Abstract:The high-precision time-frequency standard is one of the core supporting technologies for space missions such as satellite navigation systems, communication systems, deep space exploration, and space-based fundamental physics research. The two-photon transition optical frequency standard based on hot rubidium(Rb) atoms can achieve high frequency stability by locking the laser frequency to the Doppler-free narrow-linewidth atomic transition line without complex atomic cooling systems, ultra-high vacuum environments, and ultra-stable cavity pre-stabilization, so as to meet the long-term high-precision timing requirements of spaceborne and other complex working environments. This scheme has therefore attracted extensive attention and research. In this work, we develop an experimental setup of an optical frequency standard based on the two-photon transition of 87Rb atoms. A 1 556 nm laser is used as the seed source, and the frequency-doubled 778 nm clock laser excites the 5S1/2→5D5/2 two-photon transition of 87Rb atoms in a vapor cell. The 420 nm fluorescence emitted by the atoms is observed and efficiently collected using a photomultiplier tube. By increasing the laser spot diameter to suppress transit-time broadening, the transition spectral linewidth is narrowed from 1 800 kHz to 600 kHz. With a combination of a focusing lens, a light shield, and a band-pass filter, the signal-to-noise ratio of the spectral line is improved by a factor of 14 compared with the direct fluorescence collection method. After the optical frequency standard is locked, the frequency stability in the optical domain is transferred to the radio-frequency domain via an optical frequency comb and compared with an active hydrogen maser, yielding a frequency stability of 2.2×10-13/τ(1~100 s), which reaches 9.1×10-15 after an integration time of 4 000 s. The light shift coefficient of the setup is evaluated to be -6.7×10-13/mW, and the collision shift coefficient is -5.6×10-13/℃. Analysis indicates that the light shift is the dominant factor limiting the long-term frequency stability of the present setup, which can be further suppressed by reducing the drift of the power feedback loop and designing an integrated physical package. This study lays an important technical foundation for the future realization of high-performance, integrated optical frequency standards.