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• We propose a highly-sensitive phase-interrogated RI sensor based on twin-core fiber with inherent noise suppression. • It is the first time that optical fiber sensing structure is combined with digital holography technology, which works as the phase interrogation method, for measuring environmental parameters. • The background signal is reduced to 1/25 of the original noise level by using phase subtraction method within a single measurement. • Experimental results show that a short response time of about 13 s and a high sensitivity of about 835 rad/refractive index unit (RIU) are obtained. • The achieved high sensitivity is about 47 times higher than the sensitivity of a refractive index plasmonic sensor realized by enabling its quadrupole resonance in phase interrogation (17.46 rad/RIU). In this work, we propose a highly-sensitive phase-interrogated RI sensor based on twin-core fiber with inherent noise suppression. This is the first time that optical fiber sensing structure is combined with digital holography technology, which works as the phase interrogation method, for measuring environmental parameters. The proposed system consists of a Mach–Zehnder interferometer formed by interference between the reference Gauss beam and the sensing dual-beam from the twin-core fiber (TCF). In the sensing optical path, a non-core fiber (NCF) and TCF offset splicing structure works as the differential fluid RI sensing element, which provides the capability of suppressing background signal by calculating the phase difference between the two cores of the TCF. This simple and practical phase subtraction method realizes direct background subtraction within a single measurement and reduces the background signal to 1/25 of the original noise level. The fluid is continuously introduced into a fluid tube in which the sensing fiber is installed to realize in-line measurement of fluid RI. Experimental results show that a short response time of about 13 s and a high sensitivity of about 835 rad/refractive index unit (RIU) are obtained. The high sensitivity benefits from direct interaction between light path and fluid, which is about 47 times higher than the sensitivity of a refractive index plasmonic sensor realized by enabling its quadrupole resonance in phase interrogation (17.46 rad/RIU). This scheme will find many applications in little sample consumption and high sensitivity required analytical measurement owing to its merits of high sensitivity, little sample consumption, fast response, and inherent background noise suppression. [ABSTRACT FROM AUTHOR]

• Electric field intensity of multilayer of GNPs coupled with gold film was studied. • The RI sensitivity of the double-layer gold nanospheres sensor was 15747 nm/RIU. • The RI sensitivity of the double-layer gold nanorods sensor was 25642 nm/RIU. • The LOD of double-layer gold nanospheres biosensor is 10 ng/mL. • The LOD of double-layer gold nanorods biosensor is 4.6 ng/mL. The main challenge for all biosensors is how to measure the biological samples at very low concentrations. In this paper, a surface plasmon resonance (SPR) fiber biosensor using multi-layer gold nanoparticles (Au NPs)/Au film coupling to enhance sensitivity was proposed. The coupling effect between such longitudinally aligned multilayer nanoparticles increases the intensity of the local electromagnetic field and the depth of propagation to the outside. We used the finite element method to analyze the local enhancement effect of multi-layer Au nanospheres and Au nanorods, and the fabricated double-layer Au NPs (Au nanospheres and Au nanorods)/Au film coupling enhancement sensors. These two sensors' refractive index sensitivities are 15,747 nm/RIU and 25,642 nm/RIU, respectively. By changing biomolecules immobilized on the sensor's surface, the highly sensitive double-layer Au nanoparticle/Au film fiber sensor enables high-precision measurement of different biomarkers at low concentration. For detection of human IgG, the detection limit of gold nanosphere sensor is 10 ng/mL and gold nanorod sensor is 4.6 ng/mL. [ABSTRACT FROM AUTHOR]