# 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
First, the simplest method for narrowing the transmit beam of the camera is to block off three of the camera's four VCSELs, which made it easy to use a single condenser lens to collect and nearly collimate  
Figure 10: Wide-angle, high-resolution image and point cloud generated using LCPG scanning combined with narrow-FOV TOF image tiling. The test scene is between 1.9–2.3 m from the camera. Figure 9: First and second steps in converting a COTS QVGA TOF camera into a high-resolution 3D imager. Targets are approximately 2 m from the native and NFOV TOF cameras. the light from one VCSEL. Beam divergence was set to ~5.5\({}^{\circ}\). Blocking off three of the VCSELs reduces the transmitted signal power by slightly more than a factor of four. A series of polarizing beam cubes and a halfwave plate are used to vertically polarize the unpolarized light from the VCSEL without discarding half of it. This polarization of the emitted light is required for efficient operation with the PGs. The transmitter is aligned to flood the area seen by the receiver which has also been modified for a 5.5\({}^{\circ}\) FOV. The transmitted light reflected by the target is partially depolarized and some of this backscattered light is collected by a new narrow-angle one-inch receiver lens, which has approximately ten times the area of the original wide-angle lens. The received light passes through a cleanup polarizer, which serves to filter out sidelobes from the transmitter, and is focused onto the QVGA TOF FPA. The measured SNR improvement for the narrow-FOV image shown in Figure 9 is approximately 6.4 dB, which is close to our theoretical estimate of 7 dB. This estimate assumes approximately a 10\(\times\) increase in aperture area, 4\(\times\) decrease in output power, 2\(\times\) decease in signal return due to polarization, and 4\(\times\) decrease in shot noise due to using a polarizer to block half the ambient light and having a smaller detector angular subtense (DAS). This estimate does not account for Fresnel losses from individual components added to the system, which appears to add up to ~10% or another ~0.5 dB for the modified camera. Since the signal gain from using a larger aperture lens is mostly cancelled out by choosing to use 1/4 th of the transmitter's output power and using a cleanup polarizer at the receiver, the main SNR improvement is mostly due to collecting less ambient light, which reduces the shot noise. That is, the native camera collects approximately 18\(\times\) more ambient light than the modified camera, which becomes a limitation for using wide-angle TOF cameras in outdoor applications. There are various options to further increase SNR, but each option presents a tradeoff. For example, with some added complexity it is possible to harness the light from more than one VCSEL to increase the transmitter's output power, but it does not take much of an increase before the narrow beam becomes an eye hazard. A safer SNR improvement is to increase the receiver aperture. This type of SNR enhancement adds some size and weight, but modest size increases are easily accommodated by other parts of the system such as the scanner. The diameter of LCPG scanners can be extended to >20 cm if necessary. It is obvious from Figure 9 that the overall wider FOV of the native camera provides more situational awareness capability. To gain back some of the angular coverage, the next step is to integrate the modified camera with a 3-stage, one-dimensional (1D) LCPG scanner, which steers the sensor's transmitted beam and the received field of view over a 44\({}^{\circ}\) field of regard in eight 5.5\({}^{\circ}\) steps. As mentioned above, the transmitted light from the VCSEL is vertically polarized before entering the scanner. This light is converted to circularly polarized light by the first LC cell in the steering stack, which acts as a switchable quarterwave retarder. This light is then efficiently steered by the rest of the stack to the selected angle set by the halfwave LC switches in the stack.