Target Calibration: Retro-Reflection Mechanisms at 2 Micron
With the growing use of 2-micrometer eye-safe solid-state laser transmitters in coherent Doppler lidar applications, there is increased interest in the reflectance properties of hard target calibration materials at this wavelength. One of the factors to be considered is the polarization characteristic of the transmitter/receiver. The two fundamental lidar systems are either based upon the backscattering of linearly polarized or circularly polarized light. Consequently, the response of calibration materials to the particular state of polarization of the incident radiation is an important consideration. Ideally the calibration materials should have a similar reflectance response to that of the remote scattering medium that is being observed. Also the material should be most efficiently adapted to the polarization characteristics of the lidar system. There are two measurement parameters that are useful in presenting the characteristics of polarization of reflecting materials. The first is the linear polarization ratio: mu (sub l) =I (sub ol)/I (sub sl), where ol denotes opposite linear and sl denotes same linear. The second is circular polarization ratio: mu (sub c) = I (sub sc)/I (sub oc) where ol denotes opposite circular and sc denotes same circular. Notice that the ratios are inverted with respect to same polarization. It is the relative values of these polarization ratios that are used to characterize the retro-reflectance mechanism. Measurement of these parameters is conveniently obtained by using the Stokes vectors corresponding to the polarized backscatter at and near the retro-angle.