In this thesis, the optical properties of $CuInS_2:Co^{2+}$ single crystals were studied. The $CuInS_2:Co^{2+}$ single crystals were synthesized by the melt growth technique and the chemical transport reaction using iodine as a transporting medium. The lattice constants and the Raman spectra of the $CuInS_2:Co^{2+}$ single crystals were almost the same as those of undoped $CuInS_2$ single crystals. The optical spectra of $CuInS_2:Co^{2+}$ single crystals at room temperature showed two broad peaks and the higher energy peak had three sub-peaks. These peaks can be explained in terms of the electronic transitions of $Co^{2+}$ ions in $T_d$ symmetry along with spin-obit coupling effect. The crystal field parameter Dq and the Racah parameter B were found to be 400 and $540cm^{-1}$, respectively. The optical spectra of the $CuInS_2:Co^{2+}$ single crystals at 4.7 K in the 1100 - 1900 nm region showed twelve peaks and these peaks can be analyzed using additional $D_{2d}$ symmetry considerations. This result shows that the tetragonal distortion of $T_d$ symmetry in $CuInS_2$ single crystals is small. The optical band gap energies of the $CuInS_2:Co^{2+}$ single crystals decreased as the doped $Co^{2+}$ ions increased.
