# Non-Mechanical Beam Steering with Polarization Gratings: A Review
## 3 Applications of Non-Mechanical Beam Steering with PGs  
### Tiling Fields of View in Time-of-Flight and Flash Lidar Sensors  
Time-of-flight (TOF) three-dimensional (3D) imaging is one application that provides a complimentary fit for the PG steering technology. A TOF 3D imaging system generally illuminates a sizable area with modulated laser light and uses a focal plane array (FPA) to simultaneously detect the return from thousands of locations in the receiver's field of view (FOV). Thousands of points, from a QVGA (320 \(\times\) 240) TOF detector for example,  
Figure 7: Polarization dynamics during monostatic PG beam steering. (**a**) Due to the handedness flip experienced by circularly polarized light during backscattering, the returning light is in an opposite handedness state and steers off-path during return (**b**) A quarter wave can be used to transmit and receive light in a constant linearly polarized state, enabling retracing of the transmit path upon backscatter. produce a dense 3D point cloud, but the 3D image's spatial resolution is low if the points are distributed over a reasonably wide field of view. High-resolution point cloud generation over a wide angle presents problems for a TOF camera (or flash lidar), since more spatial resolution generally means more pixels within the array and thus less return signal per detector element [58]. Additionally, wider view angles cause more ambient (background) light to be collected along with the TOF signal, which increases shot noise even if the ambient noise is rejected through electronic processing. These signal-to-noise issues along with size, weight, power, and cost constraints usually force a tradeoff of resolution for coverage. Fortunately, a TOF camera (or flash lidar) can provide higher resolution and better signal-to-noise ratio (SNR) by narrowing the FOV of the camera, and the PG scanner can step both the transmit beam and receiver FOV to gain back area coverage. To examine this basic concept, we modified a commercial-off-the-shelf (COTS) QVGA TOF camera (EVK75123-60-850-1, Melexis) as shown in Figure 8. The modification was performed in three steps, which are represented in Figures 9 and 10. The first step was to characterize the native TOF camera. The second step was to modify the camera optics to narrow the transmit beam and sensor FOV, and then to test the modified configuration. The third step was to add the scanner, measure system performance and finally splice the collected 3D data into a single point cloud. This modification sequence allowed us to compare the resolution and SNR improvement obtained in each step. As comparatively shown in Figure 9, the native COTS TOF camera provides a \(320\times 240\) image over a \(60^{\circ}\times 45^{\circ}\) field of view or a \(0.19^{\circ}\times 0.19^{\circ}\) angular resolution, which is roughly the angular resolution of a person with 20/200 visual acuity. After modification of the camera's optics, the sensor's angular resolution improves to \(0.02^{\circ}\times 0.02^{\circ}\), which is nearly the angular resolution of a person with 20/20 vision. For the images and point clouds shown in Figures 9 and 10, the targets are approximately 2 m from the native and narrow-FOV TOF cameras. By narrowing the FOV, the increase in resolution magnifies the image and point cloud bringing out more detail for identifying objects. In addition to a resolution difference, the resulting point cloud from the native COTS camera has considerably more noise (i.e., more graininess with a few patches of out-of-place points spread over the plot). Depth variance/accuracy, as indicated by the roughness of the point clouds, is directly related to SNR. Figure 8: High-resolution, wide-angle TOF camera and schematic showing optical modifications to COTS camera. To understand the various factors that contribute to SNR, it is necessary to look at the optical modifications made to narrow the camera's FOV.