A modulation depth calibration in orthogonal demodulation for optical fiber interferometric sensor

In orthogonal demodulation, it is important to keep the modulation depth on pi. The real modulation depth always has deviation which affects the sampling off the peak point. A calibration method is proposed. By choosing suitable sampling points and using the orthogonal property of two components, an approximate offset can be calculated by selecting different formula from different case, which is, when the sine of the phase related to measured parameters is small, the even component sampling points are combined to calculate the offset, otherwise, the odd component sampling points are combined. The formula for the calibration is linear relation with the sampling values, and just reuse the existed values. With an iterative calibration process, the modulation depth can be controlled near to pi within a certain error degree. After this process, all the sampling points keep near to the peaks and sampling values are insensitive to small shift of the sampling time.