| Recently,the optical sensing technology based on surface plasmon resonance(SPR)effect and Sagnac interference effect have been received widely attention and investigated,owning to its merits such as high sensitivity and high stability.The special optical fiber is a kind of fiber with special properties,including strong evanescent field effect,high birefringence effect,high nonlinear effect,and controllable dispersion.On account of these characteristics,the special optical fiber has important research values and broad application prospects in the field of optical fiber sensing.Therefore,combining special optical fiber with SPR effect and Sagnac interference effect is expected to further improve the performance of optical fiber sensor.In this paper,based on the SPR effect and Sagnac interference effect,a variety of special optical fiber sensors were experimentally and numerically studied by depositing a metal film at the outer surface of the special optical fiber,filling the temperaturesensitive liquid in the air holes of the special optical fiber,and cascading structures.The specific research works are as follows:Firstly,three kinds of special optical fiber SPR sensors with external metal film coating were studied.By depositing silver film on the outer surface of the no-core fiber(NCF)to introduce SPR structure,a high sensitivity silver-coated NCF refractive index sensor was experimentally investigated.The influences of silver film thickness and analyte refractive index on SPR dip were tested and analyzed.The experimental results showed that the optical fiber sensor can realize refractive index sensing in a wide range of 1.3330-1.4107 RIU,and the sensitivity can reach 5228.21 nm/RIU,with good repeatability.In order to further improve the sensitivity,a side-polished gold-coated offset-core photonic crystal fiber(PCF)refractive index sensor was numerically studied.By setting an offset-core to enhance the leakage of the evanescent field,the sensitivity was improved.The coupling characteristics between the core mode and the SPP mode,and the variation law of the resonance wavelength with the analyte refractive index were analyzed by the finite element method.A single-polarization PCF refractive index sensor was obtained with a maximum sensitivity of up to 42000 nm/RIU.In order to further improve the mechanical strength of the PCF sensor,a PCF refractive index sensor with gold film coating on the outer surface of the optical fiber was numerically studied.High sensitivity was also achieved by providing only one layer of air holes in the cladding to enhance the leakage of the evanescent field.A dual-polarization PCF refractive index sensor was obtained,whose maximum sensitivity and fabrication tolerance reached 16900 nm/RIU and ±0.4 μm,respectively.The PCF sensor not only improved the sensitivity but also reduced the fabrication difficulty.Secondly,three kinds of special optical fiber Saganc sensors were studied.By filling the air holes of polarization-maintaining photonic crystal fiber(PM-PCF)with ethanol to improve the sensitivity,a high sensitivity ethanol-filled PM-PCF temperature sensor was experimentally studied.The influences of ethanol filling on the sensitive characteristics of the sensor were tested and analyzed.The experimental results showed that the sensitivity of the optical fiber sensor reached-2.66 nm/°C,which was 1.55 times higher than that of unfilled ethanol.In order to realize the flexible modulation of the sensor performance,an optical fiber sensor based on cascaded Sagnac interferometers was further experimentally studied.By cascading two Saganc interferometers containing polarization-maintaining fibers(PMFs),the influences of cascading on the strain/temperature sensitive characteristics of the sensor were investigated.The experimental results showed that the strain and temperature sensitivities of the optical fiber sensor were as high as 336 pm/με and-14.86 nm/°C,respectively,which was about 10 times higher than that of a single Sagnac interferometer.In order to facilitate the integration of optical fiber sensors,a thermosensitive liquid-filled PCF temperature sensor was numerically studied.The relationship between the interference dip wavelength and the phase difference,and the variation law of the interference dip wavelength with temperature were analyzed by the finite element method,and a short-type and high-sensitivity PCF temperature sensor was obtained.Finally,combining SPR effect and Sagnac interference effect,a multi-parameter optical fiber sensor was experimentally studied by connecting a SPR optical fiber sensing structure with a Sagnac optical fiber sensing structure in series.Owing to the combined action of the dual sensing effect,the optical fiber sensor can realize ultra-wide range of multiparameter sensing(refractive index,temperature,and strain)from the visible light band to the near-infrared band,and the refractive index sensing and temperature/strain sensing are independent of each other without interference.The experimental results showed that the refractive index sensing was realized in the visible light band by the sensor,and its refractive index sensitivity reached 4462.61 nm/RIU;the temperature/strain sensing were realized in the near-infrared band,and its temperature and strain sensitivities were-1.10 nm/°C and34.56 pm/με,respectively.In addition,the sensor was equiped with good sensing hysteresis of refractive index,temperature,and strain.The experiment proved that the combination of SPR effect and Sagnac interference effect can effectively expand the application field of special optical fiber sensor,and promote its development in diversified fields such as environmental protection,food safety,and agricultural breeding to a certain extent. |
