Objective  Optical fiber signal demodulation is the basis of optical fiber sensing applications, and the systems based on FBG sensing are sometimes limited by the demodulation speed when measuring the high-speed dynamically changing signal. Nowadays, the high speed demodulation method of fiber Bragg grating has become one of the main research directions of fiber demodulation. The existing FBG high-speed demodulation methods have their own advantages and disadvantages. Among them, the scanning spectral analysis system has good versatility, low cost, and more balanced performance in speed and accuracy. Therefore, improving the demodulation speed of the scanning laser fiber Bragg grating demodulation system has become the research goal of this paper.

  Methods  A variable step scanning strategy is designed to solve the problems such as many tuning times and long scanning time in the practical application of the laser based fiber Bragg grating wavelength demodulation instrument. The crest of the spectrum can be extracted by using the variable step scanning strategy with less wavelength tuning times. The variable step scanning strategy first uses OTSU algorithm to calculate the threshold and the coarse scanning step size in the initialization stage. In the demodulation stage, the global coarse scan is performed to extract the approximate range of the wave crest, and then the local point-by-point scan is performed in the sampling points exceeding the threshold. Only the spectral sampling data of point-by-point scanning in the local range needs to be uploaded to the supreme computer for peak searching, the amount of data required for peak searching is reduced.

  Results and Discussions  A FBG wavelength demodulation system based on modulated grating Y-branch (MG-Y) laser was built (Fig.3), and demodulation test was carried out after calibration of the laser. Gaussian fitting demodulation was carried out on the host computer software for the transmitted local spectral data, and the signal-to-noise ratio simulation experiment was conducted on the variable step scanning strategy. The simulation proved that the variable step scanning strategy was suitable for the situation where the signal-to-noise ratio was higher than 9.23 dB (Fig.7). The number of the sampling points and time of laser scanning in a single round demodulation process with different fixed scanning steps and variable scanning strategies are analyzed. Experiments show that the demodulation accuracy of sample data obtained with variable scanning strategies is similar to that obtained with the smallest fixed scanning steps when there is only a single grating in the channel. In contrast, the number of points to be scanned using the variable step scanning strategy is reduced by at least 90% and the scanning time is reduced by at least 90% (Tab.1), and the demodulation speed is increased by 10 times. Although the sampling number and time of laser scanning can be reduced by using a large fixed scanning step, the demodulation accuracy is greatly reduced and has no practical application value. In summary, using variable step size scanning strategy can not only reduce the scanning time, but also have little effect on the accuracy of subsequent demodulation.

  Conclusions  Through theoretical and experimental research, a variable step scanning strategy based on scanning laser type fiber grating wavelength demodulation instrument is proposed, and its high speed performance and anti-interference performance are tested. Compared with the case of using the minimum fixed step forward scanning, this strategy reduces the tuning times of single round demodulation and the amount of data collected, improves the speed of wavelength demodulation, and has similar demodulation accuracy as the case of using the minimum fixed step forward scanning. When there is only a single grating in the channel and the processing speed of the back-end demodulation algorithm is not restricted, the demodulation speed can be increased to 10 times by using the variable step scanning strategy, which can reliably meet the demand of high-speed demodulation based on the scanning laser demodulation system, and has a good application prospect.