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Applied Optics

Laser frequency stabilization via bichromatic Doppler-free spectroscopy of an 87Rb D1 line

Minhua Zhao, Xunda Jiang, Ruihuan Fang, Yuxiang Qiu, Zhu Ma, Chengyin Han, Bo Lü, Chaohong Lee

Published May 20, 2021• 19 citations
Publisher

Abstract

We demonstrate a bichromatic Doppler-free spectroscopy of an 87 R b D 1 line by using a dual-frequency, counterpropagating laser field with orthogonal linear polarizations. A reversed Doppler-free resonance dip is observed in the dual-frequency scheme, and a significant improvement of frequency discrimination curve is acquired due to the coherent population trapping (CPT) effect. The influence of the static magnetic field and laser intensity on the spectroscopy is studied in both single- and dual-frequency schemes. After locking the laser frequency to the 87 R b D 1 line in the dual-frequency stabilization scheme, the beat note fractional frequency stability is at the level of 7 × 10 − 12 at 1 s integration time. This technique can be used in various applications, such as CPT atomic clocks, laser spectroscopy, quantum optics, and laser-cooling experiments.

DOI
10.1364/ao.425694

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We demonstrate a bichromatic Doppler-free spectroscopy of an 87 R b D 1 line by using a dual-frequency, counterpropagating laser field with orthogonal linear polarizations. A reversed Doppler-free resonance dip is observed in the dual-frequency scheme, and a significant improvement of frequency discrimination curve is acquired due to the coherent population trapping (CPT) effect. The influence of the static magnetic field and laser intensity on the spectroscopy is studied in both single- and dual-frequency schemes. After locking the laser frequency to the 87 R b D 1 line in the dual-frequency stabilization scheme, the beat note fractional frequency stability is at the level of 7 × 10 − 12 at 1 s integration time. This technique can be used in various applications, such as CPT atomic clocks, laser spectroscopy, quantum optics, and laser-cooling experiments.
Coherent population trapping (CPT) magnetometers are important geomagnetic measurement tools. However, the shift and jitter of laser frequencies can seriously interfere with the measurement results of CPT magnetometers. A CPT magnetometer uses bichromatic light that reacts with alkali metal atoms to obtain magnetic field information. Therefore, traditional laser-frequency stabilization methods, which are primarily based on monochromatic light, are not suitable for CPT magnetometers. In this study, we propose a new laser-frequency stabilization method based on the Doppler-free spectrum of the87Rb D1 line produced by bichromatic circularly polarized light. First, the bichromatic Doppler-free spectrum at the87Rb D1 line was theoretically analyzed based on the energy level, which revealed that the Doppler-free spectrum produced by bichromatic circularly polarized light can exhibit a large amplitude in the geomagnetic field. Next, we utilized the bichromatic circularly polarized light to construct a self-assembly Dopplerfree spectroscopy of the87Rb D1 line to verify the proposed laser-frequency stabilization method. The experimental results demonstrate that the frequency stability within an integration period of 1s was 5.74 × 10−11. When the integration time was 100s, the frequency stability was 8.63 × 10−11. These results show that the proposed method is promising for ultra-highsensitivity CPT magnetometers, ensuring accurate measurements in practical applications.
We report on Doppler-free laser spectroscopy in a Cs vapor cell using a dual-frequency laser system tuned on the Cs D1 line. Using counter-propagating beams with crossed linear polarizations, an original sign reversal of the usual saturated absorption dip and large increase in Doppler-free atomic absorption is observed. This phenomenon is explained by coherent population trapping (CPT) effects. The impact of laser intensity and light polarization on absorption profiles is reported in both single-frequency and dual-frequency regimes. In the latter, frequency stabilization of two diode lasers was performed, yielding a beat note fractional frequency stability at the level of 3×10−12 at 1 s averaging time. These performances are about an order of magnitude better than those obtained using a conventional single-frequency saturated absorption scheme.
We present a new polarization-selective detection scheme that enables simultaneous dual-frequency laser stabilization and implementation of high-contrast coherent population trapping (CPT) resonance. The proposed configuration splits elliptically polarized light into two optical paths after atomic interaction. The component reflected from the polarizing beam splitter, containing Doppler-broadened spectral features, is employed for precise laser frequency locking. The reflected signal exhibits a frequency discrimination slope nine times steeper than that of the transmitted polarizing beam splitter component. Simultaneously, the transmitted component isolates the CPT resonance signals from the positive and negative first-order sidebands for microwave frequency stabilization. Experiment demonstrates excellent agreement with theoretical simulations derived from the Liouville equation. Compared with conventional circular polarization methods, this approach achieves a fourfold enhancement in short-term frequency stability. These advancements make the technique particularly promising for developing compact CPT atomic clocks with improved metrological performance.
The detection of narrow and high-amplitude optical resonances in atomic cells is of crucial importance for the development of compact and high-stability lasers or vapor-cell optical references. In this work, using monochromatic light and appropriate polarization settings for the counterpropagating pump and probe beams, we demonstrate the detection of enhanced-absorption sub-Doppler resonances on the D1{D}_{1}D1​ line of the Rb\mathrm{Rb}Rb atom. We report a theoretical model showing that the Zeeman coherence induced between magnetic sublevels---especially in the $F\ensuremath{\rightarrow}{F}^{\ensuremath{'}}=F\ensuremath{-}1$ systems, with FFF and ${F}^{\ensuremath{'}}$ being the hyperfine quantum numbers of the ground and excited states, respectively---plays a key role in the observed enhanced-absorption process. The theoretically predicted behavior matches the experimental observations well. The impact of light polarization, laser intensity, magnetic field, and cell temperature on the Doppler-free resonance features is experimentally investigated. For application, two identical diode lasers are frequency-stabilized onto enhanced-absorption sub-Doppler resonances. The fractional frequency stability of the laser beatnote is measured to be $1.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ at 1s1\phantom{\rule{0.1em}{0ex}}\mathrm{s}1s and $8.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ at 104s{10}^{4}\phantom{\rule{0.1em}{0ex}}\mathrm{s}104s. These results demonstrate the potential of this scheme for the implementation of a compact or even chip-scale optical frequency reference, which might find applications in instrumentation, navigation, and metrology.
The utilization of atomic or molecular spectroscopy for frequency locking of single-frequency laser to improve laser frequency stability plays an important role in the experimental investigation of optically pumped atomic magnetometers, atomic clocks, laser cooling and trapping of atoms, etc. We have experimentally demonstrated a technique for frequency stabilization of a single-frequency laser employing the bright state spectroscopy (BSS) with a rubidium atomic vapor cell. By utilizing the counter-propagating dual-frequency 795 nm laser beams with mutually orthogonal linear polarization and a frequency difference of 6.834 GHz, which is equal to the hyperfine splitting of rubidium-87 ground state 5S1/2, an absorption-enhanced signal with narrow linewidth at the center of Doppler-broadened transmission spectroscopy is observed when continuous scanning the laser frequency over rubidium-87 D1 transition. This is the so-called BSS. Amplitude of the absorption-enhanced signal in the BSS is much larger compared with the conventional saturation absorption spectroscopy (SAS). The relationship between linewidth and amplitude of the BSS signal and laser beam intensity has been investigated. This high-contrast absorption-enhanced BSS signal has been employed for the laser frequency stabilization. The experimental results show that the frequency stability is 4.4×10−11 with an integration time of 40 s, near one order of magnitude better than that for using the SAS.
We are studying a new spectroscopic method using laser-cooled rubidium atoms that allows us to lock the carrier frequency of modulated laser in the coherent population trapping (CPT) clock experiment. This method uses counter-propagation of the bichromatic laser field produced by the modulated laser. The counter-propagation cancels the CPT effect near the optical resonance and gives rise to enhanced Doppler-free absorptive resonances with high-contrast away from the transition. We demonstrate the CPT clock operation by locking the modulated laser using this resonance. We also describe a theoretical model that we have developed based on density-matrix formalism to simulate this spectroscopic scheme for accurately predicting the lineshape and amplitude of resonances under different excitation conditions. Our study suggests that this spectroscopic method is also quite suiatble for developing cold-atom based optical frequency standard.