Rubidium atomic hyperfine filter for amplitude manipulation of femtosecond frequency comb

Femtosecond mode-locked laser pulse train excitation in rubidium $5$ $^{2}S_{1/2}\rightarrow5$ $^{2}P_{3/2}$ transition is used for direct mapping of the optical frequency comb to rubidium atom velocity comb. The induced comb-like structure in the rubidium ground state hyperfine level populations is monitored by a weak cw scanning probe laser. By introducing an additional rubidium cell, a hyperfine atomic filter is realized which modifies the amplitude of the selected frequency comb lines. The hyperfine optical filter can be a pure isotope $^{85}$Rb or $^{87}$Rb cell, in which case only that isotope will affect the optical frequency comb.