Axial Micro-Strain Sensor Based on FM-FBG via Dual-Mode ML-FMF in Sensor Networks

An in-fiber axial micro-strain sensor based on a Few Mode Fiber Bragg Grating (FM-FBG) is proposed and experimentally characterized. This FM-FBG is in inscribed in a multi-layer few-mode fiber (ML-FMF), and could acquire the change of the axial strain along fibers, which depends on the transmission dips. On account of the distinct dual-mode property, a good stability of this sensor is realized. The two transmission dips could have the different sensing behaviors. Both the propagation characteristics and operation principle of such a sensor are demonstrated in detail. High sensitivity of the FM-FBG, ~4 pm/με and ~4.5 pm/με within the range of 0 με - 1456 με, is experimentally achieved. FM-FBGs could be easily scattered along one fiber. So this sensor may have a great potential of being used in sensor networks.


Introduction
In recent years, in-fiber strain sensors based on Fiber Bragg gratings (FBGs) have been paid more attention in monitoring the axial micro-strain, and their unique advantages show a high degree of accuracy, fast response, high immunity to electromagnetic interference, long-range operation capability, low cost and easy to manufacture [1]. The sensing principle of the FBG sensor is based on the demodulation of the reflection spectra in response to strain [2] [3], temperature [4] [5], surrounding refractive indexes (SRI), pressure [6] [7], acceleration [8], and tilt angle [9]. To date, a great many of sensor designs based on few mode FBGs (FM-FBG) have been reported. The FM-FBG has unique spectrum cha-How to cite this paper: Liang, X., Geng, Z.X., Li racteristics, which has more flexibility in a composite detection system. It has already been widely utilized in FBG sensors, application scenarios ranging from strain, temperature, solution concentration and refractive index [10]- [15]. Compared with the traditional temperature sensing scheme, the strain FBG sensor based on a dual mode multi-layer few-mode fiber (ML-FMF) in this paper takes into account the well characteristics of simple structure and well sensitivity.
In this work, we demonstrate an axial micro-strain sensor on the basis of an  Figure 1 shows a cross section under microscope and measured index profile of the ML-FMF, which supports only the fundamental mode and LP 11 mode groups.

Configuration of ML-FMF
By the cross section as shown in Figure 1(a), the ML-FMF is consisted of a circular core area and a cladding layer. The bright part of the core area is a high refractive index layer which is achieved by doping the germanium. The dark part of the core area is a low refractive index layer which is achieved by doping fluoride, and the part outside the core area is the cladding, composed of pure silicon dioxide, whose refractive index is 1.444. Figure 1

FM-FBG Properties and Sensing Principles
The excitation coefficients of LP 01 and LP 11 could be calculated by the overlap For simplicity, it is an assumption that the excited power of both guided modes is equal in the calculation. The theoretically calculated transmission and reflection spectral with conventional transfer matrix method is shown in Figure   2(b). The self-coupling and cross-coupling will be considered between less contrapropagating modes, which would be gotten clear Bragg transmission resonances. The modes, LP 01 and LP 11 contra-propagating modes, couple to each other subjected to the mode phase matching condition given by Equation (2).

Experimental and Discussion
The measured transmission spectrum is shown in Figure 3. As we can see on the This experiment uses FM-FBG as a sensing head to measure the micro-strain along the fiber. Figure 4 shows an axial micro-strain sensing experimental setup.
The FM-FBG is secured by two fiber clips and straightened with a certain longitudinal stress. One is fixed and the other one can be elongated by the micro-adjuster, and the change of the axial micro-strain would be measured. The light of the broadband light source (KOHERASTM, superK uersa) is injected into the FM-FBG and the output spectrum is detected using a spectrometer (YOKOGAWATM, AQ6375) to observe the effect of strain on the transmission spectrum in real time.
For uniform strain, each micro-adjuster elongates the FM-FBG at a step of 208 με, and the range of 0 με -1456 με is detected in this experiment. Figure 5(a) shows how the transmission spectrum drifts with the strain. It can be seen that the dips drift to long wavelengths. Figure 5(b) shows the relationship between

Conclusion
An in-fiber axial micro-strain sensor based on a FM-FBG inscribed in a novel dual-mode ML-FMF is demonstrated and experimentally characterized. The FM-FBG owned two resonant dips act as a sensing head, which would be one of the key techniques to achieve fiber grating distributed sensing network in internet of things. The axial strain sensitivity performs well. It is also simple, easy to manufacture, potentially low cost, and possesses a much larger measurement range.