  We introduce an analysis model, an extended Drude-Lorentz model, and apply it
to Fe-pnictide systems to extract their electron-boson spectral density
functions (or correlation spectra). The extended Drude-Lorentz model consists
of an extended Drude mode for describing correlated charge carriers and Lorentz
modes for interband transitions. The extended Drude mode can be obtained by a
reverse process starting from the electron-boson spectral density function and
extending to the optical self-energy, and eventually, to the optical
conductivity. Using the extended Drude-Lorentz model, we obtained the
electron-boson spectral density functions of K-doped BaFe$_2$As$_2$ (Ba-122) at
four different doping levels. We discuss the doping-dependent properties of the
electron-boson spectral density function of K-doped Ba-122. This new approach
is very helpful for understanding and analyzing measured optical spectra of
strongly correlation electron systems, including high-temperature
superconductors (cuprates and Fe-pnictides).
