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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Cesium active optical clock in four-level laser configuration

Yanfei Wang, Xiaobo Xue, Dongying Wang, Tonggang Zhang, Qinqing Sun, Yelong Hong, Wei Zhuang, Jingbiao Chen

Published May 1, 2012• 8 citations
Publisher

Abstract

We propose an active optical clock scheme in four-level configuration with cesium atoms. The population inversion has been established experimentally between 7S1/2state and 6P3/2state in a thermal cesium cell with a 455.5 nm pumping laser. It can be used to generate active optical clock laser output in a bad cavity laser regime at the wavelength of 1469 nm.

DOI
10.1109/fcs.2012.6243657

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We propose an active optical clock scheme in four-level configuration with cesium atoms. The population inversion has been established experimentally between 7S1/2state and 6P3/2state in a thermal cesium cell with a 455.5 nm pumping laser. It can be used to generate active optical clock laser output in a bad cavity laser regime at the wavelength of 1469 nm.
We realize the lasing of cesium active optical clock with 459 nm laser pumping, which operates deep in the optical bad-cavity regime. The ratio between cavity bandwidth and gain bandwidth is 55.08 theoretically. With 459 nm laser pumping, the population inversion occurs between the cesium 7S1/2state and 6P3/2state, and the stimulated radiation between the two states is realized. The 1469.9 nm output signal of cesium active optical clock can reach a power of 8 μW. The output power also shows a threshold character with pumping laser power and can be influenced by the Cs cell temperature. This active optical clock has an advantage in stability over the conventional optical clocks, due to its suppression of cavity pulling effect, which is also demonstrated experimentally.
We are setting up an experiment to investigate an active optical clock scheme in four-level configuration with Cesium atoms. The 1469 nm Cesium active optical clock output in a bad cavity laser regime is generated and the wavelength is measured to be 1469.500 nm by using spectrometer. The 1469 nm lasing threshold and output power is measured when changing 455 nm pumping laser power and frequency. The 1469 nm Cesium active optical clock output multi-threshold is observed when changing bad-cavity length. We also sweep the bad-cavity length and measure the 1469 nm Cesium active optical clock output frequency using a wavelength meter, proving the cavity-pulling reduction due to the bad-cavity effect.
We experimentally demonstrate the collective emission behavior and suppressed cavity-pulling effect of four-level active optical clock with Cesium atoms. Thermal Cesium atoms in a glass cell velocity selective pumped with a 455.5 nm laser operating at 6S1/2_{1/2}1/2​ to 7P3/2_{3/2}3/2​ transition are used as lasing medium. Population inverted Cesium atoms between 7S1/2_{1/2}1/2​ and 6P3/2_{3/2}3/2​ levels are optical weakly coupled by a pair cavity mirrors working at deep bad-cavity regime with a finesse of 4.3, and the ratio between cavity bandwidth and gain bandwidth is approximately 45. With increased 455.5 nm pumping laser intensity, the output power of cesium active optical clock at 1469.9 nm from 7S1/2_{1/2}1/2​ level to 6P3/2_{3/2}3/2​ level shows a threshold and reach a power of 13 μμμW. Active optical clock would dramatically improve the optical clock stability since the lasing frequency does not follow the cavity length variation exactly, but in a form of suppressed cavity pulling effect. In this letter the cavity pulling effect is measured using a Fabry-Perot interferometer (FPI) to be reduced by a factor of 38.2 and 41.4 as the detuning between the 1469.9 nm cavity length of the Cs active optical clock and the Cs 1469.9 nm transition is set to be 140.8 MHz and 281.6 MHz respectively. The mechanism demonstrated here is of great significance for new generation optical clocks and can be applied to improve the stability of best optical clocks by at least two orders of magnitude.
We present an active optical clock scheme with a four-level quantum potassium system. We calculate the population probabilities of each state using the density matrix. At the steady state, ρ 33 and ρ 55 are equal to 8.3% and 3.5%, respectively, and the population inversion between the 5S 1/2 and 4P 3/2 states is built up in the thermal potassium cell with a 404.7 nm pumping laser. According to the mechanism of the active optical clock, under the action of the 404.7 nm pumping laser, the scheme can output a 1252.2 nm quantum-limited-linewidth laser, which can be directly used as an active optical frequency standard.
Since the concept of active optical clock has been proposed, the experimental research of Rb four-level active optical clock has never been carried out, but it has great value in research and application. First the Rb atoms are pumped from ground state 5S1/2to 6P3/2excited state by a narrow-linewidth 420 nm laser, and then decay to lower energy levels by spontaneous transitions, and immediately are pumped by pumping laser again. This process continues until the population inversion is established between 6 S and 5P states. Once the population inversion is built up, a Fabry-Perot type resonator, at a bad cavity regime whose cavity mode linewidth is much wider than that of the laser gain profile, is used to realize 1367 nm active optical clock laser output. As Rb atom 1367 nm 5P-6S excited state transition is in the communication E band, it can be used as a narrow-linewidth communication band optical frequency standard, which greatly improves the communication rate and accuracy.
The active optical clock (AOC) takes advantage of the narrowed linewidth and cavity pulling suppression of the bad cavity laser, making it a candidate for the next generation of optical clock. We propose a new experimental method to measure the cavity-pulling effect of a four-level AOC based on thermal cesium atoms. After modulating the frequency of the 459 nm pumping laser, atoms in different velocity groups are pumped to the 7P1/2energy level. Therefore, population inversion is achieved in separate velocity groups, and sidebands are created in the gain profile. By measuring the central frequency of the beat spectrum between the zeroth order AOC output and the sidebands, the cavity-pulling coefficient can be calculated. This method has the advantage of not requiring any additional optical frequency reference, and can be adopted in AOC systems other than those using thermal atoms. Furthermore, our proposed method for cavity-pulling measurement can serve as a basis for the characterization of AOCs.