The theoretical analysis and experimental verification of the principle of re-entrant IOG were carried out here. The experimental setup for analog simulation was established with Sagnac Sensing Ring (SSR) length of 20m. Based on the analysis of open loop output signal of the system, the signal of re-entrant part was verified. In order to get the high order re-entrant signal, a new modulation method using square waveform which duty ratio is not equal to 0.5 was suggested, in which the corressponding order of re-entrant signal was biased at maximum sensitivity. It provides a way to solve the problems in signal readout from ARE-IOG and PRE-IOG.