## 1 Introduction  
The surface plasmon resonance (SPR) technique has been widely used in the last two decades as the detection principle for several sensors used in the biological and chemical sensing fields [1; 2; 3; 4]. The SPR phenomenon occurs at the interface between a dielectric layer and a thin metal film (usually a gold film), hit by a p-polarized light. The incident light generates electron charge density waves called plasmons. An electromagnetic wave, namely a surface plasmon wave (SPW), propagates at the metal-dielectric interface. When the p-polarized light is incident on this metal-dielectric interface in such a way that the propagation constant (and energy) of the resultant evanescent wave is equal to that of the SPW, strong absorption of light takes place as a result of the transfer of energy, and a dip at a particular wavelength (resonance wavelength) appears in the output signal. The resonance wavelength is highly dependent on changes in the refractive index of the dielectric layer. In particular, low-cost sensors based on SPR in optical fibers for the implementation of biosensors and chemical sensors constitute a very sensitive method for determining even small refractive index changes at the interface between a metal substrate and a dielectric medium (a receptor layer that interacts with a specific analyte) [5; 6; 7; 8; 9; 10; 11]. In order to improve the performances of these kinds of sensors, different configurations have been recently proposed [12; 13; 14; 15; 16]. Particularly, multimode plastic optical fibers (POFs) are suitable for low-cost SPR sensing systems. They are especially advantageous due to their simple setup and for the POFs' properties, such as excellent flexibility, easy manipulation, great numerical aperture, large diameter, and the ability to withstand smaller bend radii than glass [17]. Furthermore, the appealing properties of POFs that have increased their popularity and competitiveness for telecommunications are exactly those that are important for optical sensors based on glass optical fibers, with the addition of simpler manufacturing and handling procedures. In this context, [PERSON] et al. have presented several biochemical applications based on an SPR-POF platform, described in [17], by varying the molecular recognition element (MRE) [18; 19; 20]. Moreover, several works have been already presented on the theoretical analysis of SPR sensors in tapered optical fibers, such as [21; 22; 23; 24]. In particular, as supposed by the theory, to improve the sensitivity of the SPR-POF sensor in biosensing, tapered plastic optical fiber can be introduced, such as that already tested in nicotine detection by SPR D-shaped tapered POFS (TPOFs) combined with a specific molecularly imprinted polymer (MIP) [21]. In the SPR D-shaped OPF sensors, it has been demonstrated that when the diameter of the OPF decreases from 1000 to 250 \(\upmu\)m, the performances decrease too [25]. In fact, in an SPR D-shaped OPF sensor, obtained by a multimode OPF with a diameter of 250 \(\upmu\)m, the sensitivity is about 550 nm/RIU. Moreover, when a tapered OPF is present at the input or output of the SPR sensing area, different filtering of higher-order modes in plastic multimode fibers can be obtained [26]. In particular, the experimental results presented in [26] have shown that the tapered filter positioned after the SPR D-shaped OPF area improves the performances in terms of refractive index range and depth of the SPR curve. Consequently, in SPR D-shaped tapered OPF sensors, when the diameter in the D-shaped sensing area changes with the taper ratio and when the shape of the tapered OPF, at input/output of the SPR area, changes as well, a specific analysis is required. This work presents a detailed analysis, carried out by specific experiments, of the performances of different SPR sensors realized by D-shaped TPOFs. Finally, in these kinds of SPR D-shaped OPF sensors, the sensitivity is also a function of the D-shaped OPF parameters [27].