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Journal of Physics Conference Series

A Novel Laser Power Measurement Scheme Using Rubidium Clock

Qianqian Ji, Lei Han, Yabei Su, Xiaobo Xue, Shengkang Zhang, Junfeng Ge

Published Nov 1, 2024• 0 citations
Publisher

Abstract

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.

DOI
10.1088/1742-6596/2889/1/012053

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Physical Review Applied•Sep 18, 2024•11 citations
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.
Precise measurement of the shift (i.e. microwave frequency shift induced by the electric field of the pumping light) in a rubidium atomic clock pumped by a semiconductor laser is discussed. The spectral lineshape of the microwave resonance, which is used as a frequency discriminator for the atomic clock in the optical microwave double resonance experiment, depends strongly on the spatial distribution of the laser beam intensity, laser frequency detuning, and modulation parameters of the microwave frequency. Based on measurements of the deformation of the resonance lineshape, a self-tuning system was built to compensate for the effect of light shift. As a result of controlling the laser frequency with this system, long-term drift of the microwave frequency as low as 6.3*10/sup -13//h was obtained.>
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.
The 5S1/2 --> 7S1/2 two-photon transition of atomic rubidium, which is 100 times weaker than the 5S-5D transition, is observed with an extended-cavity diode laser and a vapor cell. Signals with a signal-to-noise ratio of 280 are obtained with a laser power of 10 mW, and the observed linewidth is 3 MHz. The laser wavelength is 760 nm and is locked on the transitions to a stability of 2 x 10(-11). For the first time to our knowledge, the isotope shift of this transition is measured to be 130(4) MHz in atomic frequency.
Experiments were carried out to evaluate the performances of a semiconductor laser pumped rubidium (87Rb) atomic clock. Two kinds of Rb gas cells were used and their performances were compared [gas cell A (natural rubidium (87Rb/85Rb =\frac{3}{7}) and buffer gases) and gas cell B (87Rb and buffer gases)]. The highest microwave frequency stabilities were estimated as3.4 \times 10^{-12} \tau^{-1/2}and2.7 \times 10^{-12} \tau^{-1/2}at the optimal gas cell temperatures of 60°C and 48°C for the gas cellsAandB, respectively (τ: integration time). The light shift, i.e., microwave frequency shift induced by laser light, was measured as -0.50 Hz/MHz and -0.11 Hz/MHz for the gas cellsAandBat their optimal operating conditions given above. As an improved experiment by utilizing high temporal coherence of the laser, a novel double resonance spectral line shape with a drastically narrower linewidth was demonstrated. A technique, similar to FM laser spectroscopy, was employed for this purpose by utilizing laser FM sidebands which are induced by microwave frequency modulation and nonlinear susceptibility of three-level87Rb atoms. The minimum linewidth obtained was 20 Hz, which can be used as a sensitive frequency discriminator for an improved87Rb atomic clock.
Abstract Modern atomic clocks based on the interrogation of an atomic transitions in the optical regions require multiple lasers at different wavelength for producing atomic ions, trapping and laser cooling of neutral atoms or atomic ions. In order to achieve highest efficiency for laser cooling or any other atomic transition, frequencies of each of the lasers involved need to be stabilized by mitigating its drifts or fluctuations arise due to ambient temperature variation or other kind of perturbations. The present article describes simultaneous frequency stabilization of multiple number of lasers, required for production and laser cooling of ytterbium (171Yb) ions, to a reference transition frequency of rubidium (Rb) atoms. In this technique, a diode laser operating at ~ 780 nm is frequency stabilized to one of the Doppler broadening-free absorption peak of rubidium atoms (85Rb) and then used as a reference frequency for calibrating a wavelength meter and subsequent simultaneous frequency stabilization of four lasers operating at different wavelengths.
Using modulation transfer spectroscopy, we achieve the frequency stabilization of a high-power 3-W blue laser at the wavelength of 420 nm to the Rb\mathrm{Rb}Rb 5S1/25{S}_{1/2}5S1/2​--6P3/26{P}_{3/2}6P3/2​ transition. The in-loop frequency stability of this laser is $1.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}/\sqrt{\ensuremath{\tau}}$, reaching $1.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ at 100 s and $7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}$ at 1000 s. An external power feedback loop is established using an acousto-optic modulator, employing the zeroth-order diffracted light for power stabilization, achieving an in-loop power stability of $1.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ at 1 s. Moreover, the continuous mode-hop free interval of this high-power laser can simultaneously cover the 5S1/25{S}_{1/2}5S1/2​--6P3/26{P}_{3/2}6P3/2​ transitions of 85Rb{}^{85}\text{Rb}85Rb and 87Rb{}^{87}\text{Rb}87Rb, with successful locking achieved for both isotopes, providing a comprehensive analysis of the Rb\mathrm{Rb}Rb atomic transitions in the blue spectral region. As an application, this 3-W 420-nm laser with excellent power and frequency stabilities is used as a repumping source for diffuse laser cooling of 87Rb{}^{87}\text{Rb}87Rb atoms, realizing a one-meter-long cold-atom cloud. This paves the way for using blue-light cooling to realize a cold-Rb\mathrm{Rb}Rb-atom active optical clock. In addition, the ultrahigh-stability 420-nm laser also finds applications in various fields such as the Rydberg atom experiment, Bose-Einstein condensation, ultranarrow-bandwidth Faraday atomic filter, and so on.