Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Intensity or Optical Power-Based Interrogation <s> Abstract A power-referenced refractometer operating in reflection mode is proposed and experimentally demonstrated based on a tilted-fiber Bragg grating (TFBG) cascaded by a reflection-band-matched chirped-fiber Bragg grating (CFBG). The optical signal reflected by the CFBG passes twice through the TFBG that enhances sensitivity of the refractometer. In addition, the optical signal is propagating all the way in the fiber core so that the extra insertion loss is low. Refractive index measurement with sensitivity up to 597.2 μW/R.I.U. is achieved within the range from 1.333 to ~1.42. The maximum detectable refractive index is ~1.45. <s> BIB001 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Intensity or Optical Power-Based Interrogation <s> It is proposed the multiplexing of optical fiber-based surface plasmon resonance (SPR) sensors deployed in a ladder topology, addressed in wavelength by combining each sensor with specific fiber Bragg gratings (FBGs) and considering intensity interrogation. In each branch of the fiber layout, the FBGs are located after the sensor and the peak optical power reflected by the FBGs is a function of the relative spectral position between the SPR sensor and the FBG resonances, with the former dependent on the refractive index of the surrounding medium. The concept is tested for the multiplexing of two SPR sensors fabricated in an etched region of a single-mode fiber showing intrinsic refractive index sensitivity up to 5000 nm/RIU, which translates into a sensitivity of ∼829 dB/RIU from the interrogation approach considered. The obtained refractive index resolution is in the order of 10−4 RIU, and the crosstalk level between the sensors was found negligible. <s> BIB002 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Intensity or Optical Power-Based Interrogation <s> A fibre optic surface plasmon resonance (SPR) sensor system for smartphones is reported, for the first time. The sensor was fabricated by using an easy-to-implement silver coating technique and by polishing both ends of a 400 µm optical fibre to obtain 45° end-faces. For excitation and interrogation of the SPR sensor system the flash-light and camera at the back side of the smartphone were employed, respectively. Consequently, no external electrical components are required for the operation of the sensor system developed. In a first application example a refractive index sensor was realised. The performance of the SPR sensor system was demonstrated by using different volume concentrations of glycerol solution. A sensitivity of 5.96·10−4 refractive index units (RIU)/pixel was obtained for a refractive index (RI) range from 1.33 to 1.36. In future implementations the reported sensor system could be integrated in a cover of a smartphone or used as a low-cost, portable point-of-care diagnostic platform. Consequently it offers the potential of monitoring a large variety of environmental or point-of-care parameters in combination with smartphones. <s> BIB003 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Intensity or Optical Power-Based Interrogation <s> A nano-scale gold-coated tilted fiber Bragg grating sensor with clear surface plasmon resonance is proposed and experimentally demonstrated. The transmission spectrum of the sensor provides a fine comb of narrowband resonances that overlap with the broader absorption of the surface plasmon and thus provides a unique tool to measure small shifts of the plasmon and identify the changes of the surrounding refractive index (SRI) with high accuracy. Meanwhile, our proposed sensor provides an amplitude interrogation method for SRI measurement, which is much simpler and cost-effective than the traditional wavelength monitoring. Experimental results obtained in sucrose solutions with different refractive indices ranging from 1.3330 to 1.3410 show that the sensitivities of our proposed sensor are 450 nm/RIU and 2040 dB/RIU by using the wavelength and amplitude interrogation methods, respectively. Furthermore, the Q-factor of amplitude interrogation signal has been increased from $\sim 70$ to $\sim 6000$ by using the narrowband cladding resonance instead of broader SPR absorption. <s> BIB004 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Intensity or Optical Power-Based Interrogation <s> Abstract A multi-sideband filtering demodulation scheme with enhanced high sensitivity was proposed to demodulate weak fiber Bragg grating signals, and a novel data processing method was employed to reduce the influence of the background noise. The relationship of the sensing sensitivity, number of sidebands, and their slope coefficients were researched theoretically, numerically, and experimentally. The results showed that the sensing sensitivity was in proportion to the number of the sidebands and their slope coefficients. The power sensitivities for single and double CWDMs demodulating FBG in the experimental system were 6.960 dB/nm and 15.187 dB/nm, respectively, which were in agreement with the theoretical results. In addition, the data processing method we used to obtain the optical power was experimentally proven to be better than the method of directly measuring the optical power because it reduces the background noise. <s> BIB005 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Intensity or Optical Power-Based Interrogation <s> Abstract Gold-coated tilted fiber Bragg gratings can probe surface Plasmon polaritons with high resolution and sensitivity. In this work, we report two configurations to interrogate such plasmonic biosensors, with the aim of providing more efficient alternatives to the widespread spectrometer-based techniques. To this aim, the interrogation is based on measuring the optical power evolution of the cladding modes with respect to surrounding refractive index changes instead of computing their wavelength shift. Both setups are composed of a broadband source and a photodiode and enable a narrowband interrogation around the cladding mode that excites the surface Plasmon resonance. The first configuration makes use of a uniform fiber Bragg grating to filter the broadband response of the source in a way that the final interrogation is based on an intensity modulation measured in transmission. The second setup uses a uniform fiber grating too, but located beyond the sensor and acting as a selective optical mirror, so the interrogation is carried out in reflection. Both configurations are compared, showing interesting differential features. The first one exhibits a very high sensitivity while the second one has an almost temperature-insensitive behavior. Hence, the choice of the most appropriate method will be driven by the requirements of the target application. <s> BIB006
Another method to interrogate plasmonic FBG-based sensors is the quantization of the intensity variations produced as a result of the coupling of light to the plasmon wave. The most typical configuration for an interrogation of the transmitted response is that shown in Figure 11b . The interrogation is carried out in a narrow band of the optical spectrum, so a tunable laser (TLS) can be used as a source, together with a photodiode (PD) as detector and an analog-to-digital converter (A/D) to obtain the desired data  BIB004 . The function of the TLS is matching the wavelength of the most sensitive mode of the fiber grating, so once the sensor is characterized it can be replaced by a common laser. This technique relies on the principle of edge filtering BIB005 so that the optical power change is produced as a result of the wavelength shift of the mode with respect to the fixed wavelength of the laser source. Several cost-effective configurations have been reported for interrogating both the transmission and reflection response of the sensors, mainly based on different associations of fiber gratings to filter the spectral region of interest BIB001 BIB002 BIB006 . It is also worth mentioning that the principle of intensity interrogation has recently been applied for the development of reduced size plasmonic optical fiber sensors interrogated with the flashlight and camera of a common smartphone BIB003 , although a fiber grating-based counterpart is yet to be investigated. Another method to interrogate plasmonic FBG-based sensors is the quantization of the intensity variations produced as a result of the coupling of light to the plasmon wave. The most typical configuration for an interrogation of the transmitted response is that shown in Figure 11b . The interrogation is carried out in a narrow band of the optical spectrum, so a tunable laser (TLS) can be used as a source, together with a photodiode (PD) as detector and an analog-to-digital converter (A/D) to obtain the desired data  BIB004 . The function of the TLS is matching the wavelength of the most sensitive mode of the fiber grating, so once the sensor is characterized it can be replaced by a common laser. This technique relies on the principle of edge filtering BIB005 so that the optical power change is produced as a result of the wavelength shift of the mode with respect to the fixed wavelength of the laser source. Several cost-effective configurations have been reported for interrogating both the transmission and reflection response of the sensors, mainly based on different associations of fiber gratings to filter the spectral region of interest BIB001 BIB002 BIB006 . It is also worth mentioning that the principle of intensity interrogation has recently been applied for the development of reduced size plasmonic optical fiber sensors interrogated with the flashlight and camera of a common smartphone BIB003 , although a fiber grating-based counterpart is yet to be investigated.