## 3 Results  
### Response to Refractive Index Variations  
The reflection spectrum of the etched GI-MMF BG at 20 \({}^{\circ}\)C for different SRI values is illustrated in Figure 3.  
Since the diameter of the GI-MMF containing the BG was only 43 \(\upmu\)m after the etching procedure, the measured reflection spectrum illustrated in Figure 3a contains a lower number of reflection peaks in the left-hand side (blue wavelength range) compared to the measured transmission spectrum shown in Figure 2, i.e., no Bragg reflection peaks below 1549 nm were measured after etching the optical fiber. In addition, at an SRI of \(n=1.47\) the reflection amplitudes of the higher order PMGs decrease, as depicted in Figure 3a. This can only be explained by modes that are confined at the core/cladding interface, i.e., when the etched fiber sensor-head is immersed into a solution with a higher RI compared to the fiber core, these modes of the higher order PMGs are coupled out of the optical glass fiber which thus results in power loss. Therefore, only modes of higher order PMGs are able to interact with the environment and thus are capable to shift the Bragg reflection wavelength depending on the RI of the surrounding. Moreover, as depicted in Figure 3a the envelope of the measured reflection spectra of the sensor varies with different SRI and thus makes the exact determination of the Bragg reflection peak wavelength for each SRI value difficult (discussed in Section 4). In Figure 3b also the response of the fabricated sensor element to RI variations is illustrated. In total of 10 measurements were performed for each refractive index solution and only the obtained mean values are depicted in Figure 3b. The corresponding sensitivity of the individual Bragg reflection peaks are summarized in Table 1.  
From the obtained response, it follows that for increasing order of the PMG, a trend toward higher sensitivity to SRI can be seen, which agrees well with the literature [7]. The value \(R^{2}\) in Table 1 specifies the coefficient of linear regression of the sensitivity for each PMG. From the obtained \(R^{2}\) values it  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline  & _P01_ & _P02_ & _P03_ & _P04_ & _P05_ & _P06_ & _P07_ & _P08_ & _P09_ & _P10_ & _P11_ \\ \hline nm/RIU & 3.04 & 0.47 & 0.98 & 0.81 & 0.71 & \(-\)0.35 & 0.47 & 0.24 & \(-\)0.21 & \(-\)0.61 & 0.22 \\ \(R^{2}\) & 0.88 & 0.12 & 0.56 & 0.49 & 0.81 & 0.11 & 0.38 & 0.12 & 0.80 & 0.07 & 0.19 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Sensitivities of the different Bragg reflection peaks to applied SRI.  
Figure 3: Reflection spectrum of etched GI-MMF with BG for different SRI values (**a**) and the response of the individual Bragg reflection peaks to different applied SRI values (**b**). _P01_-_P11_ indicate the Bragg reflection peaks in the reflection spectrum from left to right, i.e., the Bragg reflection peak of the highest PMG is marked with _P01_, whereas the Bragg reflection peak of the lowest PMG is marked with _P11_.  
follows that for a decreasing PMG order, a trend towards negligible or no sensitivity to applied RI was measured. Moreover, the PMGs _P06_, _P09_ and _P10_ show a negative sensitivity to applied RI. However, this error in measurement is attributed to the optical interrogation system (see Section 4). In Figure 4 the relative wavelength shifts of the Bragg reflection peaks _P01_ and _P11_ for different SRI are shown for the 10 measurements performed for each refractive index solution. A standard deviation of 0.064 nm has been obtained for the refractive index measurements using the Bragg reflection peak _P01_.