Eero Noponen

Eigenmode method for electromagnetic synthesis of diffractive elements with three-dimensional profiles

We extend the rigorous eigenmode theory of binary surface-relief gratings to accommodate three-dimensional-modulation profiles, to formulate a synthesis problem for doubly periodic resonance-domain diffractive elements, and to demonstrate some of the problem’s symmetry properties. Several solutions for multiple beam splitters with ~90% transmission-mode diffraction efficiency are obtained by nonlinear parametric optimization. The polarization sensitivity and the required fabrication accuracy are analyzed for some solutions.

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.

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.

Millimeter-wave beam shaping using holograms

We synthesize amplitude- and phase-type computer-generated holograms (diffractive gratings) for shaping millimeter-wave fields. We design holograms using quasi-optical back-propagation and rigorous optimization methods adopted from diffractive optics. We present experimental results from a plane-wave-generating hologram and a custom-designed field shaper at 310 GHz. Holograms can be applied, e.g., in a compact antenna test range and we propose their use for alignment purposes.

Electromagnetic theory of Talbot imaging

Abstract Electromagnetic diffraction theory is applied to investigate self-imaging of optical fields transmitted by lamellar diffraction gratings. We consider both conventional and fractional Talbot imaging of metallic Ronchi rulings and dielectric surface-relief gratings. Our rigorous approach takes fully into account diffraction inside the modulated grating structure as well as non-paraxial propagation of the electromagnetic field into the observation plane, and it is valid for any grating period. We also show that, for certain grating periods, the transmitted electromagnetic field self-images exactly, regardless of the grating profile.

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.

Guided-mode resonance filters of finite aperture

The operation of optical narrowband reflection filters based on resonance anomalies of waveguide gratings is well established for gratings of infinite extent. We investigate the properties of finite-aperture waveguide-grating resonance filters by means of rigorous electromagnetic theory and an approximate model. The rigorous approach illustrates the scattering of optical energy from the guided mode at and near the edges of the element, which leads to a reduced diffraction efficiency into the backward-diffracted zeroth order. The approximate approach provides an optical-engineering model for the estimation of the minimum grating size required to achieve a high resonance-wavelength reflectivity.

Binary high-frequency-carrier diffractive optical elements: electromagnetic theory

Using rigorous electromagnetic diffraction theory, we evaluate the potential performance and the limitations of coding diffractive optical elements in the form of a pulse-frequency-modulated carrier grating. This coding scheme employs the first diffraction order of an ultrahigh-frequency binary carrier grating, with a period below 1.5 wavelengths. We establish that the pulse-frequency-modulation structure can be designed with the standard synthesis techniques based on paraxial scalar diffraction theory. However, we had to optimize the groove depth, the aspect ratio, and the carrier period with rigorous electromagnetic theory to achieve close to 100% efficiency and the desired polarization properties. Our method is compared with another recent coding scheme that utilizes the zeroth order of a subwavelength-period pulse-width-modulated binary carrier grating.

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.

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.