Optical Line Filter and Spectrometer based Upon Two-Photon Absorption for Doppler LIDAR at 1.053 um

A receiver system for use in LIDAR consisting of an optical filter and optical spectrometer based upon two-photon absorption is proposed. The filter and spectrometer both use dispersion from excited state transitions of atomic rubidium to create a birefringent medium. The birefringent medium of the optical filter rotates the polarization of light so that it is transmitted through crossed polarizers. The birefringent medium of the spectrometer rotates the polarization of light an amount dependent upon frequency so that Doppler shift can be determined. The wavelength of operation is at 1.530 um which coincides with both the H atmospheric transmission band, and high power C-band transmitters. Since the spectrometer utilizes dispersion rather than absorption to determine frequency, there is no absorption line attenuation of the signal. With a high performance filter and spectrometer, the principle disadvantage of direct detection compared to coherent detection is mitigated. The strength of this method is that superior spatial resolution of wind, on the order of a decade, is attainable compared to coherent detection. Operating at an eye-safe wavelength, a LIDAR using the proposed receiver would bring increased detection capability of wind shear and turbulence.