Phase-related noise characteristics of 780 nm band single-frequency lasers used in the cold atomic clock

We propose a method to directly measure phase-related noise characteristics of single-frequency lasers in the 728-980 nm band based on a 120° phase difference interferometer. Differential phase information of the laser under test is demodulated via the interferometer. Other parameters related to the phase noise characteristics such as linewidth at different observation time, phase/frequency noise, power spectrum density of phase/frequency fluctuation, and Allan deviation are further obtained. Frequency noise as low as 1 Hz²/Hz can be measured using our system. Then the phase-related noise characteristics of two commercial lasers frequently used in cold atomic clocks are studied systematically by the method. Furthermore, several influencing factors and their relative evolution laws are also revealed, such as the pump current and frequency-locking control parameters. This would help to optimize the laser performance, select laser sources, and evaluate the system performance for cold atomic physics applications.