Spectral characteristics and interrogation of a fiber-optic ball resonator biosensor modulated by a tilted fiber Bragg grating

In this work, we describe the design and analyze the characteristic of a fiber-optic biosensing device formed by a fiber-tip ball resonator (BR) assisted by an in-line tilted fiber Bragg grating (TFBG). The BR-TFBG device acts as miniature fiber-tip biosensor, characterized by a broad reflection spectrum remodulated by the cladding modes of the grating that enhance the amplitude sensitivity. We demonstrate an interrogation method that improves the sensitivity on the cladding mode comb, showing a sensitivity up to 274.0 dB/RIU in amplitude by using polarization-based analysis, and 154.7 nm/RIU for wavelength shift by using a cross-correlation method. Then, we demonstrate the biosensing application of the BR-TFBG for the detection of human epidermal growth factor receptor-2 (HER2) in buffer, within 0.5 - 128.0 ng/ml concentration.