Polarimetric Fiber Laser Sensors In The Current Measurement And Fiber Laser Gyro Application

Fiber laser is a kind of lasers with fibers or doped fibers as its working media. Recently much effort has devoted to the theoretical and experimental investigations of fiber lasers due to their attractive characteristics,such as low-threshold,high efficiency,narrow linewidth,tunable output frequency,compact structure,light weight,high performance and cost ratio,as well as the stability and reliability.Fiber lasers can be widely used in many fields such as sensing,communications,medical and military areas.During these applications,polarimetric fiber laser sensor(PFLS), which detects external perturbations by a direct measurement on the beat frequency's change of the orthogonal modes in a fiber laser,has attracted more and more interests for their remarkable advantages such as high sensitivity and ease of measurement.Currently,most of the research on PFLS is to measure pressure, current,hydrostatic pressure and temperature.The research work in this thesis mainly focus on the study of two types fiber lasers for PFLS,as well as its applications in non-reciprocity measurement including current measurement and fiber laser gyro.More concretely,their design work has been carried out in a comprehensive manner and validated through experiments.The major contributions of this thesis are summarized as follows: (?)Studies on basic working principles and designs of fiber lasersThe basic working principles as well as the major characteristics of laser and fiber lasers are firstly reviewed,including the interaction between light and matter, the laser oscillation conditions and the spectrum distribution,the gain characteristic and spatial hole burning effect,the influence of external environment on the stability of laser output and the actual limits of fiber laser bandwidth;The designs of bidirectional single-mode Er-doped fiber ring laser and multi-wavelength laser is then discussed,respectively;The characteristics of dynamic grating reflection spectrum of the fiber saturable absorber is systematically analyzed. (?)Studies on polarimetric fiber laser current sensorsThree-wavelength fiber F-P cavity laser is designed by utilizing the spatial hole burning effect and the fiber saturable absorber.The design principles and eigen state of polarization of the laser is presented;The theoretical model of three-wavelength fiber lasers used as current sensors is formulated by using Jones matrix method.The analysis is emphasized on the influence of fiber birefringence at the sensing area on the current sensor,and temperature characteristics are also discussed;The design of the fiber laser is verified through experimental research,and a stable three wavelength fiber laser is thus achieved.When the laser is used in a current sensor system,a good linear relation between the beat frequency shift and the current is acquired,with a slope of 3.05KHz/A. (?)Studies on linear polarization fiber laser gyroA direction related polarizer(DRP) is proposed and designed as a non-reciprocal optical device.The theoretical analysis of the working principle and test methods of the device are introduced.Experimental results show that insertion loss is 0.61 dB,the extinction ratio is larger than 23.35 dB;A novel and stable bidirectional single-mode Er-doped fiber ring laser making use of a DRP and a combined filter based on a narrowband thin film filter and a Mach-Zehnder comb filter is realized for the first time to best of our knowledge.Taking account of the effect of fiber birefringence,the influence of the temperature change on the null-drift from the frenquency bias(NDFFB) is analyzed.When the refractive index difference is 0.01 and temperature change is 0.01℃,the NDFFB is on the order of magnitude of 10KHz;The design of linear polarization fiber laser gyro is experimentally verified.The fiber laser with a pumping power threshold of about 16 mW and a lasing wavelength of 1550 nm is realized.When the pumping power is between 20 mW and 70 mW,the fiber laser maintains single longitudinal mode operation in both CW and CCW directions.The signal to noise ratio of the CW-CCW beat signal is larger than 30dB.By adjusting the polarization controller,a good linear relation between the beat frequency shift and the cavity rotation rate is observed in a gyro system when the frequency bias is 1.68 MHz or 409 KHz respectively.The scale factor is 1.38 KHz/(deg/sec) and 1.23 KHz/(deg/sec) respectively,and the NDFFB is about 7 KHz,which are all consistent with the theoretical analysis.The phenomenon that the scale factor reduces with the decrease of the frequency bias is analyzed,and its solution is then given theoretically.A continuous-wave fiber laser gyro is achieved for the first time. (?)Studies on circular polarization fiber laser gyroA circular polarization fiber laser gyro is proposed.The principle of the circular polarizer and twisting fiber is analyzed.The fabrication methods of the twisting fiber are improved;The closed-form expression of the lock-in effect in fiber laser gyro is deduced for the first time Numerical simulation shows that the lock-in threshold of about 7.3°/s is obtained when the working wavelength is 1550 nm,the FSR and the diameter of the ring cavity is 34 MHz and 38 cm,respectively;The design of circular polarizationpolarized? fiber laser is implemented and validated for the first time.The threshold pumping power of the fiber ring laser is about 35 mW and the lasing wavelength is 1549.956 nm.A stable and continuous laser operation is observed for the bidirectional single mode Er-doped fiber laser and the laser output is larger than 0.1 mW.When the laser was used in a gym system,it is observe that the signal to noise ratio of the CW-CCW beat signal is larger than 30 dB;the scale factor is 2.67 KHz/(deg/sec);the NDFFB is about±1.5KHz;the sensitivity of the fiber laser gyro is 1°/s and the lock-in threshold is about 10°/s.These experimental results are consistent with that of the theoretical simulation,which hence validates the proposed design idea of the circular polarization fiber laser gyro Finally,the problems encountered in the experiment are analyzed and possible solutions are suggested.