Feedback method of the noise suppression in wave-mixing amplifiers based on non-linear materials with photorefractive response in a reflection grating configuration

In the photorefractive wave-mixing system, fluctuation in the signal beam intensity of the photorefractive output with a reflection grating has been analyzed by employing pump feedback method. In this method, fluctuations of the photorefractive wave-mixing process not only induce the intensity fluctuation of the mixing waves but also induce phase fluctuation of the mixing waves. Thus, the phase of the pump and signal beams at the output surface fluctuates in time around a mean value. Using such a positive feedback method of a pump beams, the relative fluctuation in the photorefractive output signal beam intensity with respect to its mean intensity can be minimized significantly without reducing its mean intensity. The factors that control the fluctuation in the signal beam intensity, such as the phase fluctuation of the output pump beam, absorption strength of the material and the feedback reflectivity of the cavity mirrors, on the relative fluctuation of the output signal intensity in the photorefractive wave-mixing systems have been studied in detail. It has been found that the fluctuation of the output signal intensity relative to its mean intensity in the photorefractive wave-mixing system can be suppressed to larger extent by taking lower value of feedback reflectivity of the cavity mirrors which could exist at a higher value of absorption strength of the photorefractive materials.     

Changes in the intensity distribution and polarization of non-uniformly polarized beams focused by a spherically aberrated lens with annular aperture

Based on the generalized Huygens-Fresnel diffraction integral and beam coherence polarization (BCP) matrix formulation, the focusing properties of a type of non-uniformly polarized (NUP) beam through a spherical aberrated lens with annular aperture are studied. The dependence of focal shift, intensity distribution, power in the bucket (PIB) and degree of polarization on the spherical aberration coefficient, obscure ratio and polarization parameters is illustrated numerically. The focusing of NUP beams by a spherically aberrated lens with circular aperture is treated as a special case, where the inner radius of the aperture is equal to zero.     

Entangled photon generation in a nonlinear microring resonator for birefringence-based sensing applications

This paper proposes a new concept of birefringence-based sensor using the entangled photon timing walk-off compensation. The superposition of nonlinear light known as four-wave mixing is introduced by the Kerr nonlinear effects type within the ring device. The possible two entangled photon pairs are randomly generated using the polarization control unit. Results obtained have shown that the entangled state walk-off of light traveling within the ring device can be compensated. This means the changes in walk-off parameters can be relatively measured to the changes in the applied physical parameters. The potential of using such a proposed system for birefringence-based sensor applications is plausible and discussed.     

Radially polarized hyperbolic-cosine-Gaussian beam

Vector beams have attracted much interest recently. In this paper, focusing properties of the radially polarized hyperbolic-cosine-Gaussian beam are investigated. Simulation results show that the focal depth increases with increasing cosine parameter in the cosine term of the beam, while focal spot decreases simultaneously, namely, superresolution occurs. Focal depth increase velocity is quicker for larger cosine parameter, while the transverse focal spot shrinks more quickly for smaller cosine parameter. In addition, for two-portion concentric piecewise radially polarized hyperbolic-cosine-Gaussian beam with π phase shift in center circle portion, focal pattern evolves considerably with increasing cosine parameter, and the evolution principle differs for different radius of the center circle portion. Focal splitting and novel focal spots may appear. This kind of vector beam can be used in optical storage, optical manipulation, and lithography.     

Design and application of phase photon sieve

The imaging principle of Fresnel zone plate and photon sieve were analyzed in this paper. The design and fabrication of phase photon sieve were discussed. The feasibility of using phase photon sieve to realize nano-lithography was analyzed, a novel lithography experiment system based on phase photon sieve was presented, which not only has higher resolution and image contrast than the Fresnel zone plate lithography but also have higher diffractive efficiency than the amplitude photon sieve lithography.     

Analysis of beam quality for the laser beams after spectral beam combining

The beam quality of the laser beams after spectral beam combining (SBC) has been analyzed in this paper. Based on the propagation model of the spectral beam combining system, the expression for the output field of an individual beam passing through the spectral beam combining system with lens aberration has been given and the intensity distribution of the combined beam has been studied by the principle of incoherent superposition. Consequently, according to the intensity second-order moments method, the M²-factor of the combined beam has been calculated and the effect of lens aberration on the characteristics of the combined beam has been analyzed quantitatively. It can be shown that the SBC system can improve the beam quality of laser array significantly, and the M²-factor of the combined beam passing through the ideal spectral beam combining system is almost the same as that of an individual beam. For the spectral beam combining system with lens aberration, the spherical aberration has greater effect on the off-axis beam than on the on-axis beam, and the beam quality of combined beam degrades with increase in lens aberration.     

Quantum effect in a generalized N-atom Dicke model

We have studied the geometric phase and the sub-Poissonian photon distribution of a generalized N two-level atoms Dicke model in the thermo-dynamical limit and the off-resonant coupling case. It is found that the geometric phase in the ground state is relative to the atom number, the coupling strength between the atom and the light field, the frequency of the electromagnetic wave and the energy difference between two levels of the atom. The photons may exhibit the sub-Poissonian distribution in the ground state.     

Interaction of a high-order Bessel beam with a submerged spherical ultrasound contrast agent shell - Scattering theory

Background and objective

Acoustic scattering properties of ultrasound contrast agents are useful in extending existing or developing new techniques for biomedical imaging applications. A useful first step in this direction is to investigate the acoustic scattering of a new class of acoustic beams, known as helicoidal high-order Bessel beams, to improve the understanding of their scattering characteristics by an ultrasound contrast agent, which at present is very limited.

Method

The transverse acoustic scattering of a commercially available albuminoidal ultrasound contrast agent shell filled with air or a denser gas such as perfluoropropane and placed in a helicoidal Bessel beam of any order is examined numerically. The shell is assumed to possess an outer radius a = 3.5 microns and a thickness of ∼105 nm. Moduli of the total and resonance transverse acoustic scattering form functions are numerically evaluated in the bandwidth 0 < ka? 3, which corresponds to a frequency bandwidth of 0-205 MHz that covers a wide range of applications for imaging with contrast agents. Particular attention is paid to the shell’s material, the content of its interior hollow region and the fluid surrounding its exterior. The contrast agent shell is assumed to be immersed in an ideal compressible fluid so the viscous corrections are not considered. Analytical equations are derived and numerical calculations of the total and resonance form functions are performed with particular emphasis on the effect of varying the half-cone angle, the order of the helicoidal Bessel beam as well as the fluid that fills the interior hollow space.

Results and conclusion

It is shown that shell wave resonance modes can be excited on an encapsulated micro-bubble. The forward and backscattering vanish for a helicoidal high-order Bessel beam. Additionally, the fluid filling the inner core affects the shell’s response significantly. Moreover, there is no monopole contribution to the axial scattering of a helicoidal Bessel beam of order m ? 1 so that the dynamics of contrast agents would be significantly altered. The main finding of the present theory is the suppression or enhancement for a particular resonance that may be used to advantage in imaging with ultrasound contrast agents for clinical applications.     

Gegenbauer expansion to model the incident wave-field of a high-order Bessel vortex beam in spherical coordinates

The aim of this short communication is to report that Gegenbauer’s (partial-wave) expansion, that may be used (under some specific conditions) to represent the incident field of an acoustical (or optical) high-order Bessel beam (HOBB) in spherical coordinates, anticipates earlier expressions for undistorted waves. The incident wave-field is written in terms of the spherical Bessel function of the first kind, the gamma function as well as the Gegenbauer or ultraspherical functions given in terms of the associated Legendre functions when the order m of the HOBB is an integer number. Expressions for high-order and zero-order Bessel beams as well as for plane progressive waves reported in prior works can be deduced from Gegenbauer’s partial-wave expansion by appropriate choice of the beams’ parameters. Hence the value of this note becomes historical. In addition, Gegenbauer’s expansion in spherical coordinates may be used to advantage to model the wave-field of a fractional HOBB at the origin (i.e. z = 0).     

Complex geometrical optics of Kerr type nonlinear media

The paper generalizes paraxial complex geometrical optics (PCGO) for Gaussian beam (GB) propagation in nonlinear media of Kerr type. Ordinary differential equations for the beam amplitude and for complex curvature of the wave front are derived, which describe the evolution of axially symmetric GB in a Kerr type nonlinear medium. It is shown that PCGO readily provides the solutions of NLS equation obtained earlier from diffraction theory on the basis of the aberration-free approach. Besides reproducing classical results of self-focusing PCGO readily describes an influence of the initial curvature of the wave front on the beam evolution in a medium of Kerr type including a nonlinear graded-index fiber. The range of applicability of the PCGO theory is discussed as well which is helpful for avoiding nonphysical solutions.