We propose and experimentally demonstrate a modulation technique for closed-loop interferometric fiber optic gyroscopes that improves its scale factor control and allows for the construction of gyroscopes with optimized angle random walk. The proposed two-level and two-period modulation is composed of two intercalated periods of the square wave modulation, one with amplitude of ϕ rad and the other with amplitude of 2π-ϕ rad. As in the two-level modulation, the angular velocity is obtained from the difference of consecutive output levels. Modulation depth error is obtained from the difference of the mean levels of the output from two consecutive periods. The proposed technique eliminates the limitation of square wave modulation: inefficient scale factor control at low angular velocities. Additionally, it requires a slower analog-to-digital converter than four-level modulation.