  We report on far infrared measurements of interplane conductivity for
underdoped single-crystal YBa2Cu3Oy in magnetic field and situate these new
data within earlier work on two other high-Tc cuprate superconductors,
La(2-x)SrxCuO4 and Bi2Sr2CaCu2O(8+d). The three systems have displayed
apparently disparate electrodynamic responses in the Josephson vortex state
formed when magnetic field H is applied parallel to the CuO2 planes.
Specifically, there is discrepancy in the number and field dependence of
longitudinal modes observed. We compare and contrast these findings with
several models of the electrodynamics in the vortex state and suggest that most
differences can be reconciled through considerations of the Josephson vortex
lattice ground state as well as the c-axis and in-plane quasiparticle
dissipations.
