Interrogation of weak Bragg grating sensors based on dual-wavelength differential detection

Abstract

It is shown that for weak Bragg gratings the logarithmic ratio of reflected intensities at any two wavelengths within the spectrum follows a linear relationship with the Bragg wavelength shift, with a slope proportional to their wavelength spacing. This finding is exploited to develop a flexible, efficient, and cheap interrogation solution of weak fiber Bragg grating (FBGs), especially ultra-short FBGs, in distributed sensing based on dual-wavelength differential detection. The concept is experimentally studied in both single and distributed sensing systems with ultra-short FBG sensors. The work may form the basis of new and promising FBG interrogation techniques based on detecting discrete rather than continuous spectra.

Flow-through micro-capillary refractive index sensor based on T/R spectral shift monitoring

“…As the wavelength positions of the reflectivity minima depend on the refractive index of the fluid, the refractive index change with respect to a reference fluid was recovered by monitoring the shift of the reflected power spectrum. This method of RI detection requires a thorough spectral analyses or, eventually, intensity monitoring but at least at two different wavelengths [13, [24][25][26].…”

Dual Wavelength Differential Detection of Fiber Bragg Grating Sensors with a Pulsed DFB Laser

“…Recently, another technique to probe the state of an FBG sensor has been proposed and demonstrated [ 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 ]. It relies on measuring the reflectivity of the grating at two, or sometimes more, fixed wavelengths, all located within the spectrum of the FBG.…”

“…The computed difference or ratio of the two or more reflectivity values can then be uniquely related to the location of the center of the spectrum. This technique has been dubbed dual-wavelength differential detection (DWDD) [ 13 ] and we shall use the same name and acronym here. Because it uses fixed wavelengths, DWDD lends itself more naturally to time-division multiplexing (TDM), where multiple identical sensors are interrogated by short optical pulses, with optical delays between the arrival times of each reflected pulse.…”

“…The two wavelengths also had to be separated by a monochromator before detection. Cheng and Xia [ 13 ] obtained a DWDD signal by taking the logarithmic ratio of the reflected signals from a tunable laser, manually switched between two wavelengths. To perform an equivalent TDM measurement, they used an electro–optic modulator to do a frequency sweep of the laser light, detecting it with an expensive network vector analyzer.…”

High-Density Distributed Crack Tip Sensing System Using Dense Ultra-Short FBG Sensors

“…Combined with the OFDR technology, the large multiplexing capacity of the DUS-FBG array facilitated long detection distance distributed sensing of 10 m, with a high spatial resolution of 1.5 mm. Even though the capability of long-distance detection with high spatial resolution is ideal, the limited demodulation accuracy, imposed by the inherent wide spectral width of individual FBGs, hinders the application of such a system for distributed crack tip sensing [25,26].…”