(a) Schematic of the experimental setup. The 510-nm coupling laser, counter-propagates with the probe laser, 852 nm, through a cesium vapor cell. The probe beam, modulated using a function generator (FG) with the modulation frequency of 1 kHz, is detected using a photodiode (PD) resulting in a modulated EIT signal. The error signal, demodulated EIT signal produced using a lock-in amplifier, is fed back to the controller of the coupling laser through PID and used to lock the coupling laser frequency to Rydberg transition. (b) The energy levels, the ground state 6S_1/2 (F = 4) (|g⟩), the excited state 6P_3/2 (F′ = 5) (|e⟩), and the Rydberg state nD_{5/2,3 /2} (|r⟩) consist of a cascade three-level system. By probing the ground state transition |g⟩→ |e⟩, the excited transition |e⟩→ |r⟩ is detected using EIT.