Since 1990s, fiber Bragg grating (FBG) sensors have well-known attributes such as electromagnetic immunity, high sensitivity, and compactness. However, expensive and unreliable wavelength interrogations nowadays still remain a barrier for their wide deployments in various sensing applications. Intensity-based interrogations, typically using linear edge filters to convert wavelength shift to simpler intensity variation, posses some advantages including much lower cost, higher speed, easier measurements, etc. However, such a concept is usually involved by a small operation rang, which directly restricts its employment in distributed measurements.

To address this problem, about one year ago, the research group of associate professor Xia Li at Wuhan National Laboratory for Optoelectronics   proposed a new wavelength interrogation technique based on crossed optical Gaussian filters, which has been published in Optics Letters in April 2015 (Vol. 40, Iss. 8, pp. 1760-1763).

With the deepening of the research, that method has been found the disadvantages of the two aspects: one is the need for an additional two filters with Gaussian filtering features, which adds the complexity of the system. The second is that the power spectral density from the used wide spectrum light source is small, which makes the limitation of sensing demodulated distance and scope.

Therefore, the research team in the near future further designs a demodulation method based on dual wavelength narrow linewidth laser incident on the weak fiber Bragg grating sensor array (Figure 1). At the input end of the demodulation system, two laser beam with a certain center wavelength interval is used. By using the weak reflection fiber grating's own Gaussian spectrum characteristic and the power difference detection, we can get a linear Bragg wavelength shift to power difference (Figure 2). And the slope of the linear relationship can be adjusted simply by controlling the wavelength interval of the dual wavelength laser. This method is not like the Gaussian matched filtering method, which has the requirements for the filter characteristics, so the whole demodulation system is relatively simple. According to the current narrow linewidth laser, the laser output power can reach more than 10dBm, thus, the method in the absence of relay amplification conditions, can support long distance sensing demodulation of more than 50km, and can achieve hundreds of weak FBGs demodulation. It can be predicted that this method will have a very good application prospect in all fields of FBG based quasi distributed sensing network.

This study, titled “Interrogation of weak Bragg grating sensors based on dual-wavelength differential detection”, was published in Optics Letters in Nov. 9, 2015 (Vol. 41, Iss. 22, pp. 5254-5257).

The work is supported by the National Science Foundation under Grant 61675078.

Fig.1. Schematic representation of dual-wavelength differential method to interrogate an FBG array 

Fig. 2. Experimental results  (a) Spectra of weak FBG and the dual-wavelength light,  (b) Initial profile in the time domain, (c) The enlarged response of the 6th grating under different strain; (d) The reflected power at two wavelengths; (e) The subtract results using the data in (d).