## 4 Optimization of Experimental Parameters
### Effect of the Laser Beam's Intensity  
As shown in Figure 6, there were three enhanced transmission signals in the spectra, and the three peaks from left to right corresponded to the Cs atom 5\(p^{6}6p\)\({}^{2}\)\(P_{3/2}\) (\(F^{\prime}\) = 5)-5\(p^{6}7d\)\({}^{2}\)\(D_{5/2}\) (\(F^{\prime\prime}\) = 6), (\(F^{\prime\prime}\) = 5), and (\(F^{\prime\prime}\) = 4) hyperfine transitions, respectively. The evolution of signals with different probe laser power and coupling laser power was studied. In Figure 6a-e, when the coupling power was set to 1.0 mW, upon increasing the power of the probe beam, the amplitude of the EIT-assisted DROP signals increased. When the power of the probe beam was set to \(\sim\)1.0 \(\mu\)W, upon increasing the power of the coupling beam, the amplitude of the EIT-assisted DROP signals also increased, as shown in Figure 6f-j, whereas the SNR of the spectral line obviously decreased when the power of the coupling laser was too weak and was gradually submerged in the noise. Upon increasing the power of the coupling or probe beam, the EIT effect strengthened. Since the DROP effect mainly depended on the power of the 852 nm beam, the optical double-resonance effect widened the spectral linewidth and hindered the separation of hyperfine-level intervals. In order to clearly distinguish the intervals among the three peaks, appropriate powers were chosen for measurement in the experiment. Figure 5: Spectral signals under different transmission modes and scanning modes: (**a**,**b**) the CP configuration; (**c**,**d**) the CTP configuration. (**a**,**c**) The 698 nm laser was adjusted to the 5\(p^{6}6p\)\({}^{2}\)\(P_{3/2}\) (\(F^{\prime}\) = 5)-5\(p^{6}7d\)\({}^{2}\)\(D_{5/2}\) (\(F^{\prime\prime}\) = 6) transition, while the 852 nm laser was scanned to obtain the two-photon transition spectra with a Doppler background. (**b**,**d**) The 852 nm laser was locked to the 5\(p^{6}6s\)\({}^{2}\)\(S_{1/2}\) (\(F\) = 4)-5\(p^{6}6p\)\({}^{2}\)\(P_{3/2}\) (\(F^{\prime}\) = 5) transition, while the 698 nm laser was scanned over the Cs 5\(p^{6}6p\)\({}^{2}\)\(P_{3/2}\) (\(F^{\prime}\) = 5)-5\(p^{6}7d\)\({}^{2}\)\(D_{5/2}\) (\(F^{\prime\prime}\) = 6, 5, 4) transitions to obtain the two-photon transition spectra without a Doppler background. We studied the evolution of signal amplitude and SNR of the EIT-assisted DROP spectra versus the laser power. It can be seen from Figure 7 that the power we chose allowed obtaining the spectra with relatively high SNR and a narrow linewidth, thereby facilitating splitting of the 5\(p^{6}\)7\(d\)\({}^{2}D_{5/2}\) (\(F^{\prime\prime}=6\), 5, 4) three-peak intervals. We can see that the amplitude linearly increases with the power. As the ratio (f\({}_{5^{\prime}-6^{\prime}}\)/FWHM\({}_{6^{\prime}}\)) can describe the relation of spectral linewidth with power, this also means the overlap of the peaks. In fact, when the ratio is 1, it means that the two peaks are indistinguishable. In summary, we chose the EIT-assisted DROP spectra with an appropriate power range. When the probe power was increased from ~0.1 uW to ~5.0 uW and the coupling power was increased from ~0.5 mW to ~1.0 mW, the three peaks did not overlap with each other. These advantages were particularly valuable for measuring the hyperfine splitting intervals and the hyperfine interaction constants. Figure 6: Evolution of the electromagnetically induced transparency (EIT) signals in the CTP configuration with (**a**–**e**) increasing probe laser power and (**f**-**j**) increasing coupling laser power. (**a**–**e**) The 698 nm laser power was ~1.0 mW, and the evolution of the signals with increasing probe laser power is shown. (**f**-**j**) The 852 nm laser power was ~1.0 uW, and the evolution of the signals with increasing coupling laser power is shown. Figure 7: The signal-to-noise ratio (SNR), amplitude and ratio (f\({}_{5^{\prime}-6^{\prime}}\)/FWHM\({}_{6^{\prime}}\)) of the cesium 5\(p^{6}\)7\(d\)\({}^{2}D_{5/2}\) (\(F^{\prime\prime}=6\)) spectroscopic signals versus.