Beam Steering research addresses the following question: Can we achieve high throughput, wide-angle laser beam steering using stacked polarization gratings and polarization selectors? And if so, what are the optimum designs to achieve lowest loss? We identified two families of beam steering: nonmechanical and mechanical.

First, in nonmechanical steering, we found an exponential increase in steering angles as steering stages are added. From the study of steering design, we found that it would be preferable to choose (a) Quasi-Ternary design if number of steering angles is most important, and (b) Supra-Binary design if throughput, ease of fabrication, and reliability at large field-of-regard are most essential. In mechanical steering, we found that Risley-PGs act similar to Risley Prisms with much smaller SWaP. We also built prototypes of nonmechanical and mechanical beam steering systems to various independent evaluators. The main contribution within the research is the demonstration of discrete nonmechanical steering with high throughput and wide-angle with four steering designs: Binary, Ternary, Quasi-Ternary, and Supra-Binary. In this work, we suggested and evaluated important non-mechanical designs, and derived governing theory of operation. We also demonstrated a new way to accomplish continuous mechanical steering with high throughput and wide-angle utilizing a pair of rotating PGs. Moreover, we demonstrated reduced chromatic dispersion of broadband source steering as utilizing compensation PGs that can adjust the color separation of diffraction.

Non-mechanical Beam Steering promises substantial benefits to optoelectronic systems such as free-space laser communications, laser weapons, laser remote sensing, and fiber-optics; however, any viable solution must offer some or all of the following: high throughput, rapid pointing ability, robustness to high intensity, and compact size. In order to achieve non-mechanical beam control, diverse approaches have been explored including: micro-lens arrays, electro-optic prisms, holographic glasses, and diffractive acousto-optic techniques.
All o
f these approaches are, however, plagued by one or more of the following limitations: low throughput, scattering, small steering angle/aperture, and large physical size. In this work, we introduced a novel beam steering device based on the polarization sensitive properties of liquid crystal Polarization Gratings (PGs). This single device is capable of diffracting incident light into one of three possible diffracted orders (0th and ±1st) according to the input polarization and the applied voltage. Based on steering designs of multiple stacked stages, we showed that it is practical to achieve a wide range of discrete steering angles, and these exhibit high throughput (e.g., > 90%) and wide field-of-regard (e.g., 90˚) compared to prior non-mechanical steering systems. Since the PGs can be formed as thin polymer films (or within thin LC cells) and they are scalable to large areas without increasing their thickness, the device provides dramatic aspect ratio. Moreover, our device can be tailored to operate at nearly any wavelength from visible to midwave−infrared. We showed the beam steering device that performs non-mechanical, wide-angle, discrete steering of a laser beam. Our discrete beam steering technology can be combined with a fine-angle continuous steering for wide-angle continuous steering.

Precision Beam Pointing is a requirement for optical systems where beam alignment and target tracking are required. Optical turrets (gimbals) can be used for the precision beam steering but their placement is very limited on high performance and small electro-optical systems such as stealth and unmanned aerial vehicles, since they can increase air resistance and observability. As this context often demands compact, robust, and cost-effective devices for beam steering, Risley Prisms, comprising two or more wedged prisms, have long been used for its high degree of accuracy and stability. Their utility, however, is often limited by small deflection angles and poor size scaling properties due to bulky prismatic elements where wide angles and modest/large apertures are required.

We introduced the Risley grating, a grating

version of the Risley prism, which can perform a continuous beam steering using a pair of polarization gratings (PGs) in independent rotation stages. By replacing wedged prisms with PGs formed in a thin Liquid Crystal (LC) layer, ultra-compact beam steering devices can be designed for virtually any size of beam. The polarization diffraction properties of PGs provide unique opportunities for beam steering with high throughputs and low levels of side lobes. Several LC grating structures (i.e., blazed or binary types) were proposed as a beam steering element. The practical use of such LC gratings, however, is limited by their poor angle performance, limited peak efficiency, and low transmittance, and they are not applicable for the Risley gratings. A similar steering operation by rotating two PGs was reported, but the Risley grating concept was not yet captured. Since two PGs can be placed with a close proximity, the beam walk-off is not an issue and multiple stages can be stacked without increasing the volume. In this work, we showed the basic concept and its operation principles of this new beam steering device based on PGs and demonstrate a Risley grating that performs continuous steering of a laser beam (at 1550 nm) with the field-of-regard (FOR) 62˚ and 89 − 92% throughput. The angle of the emerging beam from the Risley grating is described in the direction cosine space and confirmed by experimental results.

[1] Kim et al. Proc. SPIE 7093, 709302 (2008)
[2] Kim et al. Appl. Opt. 50, 17 (2011)
[3] Kim et al. Proc. SPIE 7816, 78160G (2010)
[4] Kim et al. Proc. SPIE 8052, 80520T (2011)
[5] Escuti, Kim et al. US Pat. Appl. WO2011014743 (2010)
[6] Oh, Kim et al. Proc. SPIE 7466, 746619 (2009)
[7] Oh, Kim et al. IEEE Phot. Tech. Lett. 22, 4 (2010)
[8] Kim et al. (in preparation)