Global trends towards mobility and miniaturization along with increasing demands to improve the performance and efficiency of sensor systems, especially in autonomous systems, have led to a need to decrease their size, weight, and power (SWaP). For millimeter wave (mmWave) and optical sensing, optically transparent mmWave antennas provide a solution to meet this need by offering a multi-modal, single aperture system with applications in vehicle windshields, solar cells on satellites, and drone camera lenses, among others. However, integrating these sensing systems while optimizing both the mmWave properties and optical sensing capabilities forces competing design and fabrication requirements. Currently, there also exists limited research and literature on a fabrication procedure for these micromesh structures. This paper presents a photolithographic procedure for fabricating micro-scale optically transparent mmWave transmission lines (TLs) and antennas suitable for use in the design optimization processes. It investigates the fabrication of varying line width versus line gap dimensions as well as the fabrication of different mesh unit cell designs. The fabrication procedure was designed to achieve down to 1-2 μm features and is replicable for both single-sided and dual-sided designs. Fabricated micro-mesh structures exhibit strong performance in both operating regimes, with optical transmission ranging from 80- 95% transmittance in the visible spectrum and high conductivity with sheet resistance less than 1 ohm-per-square.
KEYWORDS: Diffraction, Point spread functions, Modulation transfer functions, Transmittance, Antennas, Diffraction gratings, Optical gratings, Electrical conductivity, Transparency, Signal to noise ratio
State-of-the-art optically transparent antennas used for shared RF/EO/IR apertures typically utilize mesh conductors to simultaneously achieve high optical transmission and low RF resistivity compared to that of transparent conducting oxides (TCOs). Unlike TCOs, however, mesh conductors need to be patterned in a geometric shape – most commonly a square or rectangular periodic grid. As a result, this grid can introduce a diffraction pattern which can degrade the imaging quality of the optical system. It is shown that transmission decreases according to the geometric area covered by the mesh at normal incidence and that transmitted energy is pushed into higher diffractive orders, as expected. These effects are ultimately summarized as a decrease in the signal-to-noise and signal-to-background ratios. Performance tradeoffs in varying the line width and line spacing of a rectangular grid are shown whereas the smaller the line width and spacing, the smaller effects there are from the grid. Furthermore, methods to reduce diffractive effects –improving imaging quality - in structures by randomizing the substructure in which light interacts are presented. Randomizing tiled mesh substructures reduces periodicity to generate ultra-uniform diffraction, while maximizing conductivity of the structure. Increased randomization of a meshed aperture allows for a reduction in higher diffraction orders by 99%. By leveraging the results of this study, shared RF/EO/IR apertures can maximize their performance across the electromagnetic spectrum.
KEYWORDS: Antennas, Point spread functions, Solids, Modulation transfer functions, Transmittance, Spatial frequencies, Diffraction, Near field optics, Image resolution, Imaging systems
Optically transparent antennas integrated within an optical aperture enable unique opportunities for shared aperture radio frequency (RF)/electro-optical(EO)/infra-red (IR) sensing and communication. While optical transmission and RF efficiency are critical performance metrics for most applications, those applications with optical imaging requirements necessitate additional imaging quality considerations. This work will present initial investigations into the effects of micromesh antenna elements utilized in a common aperture. Simulated imaging performance as a function of micro-mesh design indicates that as the mesh fill factor increases, the imaging quality as well as the optical transmission decrease. Fill factor increase has also been previously associated with increased RF performance. Specifically, for a 0.5 inch diameter optical aperture with a fill factor of 0.05, a 5% reduction in peak point spread function (PSF) and a 13% reduction in contrast in group-2 of the USAF-1951 resolution chart is presented. The obstruction from the solid topology causes a significant 75% reduction in the PSF. This degradation can be used as an additional metric to evaluate and optimize a hybrid aperture.
Optically transparent antennas are an enabling technology for creating single aperture radio frequency (RF)/Electro- Optical (EO)/Infra-red (IR) sensing and communication systems. Multi-sensor information processing, simplified alignment, resiliency to jamming, and lower size, weight, and power (SWaP) are all among the potential benefits realized by a multi-modal aperture. However, the ability to have highly transmissive optical imaging, LIDAR, and/or communication systems at the same time in the same physical footprint as low loss antenna systems for RADARs, communication, and/or radiometers results in a complex set of competing requirements and engineering tradeoffs. Of course, the exact requirements will depend on the nature of the individual systems which are being utilized in the aperture. For instance, a LIDAR-RADAR common aperture will have very different requirements than a short-wave infra-red (SWIR) imaging system with a microwave communication system. This paper will address some of these competing requirements and begin to investigate some of the limiting factors in operating these systems. For example, micro-meshed conductors have demonstrated the potential as an optically transparent conductor that is well suited to both RF and EO/IR performance. However, the RF power handling capacity of the micro mesh may limit the maximum output power of the RF system. At the same time, it has been shown that the micro mesh is highly transmissive, but the effects of the micro mesh on the point spread system of the optical system have not been characterized. These considerations will be discussed in the context of a LIDAR-RADAR common aperture for autonomous vehicle applications.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.