Experiments on a semiconductor laser pumped rubidium atomic clock

Experiments were carried out to evaluate the performances of a semiconductor laser pumped rubidium (⁸⁷Rb) atomic clock. Two kinds of Rb gas cells were used and their performances were compared gas cell A (natural rubidium (⁸⁷Rb/⁸⁵Rb =frac{3}{7}) and buffer gases) and gas cell B (⁸⁷Rb and buffer gases)]. The highest microwave frequency stabilities were estimated as3.4 times 10^{-12} tau^{-1/2}and2.7 times 10^{-12} tau^{-1/2}at the optimal gas cell temperatures of 60°C and 48°C for the gas cellsAandB, respectively (τ: integration time). The light shift, i.e., microwave frequency shift induced by laser light, was measured as -0.50 Hz/MHz and -0.11 Hz/MHz for the gas cellsAandBat their optimal operating conditions given above. As an improved experiment by utilizing high temporal coherence of the laser, a novel double resonance spectral line shape with a drastically narrower linewidth was demonstrated. A technique, similar to FM laser spectroscopy, was employed for this purpose by utilizing laser FM sidebands which are induced by microwave frequency modulation and nonlinear susceptibility of three-level⁸⁷Rb atoms. The minimum linewidth obtained was 20 Hz, which can be used as a sensitive frequency discriminator for an improved⁸⁷Rb atomic clock.