# Non-Mechanical Beam Steering with Polarization Gratings: A Review
## 3 Applications of Non-Mechanical Beam Steering with PGs
### Broadband Polarization-Insensitive Passive Imaging  
In conventional passive imaging systems, the FOV is mechanically scanned over the required field-of-regard (FOR) using large gimbals. For many applications, SWaP of these mechanical beam control systems can be prohibitive. Passive imaging is a challenging application for PG beam steering, where the dispersive and polarization-sensitive nature of PGs normally require filtering out much of the available signal. To address this challenging application, we developed a compact non-mechanical shutter that uses self-compensating PGs to achieve broadband and polarization-independent operation. When combined with a custom camera lens train, an array of these zone selectors can randomly steer the FOV of a passive imaging system [61]. This technology was demonstrated in the MWIR using a lens array and non-mechanical shutters based on FLC switches and MWIR PGs. With this non-mechanical shutter array, any portion of the scene could be selected with random access, so there was no slewing (or time) penalty for disparately stepping between widespread FOVs. Despite the long MWIR waveband, the FLC switching was fast with sub-millisecond settling time, and FOV positioning was accurate and consistent (no overshoot, oscillation or hysteresis). These non-mechanical attributes maximize the available integration time of a FOV, while the polarization independence of the approach maximizes the sensitivity of the imaging sensor. The demonstrated system provided advantages over a traditional, inertially constrained, mechanically scanned system, which would need to slew and settle at a new look angle. The geometry of the non-mechanical shutters consisting of four PGs and two FLC quarter-wave switches combined into a stack is shown in Figure 14 and similar in concept to the visible shutter described in [PERSON] et al. [62]. The shutter stack operates as follows: randomly polarized light is incident on a first a grating which in this case splits the light into left hand and right hand circularly polarized components that are deflected at angles of \(\pm\)\(\alpha^{\mathrm{o}}\). A quarter-wave FLC switch pair then will either add together to form a halfwave handedness switch and the light will be deflected another \(\pm\)\(\alpha^{\mathrm{o}}\) by the remaining three gratings, or the switches will subtract, resulting in the light being guided back to the zero order and recombined into the randomly polarized state. A spacer is in place between the central grating pair and the final grating to path match the beams so that in the crossed axis (subtraction) state the light recombines in the same plane along the direction of propagation. For the case where the beam is deflected, the light that has been steered out of the field of view of the sensor is not collected. As the effects of the PGs cancel each out in the zero-order transmission state, the device is both polarization-insensitive and dispersion-compensated. Figure 13: CNR measurements through the two-axis beam steering stack. (**a**) Representative Doppler return with steered beam on target. (**b**) Measured Doppler signal with beam steered off of target, showing contribution of any leakage beam to signal. No contribution is detectable. The above shutter concept was used in a MWIR demonstration in which seven shutters (Figure 15) were arranged in an array each of which covered a 5.8\({}^{\circ}\)\(\times\) 5.8\({}^{\circ}\) FOV. The light collected by the shutter array was sequentially imaged onto the detector plane and the zones stitched into an effective FOV covering 17.5\({}^{\circ}\)\(\times\) 17.5\({}^{\circ}\). The shutter array was placed in a telescope assembly consisting of relay optics to image each of the seven zones onto an integrated focal plane array (FPA) as shown in the diagram of Figure 16. The FPA was an InSb infrared camera produced by Santa Barbara Focalplane (ImagIR-SBF184) consisting of 1024 \(\times\) 1024 pixels, a pixel pitch of 19.5 microns, and a full-frame update rate of 114 Hz.