(a) Operating time sequence of the Ramsey-CPT atomic clock. Laser: TTL trigger of the RF switch to turn the laser pulses on (high-voltage level) and off (low-voltage level) via AOM3. t_p and T are the periods when the laser pulses are turned on and off, respectively. Detection: sampling trigger of the Ramsey signal detected using PD3, which lasts for a period t m = 16 μ s . τ d is a 500 ns delay used to ensure that the laser pulse is switched on. SWPM: the 9.2 GHz microwave signal is phase-modulated by a square wave via the built-in phase modulation function where the neighboring laser pulses alternate between −90° (red) and +90° (blue). (b) Central Ramsey fringes with square wave phase modulation at phases of −90° (solid line) and +90° (dashed line) used to obtain the left and right sides at half-linewidth points L and R, respectively.