为了实现短双折射光纤环镜(Bi?FLM)应变传感器的在线测量，理论推导得出通过任意连续的2个相邻波谷波长、2个相邻波峰波长及其双折射光纤初始长度、初始双折射率和初始双折射应变系数计算双折射光纤所受应变大小的方法。选取典型通讯波长1 550 nm附近的波长，波长分辨率分别为0.01 nm、0.001 nm、0.000 1 nm时，应用提出的计算方法计算应变和误差。结果表明：波长分辨率越小，测量精度越高。波长分辨率为0.000 1 nm时，最大绝对误差为0.39%，最小绝对误差为0.01%，平均绝对误差为0.15%。进一步选取典型通讯波长1 310 nm附近的3组不同波长组合，计算的应变均与给定应变基本吻合。根据干涉光谱波谷波长、波峰波长的相对位置蕴含着应变信息的特点，与初始相角无关，可以剔除外界干扰，提高测量精度。该方法无需人为判断，有助于实现计算机在线测量；需要的信息量较小，可以实现短Bi?FLM传感器的在线测量。研究结果对Bi?FLM各类传感器实现计算机在线测量，提高测量精度具有指导意义。

In order to realize the on-line measurement of the short birefringence fiber (BF) loop mirror (Bi-FLM) strain sensors, the method of calculating the BF strain by random continuous two-adjacent-wave-valley wavelengths, two-adjacent-wave-peak wavelengths, the initial BF length, the initial BF birefringence and the initial strain dependent birefringence coefficient were deduced theoretically. The wavelengths near 1 550 nm were chosen and used to calculate the strain by the presented theoretical method when the wavelength resolution was 0.01 nm, 0.001 nm and 0.000 1 nm, respectively. The results show that the smaller the wavelength resolution is, the higher the measurement accuracy is. When the wavelength resolution is 0.000 1 nm, the maximum absolute error is 0.39%, the minimum absolute error is 0.01%, and the average absolute error is 0.15%. Three groups of different wavelength combinations near 1 310 nm are also chosen and used to calculate the strain by the theoretical method. The calculated strains are basically consistent with the given strains. According to the characteristics of strain information contained in the relative positions of two-adjacent-wave-valley wavelengths and two-adjacent-wave-peak wavelengths, which are independent of the initial phase angle, the external noises could be eliminated and the measurement accuracy could be improved. This method does not need human judgment and is helpful to realize on-line measurement by computer. In addition, this method requires less information. So it could realize online measurement of the short Bi-FLM sensors. The results could provide guidance for realizing the on-line measurement and improving the measurement accuracy of the Bi-FLM sensors.