# Non-Mechanical Beam Steering with Polarization Gratings: A Review
## 3 Applications of Non-Mechanical Beam Steering with PGs
### Steering of Monostatic Coherent Doppler Lidar for Wind Sensing  
The use of lidar for atmospheric sensing presents another good fit for the PG steering technology. For example, in coherent 3D wind sensing lidar, a small number of beam directions (typically 2-8) are measured around a fixed cone angle. A coherent Doppler lidar measurement of aerosol backscatter is taken along each line-of-sight (LOS) and angular differences between LOS permit solving for the average 3D wind vector in the sampled volume [59]. The small number of discrete steering angles spaced over a wide angular range is well suited to PG steering, especially for long-range systems requiring larger apertures. In 2016, a monostatic frequency modulated continuous wave (FMCW) lidar was built to characterize the impact of PG steering on such systems [21]. The FMCW lidar system with PG steerer is schematically illustrated in Figure 12 and described in detail in [PERSON] et al. [21]. Briefly, the system used a 1550 nm single-frequency laser diode, split into a transmit beam and local oscillator. The transmit beam was delivered via COTS fiber optic components to a telescope before passing through a 4-angle PG steering unit with 4-cm clear aperture. Similar to the TOF demonstration above, an additional LC cell was used at the exit aperture of the PG steerer and driven to act as a quarterwave retarder to ensure that light propagated to the target in a linearly polarized state and was returned to a circularly polarized state upon backscatter to the PG steerer. Figure 11: Two dimensional developed for next generation wide-angle, high-resolution TOF camera. Further size reduction is possible by building the LC components on thinner substrates. Upon re-entering the system, the collected backscattered light is separated from the transmitted light by a fiber circulator. This is necessary to split the transmit and receive beams as they are in a common polarization state. The collected light is then mixed with the local oscillator in a 50/50 beam coupler and measured by balanced optical receivers for heterodyne detection. For this study, we built a 4-angle PG beam steering assembly with 4-cm clear aperture and steering angles of \(\pm 1^{\circ}\) in azimuth and elevation. The relatively small steering angle was chosen to facilitate indoor measurements. In coherent Doppler lidar of aerosol backscatter, the signal strength of the far-field is sensitive to wavefront error [60], and so this device was fabricated using 3-mm thick glass for the PGs and 2-mm substrates for the LC cells to maintain high wavefront quality. The resulting transmitted wavefront error of the steered beams was measured by a Shack-Hartmann wavefront sensor to vary from \(\lambda/61.2\) to \(\lambda/84.5\) RMS at 1550 nm. With the index-matched assembly of three LC cells, two PGs, and anti-reflection coated end caps, the single-pass steering efficiency of the PG steerer was measured to be an average of 89% across the 4 angles, or about 0.5 dB loss. To determine the impact of the PG steering stages on the lidar, the lidar system was used to measure the Doppler return from a spinning sandpaper disk with and without the PG steerer. The coherent lidar detected the Doppler return from the target and a spectrum analyzer was used to determine the carrier-to-noise ratio (CNR). With the two-stage LCPG beam steerer, four different steer angles were available. The CNR measurements at the four steering angles were 28.9, 29.1, 29.8, and 29.4 dB, which are very close to the 29.7-dB CNR measured without the PG steerer. In this test, the variation due to target reflectivity and estimated 2% measurement error in this experiment was sufficient to mask the effect of insertion loss from the beam steering stack. In addition to investigating the PG steerer's impact on CNR, this study sought to evaluate the impact of weak side lobes which could, in theory, create spurious signals when a strong backscatterer is in the path of a side lobe.