## 5 Experimental Results and Discussion  
In Figure 8, under the conditions of P852 nm = 1.0 \(\mu\)W and P698 nm = 1.0 mW, the frequency calibration was performed by using an EOPM with a 30 MHz modulation sideband for 698 nm coupling beam. The EOPM was driven by a frequency synthesizer (Agilent Technologies E8257C, CA, USA) with a stability of \(1\times 10^{-10}\) s\({}^{-1}\). The transmission peak of 5\(p^{6}\)7\(d\)\({}^{2}\)\(D_{5/2}\) (\(F^{\prime\prime}\) = 6) served as the reference position for detecting zero detuning of the frequency. In this work, we used the EIT-assisted DROP spectra with high SNR and narrow linewidth, thereby facilitating splitting of the Cs 5\(p^{6}\)7\(d\)\({}^{2}\)\(D_{5/2}\) (\(F^{\prime\prime}\) = 6, 5, 4) three-peak intervals. These advantages were valuable for measuring the hyperfine interval and the hyperfine interaction constants. In reference [27, 29], the Cs 5\(p^{6}\)6\(s\)\({}^{2}\)5\({}_{1/2}\)-5\(p^{6}\)7\(d\)\({}^{2}\)\(D_{3/2}\), 5\({}_{7/2}\) electric quadrupole transitions were used to couple the ground state and the excited state, representing a monochromatic two-photon transition. The probability was low and the power of the laser beam was much higher. The hyperfine splitting intervals of the corresponding states were measured, and the experimental results showed that the SNRs of the obtained spectra were high due to the large splitting intervals of the Cs 5\(p^{6}\)7\(d\)\({}^{2}\)\(D_{5/2}\) states. However, for the Cs 5\(p^{6}\)7\(d\)\({}^{2}\)\(D_{5/2}\) state, the resolution was so poor that it was difficult to separate the peaks for the (\(F^{\prime\prime}\) = 6), (\(F^{\prime\prime}\) = 5), and (\(F^{\prime\prime}\) = 4) hyperfine folds in Cs 5\(p^{6}\)7\(d\)\({}^{2}\)\(D_{5/2}\) state. The frequency interval between the carrier frequency and the \(-1\)-order sideband served as the frequency calibration to measure the Cs 5\(p^{6}\)7\(d\)\({}^{2}\)\(D_{5/2}\) (\(F^{\prime\prime}\) = 6, 5, 4) frequency intervals. We used the interval between the \(-1\)-order sideband peak and Cs 5\(p^{6}\)7\(d\)\({}^{2}\)\(D_{5/2}\) (\(F^{\prime\prime}\) = 6) peak to calibrate the horizontal axis frequency in Figure 8, thus allowing the frequency intervals between states 5\(p^{6}\)7\(d\)\({}^{2}\)\(D_{5/2}\) (\(F^{\prime\prime}\) = 6, 5, 4) to be calculated. Due to the nonlinear frequency scanning of grating external cavity, a larger scanning range resulted in a greater influence of the nonlinear effect. The frequency of the selected radio-frequency signal was 30 MHz, which was close to the measured interval value of the 5\(p^{6}\)7\(d\)\({}^{2}\)\(D_{5/2}\) state, in order to minimize the influence of the nonlinear effect for scanning the laser frequency. The hyperfine splitting intervals of the 5\(p^{6}\)7\(d\)\({}^{2}\)\(D_{5/2}\) (\(F^{\prime\prime}\) = 6, 5, 4) state were measured and calibrated several times. Therefore, we used a convolution of a Lorentzian profile to fit the width of these peaks and to locate their central position. Each simulated peak had three fitting parameters corresponding to the amplitude of the signal, the central position of each signal, and the Lorentzian width. The linewidth of these transitions was about \(\sim\)3 MHz, which was a combination of the  
Figure 8: Calibration of the frequency interval with a 30 MHz radio-frequency phase modulation applied to the coupling laser beam via the waveguide-type electro-optic phase modulator (EOPM). The frequency interval between the carrier (reasonably indicated zero detuning) and the \(-1\)-order sideband (or the \(+1\)-order sideband) should be 30 MHz. The blue line displays multi-peak Lorentz fitting, while the red curve displays the experimental spectra. The three peaks at the left side belong to the \(-1\)-order modulation side band, corresponding to the (\(F^{\prime\prime}\) = 6, 5, and 4) hyperfine folds. The three peaks at the right side belong to the \(+1\)-order modulation side band, corresponding to the (\(F^{\prime\prime}\) = 6, 5, and 4) hyperfine folds.