  We measured the Josephson plasma resonance (JPR) in optimally doped
Tl$_2$Ba$_2$CaCu$_2$O$_{8+\delta}$ thin films using terahertz time-domain
spectroscopy in transmission. The temperature and magnetic field dependence of
the JPR frequency shows that the c-axis correlations of pancake vortices remain
intact at the transition from the vortex solid to the liquid phase. In this
respect Tl$_2$Ba$_2$CaCu$_2$O$_{8+\delta}$ films, withanisotropy parameter
$\gamma\approx 150$, are similar to the less anisotropic
YBa$_2$Cu$_3$O$_{7-\delta}$ $(\gamma\approx 8)$ rather than to the most
anisotropic Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystals $\gamma\geq
500$).
