Rydberg atom system can strongly respond to weak microwave signals on the electromagnetically induced transparency (EIT) effect and Aulter-Townes (AT) effect. Therefore, people want to utilize this system to detect and demodulate microwaves instead of the traditional mode. At present, there are two methods to demodulate amplitude modulation (AM) microwave signals using the Rydberg atom system, including indirect demodulation and direct demodulation. In the indirect method, the first step is to scan the probe or coupling laser frequency near the zero-detune point, and the second step is to measure the splitting peak-to-peak frequency separation in the probe transmission spectrum. The third step is to calculate the microwave electric field (E-field) strength because the above frequency separation is proportion to the microwave E-field strength.
We build a simplified Rydberg atom system model (Fig. 1) and numerically simulate the probe laser transmissivity in the Rydberg atom system when 133Cs (energy levels of 6S1/2, 6P3/2, 47D5/2, and 48P3/2) is chosen as Rydberg atom. Our simulation assumes the coupling laser Rabi frequencies separately are 2π×2.7 MHz, 2π×3.2 MHz, and 2π×3.7 MHz. Additionally, our simulation is kept under the frequency-zero-detune, which means probe and coupling laser frequencies are both locked to the energy transition frequency of the Rydberg atom. In these conditions, we conduct the following research.
By mathematical analysis we obtain the optimal linear operation point of the Rydberg atom system from the second derivative of zero (Fig. 3). When the system is operating at that point, the nonlinear distortion of AM microwave demodulation is minimum.
We study the relationship between the nonlinear distortion and the operation point in the Rydberg atom system demodulating the AM microwave signals by the direct method. First, we analyze the demodulation model of the Rydberg atom system in the frequency-zero-detune condition. Second, we calculate the first and second derivatives of the probe laser transmissivity for the microwave Rabi frequency. Utilizing the second derivatives of zero, we find the optimal linear operation point of the Rydberg atom cell in which the nonlinear distortion is the minimum in demodulating AM microwave. Third, the THD is adopted to explore the relationship between the operation point of the Rydberg atom cell, the baseband signal amplitude, and the nonlinear distortion. The simulation shows that the THD of the demodulation system with the Rydberg atom 133Cs (energy levels of 6S1/2, 6P3/2, 47D5/2, and 48P3/2) can reach -95.4984 dB, when the Rydberg atom cell is near the optimum operation point, at 2π×2.7 MHz (coupling laser Rabi frequency) and 1 mV/m (baseband signal electrical field amplitude).