Fan-out Gratings Research Articles

Analysis of errors and correction of topology of computer-generated two-dimensional fan-out gratings

A procedure is proposed and investigated for correction of direct laser writing of the topology of two-dimensional diffraction fan-out gratings with the use of a laser-lithographic system DWL66+ operating in the Cartesian system. Thin films of photoresist and chromium on a quartz substrate were used as recording materials. The required binary-phase structures were obtained by subsequent ion-plasma etching of the amplitude structures recorded by the system. The minimum feature size of the binary zones of synthesized structures was 1500´1500 nm. It has been found that, in order to obtain the stability of the reproduction of the synthesized structures and to approach these structures to the calculated ones, it is necessary to use a regime with partial overexposure of photoresist and correction of position of transition boundaries of exposed zones. The specified correction of position of transition boundaries was experimentally implemented by software, with a modification of the topology of written structures and with experimental founding the optimal diffraction fan-out gratings accounting for their response, with the maximization of efficiency in useful diffraction orders and minimization of the root-mean-square deviation between intensity maxima in useful orders. Taking into account the size of a focused spot (about 550 nm), as well as the recording materials and the regime of photoresist overexposure, it was found that the optimal correction for the displacement of the boundaries of the exposed zones amounts to 200 nm, with the boundaries being shifted to the centers (inside) of the zones.

Continuous wavelength tuning from 3.9–12 µm from an optical parametric oscillator based on orientation-patterned GaP grown on GaAs

We report for the first time nonlinear frequency conversion—specifically optical parametric oscillation—in OP-GaP layers grown by hydride vapor-phase epitaxy on 3-inch OP-GaAs templates. Continuous tuning with wavelength coverage from 3.9–12 µm was achieved by using stepped and fan-out gratings having periods from 18.0–35.2 µm, which propagated 150 µm of a 1.2-mm-thick layer before overgrowth. Anti-reflection-coated OP-GaP crystals were pumped at 1040 nm with an ultrafast Yb-fiber laser, yielding idler output powers up to 150 mW (60 mW) in spectra centered at a wavelength of 5.6 µm (10.7 µm).

Inverse design and demonstration of high-performance wide-angle diffractive optical elements.

Diffractive optical elements are ultra-thin optical components required for constructing very compact optical 3D sensors. However, the required wide-angle diffractive 2D fan-out gratings have been elusive due to design challenges. Here, we introduce a new strategy for optimizing such high-performance and wide-angle diffractive optical elements, offering unprecedented control over the power distribution among the desired diffraction orders with only low requirements with respect to computational power. The microstructure surfaces were designed by an iterative gradient optimization procedure based on an adjoint-state method, capable to account for application-dependent target functions while ensuring compatibility with existing fabrication processes. The results of the experimental characterization confirm the simulated tailored power distributions and optical efficiencies of the fabricated elements.

轨道角动量分离系统中纯相位型扇出光栅的设计

轨道角动量分离系统中纯相位型扇出光栅的设计

High efficiency geometric-phase polarization fan-out grating on silicon.

We report the design, fabrication and characterization of a 1-by-5 geometric-phase polarization fan-out grating for coherent beam combining at 1550 nm. The phase profile of the grating is accurately controlled by the local orientation of the binary subwavelength structure instead of the etching depth and profile empowering the grating to be more tolerant to fabrication errors. Deep-UV interference lithography on silicon offers an inexpensive, highly efficient and high damage threshold solution to fabricating large-area fan-out gratings than electron beam lithography (EBL) and photoalignment liquid crystals. The theoretical and experimental diffraction efficiency of the grating is 87% and 85.7% respectively. Such a fan-out grating may find application to high-power beam combining in the infrared regime.

Geometric-Phase Polarization Fan-out Grating Fabricated with Deep-UV Interference Lithography

We report the design, fabrication and testing of a highly efficient polarization fan-out grating for coherent beam combining working at 1550 nm. The grating design exploits the geometric-phase effect. Deep-UV interference lithography is used to fabricate the designed grating. Such a polarization fan-out grating demonstrates several advantages that are ideal for laser beam combining.

Analysis of multimask fabrication errors for diffractive optical elements

As design algorithms for diffractive optical elements improve, the limiting factor becomes the fabrication process. It is hoped a better understanding of fabrication errors will allow elements with greater tolerance to be designed. This is important for high-power laser fiber coupling, where hot spots lead to failure. We model seven different fan-out gratings applying misetch, misalignment, and feature rounding. Our main findings are that misetch can lead to improved results, misalignment is strongly asymmetric, and both the pi and pi/2 masks can dominate misalignment. Rounding has a r(2) dependence and potentially can be incorporated into the design stage. Finally we present some experimental data for misalignment.

High-efficiency multiple imaging in three-dimensional space

A technique of the high-efficiency formation of identical multiple images of a monochromatic object within three-dimensional space is proposed and investigated. The central points of these images are located in the points of a regular three-dimensional free-space lattice. The image replication is achieved by the spatial-frequency filtering based on the multiple uniform splitting of the angular spectrum of the object. Within each useful spectral group, all the transverse and longitudinal spectral components of the object are shifted by its own constants along the corresponding three spatial-frequency directions. To this end, a coherent optical processor is employed, which contains a special phase diffractive element in its Fourier plane. The phase function of the element is the superposition of the phase functions of a diffractive multifocal lens and a two-dimensional diffractive fan-out (Dammann-type) grating. The results of numerical simulation are given for a particular case of using a 4-level phase diffractive element and generating the wave field with a regular 3 × 3 × 3 array of main intensity maxima.

Multiwavelength operation with thin diffractive elements

It is generally accepted that diffractive elements designed for multiwavelength operation require deep surface-relief profiles. We show, however, that thin diffractive elements can be designed to operate with more than one wavelength. A novel, to our knowledge, optimization technique is introduced for this purpose. The maximum phase delay is limited to only a few multiples of 2pi, and the element can implement different functions for different wavelengths. Examples with fan-out gratings are discussed.

Rigorous coupled-wave method applied to fan-out gratings

This paper extends and generalises the rigorous coupled-wave method (for volume-phase holograms), in order to study multiplexed (fan-out) gratings recorded in the same volume. The theory is equally applicable to transmission and reflection gratings replayed with TE polarised light and is consistent with published experimental data. Reflection fan-out holograms are shown to be more efficient and have a higher replay fidelity than their transmission counterparts.

Electromagnetic analysis of fan-out gratings and diffractive cylindrical lens arrays by field stitching

The field-stitching method introduced previously [Opt. Lett.21, 1508 (1996)] allows large-period gratings containing small-scale local structure to be analyzed without requiring excessive computing power. Multiple scattering and the polarization of the electromagnetic field are taken into account by standard rigorous analysis. A variety of calculations demonstrating the accuracy and the speed of the method are made. Also, we report on the optimization by simulated annealing of fan-out gratings with periods of up to 240λ and of diffractive cylindrical lenses with a period of 1200λ, which has been made possible by field-stitching analysis.

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.

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.

High-efficiency binary fan-out gratings by modulation of a high-frequency carrier grating

A modulation scheme that uses pulse-position modulation of a high-frequency binary grating to increase the diffraction efficiency of the elements is presented. These elements are designed and fabricated with both one- and two-dimensional signals for operation in transmission or reflection modes in the visible and the infrared regions of the spectrum. A direct electron-beam lithography fabrication process capable of realizing features of ∼280 nm with a resolution of 15 nm is described in detail. Experimental results show that diffraction efficiencies of >80% are attainable.

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.

Fan-out gratings with a continuous profile

We show that it is relatively simple to design a fan-out grating with a continuous profile which has a theoretical performance equal to that of a binary profile but takes considerably less time to optimise. We demonstrate that it is feasible to produce gratings with a continuous and arbitary groove profile although present results are not as good as for some binary gratings.