  We first explain the pseudogap of high-temperature superconductivity based on
an approach of quantum optics. After introducing a damping factor for the
lifetime $\tau$ of quasiparticles, the superconducting dome is naturally
produced, and the pseudogap is the consequence of pairing with damped
coherence. We derive a new expression of Ginzburg-Landau free energy density,
in which a six-order term due to decoherence damping effect is included.
Without invoking any microscopic pairing mechanism, this approach provides a
simple universal equation of second-order phase transition, which can be
reduced to two well-known empirical scaling equations: the superconducting dome
Presland-Tallon equation, and the normal-state pseudogap crossover temperature
$T^{*}$ line.
