Where Twisted Light Could Help Detect Drones

Light can be made to carry orbital angular momentum. The wavefront spirals around the propagation axis like a corkscrew, and the beam profile takes on a characteristic doughnut shape with a dark center. This is "OAM light," and it has a property that makes it interesting for sensing rotating things: when an OAM beam reflects off a smooth rotating surface, it picks up a Doppler shift proportional…