Antti Vasara

Realization of general nondiffracting beams with computer-generated holograms

A new class of solutions to the scalar wave equation was introduced recently that represents transversely localized but totally nondiffracting fields. We show by the method of stationary phase that any of these wave fields can be realized approximately with a laser and a single computer-generated hologram. We briefly discuss various techniques for coding and fabrication of the required hologram and the associated diffraction efficiencies. Using both binary-amplitude and four-level phase holograms, we demonstrate experimentally the formation of arbitrary-order Bessel beams and rotationally nonsymmetric beams.

Propagation invariance and self-imaging in variable-coherence optics

The concept of propagation invariance in partially coherent optics is introduced. Explicit expressions are given for the cross-spectral density and the angular correlation function (cross-angular spectrum) characterizing a class of fields that are propagation invariant in the sense that their correlation properties in the space-frequency domain are exactly the same in every transverse plane. The so-called diffraction-free beams are shown to be members of this new, wider class of wave fields, which itself is a subset of a generalized class of partially coherent self-imaging fields. The existence of partially coherent propagation-invariant fields with a sharp correlation peak is verified experimentally by considering radiation from a planar J0 Bessel-correlated source.

Electromagnetic theory and design of diffractive-lens arrays

We apply the rigorous electromagnetic grating theory and an electromagnetic wave-propagation model with Kirchhoff boundary conditions to analyze and optimize the performance of arrays of cylindrical dielectric multilevel diffractive lenses. We show that the diffraction efficiency of a high-numerical-aperture lens array can be improved if the surface-relief profile is constructed with the use of rigorously optimized grating groove structures, which differ significantly from a multilevel approximation of a triangular profile.

Kinoform Phase Relief Synthesis: A Stochastic Method

We introduce a new method of finding periodic phase relief structures, which produce the desired diffraction pattern with high light efficiency (75% to 96%) and low noise (<±1%). Use is made of explicit equations for the angular spectrum of the field immediately behind such structures and of different stochastic nonlinear optimization methods.

Synthetic diffractive optics in the resonance domain

Resonance-domain diffractive optics covers the region where the characteristic feature sizes in the surface-relief modulation structure of the diffractive optical element are comparable to the wavelength of light; it may be viewed as a bridge between synthetic holography and the electromagnetic theory of diffraction gratings and rough surfaces. We consider the problem of synthesizing, in the framework of the electromagnetic theory, various types of periodic resonance-domain diffractive optical elements that utilize several diffraction orders. Parametric optimization is used to design one-to-N fan-out elements, N-to-N star couplers, and polarization-controlled optical beam splitters and switches with close to 100% efficiencies and no undesired diffraction orders in the image half-space. Reflection-type fan-out gratings with six and seven output beams are demonstrated experimentally at λ = 10.6 μm.

Optimisation and fabrication of grating beamsplitters

Optimisation of certain holographic 1 to N and 1 to N*N beamsplitters (Dammann gratings) with the aid of simulated annealing and damped least-squares algorithms is considered. Structures with N=5 to 53 uniform beams have been calculated. A photolithographic technique for fabrication of these elements is also presented, and its accuracy analysed with the aid of numerical simulations and actual test results.

Parametric optimization of multilevel diffractive optical elements by electromagnetic theory

To maximize the efficiency of dielectric diffractive optical elements, we optimized the local groove shape using the rigorous diffraction theory of multilevel surface-relief gratings.

Kinoform array illuminators in fused silica

Abstract High-efficiency (> 90%) multilevel grating array illuminators are designed using nonlinear parametric optimization and analysed using rigorous electromagnetic grating theory. Designs with eight and 16 relief depth levels are fabricated in fused silica using reactive ion etching and photolithography with three and four electron-beam generated binary amplitude masks, respectively.

Detour-phase kinoform interconnects: the concept and fabrication considerations

The concept of detour phase in computer-generated holography is generalized here to specify K-level phasetransmission functions that approximate optimized, periodic Fourier-transform-type carrier-frequency kinoforms. Combined with standard microlithographic techniques, this concept is shown to open up a possibility of generating (using K = 8) fully arbitrary space-invariant holographic interconnects characterized by negligible reconstruction error (<2%) and high diffraction efficiency (75–95%) into the desired pattern.

Rigorous diffraction analysis of Dammann gratings

Abstract Rigorous diffraction theory is applied for the first time to the analysis of periodic, binary computer-generated holographic optical fan-out elements, often called Dammann gratings. The effects of the length of the grating period on the reconstruction noise and the diffraction efficiency are analyzed and discussed. Novel schemes are proposed for producing compact and highly efficient space-invariant optical interconnects that operate in the resonance domain.