## 3 Results and Discussion
(**d–f**) are corresponding the AT splitting spectra with the microwave E-field frequency blue detuning 20 MHz from the \(57\mathcal{D}_{5/2}\)–\(58\mathcal{P}_{3/2}\) transition. The black dots represent the experimental results. The experimental data are fitted with the Voigt profiles. is \(-26\) dBm. Figure 3b shows the right transmission signal with the modulation frequency of 12 kHz. The resulting SNR has an improvement of about 4.2 times compared with Figure 3a. Then, the AM is acted on the coupling laser by an optical chopper with 600 Hz modulation, and the optimal SNR is 16.6, which is shown in Figure 3c. Furthermore, the MCM scheme is employed to improve the spectral SNR in Figure 3d, and the spectral SNR is found to be 25.7. Finally, a 5.8 times improvement is obtained with MCM compared with Figure 3a. In order to evaluate the effectiveness of the MCM scheme, a series of spectra measurements are performed. The SNR of the right transmission signals are measured with different microwave powers from \(-34\) dBm to \(-14\) dBm, which can be seen from Figure 4. The experimental data are averaged from three measurements. The error bars are the standard deviations. When the power of microwave E-field power is \(-30\) dBm, the spectral SNR measured by AM scheme (blue dots) shows a factor of 2.4 improvement compared with the modulation-free case in black dots. In addition, the spectral SNR also has an improvement of about 3.2 times by using FM (red dots) compared with a modulation-free signal. A spectrum with higher SNR can be effectively obtained by adopting a MCM scheme (green dots), which has a 4.8 times improvement. Therefore, the MCM scheme can efficiently achieve a high SNR spectral measurement of transmission signals. Figure 3: The right transmission signals of the AT splitting as a function of the coupling laser frequency detuning \(\Delta_{c}\) in Figure 2e. (**a**) no modulation (No mod); (**b**) frequency modulation (FM); (**c**) amplitude modulation (AM); (**d**) multi-carrier modulation (MCM). The black dots are the experimental results and the red lines are the fitting results with Voigt profiles. Figure 5 shows the right transmission signals of the AT splitting with microwave E-field powers decreasing from \(-26\) dBm to \(-34\) dBm under the experimental parameters of Figure 3d. The linewidth and the amplitude of the transmission signals gradually decrease as the microwave E-field power decreasing. The minimum detectable microwave power is \(-34\) dBm by MCM scheme as shown in Figure 5d, corresponding to the AT splitting interval \(\Delta f_{0}=3.7\) MHz (calculated by Equation (1)), which can be found about a factor of 2 smaller compared with on resonance case. As far as we know, the minimum AT splitting measured previously by the frequency detuning method is 6 MHz [20]. We know that the minimum AT splitting of EIT is limited by the linewidth [25], so the narrow linewidth EIT can be considered to measure the weaker microwave E-field. Therefore, a more weaker microwave E-field can be measured with a narrower EIT linewidth. Figure 4: The SNR of right transmission signals of the AT splitting versus the microwave E-field power with different modulation methods. Figure 5: The right transmission signals of the AT splitting with different microwave E-fields. (**a**) \(-26\) dBm; (**b**) \(-30\) dBm; (**c**) \(-32\) dBm; (**d**) \(-34\) dBm. The black dots are the experimental results and the red lines are the fittings with Voigt profiles.