  In recent years, unshielded atomic systems have been attracting researchers'
attention, in which decoherence is one of the major problems, especially for
high precision measurements. The nonlinear Zeeman effect and magnetic field
gradient are the main decoherence sources of atomic electron spin in
Earth-field range. Here, we propose a method to cancel out the two dominant
broadening effects simultaneously by an all-optical dynamic decoupling approach
based on Raman scattering in the 87Rb Zeeman sublevels. By adjusting the
parameters of the Raman lasers, we realize spin control along an arbitrary
direction. We analyze the state evolution of atomic spin under the Raman light
control sequence in detail. The results show that both the nonlinear Zeeman
effect and magnetic field gradient can be significantly suppressed.
