Using a complex optical orbital-angular-momentum spectrum to measure object parameters
In a proof of concept experiment, Xie et al. demonstrate the advantage of using spatial modes in their case modes that carry orbital angular momentum to unveil two characteristics of an opening angle of an object: the value of the angle and its orientation. A major goal of optical imaging is to unveil relevant features of matter. A light beam interacts with the object and a particular property of the light changes. This property can be its spatial shape, which can be fully characterized by its decomposition into modes (spatial spectrum). It constitutes a complex array of numbers that corresponds to the weight of every mode in the decomposition.
The success of their approach is based on two main ingredients. Firstly, they identify a direct link between a property of the spatial spectrum and the features of interest. In their case, the modulus of the spectrum is sensitive to the value of the angle, but insensitive to its orientation, while the phase of the spectrum is sensitive to the orientation, but insensitive to the value of the angle. Secondly, they use a technology that can reliably measure the complex spectrum of an arbitrary spatial beam with great accuracy.