In recent years, the Surface Plasmon Resonance phenomenon has achieved a lot of importance in different application fields. This is a highly sensitive technique that allows the measurement of a small refractive index variation at the boundary between a metallic layer and an encompassing dielectric medium. Once the resonance conditions are verified, a dip in the output signal, as a consequence of a strong absorption, can be observed. The dip is achieved at a particular wavelength, called the resonance wavelength, and it is linked to the refractive index at the boundary 
. This technique can be paired with optical waveguides, such as optical fibers, to attain sundry sensor systems for several application areas 
. These types of sensors have important advantages, such as low cost, rapid response, immunity to electromagnetic interference, high sensitivity, and small size.
In particular, the SPR sensor can be used as a refractometer, a bio/chemical sensor by functionalizing the surface with a particular receptor, a pressure sensor, and so on 
. Generally, the sensor system based on the SPR can be realized by using different types of optical waveguides, such as monomodal or multimodal. Thanks to the properties of the multimodal waveguides, the SPR technique can be exploited more efficiently, and, for this reason, multimodal waveguides are widely adopted for optical sensors 
. In more detail, when dealing with multimodal waveguides, the dip, caused by the SPR phenomenon, is wider when compared with that caused by the monomodal waveguides. This phenomenon is a consequence of a convolution of diverse propagating modes, each of which is linked to a distinct angle of incidence at the metal–dielectric interface 
. In such a case, despite broader SPR spectra being obtained, a greater sensitivity is achieved as well, thus leading to an overall performance improvement 
2. Physical Sensing
2.1. Magnetic Field Sensor
The magnetic field sensor is realized by using a POF patch, with a core diameter of 500 μm, paired with the SPR-POF sensor. To make the patch sensitive to the change in the magnetic field, it is coated with a ferrofluid along 20 mm of its length 
. To modify the mode profile of the input light, a magnet is used, since, by tuning the distance from the sensitive patch, it is possible to change the value of the bending force on the patch covered with ferrofluid. The distance between the magnet and the sensitive patch can be regulated by a special holder, as schematically reported in Figure 1
Figure 1. Outline of the magnetic field sensor system. A special holder is utilized to tune the distance between the magnet and the ferrofluidic-covered patch.
The sensor system is tested by keeping unchanged the refractive index on the plasmonic platform (equal to 1.332, i.e., water) and by tuning the distance between the magnet and the ferrofluidic-covered patch, thus making it possible to change the intensity of the target magnetic field. More details about this magnetic field sensor are extensively reported in 
. The analyzed magnetic field ranges from 0.15 mT to 1.20 mT, which corresponds, in terms of spacing between the magnet and the sensitive patch, to a range between 64 mm and 24 mm, with a step size of 4 mm. Therefore, once the magnet approaches the ferrofluidic-covered area, the intensity of the exited magnetic field increases.
More specifically, the magnetic field sensor system denotes a good linear response in the magnetic field intensity range between 0.28 mT and 0.75 mT. The latter reports the experimental variations in resonance wavelength calculated with respect to the resonance wavelength related to the reference (acquired without the magnet) in the analyzed magnetic field range.
An analysis, reported in Table 1, is carried out to compare the presented magnetic field platform, with other configurations already presented in the literature.
Table 1. A comparison of several magnetic field sensors based on different sensing methods.
2.2. Temperature Sensor
The temperature sensor system includes two D-shaped-POF probes which are coupled in a sequence: the first platform is characterized by a coating of thermosensitive material on the exposed POF core to make it responsive to temperature changes, while the second one consists of an SPR-POF probe 
The POF used for the realization of both the platforms is multimodal, with a 1 mm diameter. The production process of the thermosensitive platform can be summarized as follows. First, the POF is fixed with glue onto a physical support, and then it is polished with two types of lapping sheets (5 and 1 μm grits) in order to obtain the D-shaped area. On the modified POF, a layer of thermosensitive material is then deposited. For this purpose, silicone is used since its physical characteristics are dependent on the temperature 
As a proof of concept to realize the thermal POF sensor chip, an adhesive tape with a nominal thickness of 70 μm is used to control the thickness of the silicone layer. More specifically, by exploiting the adhesive tape, a channel is achieved by fixing two pieces of adhesive tape at the end of the physical support. After this, the silicone is placed on the exposed POF’s core by exploiting a spatula to level the silicone layer at the adhesive tape thickness. In this way, a silicone thickness of 70 μm is obtained. This simple approach offers the ability to realize thermal sensor chips with good reproducibility.
The sensor’s performances can be changed by using a different thickness of the silicone layer.
The presented sensor system is tested by tuning the temperature of the water deposited on the thermosensitive platform to induce a change in the physical characteristics and, consequently, in the refractive index, of the silicone layer 
. During the test, the refractive index of the solution upon the SPR D-shaped platform is fixed at 1.332 (water), and the temperature of the solution upon the thermosensitive platform varies from 20 °C to 38 °C, with a step size of 3 °C. More details about this temperature sensor are extensively reported in 
An analysis, reported in Table 2, is performed to compare the presented temperature sensor platform with other configurations already presented in the literature.
Table 2. A comparison of several temperature sensors based on different sensing methods.
2.3. Force Sensor
The force sensor system is realized by using a 1 mm POF patch immobilized into a 3D-printed support through two clamps and representing the force sensing region. The force sensitive patch is then coupled to an SPR-POF probe in a series. The holder, reported in Figure 2
, is devised and successively produced by means of a 3D printer (Photon Mono X, Anycubic®
, Shenzhen, China) 
Figure 2. A schematization of the force sensor system.
The POF patch is subject to various forces by placing various weights in the center of the sensing area. In the same way as the previous sensor systems, the refractive index of the solution on the SPR platform is fixed. By varying the applied force, the incidence angles related to the propagating modes in the following SPR-POF platform change, and so the resonance wavelength is tuned. Figure 2 reports a schematization of the abovementioned force sensor system.
The force sensor system is tested by varying the applied force to the POF patch in the range between 0 N and 0.5 N (with a step size of 0.05 N) and by fixing the refractive index of the solution on the SPR platform to 1.332 (water). More details about this force sensor are extensively reported in 
A comparative analysis in terms of resolution, reported in Table 3, is carried out by comparing different types of force sensors already presented in the literature.
Table 3. A comparison of several force sensors based on different sensing methods.
2.4. Micro-Liquid Volume Sensor
The micro-liquid volume sensor system consists of a patch of light diffusive fiber (LDF). In LDFs, the light is spread from the core, and it is diffused toward the external medium thanks to the scattering centers present in their core. The used LDF-POF is fixed in a 3D-printed tank in order to realize the volume-sensing region, and it is coupled in a sequence with an SPR-POF probe 
The tank, highlighted in the outline of the micro-liquid sensor system in Figure 3
, is printed using a 3D printer (Photon Mono X, Anycubic, Shenzhen, China), and its aim is to hold the micro-liquid volume around the fiber. The patch consists of a 1600 μm uncladded LDF-POF (manufactured by Global Engineering Network, Dosson di Casier, Italy) with a removable jacket of approximately 400 μm (total diameter 2 mm). The jacket is stripped off by a mechanical tool along 1 cm of its length; this piece of fiber without jackets represents the volume-sensitive area, and it is fixed in the tank through two holes using white silicone 
Figure 3. Outline of the micro-volume liquid sensor system.
The mode profile of the input light at the SPR platform is altered by changing the surrounding volume of liquid in the tank. In this way the resonance wavelength changes, and it is possible to detect small variations of volume 
. More details about this micro-liquid volume sensor are extensively reported in 
The presented sensor system is tested by fixing the refractive index of the solution on the SPR platform to 1.332 (water) and by changing the water volume in the tank, ranging from 0 to 5 μL (1 μL step).
Table 4. A comparison of several volume sensors based on different sensing methods.