An all-optical microwave frequency standard based on coherent population trapping (CPT) in <sup>85</sup>Rb is developed. The CPT resonances are detected by an ordinary edge-emitting diode laser in a simple optical setup. A buffer-gas mixture is carefully optimized to yield a narrow linewidth and a reduced temperature dependence of the resonance frequency. With the developed system we are able to measure ultranarrow optically induced hyperfine CPT resonances at <20 Hz, which is in good agreement with the linewidth calculated from experimental parameters. The frequency of an RF-signal generator has been stabilized to the CPT resonance between the two m<sub>F</sub>=0 magnetic sublevels. The relative frequency stability (square root of Allan variance) follows a slope of 3.5×10<sup>−11</sup> τ<sup>−1/2</sup> (1 s<τ<2000 s). The best stability of 6.4×10<sup>−13</sup> is reached at an integration time of τ=2000 s. This stability is sufficient for many high-precision applications. Frequency-shift measurements were made to evaluate the frequency dependencies on the operation parameters.
© 2003 Optical Society of America
(020.1670) Atomic and molecular physics : Coherent optical effects
(120.3930) Instrumentation, measurement, and metrology : Metrological instrumentation
(300.3700) Spectroscopy : Linewidth
(300.6260) Spectroscopy : Spectroscopy, diode lasers
(300.6320) Spectroscopy : Spectroscopy, high-resolution
Mikko Merimaa, Thomas Lindvall, Ilkka Tittonen, and Erkki Ikonen, "All-optical atomic clock based on coherent population trapping in 85Rb," J. Opt. Soc. Am. B 20, 273-279 (2003)