This manuscript is devoted to an analysis of highly sensitive absorption spectra of natural carbon dioxide and oxygen-18 enriched carbon dioxide at temperature 296 K and pressure 100 Torr and 47 Torr. The spectra have been recorded in Cavity Ring Down Spectrometer with Vertical-External-Cavity Surface Emitting Laser (VECSEL-CRDS) in the spectral range between 4295 and 4380 cm-1. In this region, the absorption of carbon dioxide is very weak what makes it interesting for astronomical research, especially for studying the atmosphere of Venus, which consists of about 96.5% of carbon dioxide. The knowledge of carbon dioxide absorption in this region provides to better analysis of thermal radiation emitted by Venus to space, so to a better spectral analysis of deep Venus atmosphere.