One GHz linewidth, 33 line per mm, wide angle imaging filter at the potassium resonant line

This paper presents a narrow linewidth, high resolution, and high quantum efficiency imaging transmission filter based on optical trapping of resonance radiation in potassium vapor. The filter can be used to image radiation over a bandwidth narrow enough to fall within a Fraunhofer dark zone in the solar spectrum, and it can be applied to the imaging of flames, plumes or discharges containing potassium. It may also be applicable to the imaging of Raman scattering from a tunable laser. The spectral and imaging properties of the filter are demonstrated with a 1 cm aperture optically thick potassium cell illuminated by a narrow linewidth tunable laser. The spectral width at the potassium D2 line wavelength, 766.5 nm, is shown to be 1 to 2 GHz (.002 nm). At the line center, the quantum efficiency is better than 60% and the imaging resolution is better than 30 line pairs per mm. By employing a 200 micron "thin" potassium vapor cell, it is also shown that the filter maintains the high quantum efficiency (~50%) and good imaging capability (~20 lines per mm) across the 2 GHz spectral bandwidth of the cell. The "thin" cell has an out-of-band rejection of better than 1000. Its operation is demonstrated with a tunable laser as well as with broad band light from a potassium lamp and from a potassium chloride seeded flame.