Abstract: A Voigt optical filter. Crossed polarizers are provided at the entrance and exit of a vapor cell continuing an alkali metal vapor. A magnet provides a magnetic field at 90.degree. with the axis of the cell and 45.degree. with each polarizer direction. The only light which passes through the filter is light within a narrow frequency band near the atomic transitions (absorption peaks) of the alkali metal vapor.

Abstract: A laser Doppler velocimeter. A laser produces a single frequency beam which is split into a probe beam and a reference beam. The probe beam is directed at a moving target and a portion of the light reflected from the target is collected, formed into a beam. Both the reference beam and the reflected beam passed through an atomic line filter in which a vapor cell is subjected to a constant magnetic field and a second magnetic field which is being oscillated. Light of the reference beam and the reflected beam passing through the filter is detected and analyzed at the frequency of oscillation of the second magnetic field and at twice that frequency. The speed of the target can be determined from measurements with respect to each beam of the amplitudes at these two frequencies.