Optimizing optical setup for transmission spectra shift-based measurement of DNAs in hollow-core photonic crystal fibers

Hollow-core photonic crystal fibers have shown promising potential for label-free DNA detection, relying on a multi-step functionalization of their inner surface to capture target DNA selectively. This process forms a bio-layer altering the fiber’s cladding thickness, causing a shift in the transmission spectrum and allowing a label-free detection with just an attenuation measurement. However, it is crucial to ensure the excitation of the fundamental mode (FM) at each functionalization step. The current optical setup has limitations: achieving FM excitation is difficult, and mode verification relies on a camera that averages modes over the range of wavelengths. In this paper, the first issue is addressed by adding two mirrors between the light source and the fiber so that the angle of light entering the fiber can be controlled, avoiding the excitation of the high order modes (HOMs) and making the setup more stable and flexible. The second issue is solved by using the band-pass filters before the camera so that the FM excitation can be checked at specific wavelengths of greater utility for the detection process. The experiments have shown that the mirrors allowed the excitation of a range of different modes, and the filters were found to be useful in improving the sensing accuracy.