Patent ID: 7876445

Claim:
A tunable optical spectrometer, acting upon a test beam, comprising: (a) a gaseous substance; (b) a circularly polarized reference laser beam supplied by a reference laser; wherein the gaseous substance and circularly polarized reference beam combine to form a birefringent medium as a result of a two-photon-absorption line for one circularly polarized component of the test beam; whereby the test beam polarization is rotated as it propagates through the birefringent medium; (c) a magnetic field supplied by a magnet that permeates the gaseous substance, shifting the frequency location of the two-photon-absorption line; wherein the test beam propagates through the birefringent medium twice, once where the test beam propagates through birefringent medium in a direction predominately the same as the direction of the magnetic field, and once where the test beam propagates through the birefringent medium with a propagation direction predominately opposite to the direction of the magnetic field; whereby the polarization state of the test beam after propagating though the birefringent medium twice, relative to polarization state of the test beam before propagating through the birefringent medium, is dependent upon test beam frequency.