Microhole-pair hollow core fiber Fabry–Perot interferometer micromachining by a femtosecond laser

•A sensitive and high-response microhole-pair (MP) hollow-core fiber (HCF) Fabry–Perot (FP) interferometer (MPHCFFPI) by microdrilling two holes symmetrically on the sidewall of the HCF via a femtosecond laser micromachining technique.•The proposed method can overcome the problems of non-parallel walls of the microcavity/microhole and rough surface of the FB cavity to achieve a high extinction ratio of optical interference.•Only 0.005 s is needed to completely fill the HCF cavity with alcohol via the capillary action of the proposed microhole-pair to demonstrate its effectiveness of the sensing configuration. This study presents a sensitive and high-response microhole-pair (MP) hollow-core fiber (HCF) Fabry–Perot (FP) interferometer (MPHCFFPI) by microdrilling two holes symmetrically on the sidewall of the HCF via a femtosecond laser micromachining technique. The dimension of FP microcavity in the MPHCFFPI is length of the HCF. Only 0.005 s is needed to completely fill the HCF cavity with alcohol via the capillary action of the MP. The MPHCFFPI is developed to measure the thermo-optic coefficients (TOCs) and refractive indices (RIs) of liquids to investigate its effectiveness and accuracy. The TOCs of the deionized water, ethanol, and acetone are accurately determined as −1.076×10−4, −3.11×10−4, and −4.28×10−4 °C−1, respectively. The measured RIs of liquids ranging from 1.3 to 1.6 also highlight that the proposed fiber sensor can achieve higher RI than the silica fibers.