| With the rapid development of optical fiber communication,internet of things and sensing detection technology,optical fiber devices have become a research hotspot in the field of new optoelectronic devices due to their advantages such as low additional loss,high reliability,easy integration and compatibility.Microstructured fibers have attracted people’s attention for their flexible structural design and excellent performance.With the maturity of fiber microfabrication and post-processing technologies,combining surface plasmon resonance(SPR)with microstructured fibers through metal coatings can further enhance or extend the performance of fiber optic devices.In this paper,based on the optical fiber transmission theory and the principle of surface plasmon resonance,several side-polished microstructured optical fiber sensors were designed and their performances were numerically analyzed.Several refractive index sensors based on the core and cladding modes excited SPR effect were prepared experimentally,and the sensitivity,stability and applicable spectral range of the sensors were studied.The main contents of the paper are as follows:Firstly,three microstructured optical fiber sensors with different core structures and side-polished microgroove coating were designed.The sensing channel was constructed by the optical fiber side polishing method,forming the evanescent field leakage window,and the gold film was deposited on the surface of the groove to stimulate the SPR effect,so as to realize the high-sensitivity detection of the object to be tested.The finite element method was used for numerical calculation,and the influence of fiber structure parameters,polishing depth and gold film thickness on the confinement loss of the fiber was analyzed.Based on the double-side polished structure,the core mode field can leak out and overlap with the gold film,thereby enhancing the SPR effect,and realizing the polarization transmission and refractive index sensing near the communication wavelength of 1550 nm.The structural design based on the microporous core can effectively control the core mode field,enhance the resonance coupling between the core mode and the surface plasmon mode,and realize the sensing detection of dual sensing peaks,high sensitivity,high resolution and high quality factor.The structure design based on the liquid crystal core can provide the core mode field that changes with temperature,and realize the dual detection of temperature and refractive index.The design of these side-polished microgrooved-coated microstructured fibers provides a new idea for high-sensitivity,multi-parameter optical fiber sensing.Secondly,a side-polished optical fiber sensor based on silver/graphene coating was designed and fabricated.The self-made optical fiber polishing system was used to realize the side polishing of singlemode and multimode fibers.The variation of the output power with the polishing depth was explored,and the influence of the polishing depth on the SPR effect and transmission spectrum of the fiber was summarized.A side-polished single mode fiber with a polishing depth of 57.91 μm and a polishing length of 3 mm was obtained,and a side-polished single mode fiber SPR sensor decorated with silver/graphene double-layer film was prepared by silver mirror reactive coating and wet transfer of graphene.The graphene film increased the sensitivity of the SPR sensor from 2636 nm/RIU to 2903 nm/RIU;the silver/graphene film SPR sensor made of side-polished multimode fiber obtained a resonance peak located in the near-infrared wave band(800-1100 nm),the sensitivity was 3001 nm/RIU sensing effect.The graphene film can not only enhance the SPR effect and adjust the resonant peak position of the sensor,but also protect the silver film and resist oxidation.Finally,an outer surface-coated microstructured optical fiber sensor based on the resonant coupling of the cladding mode and the surface plasmon mode was proposed.The characteristics of the sensor are as follows: there is no need to polish the side of the optical fiber,and the silver film and graphene were directly deposited on the outer surface to construct the multimode fiber-microstructured fiber-multimode fiber sensing structure.Excitation of the SPR effect was achieved using evanescent field leakage of cladding modes in microstructured fibers.The experimental and numerical analysis found that the silver film effectively stimulates the SPR effect,and graphene can greatly promote the transfer of the cladding energy to the interface to enhance the SPR effect.Adding graphene film increased the sensing sensitivity from 2686 nm/RIU to 2917 nm/RIU.The silver/graphene bilayer film-modified microstructured fibers sensor has high sensitivity,good repeatability and temperature stability,and is expected to be applied in biomedicine and environmental monitoring. |
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