Figure 6. Schematic of the two-step filtering system.

The picosecond laser was first filtered by an Andor Shamrock 500i spectrometer system with a 1,200-line-per-mm grating and further narrowed by a fused silica etalon. After two-stage filtering, the linewidth of the pulsed laser was around 1.65 GHz. The third part was used to generate the delay between two pulses in the Ramsey sequence. The beam was first split by a 50:50 beam splitter (BS). One arm of the beam was delayed and combined with the other undelayed pulse by another 50:50 beam splitter. The beam was guided by single mode fiber (SMF).