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Frontiers in Physics

Active optical clock lasing on the Cs 7S1/2-6P3/2 transition under a weak magnetic field

Tiantian Shi, Jianxiang Miao, Jia Zhang, Jingbiao Chen

Published Aug 12, 2022• 3 citations
Publisher

Abstract

In the bad-cavity limit, the collective atomic dipole is highly coherent, resulting in the phase information of an active optical clock (AOC) laser primarily stored in the atomic gain medium. Therefore, compared with the good-cavity laser, the sensitivity of an AOC laser to cavity fluctuations is greatly reduced, as characterized by the suppressed cavity-pulling effect. In this work, the AOC lasing on the cesium 7S 1/2 -6P 3/2 clock transition with a natural linewidth of 1.81 MHz under a weak magnetic field is achieved. We calculate the Zeeman spectra of upper and lower states of clock transition, and measure the beat-note spectrum between different Zeeman-sublevel transitions of 7S 1/2 -6P 3/2 . Moreover, the cavity-pulling, temperature, power, and linewidth characteristics of the AOC laser are demonstrated under a weak magnetic field. Such an emerging laser can be applied as a narrow-linewidth local oscillator, as well as an active optical frequency standard, which is promising for the field of precision measurement.

DOI
10.3389/fphy.2022.967255

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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.

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In the bad-cavity limit, the collective atomic dipole is highly coherent, resulting in the phase information of an active optical clock (AOC) laser primarily stored in the atomic gain medium. Therefore, compared with the good-cavity laser, the sensitivity of an AOC laser to cavity fluctuations is greatly reduced, as characterized by the suppressed cavity-pulling effect. In this work, the AOC lasing on the cesium 7S 1/2 -6P 3/2 clock transition with a natural linewidth of 1.81 MHz under a weak magnetic field is achieved. We calculate the Zeeman spectra of upper and lower states of clock transition, and measure the beat-note spectrum between different Zeeman-sublevel transitions of 7S 1/2 -6P 3/2 . Moreover, the cavity-pulling, temperature, power, and linewidth characteristics of the AOC laser are demonstrated under a weak magnetic field. Such an emerging laser can be applied as a narrow-linewidth local oscillator, as well as an active optical frequency standard, which is promising for the field of precision measurement.
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.
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.
Two feasible schemes are proposed by laser cooling of alkali atoms to realize the narrow-linewidth, continual superradiant lasing. Utilizing the D1 gray molasses to realize a cloud of cold Cs atoms for the two-level Faraday active optical clock (AOC), the effect of Doppler broadening on the transmission bandwidth of Faraday atomic filter will be effectively suppressed to the level of natural linewidth. Thus, the influence of mechanical or thermal vibrations of cavity mirrors on the clock laser will be decreased by about 102 times, which is expected to further narrow the laser linewidth by two orders of magnitude compared with the thermal atomic Faraday AOC to Hz level. Moreover, the four-level AOC based on the narrow linewidth cooling of Cs atoms using the 6S-7P transition is proposed. The 455 nm laser at 6S1/2-7P3/2transition plays two roles simultaneously, one is the cooling laser for a magneto-optical trap (MOT), another is the pumping laser for population inversion of 1470 nm clock transition, which is helpful to improve the compactness of the system. In the four-level structure, the frequency of pumping laser is far detuning from that of clock laser, therefore, the light shift resulting from the 455 nm laser is negligible. This four-level AOC based on cold atoms will further reduce the collision shift in thermal atom system to achieve a high-precision optical frequency reference.
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 have experimentally demonstrated the active optical clock with a good suppression to cavity-length noise compared with the passive one. However, the frequency stability of the four-level active optical clock for thermal atoms is still limited by the residual cavity-pulling effect. In this paper, we preliminarily realized the cavity-length stabilization in 1064/1470 nm dual-wavelength good-bad cavity system. The synchronous cavity-length change between two dual-wavelength systems is realized by using the phase locking technique of 1064 nm goodcavity lasers. Therefore, the beating linewidth of 1470 nm badcavity lasers will also be reduced considering that the 1064/1470 nm dual-wavelength lasers share the one cavity. The tracking accuracy of 1064 nm good-cavity lasers is better than 3×10-16at 1 s after phase-locking, and the linewidth of 1470 nm bad-cavity laser is reduced to 77 Hz. The linewidth of active optical frequency standard is still limited by the light shift, collision shift and Zeeman shift. The beating signal of magnetic sublevels corresponding to the Cs 1470 nm clock transition is observed under Zeeman effect. Further research is needed, such as, the design of vacuum chamber and magnetic shielding, to reduce the linewidth of the continuous 1470 nm active optical frequency standard.
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.