Abstract

Since 1992, when Laguerre-Gaussian (LG) beams were shown to carry orbital angular momentum (AM), a growing number of applications have been proposed, including quantum information, data storage and communications in free space [1]. Orbital AM is found in beams with helical profile in the phase and is, indeed, a property associated with the field spatial distribution. This is different from the spin AM found in any elliptically polarized beam and due to the vectorial character of the electromagnetic field. AM can be also distinguished in intrinsic and extrinsic when the effects of changing the rotation axis are considered, as well-known in classical mechanics. Interestingly, the total AM of a field in the direction of the linear momentum is always unchanged on shifting to a parallel axis, irrespective of whether it comprises spin AM, orbital AM, or a mixture of the two. This is surprising as the orbital AM of LG beams is due to their helical phase distribution about the beam axis, the position of which coincides with a phase singularity. Hence their cylindrical spatial distribution indicates a natural rotation axis and leads us to suspect that orbital AM might have an extrinsic quality. Moreover, the sensitivity to misalignment in the detection of the orbital AM suggests again an extrinsic quality. Intimately related issues are the possibility to engineer the AM spectrum seeding off-axis vortices or more complex spatial profiles in light beams. In this work we resolve this apparent inconsistency by pointing out that the orbital AM has an intermediate and distinct character between intrinsic and extrinsic, it is quasi-intrinsic [2].

© 2007 IEEE