A toxic gas sensor or device which is a cavity ring-down spectroscopy device having two or more mirror components. Each of the mirror components has two Brewster windows attached to it. The Brewster windows are resistant to toxic gases and together with the respective mirror form a hermetically sealed volume for the mirror surface to protect it from the environment or test gases. The Brewster windows may have a heating mechanism to remove contaminants, condensation, and provide temperature stabilization and other beneficial features.