  In the optical pumping systems based on the pump-probe arrangement, the spin
polarization of the atoms is generally monitored utilizing the probe laser
beam, in which way an extra perturbation must be introduced and thus affects
the normal operation of the sensors. By investigating the absorption rate of
the circularly polarized pump laser, here we demonstrate the feasibility of
extracting the electron-spin polarization from the transmitted pump laser
intensity. We experimentally validate the method in a spin-exchange relaxation
free (SERF) magnetometer and the results are in excellent agreement with the
theory. The scheme operates in a silent mode and features a real-time
observation. We also study the corresponding magnetic field response of the
SERF magnetometer and a term arising from the diffusion effects has been added
to the original model to explain the discrepancy of the response.
