# SARS-CoV-2 Detection Using Optical Fiber Based Sensor Method
## 4 Fiber-Optic Sensing for COVID-19
### Fiber-Optic Sensing and Signal Processing  
It is well known that propagation of light via total internal reflection inside optical fiber is influenced by variation in the refractive index. Besides, the intrinsic evanescent wave in case of optical fiber is pinned to the core during removal of the cladding portion, which further facilitates interaction with the surrounding medium. A change in surroundings results in refractive index variation, which eventually leads to modulated light intensity. When relevant functionalities corresponding to the analyte are impregnated, the intensity fluctuation becomes more obvious. The exposed area is subsequently impregnated with appropriate functions. It is desirable to use noble metal nanoparticles that further immobilize with appropriate bio receptors/bio linkers and amplify the interaction signal, eventually leading to a greater metrics performance (intensity, absorbance, etc.). Antibodies/proteins might be used as bio receptors/bio linkers for SARS-CoV-2 in this suggested catheter-like probe. When the probe is exposed to the virus (analyte), it undergoes an intensity modulation. The behavior is explained in Figure 4 with the presence of metallic nanoparticles present on the upper layer. However, this can be a quite cost-consuming method so utilization of nanoparticles should be selected in a way that is economical and appropriate.  
The signal processing through the probe can be assessed through many methodologies, namely intensity modulation, wavelength division multiplexing, frequency multiplexing, etc. However, the intensity modulation is the quickest and most stable solution.  
Considering detectors to be extremely accurate, if we consider the practicality of the schemes, the intensity-based interrogation approach might be considered the greatest fit because it requires the least amount of logistics; when the proposed configuration is complete, positive samples (infected) are tested in a controlled environment. Similarly, the same setup can also be made for samples without any infection. Accordingly, the Positive (SARS-CoV-2 infected) and negative (SARS-CoV-2 not infected) profiles can be correlated in order to set a threshold for positive cases identifications, thereby approximating the calibration procedure.  
The decision is expressed in the following way:  
\[I_{v}>I_{vthreshold}\ \Rightarrow\ \text{SARS}-\text{CoV}-2\ \text{is positive} \tag{1}\]  
\[I_{v}<I_{vthreshold}\ \Rightarrow\ \text{SARS}-\text{CoV}-2\ \text{is negative} \tag{2}\]  
where \(I_{v}\) represents the attained intensity of the sample or swab under test while \(I_{vthreshold}\) shows the threshold intensity.  
The proposed probing can be used in both oropharyngeal swab and nasopharyngeal swab methods. When the probe encounters the swab, modulated intensities are triggered. If the attained intensity \(I_{v}\) abides by Equation (1) then the sample is COVID-19 positive while if \(I_{v}\) is below \(I_{vthreshold}\) than the sample is declared COVID-19 negative.  
Similarly, the viral load is included in the proposed system based on the modulated response and calibration from the NP standard. The handheld unit can be equipped with wireless capability that can be incorporated into this method. The two approaches are quite viable/adoptable, depending on the receptor for functionalizing the probes. Themodulated responses acquired with respect to the swabss are used to establish the cases' positivity. The optimization of the specimen with several species of viruses belonging to the SARS-CoV-2 family will be carried out before it is used for direct sample analysis. If the modulated response is above the threshold, the diagnosis is complete, enabling avoidance of false-positive results at the same time. This step is shown as Figure 4 and labeled as step (D).