Identical weak fiber Bragg gratings (FBGs) based quasi-distributed sensors have wide range of applications in large civil construction, manufacturing industry, defense industry and so on. The use of weak gratings eliminates the capacity limitation caused by the limited optical source bandwidth, allowing for the multiplexing of a large number of sensors. In addition, weak gratings are also greatly facilitated by the well-established on-line grating writing technologies, through which hundreds or even thousands of identical gratings can be easily fabricated along a single optical fiber. Currently, time-division multiplexing (TDM) is the most common method to multiplex identical weak gratings. It is also feasible to improve the capacity by combining two or three multiplexing methods together, such as TDM/WDM, and the multiplexing of thousands of gratings have been demonstrated through this concept. The interrogation performance is another key factor for a large sensing network, which can directly determine the sensing performance. However, for traditional wavelength based interrogation methods using the active spectrum-detection devices, such as Fabry–Pérot filters and tunable lasers, the measurement typically suffers from a low response rate, due to the inevitable mechanical wavelength scanning and spectrum peak (or notch) search processes. Generally, the total interrogation time for all the sensors is more than a minute.
In practice, the intensity-based interrogation technique is still of great interest because of its simplicity, potential cost-effectiveness, much faster and more robust measurement. However, the signal power and SNR are typically low in TDM sensing network with weak gratings,it is still challenging to implement them in distributed sensing applications. More importantly, they generally suffer from a small wavelength operational range, for example, the dynamic range of edge filtering methods is low , and that of matched filtering approaches for conventional FBGs is usually less than 1nm . This directly prevents their use in some hybrid multiplexing sensing networks (i.e. WDM/TDM), where the wavelength channels that need to be detected can usually cover a large wavelength band (usually tens of even hundreds of nm) .
To address this problem, the research group of professor Xia Li at Wuhan National Laboratory for Optoelectronics proposed a demodulation method based on multi-wavelength narrow linewidth laser incident on the weak fiber Bragg grating sensor network (Figure 1).
We report a large-scale multi-channel fiber sensing network, where ultra-short FBGs (USFBGs) instead of conventional narrow-band ultra-weak FBGs are used as the sensors. In the time division multiplexing scheme of the network, each grating response is resolved as three adjacent discrete peaks. The central wavelengths of USFBGs are tracked with the differential detection, which is achieved by calculating the peak-to-peak ratio of two maximum peaks. Compared with previous large-scale hybrid multiplexing sensing networks (e.g., WDM/TDM) which typically have relatively low interrogation speed and very high complexity, the proposed system can achieve interrogation of all channel sensors through very fast and simple intensity measurements with a broad dynamic range. A proof-of-concept experiment with twenty USFBGs, at two wavelength channels, was performed and a fast static strain measurements were demonstrated, with a high average sensitivity of ~0.54dB/μεand wide dynamic range of over ~3000με. The channel to channel switching time was 10ms and total network interrogation time was 50ms.
This study, titled “TDM interrogation of intensity-modulated USFBGs network based on multichannel lasers”, was published in Optics Express in Jan. 23, 2017 (Vol. 25, No. 22, pp. 670-680).
The work is supported by Major Program of National Natural Science Fundation of China 61290315 and the National Science Foundation under Grant 61675078.
Fig.1.Schematic representation of TDM differential method to interrogate FBG network
Fig.2. Experimental results (a):power response under different strain (b):spectrum under different strain，(c): The reflected power at three wavelengths and the substract results