The quality of the cold atoms sample plays an important role to improve the performance of cesium atomic fountain clocks. The preparation of cold atoms sample has a strict requirement for cooling lasers characteristics such as power stability, polarization, collimation, uniformity and verticality. This work implements a design of an expander system of high stable laser beam for the cold atom preparation. The cooling laser characteristics can be adjusted by the design, and the use of non-magnetic materials to avoid the magnetic field on the impact of cold atom. The cold atom sample with the number of 2×10<sup>8</sup> and temperature of 5μk was obtained by expander system. The result meets the requirements of the preparation of cold atoms for cesium atomic fountain clocks.
We demonstrate frequency stabilization of DFB laser via modulation transfer spectroscopy, which avoids the interference of Doppler absorption spectrum. By modulating the frequency of laser beam(pump laser beam), the other laser beam(probe laser beam) overlap pump laser beam is modulated as the same frequency by nonlinear interaction. By demodulating the signal from detection beam, an error signal is generated through the low pass filter. The frequency is stabilized at the 0 point position of the MTS frequency discrimination curve by PID loop which shows frequency fluctuation is less than 149 kHz, and the relative frequency stability is improved by nearly two orders of magnitude.