(a) Single-sideband phase noise of the 9.2 GHz microwave synthesizer (black solid line) measured using a signal source analyzer (E5052B and E5053A, Keysight) and fitted curves in two frequency ranges (pink dashed line: 1 kHz–100 kHz; blue solid line: 100 kHz–1 MHz). (b) Measured laser relative intensity noise. Blue line: measured noise with the laser frequency locked to the atomic resonance line, which is the sum of the AM–AM noise and the FM–AM noise. Red line: measured noise when the laser frequency is detuned away from the atomic resonance line, which is mainly AM–AM noise. Black line: measurement floor of the PD and the fast Fourier transform analyzer (SRS785, SRS) with no incident light.