An optical system for performing an absorption measurement of a medium sample includes a laser source configured to output a laser beam having a wavelength corresponding to an absorption region of interest; a ringdown cavity comprising a chamber configured to receive the medium sample, an input mirror at an input end, an output mirror at an output end, and an optical axis that extends through the centers of the input mirror and the output mirror; a coupling device configured to couple the laser beam through the input mirror into the chamber; and a detector optically coupled with the cavity, and configured to detect an intensity of light of the wavelength corresponding to the absorption region of interest that extends through the output mirror, wherein a cavity geometry of the cavity increases the re-entrant condition of the cavity relative to a conventional cavity comprised of two spherical mirrors.