In this thesis the optical preporties of the $Zn_{1-x}Co_xSe$ and the Co impurity effects on the ZnSe host crystal is studied. $Zn_{1-x}Co_xSe$ (x ＜ 0.02) polycrystals were prepared by the melt growth technique. There are many peaks appeared in PA absorption spectra in the visible and infrared regions. Among them the weak peaks at $15576cm^{-1}$, $16385cm^{-1}$ and $17212cm^{-1}$ are observed and we assign these peaks to the forbidden transition from the ground state $^4A_2$ to $^2T_2$ and $^2T_1$, $^2E$ $^2T_1$ states, respectively. From the energies of transitions of $Co^{2+}$ in ZnSe the crystal field parameter $\delta$ is obtained to be $3864cm^{-1}$, the Racah parameter B $587cm^{-1}$ and β 0.56. And from the results of XRD and Raman spectroscopy it was found that the lattice constant of the crystal increases linearly to x = 0.0086 and then saturates due to the size effect of the incorporated Co atom as its amount increases. It is also observed that the $TO_2$ and $LO_1$ Raman modes in $Zn_{1-x}Co_xSe$ soften in x ＜ 0.0086 by the addition of Co atoms. The oscillator strength, which is calculated using the energies of $TO_2$ and $LO_1$ phonon modes, decreases as the content of cobalt increases. There is a linear relationship between the ionicity and the oscillator strength with the slope of $-7.02\times10^{15}(\sec^{-1}$). From these observations, it is clear that the instability of $Zn_{1-x}Co_xS$ crystal is mainly due to the ionicity difference between Zn and Co atoms.
