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

Faraday Laser with Cavity Mode Locked for Optical Pumped Rubidium Atomic Clock

Tianyu Liu, Xiaolei Guan, Dayong Chen, Jianxiang Wang, Duo Pan, Jingbiao Chen

Published Jul 7, 2021• 0 citations
Publisher

Abstract

We measure the light shift of the rubidium atomic clock under interaction of a 780 nm laser, which has potential application in the laser power stabilization. At the same time, in order to improve the performance of rubidium clock, we demonstrate a Faraday laser of which the cavity mode is locked to the peak of the transition spectrum, thus suppressing the otherwise existed power and frequency noise caused by the cavity mode fluctuations. This Faraday laser is suitable for the optical pumped rubidium atomic clock with the frequency being stably on resonance with the rubidium 780nm transition, and has potential applications on the cesium beam clocks and fountain clocks.

DOI
10.1109/eftf/ifcs52194.2021.9604274

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We measure the light shift of the rubidium atomic clock under interaction of a 780 nm laser, which has potential application in the laser power stabilization. At the same time, in order to improve the performance of rubidium clock, we demonstrate a Faraday laser of which the cavity mode is locked to the peak of the transition spectrum, thus suppressing the otherwise existed power and frequency noise caused by the cavity mode fluctuations. This Faraday laser is suitable for the optical pumped rubidium atomic clock with the frequency being stably on resonance with the rubidium 780nm transition, and has potential applications on the cesium beam clocks and fountain clocks.
Dual-frequency (DF) lasers with specific frequency intervals have crucial applications in geometric measurement, optical microwave generation, quantum precision measurement, etc. Here, we report a 780 nm DF rubidium Faraday laser, with two modes’ frequencies corresponding to the 85 Rb and 87 Rb D 2 transitions. The microwave signal generated from the two laser modes’ beat has a linewidth of 671 Hz and is tunable from 4.7–4.95 GHz to 5.94–6.7 GHz by adjusting the diode current. Moreover, we use modulation transfer spectroscopy to lock one of the laser modes to the 87 Rb hyperfine transition, and the frequency stability improves nearly an order of magnitude. The noise characteristics are also measured, where the phase noise is suppressed over 25 dB at 100 Hz offset compared to the microwave signal generated by two single-mode lasers’ heterodyne beat. This 780 nm DF rubidium Faraday laser has two coherent modes with frequencies related to Rb transitions, which provides a promising and convenient method for compact atomic physics applications, such as coherent population trapping clocks, atom gravimeters, and so on.
Summarywe report on the development of a power efficient Faraday laser working at 780nm Rb transition using an atomic vapor Faraday filter as a frequency-selecting element. In the routine linear-cavity of Faraday laser, a partial reflectivity cavity mirror was used as output coupler. In this work, a high-reflection mirror (R>99.98%) replaces the partial reflectivity cavity mirror, the transmitted light of the Faraday optical filter is fully used as feedback beam and for frequency selection, while the light with polarization perpendicular to the transmitted part from the second polarizing beam splitter (PBS2) of Faraday anomalous dispersion optical filter (FADOF) is output. Single frequency laser is obtained at 780 nm Rb transition over a range of laser diode (LD) current from 60 to 140 mA. Such a single-frequency laser will be an excellent optical frequency standard by frequency stabilization technique.
Abstract To measure microwatt-level laser power, this paper presents a method based on the atomic light shift effect. A 795 nm semiconductor laser is directed into a rubidium atomic clock with an output frequency of 10 MHz. To minimize the influence of laser frequency on power measurements, an experimental setup is implemented to precisely lock the laser frequency to the wavelength meter. After stabilizing the laser frequency, we analyze its correlation with the output frequency of the rubidium atomic clock. The experimental results indicate that, during free-running, the laser undergoes a frequency drift of -90 MHz over 2 hours, which is subsequently reduced to ±0.9 MHz over 40 hours through frequency locking. Moreover, by fixing the laser wavelength at 794.99 nm, we observe an increase in the atomic clock frequency shift with rising laser power. Specifically, a laser power of 2 μW corresponds to a frequency shift of 0.13 mHz at 10 MHz. This study achieves quantum measurement of weak laser power, significantly enhancing the accuracy of low-power laser measurements.
One impediment to achieving the promise of enhanced laser-pumped gas cell clock performance resides with the light-shift effect, a phenomenon whereby diode laser optical frequency fluctuations are mapped onto the atomic clock's output frequency. The alternative to tightly stabilizing the diode laser in order to overcome this problem is to develop means of reducing the actual magnitude of the light-shift. Here, we briefly review two strategies for accomplishing this, and introduce a new one based on optical pumping with laser-induced-fluorescence (LIF). The advantage of LIF optical pumping is that the clock's output frequency is insensitive to laser frequency fluctuations. Though LIF optical pumping creates a dependence of clock frequency on laser intensity variations, it is to be noted that diode lasers have very low intensity noise. Consequently, with regards to the light-shift effect the LIF strategy takes advantage of an inherent diode laser strength.
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.
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.