Abstract: Optical frequency standards stabilized by sub-Doppler laser spectroscopy have achieved short-term frequency stability better than 10^-13 level in different atomic and molecular ensembles.However,further improvement in the long-term frequency stability of existing thermal-atom optical frequency standards is challenging due to the influence of collision and Doppler frequency shifts.To address this issue,a cold-atom optical frequency standard based on long-strip diffuse laser cooling is proposed.It innovatively combines cold atoms with the frequency modulation spectroscopy (FMS) stabilization technique,and uses diffuse laser cooling to prepare cold atomic cloud up to 50 cm in length,which significantly increases the number of effective atoms.By probing the cold-atom FMS and locking the output frequency of a 780 nm external cavity diode laser to the ⁸⁷Rb 5²S_1/2（F=2）→5²P_3/2（F=3） cycling transition,the cold-atom optical frequency standard ultimately achieves an in-loop short-term frequency stability of 5×10^-15/τ.In the future,this cold-atom optical frequency standard is expected to be developed into a space cold-atom clock or a wavelength standard,promoting continuous progress in the field of precision measurement.

Key words: Optical frequency standard, Diffuse reflection, Frequency Modulation Spectroscopy(FMS)