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For SERF magnetometer, slow diffusion is one of the key operations required to achieve multi-channel and multi-axis measurement. Small diffusion distance over a polarization lifetime allows for the measurement of each separate region of a singular cell to be considered independent. Especially for multi-channel measurement, the spatial resolution is mainly limited by the diffusion distance, which is positively correlated to the diffusion constant. Therefore, the precise determination of the diffusion constant is significant for further optimization of the magnetometer’s spatial distribution and beneficial for improving the performance of spatial resolution. Herein, we proposed an in-situ measurement method of diffusion constant under large-scale cell condition in dual-beam magnetometer. With consideration of the diffusion effect, we established the distribution of polarization in the plane vertical to the pump beam. Then we analyzed the dependence of the optical rotation angle on the diffusion constant and the pump beam facular radius from both experiment and numerical simulation perspectives. Based on this, we realized the in-situ measurement of the diffusion constant through the detection of the optical rotation angle under different pump beam facular radii. In our experiment, we adopted a pump beam with great optical power to sufficiently polarize the alkali atoms and derive the diffusion constant at 1.6 cm2/s through formula fitting. This method can achieve an in-situ measurement of the diffusion constant and be generally applied to different cell conditions or measurement modes. Furthermore, it can provide guidance on further improvement in the spatial resolution of SERF magnetometer in multi-channel measurement, as well as the distribution of pump or probe beam in multi-axis measurement within a singular cell.
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