Low-Concentration Bilirubin Detection Based on Photothermal Effect With Parallel Vernier Effect of Optical Fiber Fabry–Pérot Cavity

The rapid advancement of industrialization, urbanization, and population aging has led to an increase in the prevalence and mortality of chronic diseases, placing a significant burden on public health. Serum bilirubin levels are crucial indicators for diagnosing various chronic diseases, particularly in adults and the elderly. Therefore, developing high-performance sensors for the online detection of bilirubin is of great research significance. This article proposes a parallel Fabry-Pérot (FP) cavity sensor, where FP cavities are formed by single-mode fibers (SMFs) at both ends. The vernier effect is achieved by parallelizing dual FP cavities with similar but different cavity lengths. Bilirubin detection is performed utilizing its photothermal effect. When the sensor operates, bilirubin in the sensing zone produces a thermal effect under 405 nm laser irradiation, causing the FP air cavity to expand and the interference spectrum drift, thereby enabling the detection of low bilirubin concentrations in serum. Experimental results demonstrate that the sensor has a sensitivity of 5.98 nm/( $\mu $ mol/L) within a linear range of 2– $10~\mu $ mol/L and a detection limit as low as $0.00334~\mu $ mol/L. The proposed sensor features high sensitivity, low detection limit, and strong robustness, indicating its potential application in the early diagnosis and prognosis of chronic diseases such as Alzheimer’s.