Wave theory based analysis with radially polarized Bessel-Gauss beam: a method for fiber optic cortisol biomarker detection with high sensitivity and resolution using wavelength interrogation technique

In this study, an analytical methodology based on wave theory is presented in a multilayered SPR fiber optic biosensor which detects cortisol, an important stress biomarker, using a radially polarized Bessel-Gauss (RPBG) beam. The analytical model is first illuminated using a Gaussian (G) beam, and the outcomes are compared with experimental data that has already been published. The findings are correlated favourably with the assessed data presented by Usha et al. Biosens. Bioelectron. 87, 178–186 (2017). Following the confirmation of the suggested theory, the method has been modified to include Gaussian, radially polarized Gaussian (RPG), and RPBG beams as source inputs for the suggested configuration. The design of this idea features a no-core fiber (NCF) integrated between two higher-order mode fibers. The efficacy of the RPBG beam within this waveguide structure is validated using eigenmode expansion (EME) analysis in mode solutions software (Lumerical’s Inc.), ensuring the accurate evaluation of light propagation. Initial investigations with a Gaussian beam demonstrated a sensitivity of 182.8 nm/ng/mL, 4945.23 nm/RIU, and 16,059.4 dB/RIU, which is 2.2 times superior to the conventional Gaussian beam-based sensors. Next, employing an RPG beam, these values increased to 202.7 nm/ng/mL, 8844.31 nm/RIU, and 76,683.33 dB/RIU, respectively. A further extension, incorporating an RPBG beam, improves the sensitivities with 241.85 nm/ng/mL, 16,515.12 nm/RIU, and 195,951 dB/RIU, with an enhanced resolution of 5.1 × 10−⁸, surpassing the Gaussian-based sensor by a factor of 7.36. This implementation of an RPBG beam significantly boosts the sensitivity in fiber optic cortisol biomarker detection. Here, the limit of detection (LOD) is 0.001 ng/mL, and significant spectral shifts are observed when the cortisol biomarker concentration ranges from 0 to 2.5 ng/mL. Thus, the early identification of psychological stress through optical measurement may be facilitated by the attainment of improved sensitivity and resolution at lower concentrations. This sensor enables non-invasive cortisol detection through saliva analysis, offering a practical and scalable biosensing platform with high sensitivity for biomedical diagnostics and occupational health monitoring.