  The Josephson plasma resonance in layered superconductors with alternating
critical current densities is investigated in a low perpendicular magnetic
field. In the vortex solid phase the current densities and the squared bare
plasma frequencies decrease linearly with the magnetic field. Taking into
account the coupling due to charge fluctuations on the layers, we extract from
recent optical data for SmLa_{1-x} Sr_x CuO_{4-delta} the Josephson penetration
length lambda_{ab} approximately 1100 A parallel to the layers at T=10 K.
