Towards Mode-Multiplexed Fiber Sensors: An Investigation on the Spectral Response of Etched Graded Index OM4 Multi-Mode Fiber with Bragg grating for Refractive Index and Temperature Measurement  
[PERSON]  
1 Hannover Centre for Optical Technologies (HOT), Leibniz University Hannover, 30167 Hannover, Germany; [PERSON]SS] (Y.Z. ); [EMAIL_ADDRESS] (B.W.R. )1 Hannover Centre for Optical Technologies (HOT), Leibniz University Hannover, 30167 Hannover, Germany; [PERSON]SS] (Y.Z. ); bernhard.rothe@hot.uni-hannover  
and the additional mode-multiplexing would allow the discrimination between the different Bragg reflection of each BG structure. This, for instance, could allow the simultaneous measurement of the Surrounding Refractive Index (SRI) and temperature using only one BG structure, since different modes depends differently on the SRI. Recent work by [PERSON] et al. [4] demonstrated the capability of this concept by showing the simultaneous measurement of refractive index (RI) and temperature using an etched Few-Mode Fiber (FMF) with BG, termed FMFBG. The main mechanism of FMF follows that although BGs are insensitive to the SRI, when the cladding is sufficiently reduced, the evanescent field could interact with the surrounding media and the effective RI of the core would depend on the RI of the surrounding media. This has a direct impact on the resonance wavelength of the grating and thus the sensor is then sensitive to the SRI. Due to the different mode field distribution in the core and cladding region (and thus the effective refractive indices, \(n_{\text{eff}}\), of the modes depend differently on the SRI), the Bragg wavelength of the _LP01_ and _LP11_ modes depend differently on applied SRI and temperature (the temperature sensitivity of the modes depends on thermally induced refractive index change and the thermal expansion coefficient [5]) and this feature allows simultaneous detection of these two parameters using only a single sensor element. Compared to step-index MMF, Graded-Index (GI)-MMF have the unique property that some of the modes propagating inside the fiber have almost identical propagation constants, which causes the formation of principle mode groups (PMGs) [3]. This unique feature leads to the grouping of all modes propagating inside the GI-MMF into only a few PMGs, where each PMG is characterized by a single mode propagation constant. Consequently, when a BG is inscribed in a GI-MMF the corresponding transmission and reflection spectrum consists of only several dips and peaks, respectively [6]. In addition, lower order PMGs contain modes which are confined more in the center of the core whereas higher order PMGs contain modes that are confined at the core/cladding interface [7]. Therefore, when the cladding material of the GI-MMF is removed, higher order PMGs are thus more sensitive to RI changes of the surrounding compared to lower order PMGs [7] and thus this kind of fibers could form the basis for mode-multiplexed FOS concepts. The work presented herein investigates whether GI-MMFs can be applied for mode-multiplexing in FOS applications and the simultaneous measurement of SRI and temperature. A schematic of the proposed fiber optic sensor-head is shown in Figure 1. By etching the fiber optic sensor-head the higher order PMG become sensitive to applied SRI variation and by inscribing the BG the higher order and lower order PMG can be discriminated at the optical detector by observing the individual Bragg reflection peaks. The advantage of this approach is that only a single sensor element would be required for the simultaneous detection of these two parameters and due to the relatively large core diameter of 50 \(\upmu\)m (in case of the OM4 GI-MMF) the sensor elements remains stable even after etching compared to FMF (core etched down to 17 \(\upmu\)m [8]) and SMF (core etched down to 3.4 \(\upmu\)m [9]) approaches and thus more applicable to distributed FOSs in the field.