Wigner function for nonparaxial wave fields

The generalized optical transfer function and the spectral correlation function are investigated for nonparaxial two-dimensional wave fields. The angle-impact marginal of the four-dimensional Wigner function is derived directly. For focused wave fields of semiangle greater than 90 degrees, the spectral correlation function exhibits overlapping and interference. For focused wave fields for which the semiangle is known to be less than 180 degrees, the magnitude and phase can be recovered directly from knowledge of the intensity in the focal region.     

Wigner functions for nonparaxial, arbitrarily polarized electromagnetic wave fields in free space

New representations are defined for describing electromagnetic wave fields in free space exactly in terms of rays for any wavelength, level of coherence or polarization, and numerical aperture, as long as there are no evanescent components. These representations correspond to tensors assigned to each ray such that the electric and magnetic energy densities, the Poynting vector, and the polarization properties of the field correspond to simple integrals involving these tensors for the rays that go through the specified point. For partially coherent fields, the ray-based approach provided by the new representations can reduce dramatically the computation times for the physical properties mentioned earlier.     

Wigner functions for paraxial and nonparaxial fields

The Wigner function gives an intuitive representation of coherent and partially coherent wave fields in the frequency domain, which takes the form a ray weight distribution. However, this weight distribution is conserved along rays only for paraxial or significantly incoherent fields. Generalizations of the Wigner function have been defined for propagation through transparent homogeneous media that are exactly conserved along rays for any angular spread and state of coherence. A series expression is derived here for calculating these nonparaxial generalizations starting from the standard Wigner function.     

Ambiguity function and phase-space tomography for nonparaxial fields

A nonparaxial generalization of the ambiguity function that retains several properties of its paraxial counterpart is presented, in both two and three dimensions. This generalization is used to extend into the nonparaxial regime a scheme for the recovery of the coherence properties of scalar partially coherent fields in two-dimensional space.     

Hybrid numerical scheme for time-evolving wave fields

Many problems in geophysics, acoustics, elasticity theory, cancer treatment, food process control and electrodynamics involve study of wave field synthesis (WFS) in some form or another. In the present work, modelling of wave propagation phenomena is studied as a static problem, using finite element method and treating time as an additional spatial dimension. In particular, WFS problems are analysed using discrete methods. It is shown that a fully finite element-based scheme is very natural and effective method for the solution of such problems. Distributed WFS in the context of two-dimensional problems is outlined and incorporation of any geometric or material non-linearities is shown to be straightforward. This has significant implications for problems in geophysics or biological media, where material inhomogeneities are quite prevalent. Numerical results are presented for several problems referring to media with material inhomogeneities and predefined absorption profiles. The method can be extended to three-dimensional problems involving anisotropic media properties in a relatively straightforward manner. Copyright © 2006 John Wiley & Sons, Ltd.     

New method for nonparaxial beam propagation

A new method for solving the wave equation is presented that is nonparaxial and can be applied to wide-angle beam propagation. It shows very good stability characteristics in the sense that relatively larger step sizes can be taken. An implementation by use of the collocation method is presented in which only simple matrix multiplications are involved and no numerical matrix diagonalization or inversion is needed. The method is hence faster and is also highly accurate.     

Measurement of Helmholtz wave fields

A simple formalism is found for the measurement of wave fields that satisfy the Helmholtz equation in free space. This formalism turns out to be analogous to the well-known theory of measurements for quantum-mechanical wave functions: A measurement corresponds to the squared magnitude of the inner product (in a suitable Hilbert space) of the wave field and a field that is associated with the detector. The measurement can also be expressed as an overlap in phase space of a special form of the Wigner function that is tailored for Helmholtz wave fields.     

Propagating free-space nonparaxial beams

We use a method based on the simultaneous combination of the propagation operator and the Fourier transform with arbitrary index in propagating the transverse component of a nonparaxial beam in free space from an arbitrary initial transverse field structure. Being an iterative method, this approach can easily be implemented computationally. As an example of its efficiency, we derive the closed-form nonparaxial corrections to a Bessel-Gaussian beam, showing that our results differ strongly from those reported previously. The validity of our approach is supported by an analysis of the paraxiality estimator recently introduced in the literature.     

A plane wave expansion theorem for cylindrically radiated fields

Summary An asymptotic expansion for two-dimensional outwardly radiating fields is developed from an integral representation of these fields by means of a saddle point integration. The expansion is given in terms of inverse powers of the distance from a point in a fixed region to a point in a circular neighborhood at a large distance from that region. The coefficients are expressed in terms of plane waves and linear combinations of derivatives of plane waves with respect to angle of incidence. The theorem may be employed in scattering problems in reducing scattering of arbitrary two-dimensional fields by arbitrary cylinders to scattering of plane waves by the arbitrary cylinders. Research supported by contract AF19 (628) 3868 with Air Force Cambridge Research Laboratories. The initial research was carried out while one of the authors (N. R. Zitron) was at Harvard University.     

Higher-order extrema in two-dimensional wave fields

Higher-order extrema with topological indices greater than unity are discussed. Explicit constructions are given for their wave functions, and simple geometric rules are presented for analysis of their topological indices. Experimental means for verifying the theory with use of Gaussian laser beams are considered, unusual properties of optical vortices constructed from this new type of critical point are described, and applications to topologically based optical arithmetic are suggested.     

Propagation-invariant wave fields with finite energy

Propagation invariance is extended in the paraxial regime, leading to a generalized self-imaging effect. These wave fields are characterized by a finite number of transverse self-images that appear, in general, at different orientations and scales. They possess finite energy and thus can be accurately generated. Necessary and sufficient conditions are derived, and they are appropriately represented in the Gauss-Laguerre modal plane. Relations with the following phenomena are investigated: classical self-imaging, rotating beams, eigen-Fourier functions, and the recently introduced generalized propagation-invariant wave fields. In the paraxial regime they are all included within the generalized self-imaging effect that is presented. In this context we show an important relation between paraxial Bessel beams and Gauss-Laguerre beams.     

Step-transition perturbation approach for pixel-structured nonparaxial diffractive elements

An extension of an approximate step-transition perturbation method is presented that permits numerically efficient diffraction analysis of pixel-structured surface profiles in the nonparaxial domain. Comparison with the rigorous diffraction theory of gratings shows that the method is reasonably accurate provided that the pixel size exceeds approximately two wavelengths even if the structure contains isolated pixels.     

Aberrations of diffracted wave fields: distortion

Near-field diffraction patterns are merely aberrated Fraunhofer diffraction patterns. These aberrations, inherent to the diffraction process, provide insight and understanding into wide-angle diffraction phenomena. Nonparaxial patterns of diffracted orders produced by a laser beam passing through a grating and projected upon a plane screen exhibit severe distortion (W311). This distortion is an artifact of the configuration chosen to observe diffraction patterns. Grating behavior expressed in terms of the direction cosines of the propagation vectors of the incident and diffracted orders exhibits no distortion. Use of a simple direction cosine diagram provides an elegant way to deal with nonparaxial diffraction patterns, particularly when large obliquely incident beams produce conical diffraction.     

Diffraction tomography in reconstructing marginal spatial frequency spectra for wave fields

Measurement Techniques -     

On the angular-spectrum representation of multipole wave fields

The closed-form expression of the angular spectrum of multipole fields, both scalar and vectorial, of any order and degree, evaluated across a plane orthogonal to an arbitrary (fixed) direction, is provided. Such a result has been obtained by starting from the Weyl representation of multipole fields and using suitable transformation rules. Moreover, as far as the vectorial case is concerned, knowledge of the (vectorial) transverse angular spectrum allows one to gain some insight into the polarization structure of the multipole fields evaluated across a typical plane. Such information could be useful, for instance, in those problems dealing with the interaction between planar partially reflecting surfaces and waves.     

Topological phase in a nonparaxial Gaussian beam

An analysis is made of the structure and evolution of the singularities of a nonparaxial Gaussian beam. It is shown that a Gaussian beam may be represented by a family of straight lines lying on the surface of a hyperboloid and that the wavefront of this beam is a function of a point source situated at a point on the z axis with the imaginary coordinate iz ₀. The argument of this complex function is the topological phase of the beam which characterizes the rotation of the wavefront. The singularities of a nonparaxial Gaussian beam are located in the focal plane and are annular edge dislocations. Dislocation processes near the constriction of the Gaussian beam only occur as a result of aperture diffraction. Pis’ma Zh. Tekh. Fiz. 25, 14–20 (November 26, 1999)     

Born approximation for the nonparaxial scalar treatment of thick phase gratings

The conventional design of phase gratings or kinoforms with a paraxial transmission function is restricted to the paraxial domain and thin elements. Therefore, the design and analysis of thick phase-relief structures require a nonparaxial theory, as given by the Born approximation. The Born approximation is derived as an extension of the scalar thin-element theory, which is applicable for thick elements with large propagation angles. As an example, general prism gratings on curved surfaces are treated.     

Computation of quasi-discrete Hankel transforms of integer order for propagating optical wave fields

The method originally proposed by Yu et al. [Opt. Lett. 23, 409 (1998)] for evaluating the zero-order Hankel transform is generalized to high-order Hankel transforms. Since the method preserves the discrete form of the Parseval theorem, it is particularly suitable for field propagation. A general algorithm for propagating an input field through axially symmetric systems using the generalized method is given. The advantages and the disadvantages of the method with respect to other typical methods are discussed.     

Control of ion beams by surface ion-acoustic wave fields

It is shown for the first time that surface ion-acoustic waves may be used for polishing, etching, and depositing coatings. The proposed method of treatment is simple and convenient since the apparatus required is not cumbersome and is easy to fabricate. Compared with conventional ion beams, the proposed method of surface treatment can deliver high particle currents at the surface, which should enhance the efficiency of the material treatment. In addition, the excited surface ion-acoustic waves are natural waves, which reduces the energy consumption. Pis’ma Zh. Tekh. Fiz. 24, 6–11 (February 12, 1998)