Nowadays, GaN-based multi-layer materials is developing fast, and it is important to know their
interface and optical properties for devices design and fabrication. In this letter, the transmission
spectra are analyzed, and the dependence of the transmission spectra on parameters of the samples is
discussed. Sequentially, we obtain the transmission spectra of a series of GaN-based samples.
Simulation of the transmission spectra is done and useful information is extracted. For one of our
samples, the refractive index varies a little between 2.518 and 2.305 with the wavelength from 400 to
800nm, while the extinction coefficient is 6x-10<sup>-9.5</sup>exp(2700/λ), and the thickness is 2860nm. Finally,
we get the dispersion relationship of the GaN and AlGaN films, and it is compared with the results of
some other research groups.