  The effect of the transversality condition on the quantization of the photon
orbital angular momentum is studied. The quantum gauge that is deduced from the
transversality condition is shown to be a Berry gauge. It determines a
helicity-dependent reference point relative to which the position is
canonically conjugate to the momentum. As a result, the photon orbital angular
momentum about the origin of the laboratory reference system split into two
parts. One is the orbital angular momentum of the photon about the reference
point, which is canonical. The other is the orbital angular momentum of the
photon concentrated at the reference point, which is dependent on the helicity.
Only when the Berry-gauge degree of freedom of a paraxial beam is perpendicular
to the propagation direction, does the total orbital angular momentum reduce to
its canonical part. One of the observable effects of the Berry-gauge degree of
freedom is also clarified.
