A potassium Faraday filter based narrowband lidar for day and night observation of the mesopause region

Since most metal lidars at present are operated at night, daytime measurements are badly needed, without which many important geophysical phenomena can not be properly studied. Daytime operation of a Na lidar has been implemented by different research groups with varied success. The first daytime temperature measurements using sodium atoms as tracer were carried out by Chen et al. using a Faraday filter. Although sodium atoms are preferred tracers due to its abundance, potassium atoms have also been used for mesopause temperature measurements as the result of a recently developed all solid-state, narrowband Alexandrite laser which has a longer pulse width. In order to allow daytime measurements, a narrowband filter is obviously needed. That a circular birefringent, dichroic medium between crossed polarizers could be used to reject considerable skylight background was demonstrated by Ohman before the laser was invented. We have constructed a potassium Faraday filter with a 1 inch long potassium vapor cell that may be used in observatories with a narrowband K lidar for daytime temperature and wind measurements. By choosing an axial magnetic field in an appropriate range and adjusting the oven temperature, we have "engineered" the Faraday filter to have a very narrow passband and a respectable peak transmission for the K D, transition at 770 nm. In this paper, we will discuss the development, measurement, and performance evaluation of a potassium Faraday filter in skybackground rejection, as compared to a comparable sodium Faraday filter and a 10 GHz wide Pabry-Perot interferometer, as well as the proposed integration of a K Faraday filter into a narrowbad potassium lidar receiver for daytime temperature, line-of-sight wind, and potassium density measurements in the mesopause region (80-110 km in altitude).