Spin and Orbital Angular Momenta of Electromagnetic Waves in Free Space
  We derive exact expressions, in the form of Fourier integrals over the (k,w) domain, for the energy, momentum, and angular momentum of a light pulse propagating in free space. The angular momentum is seen to split naturally into two parts. The spin contribution of each plane-wave constituent of the pulse, representing the difference between its right- and left-circular polarization content, is aligned with the corresponding k-vector. In contrast, the orbital angular momentum associated with each plane-wave is orthogonal to its k-vector. In general, the orbital angular momentum content of the wavepacket is the sum of an intrinsic part, due, for example, to phase vorticity, and an extrinsic part, r_CM x p, produced by the linear motion of the center-of-mass r_CM of the light pulse in the direction of its linear momentum p. 
Masud Mansuripur
physics.optics math-ph math.MP physics.class-ph