| Fiber-optic sensors have moved out of the laboratory and are widely used in defense,transportation,and medical applications.Compared with general single-mode fibers sensors,polarization-maintaining fibers sensors not only have the advantage of resisting external interference and maintaining stable transmission of polarization state,but also have more parameters such as birefringence,polarization principal axis,and mode coupling,and can be used for more parameters sensing.When Polarization-maintaining fiber is used as a sensor,new birefringence will be introduced.Therefore,as the basic sensing mechanism of polarization-maintaining fiber sensor,the double polarization effect between the inherent birefringence and the induced birefringence caused by the measurement parameters must be considered.In this paper,the concept of quaternion are introduced and applied in the field of polarization optics.It is of great theoretical significance to thoroughly understand the double polarization effect,which is the basis and prerequisite for the research of polarization-maintaining fiber-optic sensors.Another important issue of polarization-maintaining fiber-optic sensors is that the performance of the polarization-maintaining fiber is degraded and the performance of the sensor is degraded.The most important factor is polarization-mode coupling or polarization crosstalk.Therefore,it is important to thoroughly study the influence of polarization mode coupling caused by a variety of factors on the transmission of polarization states in polarization-maintaining fibers,and to realize the distributed,high-precision,and high spatial resolution measurements of polarization-mode coupling coefficients of polarization-maintaining fibers.Polarization-maintaining fiber-optic sensors can be classified into three types:point,quasi-distributed,and distributed.Distributed polarization-maintaining fiber-optic sensors have the dual advantages of both distributed sensing and polarization-maintaining fiber sensing,which means that the measurement parameters will be more,and measurement results will be more stable.The main technical solution of polarization-maintaining fiber distributed sensors is polarization-based OTDR technology.However,for traditional P-OTDR technologies,the polarization state detecting is closely related to the pulse width,and the length corresponding to the pulse width must be less than one quarter of the beat length.Since the polarization-maintaining fiber has a very short beat length(in the order of mm),according to the traditional technology,the pulse width of the distributed polarization-maintaining fiber distributed sensor should be in the order of sub-ps or even fs,which is very difficult to achieve.Therefore,as an actual sensor,it is necessary to break this constraint.In this paper,a polarization-maintaining fiber-optic distributed sensor aligned with the polarization principal axis is proposed,and distributed measurement of a polarization-maintaining fiber sensor is realized.In a P-OTDR system,achieving low noise amplification of the Rayleigh backscattered light becomes a key issue.Therefore,in-depth study of the EDFA's amplification mechanism and suppression of ASE noise,and achieving low-noise amplification of weak signal light are very important to improve the measurement distance and measurement accuracy of a P-OTDR systems.This paper contains a lot of research work on the above issues.The innovative results obtained during the research process are as follows:1.The quaternion method was adopted and the stress-type PMF was taken as an example to theoretically analyze the multiple birefringence synthesis in PMF.Finally,the conclusion that"the synthesis of birefringence induced by multi-polarization-effect in PMF satisfies the vector superposition principle."was deduced.2.A polarization-maintaining fiber stress sensor structure was proposed.Based on the elastic-optical effect in PMF,the equivalent step fiber method was used to derive the relationship between the stress magnitude and the dielectric constant of the fiber.Ignoring the loss of the fiber,the four factors(i.e.,intrinsic birefringence,SOP of the incident light,direction and magnitude of the external stress)that determine the output SOP of PMF were studied by using the quaternion method,ultimately yield a linear relationship between the direction of the stress principal axis and the direction of the external stress.Based on different types of fibers,the maximum angle measurement error is less than 2.24%.This scheme fills the gap in the measurement of the stress direction based on the optical fiber.In theory,it is feasible to simultaneously measure the magnitude and direction of external stress based on the SOP of the output light.In addition,an experimental study was performed on the deformation recovery characteristics of different coated optical fibers,and it was found that the metal coated optical fiber is more suitable for the scene where the stress changes rapidly than the ordinary coated optical fiber.3.A polarization-maintaining fiber twist sensor structure is proposed.The quaternion method is used to study the relationship between the fiber twist and the evolution of the SOP.It is found that the twisting principal axis are independent of the intrinsic birefringence or the length of twisting,and only depend on the direction of twisting.In addition,the twisting coefficients of different types of fibers were measured experimentally,and a scheme for measuring the twist angle of optical fibers based on the evolution trajectory of SOP was proposed.The proposed scheme has a simple structure,low cost,and high measurement accuracy.4.A distributed sensor structure based on polarization-maintaining fiber is proposed.Based on the classical theory of power mode coupling,we derive that the PMC coefficient is obtained by solving the polarization ER,which is calculable by the SOP of Rayleigh backscattered light.The scheme realizes the distributed measurement of the multi external stress points of the PMF.The feasibility and repeatability of the scheme are verified by experiments,with spatial resolution of I meter and positioning accuracy of 0.5 meters.Compared with the traditional incomplete P-OTDR,the proposed complete P-OTDR has higher measurement accuracy and spatial resolution.5.A method for measuring the mode coupling of sensitive optic fiber ring is proposed.Using the three-point quaternion method,it is theoretically proved that the PMC coefficient between the fiber sections can be calculated by the three Stokes vectors corresponding to the three adjacent points on the fiber.The comparison results of the forward and backward,the fast and slow axes in the fiber with different lengths verified that the scheme has high feasibility and repeatability,with spatial resolution of 1 meter.The feasibility of distributed measurement and positioning of the systen to achieve principal axes misalignment of polarization-maintaining fibers was also verified.This achievement provides a novel scheme for accurately measuring the PMC of the PMF ring and improving the positioning accuracy of the coupling point,and has important reference significance for improving the performance of the fiber optic gyroscope. |
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