by the process of field self-organization inside a multimode laser cavity, without requiring electronic feedback, spatial light modulators or phase-conjugation crystals. [ABSTRACT FROM AUTHOR]

In this paper, we suggest a doubly degenerate four-level system, in which the transition takes place between the hyperfine energy 52S1/2F = 1 and 52P3/2F = 2 in rubidium 87 D2 line, for studying atomic phase grating based on the cross-Kerr and phase conjugation effects. The phase grating with high efficiency can be obtained by tuning phase shift Φ between the coupling and probe field, when the coupling intensity is much stronger than the strength of probe field. Under different coupling intensities, a high diffraction efficiency can be maintained. A new and simple way of implementing phase grating is presented. However, in such an atomic system, two main limitations must be taken into account. First, the independence between steady state probe susceptibility and the coupling intensity, when the population decay rate is larger than the Rabi frequency of the coupling field, cannot result in diffraction grating; second, the sample to be prepared should not be too long. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, FWM power has been estimated and mitigated by using Optical Phase Conjugator (OPC) in the middle of the optical fiber span which strongly suppresses the FWM power by introducing destructive interference between the first and second halves of the fiber optic link. −20dBm FWM power has been suppressed by using OPC which is in close agreement with the calculated FWM power of −21.74dBm. Further, symmetrical dispersion compensation has been employed to negate the accumulated chromatic dispersion in a considered 32 channel, Terabit capacity, long haul DWDM soliton link. [Copyright &y& Elsevier]

The dependences of the reflection (transmission) coefficients and the half-width of the spatial frequency band on the pump-wave intensity are obtained for an object (signal) wave in degenerate and quasi-degenerate four-wave mixers on resonant and thermal nonlinearities. The effect of the difference of the pump-wave frequencies on the phase-conjugation quality is studied at relatively high reflection coefficients. [ABSTRACT FROM AUTHOR]

A possibility of almost perfect phase conjugation in electrically thin nonlinear sheets is shown for the normal incidence of the signal wave, using either symmetrical or asymmetrical particles and nondegenerate interaction with slightly different frequencies of the incident and phase-conjugated waves. Under these conditions, the use of linear and nonlinear Huygens sources for both linear (signal) and nonlinear (phase-conjugated) waves ensures zero or very small reflection for both of them. In perspective, this can lead to realization of reflectionless superlens based on three-wave interactions, as well to other applications based on reflection-free and electrically thin nonlinear layers. [ABSTRACT FROM AUTHOR]

Abstract: We numerically investigate two multi-stage feed-forward optical carrier phase estimation (CPE) methods for coherent optical 64-QAM signals, which are QPSK partitioning with maximum likelihood (ML) method and QPSK partitioning with blind phase search (BPS) method. Both methods are proved to be possible solutions for CPE of 64-QAM signals, with good laser phase noise tolerance and simple complexity. For QPSK partitioning with ML method, we creatively introduce cascaded multi-stage ML following QPSK partitioning, which leads to further improved laser phase noise tolerance compared to 1-stage ML condition. For QPSK partitioning with BPS method, BPS with 8 or 12 test phases and test phase range of π/4 is found to be suitable for practical application, because it can achieve good laser phase noise tolerance while keeping low complexity, and the use of BPS with more test phases in this method has limited advantage over single stage BPS with the same test phases. [Copyright &y& Elsevier]

Gordon and Mollenauer (G-M) proposed in 1990 that the interplay between the nonlinear Kerr effect and amplified spontaneous emission noise can generate an enhanced level of noise and degrade the performance of optical phase-modulated systems. We systematically investigate the G-M effect in 112 Gbit/s coherent dual polarization quaternary phase shift keying (DP-QPSK) systems through comprehensive simulation. The results show that in the presence or absence of inline dispersion compensation, the G-M effect will seriously damage the system performance. Some of our important conclusions are listed as follows: with the increase in the input power into each span, the G-M effect is enhanced simultaneously; the smaller the span length, the stronger the G-M effect caused by more inter-span interaction; for non-zero dispersion-shifted fibers, different coefficient values will cause a similar amount of G-M effect penalty; by comparing the system performance with different dispersion compensation ratios, one can conclude that different residual dispersions of the same magnitude, no matter whether they are under-compensated or over-compensated, can derogate the overall system performance to a similar degree. In brief, the G-M effect cannot be ignored. We also have a short discussion on how to reduce and compensate for the G-M effect in DP-QPSK systems. [ABSTRACT FROM AUTHOR]

Phonon lifetime is a significant parameter in the process of stimulated Brillouin scattering (SBS). In the present study, SBS slow light technique is used to measure phonon lifetime. Brillouin bandwidth is divided into natural, spontaneous and stimulated bandwidth. Compared with the traditional heterodyne-detection and pump-probe techniques, the natural Brillouin bandwidth can be obtained by SBS slow light technique, which equals the reciprocal of phonon lifetime and has no relations with the pump power. Another advantage of this technique is that the effect of polarization can be excluded. The natural Brillouin bandwidth is measured to be ~50 MHz and the phonon lifetime ~3.2ns in the conventional single-mode fiber (SMF) at room temperature and zero strain. The obtained results are guidable in applications where the phonon lifetime information is a requisite such as phase conjugation and pulse compression. [ABSTRACT FROM AUTHOR]

Abstract: Midlink optical phase conjugation (M-OPC) based on four wave mixing (FWM) in semiconductor optical amplifier (SOA) can be used to compensate for the fiber nonlinearity effect in CO-OFDM systems. Simulation of 40Gb/s CO-OFDM systems without optical dispersion compensation is presented. For a single-channel system, even greater benefits are obtained and the nonlinear limits are significantly decreased: the maximum Q can be increased by 3dB for all transmission distances greater than 2400km, the nonlinear threshold (NLT) (for Q >10dB) is increased by >4dB. Due to cross phase modulation (XPM) from neighbouring WDM channels, the maximum Q and NLT are increased by about 1.1dB and 1dB for 50-GHz-spacing WDM systems separately. These results show that M-OPC can mitigate the fiber nonlinearity impairment for both single-channel and WDM systems. [Copyright &y& Elsevier]

Kerr nonlinearity imposes a limit on the achievable transmission performance and capacity of optical fibre communication links. We show that the nonlinear distortions of a pair of phase-conjugated twin waves are essentially anticorrelated, so cancellation of signal-to-signal nonlinear interactions can be achieved by coherently superimposing the twin waves at the end of the transmission line. We demonstrate that by applying this approach to fibre communication, nonlinear distortions can be reduced by >8.5 dB. In dispersive nonlinear transmission, the nonlinearity cancellation additionally requires a dispersion-symmetry condition that can be satisfied by appropriately predispersing the signals. By using these techniques we succeed in transmitting a 400 Gb s−1 superchannel over 12,800 km of fibre. We further show a connection between the nonlinearity cancellation and a nonlinear noise squeezing effect. The concept of using phase-conjugated twin waves to suppress nonlinear interactions may prove beneficial in other physical systems governed by the nonlinear Schrödinger equation. [ABSTRACT FROM AUTHOR]

An approach to the analysis of the phase conjugation by stimulated light scattering that is alternative to the mode theory of 3D hologram, which is usually applied for these purposes, has been studied. As is known, the use of the mode theory is based on the phenomenological gain factor of the Stokes wave, which is induced by a pump wave in a nonlinear medium. The approach developed in this work directly considers the interaction between plane components of the pump and Stokes waves via hypersonic gratings of the dielectric permittivity, which the pump and Stokes waves induce in the medium. It has been shown that, as a result of the phase conjugation, two hypersonic gratings participate equally efficiently in the interaction between any pair of plane components of the pump and Stokes waves, with one of these gratings being excited by the interacting waves themselves. The other grating in each such pair is unambiguously identified with a simplest vector diagram. Conditions have been analyzed under which numerous other gratings, which could also participate in the interaction of each pair of plane components of the pump and Stokes waves without violating the Bragg conditions and the joining conditions of the transverse components of the wave vectors at the interface between the two media, contribute negligibly to stimulated light scattering. It has been shown that, if the pump is spatially coherent, the considered approach yields the same results as the standard mode theory does. In October 2011, I reported the results of this work at the Vavilov State Optical Institute at the Memorial Meeting 'Half a Century of New Optics in Russia: Lasers, Nonlinear Optics, and Optical Holography' and at the International Conference 'Laser Optics-2012' (June 2012). [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the optical phase conjugation (OPC) using degenerate four wave mixing (DFWM) technique in a nonlinear DR1 chromophore doped in polymer PMMA thin films (by 8% W/W) is experimentally investigated. A CW Nd:YAG laser (532nm) with 50mW incident power has been used for (DFWM) technique to produce optical phase conjugation beam. The optimized reflectivity (R) relates to signal, forward and backward pump intensity titled with parameters and . Furthermore, we have discussed temporal behavior and the maximum of phase conjugation reflectivity. The photo-induced reorientation of the DR1 by trans–cis–trans photoisomerization is the dominant mechanism for the OPC wave generation. [Copyright &y& Elsevier]

Imaging by phase conjugation through diffusing media is performed by using parametric amplification in a type II crystal, at a wavelength included in the therapeutic window. By nature, the method ensures imaging in a time far below the decorrelation time of in vivo biological tissues. A systematic comparison of performance with direct imaging is provided. [ABSTRACT FROM AUTHOR]

Abstract: Digital optical wavefront reconstruction for 3D display at tens or hundreds centimeters scale with viewing angle over 100° calls for resolutions at sub-wavelength scale. However the pixel pitches of spatial light modulators (SLM) are at micrometer or tens of micrometer scale. When people generated 3D images by digital holography using SLMs they had to make the interference patterns very sparse, resulting in small image size or narrow viewing angle. The paper employed an adiabatic waveguide taper made of quantities of single-mode waveguides to decompose complicated light fields into simpler forms so that complete digital optical phase conjugation (CDOPC) could be performed using low resolution SLMs. Then the paper introduced an optical transforming unit to project the reconstructed small wavefronts over a large volume while maintaining large viewing angle. The paper shows both theoretically and experimentally that 3D display systems based on CDOPC could create large size 3D images with large viewing angle and diffraction limited image quality. Besides, CDOPC might also be widely used for 3D measurement, optical tweezers, high energy laser beam forming, etc. [Copyright &y& Elsevier]

We study the electronic properties of rippled freestanding graphene membranes under central load from a sharp tip. To that end, we develop a gauge field theory on a honeycomb lattice valid beyond the continuum theory. Based on the proper phase conjugation of the tight-binding pseudospin Hamiltonian, we develop a method to determine conditions under which continuum elasticity can be used to extract gauge fields from strain. Along the way, we resolve a recent controversy on the theory of strain engineering in graphene: There are no K-point-dependent gauge fields. We combine this lattice gauge field theory with atomistic calculations and find that for moderate load, the rippled graphene membranes conform to the extruding tip without a significant increase in elastic energy. Mechanical strain is created on a membrane only after a certain amount of load is exerted. In addition, we find that the deformation potential-even when partially screened-induces qualitative changes on the electronic spectra, with Landau levels giving way to equally spaced peaks. [ABSTRACT FROM AUTHOR]

Abstract: Passive-phase conjugation (PPC) achieves the pulse compression for the time delayed arrivals. This property has been used in underwater acoustic communications. In a time-varying channel, the temporal focusing degrades with time evolution. The block-based approach is implemented to avoid the impact of channel variations. The basic idea is that the channel is assumed constant within each block of a short time interval, and the channel is updated using detected symbols in the previous block. Using real data collected in a recent sea trial, this paper investigates: (1) the communications using PPC processing, where the block-based approach is used, (2) the matching pursuit (MP) algorithm exploiting the channel sparseness. It is found that the MP algorithm improves performance of the communications using PPC processing. [Copyright &y& Elsevier]

Measurements of the sound absorption coefficient in test objects containing solid microparticles randomly distributed over the object volumes are carried out. Two methods are used for this purpose: the standard echo-pulse insert-substitution method and a modified method using phase conjugation of ultrasound. The test objects are made from gelatin, and the size of the particles introduced in it is chosen to allow measurements in both the long- and medium-wavelength scattering modes of the probing beam. It is shown that, in the first scattering mode, in which the presence of particles causes additional viscous and temperature losses, the two aforementioned methods give identical results. In the second scattering mode, in which the dominant mechanism of additional loss is elastic scattering, the use of phase conjugation allows an almost complete reconstruction of the scattered field and, hence, a more reliable upper estimate for the coefficient of ultrasonic absorption in the test objects. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the coherent beam combination technique using SBS-PCM for a high repetition rate laser will be demonstrated. It is a novel concept capable of achieving a high energy and high repetition rate laser. A phase conjugation mirror and beam combination laser using a self-phase-controlling technique for coherent beam combination is introduced in this paper. And, a practical demonstration of the coherent beam combination laser using SBS-PCM is presented. With a wave-front dividing scheme, the relative phases are measured to be less than λ/26 even when the amplifier is operating. The coherent beam combination laser using SBS-PCMs has the additional advantage of providing accurate high speed/high rate injection of the fusion target in the LIFE reactor system. The coherent beam combination of 25 100J/10Hz laser modules for the 2.5kJ/10Hz laser beam line for the fusion energy driver is suggested, which is available with the current laser technologies. [Copyright &y& Elsevier]

Abstract: A scheme for a high-speed wavelength encoded all-optical S–R flip-flop (or a digital memory cell for storing of optical bits) based on wavelength conversion (MZI) in semiconductor optical amplifier (SOA) and phase conjugation system (PCS) is proposed. The switching action of semiconductor optical amplifier (SOA) does not give too high operational speed because of electrical pumping power. But optical phase conjugation mechanism gives us ultrahigh operational speed. So, joint use of them gives rise to a more high speed system comparatively to only SOA based switches. Here two logic states of the whole system is encoded by two wavelengths as well as frequencies, since the information in the bit is unaffected throughout the communication not having regard to the loss of light energy due to reflection, refraction polarization, etc. [Copyright &y& Elsevier]

Piezoelectric flextensional actuators (PFAs) are an efficient alternative to systems that demand nano-positioning of devices, such as in nanotechnology. Optical techniques constitute an excellent choice for contactless measurement of nano-displacements. In particular, optical interferometry constitutes an adequate choice for characterizing PFAs. There are several types of interferometers, as well as optical phase demodulation methods, used in practice. One interesting class of demodulation methods uses the spectrum of the photo-detected signal and its intrinsic properties when there is a harmonically varying time-domain modulating signal. In this work, a low cost homodyne Michelson interferometer, associated with simple electronic circuits for signal conditioning and acquisition, is used. A novel dynamic phase demodulation method, named Jm&Jm + 2, is proposed, which uses only the magnitude spectrum of the photo-detected signal, without the need to know its phase spectrum. The method is passive, direct, self-consistent, without problems of phase ambiguity and immune to fading, and presents a dynamic range from 0.45 to 100 rad displacements (between 22.6 nm and 5 µm, for λ= 632.8 nm). When applied to the measurement of half-wave voltage in a proof-of-concept Pockels cell, it presents errors smaller than 0.9% when compared to theory. For the estimation of PFA displacement, it allows the measurement of linearity and frequency response curves, with excellent results [ABSTRACT FROM AUTHOR]

We propose a new method for characterizing optical phase modulators based on phase modulation-to-intensity modulation (PM-to-IM) conversion in dispersive fibers. The fiber dispersion spectrally alters the relative phasing of the phase-modulated signal and leads to the PM-to-IM conversion, which is extended to measure the modulation efficiency of optical phase modulators. In the demonstration, the frequency-dependent modulation index and half-wave voltage are experimentally measured for a commercial phase modulator. Compared with conventional methods, the proposed method works without the restriction of small-signal operations, and allows swept-frequency measurement with high resolution and accuracy by using a vector network analyzer. [ABSTRACT FROM AUTHOR]

Fluorescence imaging has revolutionized biomedical research over the past three decades. Its high molecular specificity and unrivalled single-molecule-level sensitivity have enabled breakthroughs in a number of research fields. For in vivo applications its major limitation is its superficial imaging depth, a result of random scattering in biological tissues causing exponential attenuation of the ballistic component of a light wave. Here, we present fluorescence imaging beyond the ballistic regime by combining single-cycle pulsed ultrasound modulation and digital optical phase conjugation. We demonstrate a near-isotropic three-dimensional localized sound-light interaction zone. With the exceptionally high optical gain provided by the digital optical phase conjugation system, we can deliver sufficient optical power to a focus inside highly scattering media for not only fluorescence imaging but also a variety of linear and nonlinear spectroscopy measurements. This technology paves the way for many important applications in both fundamental biology research and clinical studies. [ABSTRACT FROM AUTHOR]

We demonstrate a method for phase conjugating fluorescence. Our method, called reference free digital optical phase conjugation, can conjugate extremely weak, incoherent optical signals. It was used to phase conjugate fluorescent light originating from a bead covered with 0.5 mm of light-scattering tissue. The phase conjugated beam refocuses onto the bead and causes a local increase of over two orders of magnitude in the light intensity. Potential applications are in imaging, optical trapping, and targeted photochemical activation inside turbid tissue. [ABSTRACT FROM AUTHOR]

We present an imaging modality capable of providing high-speed optical dispersion measurements of live cells. The technique permits wide-field measurement of the optical phase shifts introduced by a sample for multiple discrete wavelengths in a single image capture. Utilizing spatial modulation and the wavelength dependence of the interference-fringe spacing, average refractive index as a function of wavelength is obtained, yielding optical dispersion measurements of the sample under observation. Because of its simple and low-cost design, the technique can be readily integrated into a standard microscope to collect additional diagnostic information about biological cells. [ABSTRACT FROM AUTHOR]

A theory of two-photon phase-sensitive amplifier based on two-photon cascade is presented. The atomic coherence is produced by means of two dipole-allowed transitions driven by coherent fields; a practical alternative versus scheme employing coupling between dipole-forbidden levels. The regime of parameters under which a laser signal can be phase locked and amplified is discussed through the corresponding Fokker-Planck equation. [ABSTRACT FROM AUTHOR]

Free space transmission of an on-off modulated sinusoidal signal through a phase conjugating lens (PCL) is theoretically examined using a combined time/frequency domain approach. The on-off keyed (OOK) signal is generated by a dipole antenna located in the far-field zone of the lens. The PCL consists of a dual layer of antenna elements interconnected via phase conjugating circuitry. We demonstrate that electromagnetic interference between antenna elements creates spatially localised areas of good-quality reception and zones where the signal is significantly denigrated by interference. Next, it is shown that destructive interference and packet desynchronisation effects critically depend on bit rate. It is also shown that a circular concave lens can be used to produce high-quality signal reception in a given direction while suppressing signal reception in all other directions. The effect that the bandwidth of the phase conjugating unit has on the transmitted signal properties for the cases of high and low bit rate OOK modulation are studied and a signal quality characterisation scheme is proposed which uses cross-correlation. The results of the study yields understanding of the performance of phase conjugating arrays under OOK modulation. The work suggests a novel approach for realising a secure communication wireless system. [ABSTRACT FROM AUTHOR]

Simulations performed with the particle-in-cell code Calder Circ show the feasibility of injection and acceleration of electrons in the laser wakefield created by few-femtosecond laser pulses with moderate energy at the fewmJ level. A detailed study of the effect of the carrier-envelope phase of the pulse on the injection is presented. It is shown that using ionization injection with nitrogen as the target gas, the control of the optical phase allows production of high-quality and shot-to-shot stable electron beams. The electron bunches obtained have a relative energy spread of a few per cent, a bunch duration in the sub-fs domain, a divergence close to 10 mrad and a peak energy in the 10MeV range, and could be produced in the near future at kHz repetition rates. [ABSTRACT FROM AUTHOR]

The interaction between a vortex and an edge dislocation nested in a cos‐Gaussian beam passing through a tilted lens was studied analytically and numerically. It is shown that in the presence of the tilted lens, by varying the cos‐part parameter of the beam, the slope of the edge dislocation, or the tilt coefficient of the lens, the topological charge is conserved for the case of the off‐axis edge dislocation, while charge conservation does not hold true for the case of an on‐axis edge dislocation, irrespective of the on‐axis or off‐axis vortex. The relation between the transverse positions (x, y) of vortices and the tilt coefficient is linear, and is interpreted. [ABSTRACT FROM PUBLISHER]

In this study, the authors analyse the temporal and spatial compression characteristics of retrodirective antenna systems wherein active phase conjugation (APC) is deployed. It is shown that under the APC Hermitian channel matrix properties result and these lead to opportunities for both temporal and spatial signal compression. The effects of additive channel noise, as well as compression assistance given by multipath richness are described. Some general properties of retrodirective antenna systems with respect to linear and circular polarised pilot signals are also discussed. The scheme elaborated here is compliant with a generic channel model and can therefore be easily incorporated into any readily available communication systems analysis or channel modelling software. The work presented in this study provides a theoretical foundation for the previous experimental findings in APC systems. [ABSTRACT FROM AUTHOR]

Abstract: A number of canted undulators will be installed at the ESRF. Permanent Magnet Steerers will be used for providing canting angles up to 5.4mrad. These steerers have been optimized in view of the limited space available and the required field quality. The magnetic interaction between the steerers and the undulators may lead to additional optical phase errors: auxiliary poles have been added to obtain a fast decrease of the fringe field. The transverse homogeneity of the magnetic field integral is another strong design constraint. Good homogeneity was reached by optimizing the profiles of the iron poles. Eight steerers have been constructed and magnetic measurements have been performed. [Copyright &y& Elsevier]

Abstract: In the present formulation using hydrodynamic model and coupled mode scheme of plasmas the interaction length necessary to achieve optical phase conjugation is obtained from steady-state Brillouin gain coefficient for a semiconductor crystal. The analytical investigation of steady-state gain of stimulated Brillouin scattering (SBS) is made by assuming that the SBS is resulted from the nonlinear interaction of an intense electromagnetic wave with acoustic perturbation internally generated due to acousto-optic property of the semiconductor crystal. Numerical estimates confirm that when cyclotron frequency is tuned with pump frequency, interaction length is found to be nearly smaller than that obtained in absence of magnetic filed. Pump intensity and free carrier concentration both helpful in reducing the required interaction length. [Copyright &y& Elsevier]