Since 1990s, fiber Bragg grating (FBG) sensors have well-knownattributes 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.
Toaddressthis problem, about one year ago, the researchgroupof associate professorXia Li atWuhan National Laboratory for Optoelectronics proposed a newwavelengthinterrogation technique based oncrossed opticalGaussian filters, which has been publishedinOptics Letters inApril2015 (Vol.40,Iss.8, pp. 1760-1763).
With the deepening of the research, thatmethod hasbeen foundthe disadvantages of the two aspects: one is the need for an additional twofilters with Gaussian filtering features,whichaddsthe complexity of the system. The second isthat the power spectral densityfrom the used wide spectrumlightsourceis small,whichmakes thelimitation of sensing demodulateddistanceandscope.
Therefore, the research team in the near future further designsa demodulation methodbased ondual 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 Gaussianspectrum characteristic and the power difference detection, we can get a linearBraggwavelengthshiftto 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 notlike theGaussianmatched filtering method, which has the requirementsfor the filter characteristics, so the whole demodulation system is relatively simple. According to thecurrentnarrow linewidth laser,thelaser 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 weakFBGsdemodulation. It can be predicted that this method will have a very good application prospect in all fields ofFBG basedquasi distributed sensing network.
Thisstudy,titled“Interrogation of weak Bragg grating sensorsbased on dual-wavelength differential detection”, waspublished inOptics Letters inNov. 9,2015 (Vol.41,Iss.22, pp.5254-5257).
Thework is supported by the NationalScience Foundationunder Grant61675078.
Fig.1.Schematic representation ofdual-wavelength differential methodto interrogate anFBGarray
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 6thgrating under different strain; (d) The reflected power at two wavelengths; (e) The subtract results using the data in (d).
