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IEEE Transactions on Instrumentation and Measurement

Laser-Frequency Stabilization Using Doppler-Free Spectrum of 87Rb D1 Line Produced by Bichromatic Circularly Polarized Light

Pengcheng Du, Xianyang Huo, Feifan Yang, Jiahao Yuan, Jin Li

Published Jan 1, 2025• 1 citation

Abstract

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.

DOI: 10.1109/tim.2025.3557829

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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 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.

We present an experimental and theoretical investigation of the coherent population trapping (CPT) resonance excited on the D1 line of 87 Rb atoms by bichromatic linearly polarized laser light. The experimental results show that a lin ‖ lin transition scheme is a promising alternative to the conventional circular—circular transition scheme for an atomic magnetometer. Compared with the circular light transition scheme, linear light accounts for high-contrast transmission resonances, which makes this excitation scheme promising for high-sensitivity magnetometers. We also use linear light and circular light to detect changes of a standard magnetic field, separately.

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

{D}_{1}

line of the

\mathrm{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

F

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

1\phantom{\rule{0.1em}{0ex}}\mathrm{s}

and $8.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ at

{10}^{4}\phantom{\rule{0.1em}{0ex}}\mathrm{s}

. 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.

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 microwave frequency standard based on coherent population trapping (CPT) in the 87Rb D1 line. The CPT spectrum is obtained using two Raman lasers with a 6.8 GHz frequency offset by injection locking of a master laser to a slave laser. We have constructed an atomic clock employing a 5 cm long Rb vapor cell confined with 6.67 kPa neon buffer gas at 70 °C. Using this system, we improve the CPT contrast through the elimination of undesired off-resonant fields created by the direct modulation method. We measured the frequency shift of the CPT signal as a function of the temperature of the Rb cell and estimated it to be approximately 1.3 ×10−9/K. The frequency of a 10 MHz crystal oscillator has been stabilized to the CPT spectrum between the two ground states in 87Rb. The relative frequency stability is approximately 2.3×10−12 for an average time of 68 s.

We have experimentally demonstrated that magnetic-field-insensitive coherent-population-trapping (CPT) resonances are generated between the ground hyperfine levels on the

{D}_{1}

line of

^{133}\mathrm{Cs}

using a two-photon $\mathrm{\ensuremath{\Lambda}}$ scheme excited by lin||lin polarizations. The frequency shift of the CPT resonance is 0.04 Hz for the deviation of $1\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{T}$ at a ``magic'' magnetic field of $139\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{T}$ and is 50 times smaller than that of the conventional clock transition at a typical bias magnetic field for the clock operation. The amplitude of the CPT spectrum excited by lin||lin polarizations is enhanced as the excitation intensity increases and is well explained by the three-level model without trap levels. Thus, the CPT resonance on the

{D}_{1}

line of

^{133}\mathrm{Cs}

excited by a simple lin||lin scheme will be one of the best candidates for frequency reference of miniature atomic clocks.