Imaging Filter for the Detection of Potassium D Lines Radiation

We present a narrow linewidth, high resolution, and high quantum efficiency optical transmission filter, which can be used to image atomic potassium line radiation from flames, plumes or discharges over a bandwidth narrow enough to fall within a Fraunhofer dark zone in the solar spectrum. Our previous work demonstrated that resonant absorption and subsequent fluorescence near the entrance window of an optically thick potassium cell acts as a 1 GHz wide (.002 nm) imaging screen transmission filter with high quantum efficiency (> 60%). With laser illumination at the potassium absorption line center, this screen has an outstanding imaging capability (33 m resolution). The work presented here focuses on using an ultra-thin (200 micron) potassium vapor cell to maintain image quality for in-band but off-resonant photons that are present with broad band radiation such as that from flames, plumes and discharges. In a thick cell, those photons are absorbed farther away from the entrance window and lead to background luminosity and image blurring. By employing a thin cell, it is shown that the filter maintains the high quantum efficiency (50%) and good imaging capability (20 lines per mm) with broad band light, has a transmission linewidth of between 1.5 and 2 GHz (0.003-0.004 nm), and an out-of-band rejection of better than 1000. Performance is demonstrated using a potassium lamp and a potassium chloride seeded flame.

Overview