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

Active Optical Clock Based on Laser Cooling of Alkali-metal Atoms

Tiantian Shi, Duo Pan, Wei Zhuang, Xiaolei Guan, Jianxiang Miao, Jia Zhang, Jingbiao Chen

Published Jul 7, 2021• 2 citations
Publisher

Abstract

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.

DOI
10.1109/eftf/ifcs52194.2021.9604323

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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 proposed a scheme for realizing active Faraday optical frequency standards using diffuse laser cooling of alkali-metal atoms. A 1-m-long cold87Rb atomic cloud was prepared within a ϕ 2 cm ×$l$1 m vacuum glass tube, featuring a maximum optical density of 4. By utilizing this setup to construct the Faraday anomalous dispersion optical filter, we are able to achieve an ultra-narrowband laser gain close to the natural linewidth of the atomic transition and corresponding to the hyperfine transition frequency. This ultra-narrowband laser gain can directly determine the output frequency of the active Faraday optical frequency standard operating in the “bad cavity” regime, significantly suppressing the cavity-pulling effect. Furthermore, due to the suppression of atomic collision and Doppler frequency shifts by the cold atoms, it is expected to further narrow the laser linewidth of the cold-atom-based active Faraday optical frequency standard to Hz level.
The cavity-pulling suppression advantage of active optical clocks (AOCs) helps to realize atomic clocks with superior performance. However, realization of continuous-wave cold-atom AOCs is a highly challenging task. In this work, we proposed an AOC scheme with diffuse-laser-cooled atoms. Combining the advantages of diffuse laser cooling to easily prepare cold-atom cloud with large volume on the scale of meters and the four-level structure to greatly reduce the light shift, it is expected to realize the continuous output of AOC superradiant signals. We experimentally realized a 100-cm-long cold87Rb atom cloud as the gain medium. The 1367 nm AOC superradiant signal with a power of 0.3μW\mu \mathrm{W}μWhas been achieved without resonant cavity feedback. In the future, we will continue to advance the experiment and also conduct research on 420 nm blue light cooling.
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.
We propose the mechanism of an active Faraday optical clock, and experimentally demonstrate an active Faraday optical frequency standard based on narrow bandwidth Faraday atomic filter by the method of velocity-selective optical pumping of cesium vapor. The center frequency of the active Faraday optical frequency standard is determined by the cesium 6 (2)S(1/2) F=4 to 6 (2)P(3/2) F'=4 and 5 crossover transition line. The optical heterodyne beat between two similar independent setups shows that the frequency linewidth reaches 281(23) Hz, which is 1.9×10(4) times smaller than the natural linewidth of the cesium 852-nm transition line. The maximum emitted light power reaches 75 μW. The active Faraday optical frequency standard reported here has advantages of narrow linewidth and reduced cavity pulling, which can readily be extended to other atomic transition lines of alkali and alkaline-earth metal atoms trapped in optical lattices at magic wavelengths, making it useful for new generation of optical atomic clocks.
Known as a unique optical clock, active optical clock has prior advantages to surpass the limitation of the conventional optical clock in potential stability. In this paper, we proposed an improved Cesium active Faraday optical frequency standard with 459 nm laser pumping. This scheme reduces the residual Doppler broadening in previous results with 852 nm laser pumping. The transmission is 2.4% and the bandwidth of the transmission peak is 12.3 MHz. It greatly narrows the bandwidth of Faraday anomalous dispersion optical filter and decreases the length of the bad cavity used in Cesium active Faraday optical frequency standard. This will be very helpful for further suppression of cavity pulling effect.
Active optical clocks, a new class of optical atomic clocks, work through stimulated emission lasing, a unique mechanism that is distinct from laser absorption spectroscopy of conventional optical clocks. With coherent weak optical feedback from a Fabry-Perot resonator operating in the bad-cavity regime, active optical clocks sustain a lasing oscillation at an atomic transition frequency. They are used directly as quantum optical frequency standards with expected stability better by about two orders of magnitude than that of the best clocks. This review starts with a simplified account of historical developments and the limitations of critical techniques in optical atomic clocks. After an introduction of the basic mechanism underlying active optical clocks, we discuss the most recent experimental results for several configurations and experimental set-ups of active optical clocks. These include those based on a 2-level atomic beam and 3-level laser-cooled trapped-atom system, those based on a 4-level atom scheme, and the Faraday active optical clock. Moreover, following the successful lasing and preliminary results on a Faraday active optical frequency standard with Faraday atomic filter using thermal Cs cell, we describe an experimental scheme of a Faraday active optical clock with a Faraday atomic filter using magic-lattice-trapped Sr atoms at the 689-nm transition, and Faraday optical clocks operating in the good-cavity regime. We conclude with a comprehensive comparison of the different active optical clocks, prospects for active Faraday optical clocks, and potential applications of active optical clocks.