# Non-Mechanical Beam Steering with Polarization Gratings: A Review
## 2 Non-Mechanical Beam Steering with PGs
### PG Fundamentals from a Beam Steering Perspective
These traits present a challenge for designing non-mechanical steering systems that cover a wide angular field with a large number of steering angles. In addition to large steering angles, the apertures achievable with PGs also make them particularly interesting for non-mechanical beam steering applications. Many alternatives, including liquid crystal on silicon (LCOS), electro-optic crystals, liquid crystal waveguides, micro-electro-mechanical systems (MEMS) mirrors, and electrowetting systems, are limited in aperture scalability [1]. The benefits of non-mechanical steering are particularly appealing when random access scanning is beneficial or for size, weight, and power (SWaP)-constrained large-aperture applications [21], where mechanical solutions are generally both bulky and power intensive. Figure 2: Spectral and angular bandwidth of PGs. **(a)** The impact of incident angle on PG diffraction efficiency for PGs with diffraction angles of \(\pm 3^{\circ}\), \(\pm 6^{\circ}\), and \(\pm 12^{\circ}\) and 1550 nm design wavelength. **(b)** Spectrum of the zero-order light transmitted through a PG designed for 1550 nm as a function of wavelength. The minimum represents the wavelength of peak diffraction. The combination of these six properties:  
* near 100% diffraction efficiency;
* polarization-selectable diffraction angle;
* wide acceptance angle;
* scalable aperture;
* transmissive operation;
* thin element thickness;  
is specific to PGs and enable the layering of PGs with polarization handedness switches, such as liquid crystal (LC) variable retarders, to provide non-mechanical beam steering over wide angles with a thin, efficient, transmissive optic [12; 13; 17; 22; 23; 24]. In practice, a non-mechanical beam steering assembly can be realized by index-matching and bonding together a polarization handedness switch and a PG as a functional unit, which we will refer to as a single steering stage. A single stage provides binary non-mechanical steering of light to the +1 and -1 diffracted orders of that grating. Multiple stages with progressively increasing PG diffraction angle can be index-matched and bonded together. In such an assembly, the diffraction angle of each grating either adds to or subtracts from the cumulative deflection angle of the light as it traverses the assembly depending on the state of each handedness switch. In this manner, PGs and polarization handedness switches produce non-mechanical beam steering in discrete steps over a potentially large field of regard, where the maximum number of steps is \(2^{N}\), where \(N\) is the number of stages. For two-dimensional steering, stages can be oriented such that the PG fringes are at angles to one another. For example, a 90\({}^{\circ}\) rotation between stages produces a rectilinear grid of steering angles and a 120\({}^{\circ}\) rotation produces a hexagonally packed grid of angles. For fabrication, numerous ways have been published to create the required PG alignment patterning [25; 26; 27], and the subsequent coating and curing of the reactive mesogen (i.e., LC polymer) are familiar processes in the fabrication of consumer LC devices. Therefore, this layered structure of laminated LC- and LC polymer-based components has the advantage of leveraging many fabrication processes developed for the consumer LC display industry, potentially reducing the barriers to inexpensive volume production. The primary limitation of PG approach to non-mechanical beam steering is that the scanner is non-continuous. For many applications this is not a problem and three such applications are provided later in this paper. Where a system needs continuous steering, the PG approach can be paired with either a continuous non-mechanical device such as an LC OPA [10] or LC waveguide scanner [28] or a low-mechanical device such as a MEMS or fast steering mirror (FSM). Additionally, electrically tunable PGs with variable pitch have recently been demonstrated [29; 30], as well as mechanically stretchable pitches [31], though early examples of both are still limited in aperture.