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Chinese Optics Letters

Laser frequency locking method for Rydberg atomic sensing

Kai Yang, Ruiqi Mao, Qiang An, Zhanshan Sun, Yunqi Fu

Published Nov 16, 2022• 22 citations
Publisher

Abstract

Based on the Rydberg cascade electromagnetically induced transparency, we propose a simultaneous dual-wavelength locking method for Rydberg atomic sensing at room temperature.The simplified frequency-locking configuration uses only one signal generator and one electro-optic modulator, realizing real-time feedback for both lasers.We studied the effect of the different probe and coupling laser powers on the error signal.In addition, the Allan variance and a 10 kHz amplitudemodulated signal are introduced to evaluate the performance of the laser frequency stabilization.In principle, the laser frequency stabilization method presented here can be extended to any cascade Rydberg atomic system.

DOI
10.3788/col202321.021407

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Laser Physics•Dec 7, 2016•11 citations
Based on the Rydberg cascade electromagnetically induced transparency, we propose a simultaneous dual-wavelength locking method for Rydberg atomic sensing at room temperature.The simplified frequency-locking configuration uses only one signal generator and one electro-optic modulator, realizing real-time feedback for both lasers.We studied the effect of the different probe and coupling laser powers on the error signal.In addition, the Allan variance and a 10 kHz amplitudemodulated signal are introduced to evaluate the performance of the laser frequency stabilization.In principle, the laser frequency stabilization method presented here can be extended to any cascade Rydberg atomic system.
We present a laser frequency locking to Rydberg transition with electromagnetically induced transparency (EIT) spectra in a room-temperature cesium vapor cell. Cesium levels 6S 1/2 , 6P 3/2 , and the n D 5/2 state, compose a cascade three-level system, where a coupling laser drives Rydberg transition, and probe laser detects the EIT signal. The error signal, obtained by demodulating the EIT signal, is used to lock the coupling laser frequency to Rydberg transition. The laser frequency fluctuation, ∼0.7 MHz, is obtained after locking on, with the minimum Allan variance to be 8.9 × 10 −11 . This kind of locking method can be used to stabilize the laser frequency to the excited transition.
We herein develop and demonstrate a stable frequency-locking scheme for Rydberg atomic experiments. We use the Zeeman effect to modulate the three-level ladder-type Rydberg electromagnetically induced transparency (EIT) signal to lock the laser frequency of the coupling light for transition from its intermediate state to a Rydberg state. The effects of polarization of the probe and coupling lights, and the amplitude of the AC modulated magnetic field B 0 on the EIT and the corresponding dispersive error signal, are both analyzed. The results show that both the EIT signal and dispersive error signal are the strongest when the polarizations of coupling and probe fields are circular and equal. The signal-to-noise ratio of the dispersive error signal increases with B 0 . The slope of the dispersive error signal increases first and then decreases with B 0 , which is related to the increase of the EIT linewidth caused by the higher B 0 . The linewidth of the laser is significantly less than 500 kHz after frequency locking, which satisfies the requirements of most experiments involving Rydberg atoms. The method proposed herein can generally be applied to any cascade system of Rydberg atoms.
We established a theoretical model for the ladder-type EIT laser frequency stabilization method based on the analysis of the system response. By using the method of deriving the density matrix equations, we obtained the density matrix elements of the three-level system and subsequently derived the system’s response function. Based on this, we revealed the complex interference mechanism of the modulated probe light in the atomic medium. Through quantitative analysis of this process, we obtained the output signal, which carries information about the frequency offset of the coupling light, after the probe light passes through the atomic medium. The error signal obtained after demodulation can then serve as the reference signal for the closed-loop feedback system. To make the error signal more intuitively reflect its variation with the coupling light frequency offset, we introduced the equivalent transfer of the modulation frequency based on the locking relationship between the probe light detuning and the coupling light detuning. This paper provides a deep analysis of the impact of the probe light modulation frequency on the error signal and experimentally validates our theoretical analysis results. Finally, we select a modulation frequency of 10 MHz to lock the coupling light and compare the Allan variance before and after locking. The minimum value of σ ( τ ) was reduced from 4.62×10 −10 before locking to 1.70×10 −10 after locking.
Frequency locking to reference atomic lines using Rydberg electromagnetically induced transparency (EIT) has been recently introduced as an inexpensive and reliable technique for laser frequency stabilization. In this work, we carry out a systematic study of this technique using heterodyne beat spectroscopy. Two different commercial semi-conductor lasers are locked to the same reference frequency using EIT locking, and their relative frequency stability is analyzed and continuously monitored in real time. A substantial improvement in the laser frequency stability is achieved through searching for the optimal proportional–integral settings and EIT probe laser powers. The results show that the cutoff frequency of the beat signal can be lowered to less than 500 kHz. We also compare the frequencies of free running lasers with that of a locked laser and characterize their frequency drifts. This study is important in assessing the use of Rydberg EIT locking in atomic electrometers.
Two-photon frequency stabilization is a critical step for experiments related to the Rydberg-state electromagnetically induced transparency (EIT) effect, as it exploits both the high optical nonlinearity of the EIT effect and the strong electric dipole-dipole interaction among Rydberg-state atoms. Fluctuations in the two-photon frequency cause decoherence in the Rydberg-EIT system, degrading its optical nonlinearity. Here, we propose and experimentally demonstrate a counterintuitive method to enhance the stability of the two-photon frequency of probe and coupling using atomic transitions. This method leverages increased probe power or intensity beyond the power broadening threshold. This improvement in the two-photon frequency stability consequently reduces the decoherence rate of Rydberg-EIT in laser-cooled atoms. By employing the double-resonance scheme with a moderate 480-nm coupling power of 64 mW or a Rabi frequency of $2\ensuremath{\pi}\ifmmode\times\else\texttimes\fi{}4.1$ MHz, the resulting fluctuation of the two-photon frequency lock was 170 kHz at an optimum 780 nm probe Rabi frequency of $2\ensuremath{\pi}\ifmmode\times\else\texttimes\fi{}15.6$ MHz. Concurrently, the observed frequency fluctuation-induced decoherence rate was $2\ensuremath{\pi}\ifmmode\times\else\texttimes\fi{}12$ kHz at a moderate coupling Rabi frequency of $2\ensuremath{\pi}\ifmmode\times\else\texttimes\fi{}4.8$ MHz for the Rydberg-EIT transition with cold atoms. This work represents a pivotal advancement, as it furnishes sufficient stability in the two-photon frequency to meet the demands of most Rydberg-EIT applications.
Abstract We demonstrate a robust method of direct laser frequency locking on the Zeeman sublevel transitions between an intermediate state and Rydberg states, with continuously tunable frequency range from −35 MHz to +35 MHz, which is based on electromagnetically induced transparency (EIT) spectra of nondegenerate Zeeman sublevels in a Rydberg cascade system. With a small axial magnetic field, the EIT spectrum will split into two individual sub-peaks due to the Zeeman Effect of three energy levels, including the 133 Cs 6S 1/2 , 6P 3/2 and n l Rydberg states which form the cascade system. It is shown that the coupling field, corresponding to the transitions between the Zeeman sublevels of the intermediate state and Rydberg state, can be locked arbitrarily on any one of the two EIT sub-peaks. The frequency stability of locked lasers is bounded by 0.81 MHz. The root of Allan variance of the frequency reaches a minimum of 2.06 × 10 − 8 for an averaging time of 512 s.