P. Ehbets

Continuous-relief diffractive optical elements for two-dimensional array generation

Continuous surface-relief diffractive optical elements for two-dimensional array generation (fan-out) are designed and fabricated. Separable and nonseparable solutions for the two-dimensional element design are compared. The phase-grating microstructures are generated by laser-beam writing lithography in a single exposure step and converted to nickel shims by electroplating, enabling low-cost replicas to be produced by using laboratory and commercial replication processes. Results are presented for a 9 x 9 fan-out diffractive optical element with a measured efficiency of 94% and an overall uniformity within +/-8%; replicas in epoxy have the same efficiency and a uniformity of +/-15%.

High-efficiency continuous surface-relief gratings for two-dimensional array generation

Continuous surface-relief phase gratings for two-dimensional (2-D) array generation have been realized by laser-beam writing lithography. For a 9 × 9 fan-out element, a diffraction efficiency of 94% and a uniformity of better than ±8% have been achieved. These are, to our knowledge, the best published results for 2-D surface-relief fan-out elements. Separable and nonseparable solutions for the design of 2-D fan-out elements are discussed.

Improvements in diffraction efficiency of gratings and microlenses with continuous relief structures

The fabrication of diffractive optical elements by laser-beam writing or gray-tone technology leads to continuous-relief phase elements. The diffraction efficiency of such elements is limited by the resolution of the process. In this paper, we compare the continuous-relief elements with the multilevel elements fabricated by binary technology. In particular, we will show that for similar sampling resolution of the ideal phase function, the continuous profiles have higher efficiencies than the multilevel profiles if the designed phase modulation is 4π and more.

Beam shaping of high-power laser diode arrays by continuous surface-relief elements

Abstract A breadboard for beam shaping of high-power laser diode arrays (LDAs) has been realized. The coherent beams are added with the aid of a continuous surface-relief fan-in element. It results in a nearly symmetric single lobed beam of collimated light with maximum conversion efficiency. The theoretical efficiency is determined to be 96·7%. Experimentally, one third of the total power is now in the central peak.

High-carrier-frequency fan-out gratings fabricated by total internal reflection holographic lithography

Total internal reflection (TIR) holographic lithography is applied to the fabrication of binary diffractive optical elements with submicrometer surface relief features. The recording conditions for the intermediate TIR volume hologram, used for high-resolution proximity printing, are discussed. In particular, the fabrication of efficient high-carrier-frequency fan-out gratings is considered and experimental results are presented for an off-axis 9 x 1 fan-out element in photoresist with a carrier frequency of 1000 lines/mm.

Continuous-relief fan-out elements with optimized fabrication tolerances

The design of kinoform fan-out elements with high efficiency and reduced sensitivity to vertical profile scaling errors is presented. We start from a high-efficiency continuous-phase fan-out solution and optimize the position of the 0-2π transitions in the phase function, in order to achieve a high fabrication-error tolerance. The sensitivity of Fourier-transform and focusing fan-out elements to vertical etch-depth errors is analyzed. The limitations for the fabrication of such structures by laserbeam writing are discussed. In particular, the influence of the finite writing-spot diameter on the fan-out performance is investigated. Design rules for fan-out elements, which consider fabrication constraints, are derived. Experimental results are presented for cylindrical focusing fanout elements with small uniformity error (2%) and weak profile scaling dependence.

Fan-out elements recorded as volume holograms: optimized recording conditions

The recording of efficient fan-out elements as volume holograms is investigated by using the coupled-wave theory. In contrast to the results published in the standard literature, we find that the efficiency and the uniformity of regular fan-out elements depend strongly on the relative phases of the object waves, at least, if the thickness of the hologram is less than ~50 wavelengths. High efficiency and uniformity can be achieved by optimized recording conditions. At the same time, the required dynamic range of the holographic material becomes minimum.

Diffractive optical elements for space communication terminals

The potential of diffractive optical elements for advanced laser communication terminals has been investigated. Applications include beam shaping of high- power laser diode arrays, optical filter elements for position detection and hybrid (refractive/diffractive) elements. In addition, we present a design example of a miniaturized terminal including diffractive optics.

TIR holography analyzed with coupled wave theory

Abstract A coupled wave model is presented to describe the diffraction behavior of thick total-internal reflection (TIR) holograms. We have transformed the coupled wave equations into a transfer matrix formalism, enabling a general treatment of the boundary conditions. The off-Bragg characteristics of TIR holograms is found to be completely different from normal volume holograms. The diffraction efficiency depends strongly on the phase of the total internal reflection and the optical pathlength through the hologram. High efficiency at the same angle as used for the recording requires careful control of all parameters.

Interferometric fabrication of modulated submicrometer gratings in photoresist

Interferometric recording is applied to the fabrication of modulated submicrometer gratings in photoresist.High diffraction efficiency requires optimized recording conditions, which are obtained by the use of an on-axis continuous surface-relief grating for the generation of the object beam. The optimized phase function is copied into the resist layer by means of a self-aligned two-step recording process with an intermediate copy in a volume photopolymer hologram. As a result, we demonstrate high carrier frequency surface-relief off-axis fan-out gratings for illumination in transmission with visible light.