Patent ID: 7701381

Claim:
An apparatus, comprising: an orbital angular momentum (OAM) source comprising an electromagnetic (EM) source that generates an electromagnetic beam in an initial state having zero orbital angular momentum (OAM) and a first diffractive element that converts the beam into a source beam having a prescribed source state with one or more non-zero OAM components; a tag comprising a second diffractive element on a friendly hard target, said tag matched to said OAM source to reflect the source beam to transform the non-zero OAM components of the prescribed source state to a different prescribed return state in a return beam; and a receiver that measures the OAM components of the return beam and with knowledge of the prescribed source state of the source beam and the transformation implemented by the OAM tag compares the measured OAM components to the nonzero OAM components of the prescribed source state to identify the tag.