  Using dispersion relation technique and experimental data, a coupled channel
analysis on $\gamma\gamma\to\pi\pi$ process is made. Di-photon coupling of
$f_0(600)$ and $f_0(980)$ resonances are extracted and their dynamical
properties are discussed. Especially we study the physical meaning of the
coupling constant $g^2_{\sigma\pi\pi}$, which maintains a negative real part as
determined through dispersive analyses.
