Problem of Fano Resonance Characterization in Ring $\pi$ -shift Fiber Bragg Grating Biosensors

Problem of Fano resonance characterization in optical biosensors of the refractometric type based on ring $\pi$ -shift fiber Bragg grating is raised. The use of this type biosensors makes it possible to increase the sensitivity of detecting changes in the refractive index of the medium under investigation to a level ${(1-2)\mathrm{x}10^{-9}}$ RIU. The given sensitivity values are achieved due to the formation of a high-quality Fano resonance in the grating with the contour width at half-height of 1–5 pm. However, the possibility of obtaining significant gains in metrological characteristics poses the problem of characterizing such narrowband resonances, since the existing optical spectrum analyzers and interrogators do not have the appropriate resolution. To solve this problem, a microwave photonics method of three-frequency probing of an ultra-narrowband and asymmetric Fano contour with symmetric amplitude-modulated radiation and unbalanced amplitude sideband components is proposed. Previously one can use microwave photonic method of dual-frequency symmetric balanced probing with equal amplitudes of components and the suppressed carrier for solving such problem. Unlike it, using of proposed method, resolve to get unambiguous determination of the center frequency, maximum amplitude and quality of Fano contours, as well as the possibility of significantly simplifying the technique of collecting information about their parameters. It should be noted that decision has also to be true for the characterization of Fano resonance with its symmetrical realizations. This article consistently reflect the questions of posing the problem and discussion of ways for it decision.