The output waveforms of a bidirectional optical fiber chaotic ring laser based on semiconductor optical amplifier (SOA) present frame structures. The adjacent-frame waveforms have the characteristic of similarity, which declines when the laser suffers an external disturbance. In order to obtain waveforms with high similarity and good consistency for accurate detection of disturbances, experimental investigations on various factors which can affect the adjacent-frame similarity of the laser are carried out, including the performances and drive current of SOAs, symmetry of system structure, output coupling ratios and optical fiber ring lengths. The similarity levels are investigated by the mean values of the adjacent-frame cross-correlation peaks. The consistency degrees are measured by the maximum deviation of the peaks. The results show that the booster optical amplifier (BOA) with the polarization-maintaining property is the most suitable active device. The bigger the drive current, the better the adjacent-frame similarity. By only adjusting polarization controller to make the output chaotic waveform look like sine signal with continuous narrow-band spectrum, the symmetry of system structure, coupling ratios and optical fiber ring lengths will not affect the adjacent-frame similarity. However, the ring lengths affect the system sensitivity. Moreover, the system stability is also investigated. The adjacentframe similarity fluctuates at 10-3 level within about five minutes.