  It is well-known that electric spin angular momentum and electric orbital
angular momentum are conserved under paraxial propagation of travelling waves
in free-space. Here we study the electric and magnetic angular momentum in
counter-propagating waves and show both theoretically and experimentally that
neither component alone is conserved except in special cases. We attribute this
non-conservation to spin-spin and orbit-orbit coupling between the electric and
magnetic fields. This work generalises previous findings based on travelling
waves, explains the apparent spin-orbit coupling in counter-propagating
paraxial light, and broadens our understanding of angular momentum conservation
in arbitrary structured light waves.
