# Non-Mechanical Beam Steering with Polarization Gratings: A Review
## 1 Introduction  
Dynamic optical beam steering is critical to many optical technologies, such as laser radar (lidar), laser illuminating beacons and designators, and free-space optical communications. To date, commercial beam steering technologies have been dominated by mechanical systems, such as gimbals, mirror galvanometers, and rotating prisms. However, the past thirty years have seen significant development in non-mechanical alternatives as a way to reduce size, weight, and power demands while improving robustness [1, 2]. In the early 2000's, interest increased in a variety of non-mechanical and low-mechanical beam steering approaches to reduce the size, weight and power (SWaP) requirements of electro-optic-based systems such as long-range lidar for drone platforms and free-space optical links for satellite-to-satellite communications. For these types of systems, larger aperture optics are preferred to reduce the performance and SWaP requirements of critical system components such as the laser source. Larger apertures reduce the divergence of a transmitter's beam and allow a receiver to collect more light from a specific direction, thus increasing collected signal and reducing noise from background radiation. The improvement in optical gain and resolution/directivity is frequently advantageous in relation to other system tradeoffs, except when scanning the system. Mechanically steering large-aperture optics introduces significant SWaP issues for small platforms, as well as presenting increased reliability and maintenance issues. Therefore, new technologies were sought to find a low-SWAP, non-mechanical (or solid state) approach that could provide wide-angle steering for larger optical apertures (i.e., a non-mechanical scanner with a large angle-aperture product). As we will show, polarization gratings (PCs) offer an angle-aperture product comparable to mechanical systems and larger than most non-mechanical and solid-state approaches, including acousto-optic (AO), electro-optic (EO), optical phased array (OPA), EO polymer/liquid crystal (LC) waveguides and MEMS devices. The angle-aperture product, also known as the Lagrange invariant, is an optical constant that a beam steerer provides. The nature of the invariant dictates that the maximum scan angle can be traded for aperture size through optical magnification. For example, a MEMS scanner with a 1-mm mirror capable of scanning over a 60\({}^{\circ}\) field of regard in two dimensions (2D) can be scaled through magnification to emulate a 1-cm aperture steered over a 6\({}^{\circ}\) field of regard. However, a 1-cm aperture is not sufficient for most long-range detection applications and 6\({}^{\circ}\) 2D scanningis not a large field of regard compared to mechanical systems. As a result, an important aim of non-mechanical beam steering development has been to achieve an angle-aperture product that supports steering apertures of \(>\)5 cm over as large a field of regard as possible with high efficiency. As research in this field began, there were only a few non-mechanical options that theoretically could provide sufficient angle-aperture product. One option was to use segmented, birefringent solid crystal elements (such as LiNbO\({}_{3}\) or YVO\({}_{4}\)) to form a large-aperture, polarization-controllable beam deflector and then stack these deflector stages together with EO-switches to form a discrete (or digital) beam scanner [2]. This approach is limited by the size, weight, and cost of the resulting beam deflector elements when scaled to sizable apertures. Additionally, significant beam walk-off results from the thickness of the individual deflector elements. This beam walk-off effectively reduced the scanner aperture for larger steer angles, which is undesirable. Another approach used volume Bragg gratings written in photo-thermal-refractive glass. This approach used LC OPAs to steer the beam into the volume grating structure, which then steered the beam into a new angle depending on the angle of arrival set by the OPAs [3,4].