  We study how the entanglement spectrum relaxes to its steady state in
one-dimensional quadratic systems after a quantum quench. In particular we
apply the saddle point expansion to the dimerized chains and 1-D p-wave
superconductors. We find that the entanglement spectrum always exhibits a
power-law relaxation superimposed with oscillations at certain characteristic
angular frequencies. For the dimerized chains, we find that the exponent $\nu$
of the power-law decay is always $3/2$. For 1-D p-wave superconductors,
however, we find that depending on the initial and final Hamiltonian, the
exponent $\nu$ can take value from a limited list of values. The smallest
possible value is $\nu=1/2$, which leads to a very slow convergence to its
steady state value.
