| The optical fiber sensor is a new type of sensor with using light as the carrier and using fiber as medium.Compared with conventional sensors based on electrical techniques,the optical fiber sensors have merits such as anti-electromagnetic interference,high corrosion-resistance,high sensitivity,thus the sensors have been widely used in many fields such as mechanical engineering,chemical industries,petroleum storage and structural health monitoring.In recent years,the optical fiber sensors have been the focus of research at home and abroad.The fiber modal interferometers is an important research direction of the optical fiber sensing technology with numerous advantages such as high degree of integration,high sensitivity,and easy to multi-parameter measurement.This paper mainly studies the sensing characteristics of the new fiber modal interferometers and the main subjects are no-core fiber(NCF)modal interferometer,multi-core fiber(MCF)modal interferometer,and multimode fiber(MMF)-MCF-MMF modal interferometer.And the spectral characteristics of the fiber modal interferometers are studied in detail.The measurement of different physical parameters of the fiber modal interferometer is verified by experiments.The paper focuses on the cross sensitivity of different parameters of the new fiber modal interferometer.Multi-parameter sensors have been achieved by using the sensing property of fiber grating.Also,the measuring principle and error of the multi-parameter sensors are analyzed.By using the sensing and filtering property of new fiber modal interferometers,fiber optic laser sensors which can measure different physical parameters are realized.The main research contents and innovation points are as follows:1.The NCF modal interferometer has been fabricated experimentally.The device consists of a segment of NCF spliced between two single-mode fibers(SMFs).The sensing properties of the strain and refractive index(RI)are studied experimentally.By using RI sensing property of NCF,a liquid level sensor is realized by fixing the NCF to the U steel rule.Using the curvature sensing property of NCF,the NCF is fixed on the aluminum wire,and the current sensor based on the Ampere force and NCF is realized.The interferometer has the advantages of simple configuration and high mechanical performance.2.The MCF modal interferometer has been proposed.The device consists of a segment of MCF spliced between two SMFs.When the device is kept straight,an obvious interference pattern appears in the transmission spectrum.The maximum fringe visibility of the interference resonance dips exceeds 23 dB.The spectral characteristics and sensing properties of the interferometer are studied in detail.The device is used to measure the strain,temperature,RI,curvature,and so on.The device shows good linearity for the above parameters.The MMF-MCF-MMF modal interferometer has been proposed.MMFs at both ends of the MCF are used as a beam splitter and combiner,respectively.By comparing with the MCF modal interferometer,the spectral characteristics of the device are optimized,and the spectral loss is not increased.The sensing properties of the device are studied.The interferometer is used to measure the strain,temperature,RI,and so on.The interferometer has the advantages of high fringe visibility and simple fabrication process.3.A liquid level sensor based on an asymmetric SMF modal interferometer has been proposed.The device can be realized by concatenating a fiber taper and a lateral-shifted junction.By using the temperature sensing property of fiber Bragg grating(FBG),the liquid level sensor with dynamic temperature compensation can be achieved.For 10 pm wavelength resolution,a resolution of 0.15 cm in liquid level and 1.01? in temperature can be achieved.The sensor is based on SMF,which has advantages of low cost and high operability.4.An optical fiber sensor capable of measuring strain and temperature simultaneously has been proposed and demonstrated.The key component is a NCF followed by a FBG,which helps to resolve the crosstalk issue.The interference fringe of the NCF and the Bragg wavelength of the FBG would shift with the variation of strain and temperature.Since the NCF and the FBG have different sensitivity responses to strain and temperature,it is possible to measure strain and temperature simultaneously by simply monitoring the wavelength shifts.For 10 pm wavelength resolution,the resolution of the sensor is 8.4?? and 0.78? in strain and temperature,respectively.The prototype might be attractive due to its high resolution and simple fabrication process,which are desirable features in dual-parameter measurement.5.An optical fiber magnetic field sensor based on the NCF and magnetic fluid(MF)is proposed and demonstrated since the NCF has no cladding,and is sensitive to RI.The NCF is sealed in a capillary tube which is filled with MF and sealed at both ends with epoxy glue.The transmittance of the NCF that is sealed in a capillary with MF is highly sensitive to the surrounding magnetic field.A FBG is written in the output SMF.By using the temperature sensing property of FBG,the magnetic field sensor with dynamic temperature compensation can be achieved.When the proposed sensor operates within the linear range and the wavelength resolution of 10pm,the proposed sensor can provide the magnetic field resolution of 1.09 mT and the temperature resolution of 0.84?.The magnetic field sensor has the advantages of high sensitivity and temperature compensated,which are desirable features in magnetic field measurement.6.An optical fiber sensor capable of measuring RI,strain,and temperature simultaneously has been proposed and demonstrated.The sensor is formed by the integration of a MCF with a fiber FBG By monitoring the wavelength shifts of three dips,simultaneous measurement of RI,strain,and temperature can be achieved.For 10 pm wavelength resolution,the resolution of the sensor is 0.0004 RIU,11.06??,and 0.17? in RI,strain,and temperature,respectively.The sensor can also measure the liquid level,strain,and temperature simultaneously.For 10pm wavelength resolution,the resolution of the sensor is 0.034 cm,11.06??,and 0.17? in liquid level,strain,and temperature,respectively.Compared with the existing three parameter optical fiber sensors,the sensor has the advantages of high fringe visibility and simple information processing.7.Fiber optic laser sensors using fiber ring cavity laser based on NCF modal interferometer and MCF modal interferometer have been proposed,respectively.The modal interferometer is inserted in the fiber ring laser cavity,which acts as a band-pass filter and the sensing component simultaneously.The interference fringe of the modal interferometer would shift with the variation of monitoring parameters and result in the change of lasing wavelength changes.Thus,it is available to measure these parameters by simply monitoring the variation of lasing wavelength.The fiber optic laser sensor not only has the advantages of optical fiber sensor,but also has the advantages of high resolution and high signal-to-noise ratio,which are desirable features in ultra-long distance,high-precision measurement fields. |
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