Fiber grating sensing demodulation made new breakthroughs

Since the 90s of the last century, fiber grating sensing technology based on Bragg grating has drawn extensive attention of scientists and carried out a great deal of research. Although large-scale, fast and cheap grating production techniques and their processes are mature, the more expensive optical demodulation devices, however, still limit their wide application in a series of important fields such as industrial, chemical and building structure inspection. In addition, compared with the conventional optical domain demodulation method, the optical filter is used to convert the drift of the grating wavelength into the change of the power. The demodulation method has the advantages of low cost, simplicity and fast response.

Previously, the research group led by Associate Professor Xia Li of the optical communications and optical network used a cross-gaussian filtering method to demodulate FBG. The research results have been published in the journal Optics Letters (Vol. 40, Iss. 8, pp. 1760- 1763). As the research progresses, it is found that there are two disadvantages of this method: one is the need of adding two extra filters with Gaussian filter characteristics, which adds to the complexity of the system; the other is the use of a wide spectrum light source with a low power spectral density , Making the sensing demodulation distance and range limited.

Therefore, in the recent work, the research group further designed a weak FBG sensing array demodulation method based on narrow-line laser with double-wavelength and narrow linewidth (Figure 1). At the input of the demodulation system, two lasers with a certain central wavelength interval are used for the incident. Utilizing the Gaussian spectral feature of the weakly reflecting fiber grating and detecting the power difference, it is just a linear conversion from Bragg wavelength to power difference (Figure 2). And the slope of their linear relationship can be easily adjusted by controlling the wavelength spacing of dual-wavelength lasers. This method does not like the Gaussian Matching filter as the filtering characteristics of the filter requirements, so the entire demodulation system is relatively simple. According to the current narrow-line laser, the output power of the light source can be easily reached to 10 dBm or more. The method can support sensing demodulation distances up to 50 km without relay amplification and can reach up to one fiber Hundreds of weak anti-grating simultaneous demodulation effect. It can be predicted that this method will have very good application prospect in all fields of Bragg grating quasi-distributed sensing network.

The research result "Interrogation of weak Bragg grating sensors based on dual-wavelength differential detection" is published in Optics Letters (Vol. 41, Iss. 22, pp 5254-5257).

This work has been funded by the National Natural Science Foundation of China (61675078).

(c): magnification response spectrum of the sixth grating under different stress conditions; (d): reflection intensity distribution at two wavelengths; (e)

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