Laser frequency stabilization played a crucial role in optically pumped Cs beam frequency standard [
Acta Photonica Sinica, Volume. 51, Issue 6, 0614003(2022)
Laser Frequency Stabilization by Two-peak Fluorescence Spectroscopy Induced by Doppler Effect with Laser at Oblique Incidence
Improving the stability of laser frequency by reducing frequency fluctuation can enhance the performance of the optically pumped Cs beam frequency standard. It has developed various methods to stabilize the laser frequency, such as the Saturated Absorption Spectroscopy (SAS), Polarization Spectroscopy (PS), Dichroic Atomic-Vapor Laser Lock (DAVLL), Frequency Modulation (FM) spectroscopy, Doppler-free bichromatic spectroscopy. These methods above are mostly based on the thermal vapor cell in a system which is kind of complicated and facing the problems of the temperature of vapor, the light power and the magnetic field, which caused the unstable frequency of laser. For compact optically pumped Cs beam frequency standard, reliability is an important thing that need to be considered, which is probably easier to realize through a simple structure. Therefore, the method of laser frequency stabilization by laser-induced (one-peak) fluorescence spectroscopy is developed. This method has enhanced the system reliability, but introduce a problem of linewidth broadening, which caused by divergent atomic beam.In this paper, we propose a method to stabilize the 852 nm laser frequency by fluorescence spectroscopy with two peaks, which have a simple and reliable structure, and can reduce the Doppler effect caused by divergent atomic beam. The fluorescence spectroscopy with two peaks is induced by two laser beams and atomic beam at oblique incidence. The two laser beams propagate contrarily and coincide exactly with each other. After fluorescence collection and photoelectric conversion, the two-peak fluorescence spectroscopy is formed. By changing the oblique angle θ,
0 Introduction
Laser frequency stabilization played a crucial role in optically pumped Cs beam frequency standard [
For compact optically pumped Cs beam frequency standard,reliability is an important thing that need to be considered,which is probably easier to realize through a simple structure. And the method of laser frequency stabilization by laser-induced(one-peak)fluorescence spectroscopy is developed[
In this paper,we present a method which has a very simple and reliable structure to stabilize the laser frequency by Two-Peak Fluorescence Spectroscopy(TPFS). The results indicate that the performance of this method is better than the method of laser frequency stabilization by One-Peak Fluorescence Spectroscopy(OPFS). With this method,we get the narrower half-width of the fluorescence signal,and the better laser frequency stability,which is suitable for a compact optically pumped Cs beam frequency standard. In the next,we will show the details of this method
1 Experimental setup
The experimental setup for stabilizing the laser frequency by TPFS is illustrated in
Figure 1.Schematic of experimental arrangement
The Cs beam tube is arranged in the experiment with the atomic beam is un-perpendicular to the laser beam,as shown in
Relevant hyperfine energy levels of Cs atoms associated with the experiment are shown in
Figure 2.Relevant energy levels of Cs atoms
2 Experimental results and discussion
The TPFS of cycling transition is obtained by oblique-incidence laser at different angles,as shown in
Figure 3.Measured TPFSs of Cs-D2 line induced by oblique-incidence laser at different angles
The half-width(as marked by a pair of arrows in
Figure 4.Frequency fluctuation of error signal without and with the locking circuit operating
Figure 5.The Allan deviation of the stabilized 852 nm laser by TPFS and OPFS separately
Figure 6.The OPFS of Cs-D2 line. The under line is the calibration reference
In this experimental scheme,the locking point will be located at the center of the atomic transition when the laser power of two beams is the same and the two beams coincide,which is easy to realize. The laser frequency is locked in the 6S1/2,F=4↔6P3/2,F'=5 transition of the Cs-D2 line. Frequency fluctuation can be estimated by the voltage of the error signal.
The laser frequency stability is obtained by the self-evaluation method,and the result of Allan deviation is shown in
In experiment,the temperature fluctuation of the oven is controlled within ±0.1 ℃,so the relative fluctuation of beam flux is about 0.6%. The oven and Cs atomic beam are shelled by three magnetic shielding material layers,with a residual magnetic field of less than 1 mG. The measured vacuum degree is better than 10-5 Pa,which will not cause a collision between atoms. Besides,the center of TPFS is not affected by the intensity of laser or beam flux. The influence of the above factors on frequency stability has been reduced to negligible level. So,the flicker floor is probably caused by the light beam angle shift,which will limit the medium- and long-term frequency stability [
For OPFS,the measured linewidth is about 42.5 MHz,as shown in
Replacing TPFS by OPFS,keeping the amplitude of dispersive signals roughly same and the other parameters invariable,we obtain the error signal after the locking circuit operating with the frequency fluctuation of about 100 kHz peak-peak. In
By comparing these two methods,its results have been shown in
For the generation of TPFS,further work can be done to simplify the arrangement of the experiment. As shown in
Figure 7.The simplified experimental arrangement for generating the TPFS
3 Conclusion
In conclusion,we have presented a very simple scheme to stabilize the laser frequency by TPFS,and the performance of this method is better than the performance of laser stabilization by OPFS. The linewidth of TPFS is tunable,and we get 16.3 MHz,18.7 MHz,21.7 MHz,24.4 MHz and 26.9 MHz at the different oblique incidence angles separately. The frequency fluctuation of stabilized laser is about 70 kHz peak-peak by TPFS with a linewidth of about 24.4 MHz,and the frequency instability is 2.7×10-11 at 1 s,and less than 2.0×10-12 at 500 s,which are fulfill the requirements of the compact optically pumped Cs beam frequency standard
[24] Xingwen ZHAO, Haijun CHEN, Lin YANG et al. Stabilizing the optical frequency by laser-induced fluorescence in optically pumped Cs beam frequency standard. Journal of Astronautic Metrology and Measurement, 40, 17-22(2020).
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Zhongzheng LIU, HaiJun CHEN, Jun CAI, Jinjun FENG. Laser Frequency Stabilization by Two-peak Fluorescence Spectroscopy Induced by Doppler Effect with Laser at Oblique Incidence[J]. Acta Photonica Sinica, 2022, 51(6): 0614003
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Received: Apr. 2, 2022
Accepted: May. 16, 2022
Published Online: Sep. 23, 2022
The Author Email: LIU Zhongzheng (bdwx1234@sina.com)