Fig. 3. Dispersion profiles and related spectral losses of the SPR D-shaped PCF with two gold slabs. The case of a constant refractive index of 1.36 and corresponding spectral losses are represented by dotted lines. The normal dispersion and anomalous dispersion regimes with the related spectral losses are represented by dashed lines and solid lines, respectively. is decreased for the normal dispersion. On the contrary, the anomalous dispersion curve leads to a larger difference in the resonance wavelengths. We performed additional numerical calculations to express the difference in the resonance wavelengths (regarded as Δλpeak ) as a function of the slope of the dispersion curve (dn/dλ). The net result is depicted in Fig. 4, where the computed data (solid blue dots) is fitted by a linear curve (green solid line) with a determination coefficient of R 2 0.86. All refractive indexes lie within the interval (1.25 - 1.40) for optical wavelengths ranging from 540 nm to 670 nm. From the linear fitting shown in Fig. 4, the SPR D-shaped PCF sensor interrogates the Fig. 4. Distance Δλpeak between resonance wavelengths as function of the first-order dispersion dn/dλ. The several dots at null dispersion are nearly at Δλpeak 68 nm and were calculated for different average refractive indexes. 66 IEEE Instrumentation & Measurement Magazine = August 2021