Microstructured Fiber-Optic Sagnac Interferometer Fabricated by Femtosecond Laser Micromachining for Optofluidic Sensing

A novel fiber-optic Sagnac interferometer (FSI) based on a four-leaf clover microstructured optical fiber (MOF) is proposed for optofluidic sensing. The FSI is formed by sandwiching a section of MOF with a side microchannel between two single-mode optical fibers. By using femtosecond laser micromachining technology, micropores for fluid inlet and outlet are precisely etched above the air hole on the one side of the long axis of the MOF fiber core for the transmission of fluid inside the optical fiber. The fluid changes the birefringence characteristics of the MOF, producing a Sagnac effect. Then, the birefringence characteristics, spectral characteristics, and sensitivity characteristics of the designed optofluidic FSI are studied theoretically and experimentally. The experimental results indicate that for analytes with a refractive index (RI) near 1.33, the optofluidic FSI has a sensitivity of 1085 nm/RIU with a linearity of 0.999, which is basically consistent with that obtained by theoretical simulation. The optofluidic sensor has excellent optofluidic sensing performance and good repeatability. In addition, the optofluidic sensor fabricated using femtosecond laser processing technology demonstrated excellent reproducibility and good robustness. The proposed all-in-fiber optofluidic sensor, which combines microfluidics with fiber-optic sensing technology, has great development potential in the field of ultralow-volume, label-free biochemical molecule detection.