A tunable optical spectrometer includes a medium configured to perform polarization rotation within a frequency band on a linearly polarized test beam, wherein the medium is circularly birefringent, and wherein the polarization rotation is achieved based on two-photon-absorption. The medium includes a gaseous substance, a first reference laser beam having a first reference frequency, and a second reference laser beam having a second reference frequency, wherein the first reference laser beam and the second reference laser beam have a same circular polarization state.