# Recent Advances in Plasmonic Sensor-Based Fiber Optic Probes for Biological Applications
## 6 FOPS Geometrical Configuration and Techniques
### Conventional Optical Fiber Sensors  
Conventional fibers are modified to trap the light inside the core, which is in direct contact with the surrounding medium and hence acts a sensor, as sketched in Figure 7a-h [46,53]. All or part of the cladding of the fiber is removed by chemical itching or side polishing for the sensing applications. These uncladded or D-shaped optical fiber sensors are established and studied in the operating wavelength range from 500 to 800 nm [46]. The incident light launched into the fiber core, i.e., fundamental mode, at a certain wavelength can couple with the plasmonic mode. Then, the particular effective refractive index of the analyte is monitored for the characteristics of the transmission light. Nevertheless, achieving the phase matching condition is challenging. The coupling between the effective refractive index of the plasmonic mode and the effective refractive index, \(n_{eff}\), of the core mode takes place at higher frequencies. Tuning the phase-matching condition in the desired wavelength has been achieved with multimode fibers by coupling the effective refractive index of higher-order modes and effective refractive index of the plasmonic mode [25,54,55]. The tapered optical fibers, as shown in Figure 7a, are produced by removing the cladding of the fiber [56]. The unclad part is fixed with translational stages at both the ends. The center part of the unclad fiber is heated with burning gas and the fiber is slowly pulled at both ends to reduce the waist diameter of the fiber. A thin metal layer is placed on the tapered region and exposed to the surrounding for sensing [56]. Such unclad/etched/tapered structured sensors have achieved a maximum sensitivity of 20,000 nm/RIU in RI (refractive index)ranging from 1.415 and 1.429 [57]. A RI sensor based on dual-core photoniccrystal fiber (PCF) has exhibited the highest sensitivity of 22,983 nm/RIU [58]. The so-called N-layered matrix method is also followed for the unclad/etched structure sensor configuration, as elucidated in Figure 7b [22]. In the N-layered matrix method, to analyze the reflectance and transmittance properties, the following three methods are surveyed, namely, the field tracing method, the resultant wave method, and the transfer matrix method [59]. The characteristics of better sensitivity with enhanced detection accuracy and figure of merit are endowed by this transfer matrix method [22; 60]. In recent years, the fabrication techniques provide several geometries for the metal components, as shown in Figure 7c, which are used for tailoring the modal dispersion of the plasmons and to increase their coupling with the fundamental mode [61]. The metallic wires support multiple modes, which are responsible for the increased number of resonant peaks that result in the sensor with different values of refractive index detection range and operating wavelengths [62]. Nanocoated D-shaped optical fibers were constructed for sensing applications with a sensitivity of 1566 nm/RIU [63]. A single mode D-shaped fiber is used for sensing the RI monitoring of liquid with linearity up to 0.984 [64]. The multi-D-shaped optical fiber has been established by forming several D-sections in an optical fiber as a highly sensitive RI sensor with the resolution of 1.27 \(\times\) 10\({}^{-3}\)-3.13 \(\times\) 10\({}^{-4}\) RIU [65].