Gauge-Invariant Spin and Orbital Angular Momentum of Laguerre--Gaussian Laser

We adopt a gauge-invariant definition to calculate the spin and orbital angular momenta of a so-called lth order Laguerre--Gaussian laser. The results reveal that photons on the axis of the beam may carry an orbital angular momentum of (l-1) besides l per photon. For the spin, we obtain a more reasonable expression proportional to the beam intensity instead of the gradient of the intensity as previously derived. We also discuss how to experimentally discriminate the angular momentum expressions given here and those commonly accepted in the literature.