Pump Induced Birefringence In Dual-polarization Fiber Grating Lasers

Optical fiber sensors are used in many engineering fields because of their numerous advantages such as high sensitivity,high resolution,low transmission loss,fast response,easy networking.In recent years,dual-polarization fiber grating lasers have been widely investigated for sensing applications.Such lasers are highly sensitive to transverse load which can easily change the fiber birefringence and hence the beat note frequency.Therefore,such lasers can be exploited as photonic sensors by monitoring the beat frequency change.Photonic sensors for the measurement of electricity,magnetic field,displacement,and even acoustic waves have been implemented with this kind of sensing elements,through proper transducer design.Because measurands interact with dual-polarization fiber grating lasers through birefringence,the stability of intrinsic birefringence of lasers is critical to measurement accuracy.In this thesis,the impact of pump induced birefringence on dual-polarization fiber grating lasers sensors has been studied with focus on beat note frequency variation versus polarization of pump.Moreover,two methods to reduce pump induced birefringence are proposed.The main contents of this thesis are as follows:Firstly,this thesis introduces the theory of birefringence and its representation methods with elaboration of the origin of birefringence phenomenon.The theoretical results show that the pump induced birefringence is a result of the interplay among pump induced refractive index change,laser dynamics and anisotropy inside fiber lasers.In addition,the relationship between the birefringence and the beat frequency is analyzed,and we point out that the beat frequency sinusoidally changes with the polarization angle of the pump light.Secondly,the method to control the polarization of a pump light is studied.Several methods to describe polarized light and its conversion are presented with comparison and analysis,including Jones vector,Stokes vector,Jones matrics,Mueller matrices,Poincarè Sphere and quaternions.By using a manual polarization controller composed of rotatable waveplates to adjust the pump light polarization state,the input of any pumped light polarization can be converted to a desired output polarization,which can be intuitively displayed on a PoincarèSphere.Lastly,pump induced birefringence inside dual-polarization fiber lasers is investigated with theoretical analysis and experiment demonstration for different pumping wavelengths.For Erbium-doped fiber lasers,consistent with theoretical analysis,a pumping scheme at 1480 nm is better than that at 980 nm in lower pump induced birefringence.Moreover,we further verified an injection of a green laser at 532 nm can reduce the pump induced birefringence for pumping at980 nm.Injection at 532 nm results in similar effect as that of annealing through heat treatment to reduce anisotropy and hence reduce pump induced birefringence.