Study On Fiber Laser And Fiber Sensor Based On Tunable Sagnac Loop

As the main media of information transfer in the internet age,optical fibers and optical fiber technologies develop dramatically.More importantly,their applications have been spread in fiber optic communications,fiber-optic sensing,aerospace and many other important areas.In this regard,fiber lasers and sensors become a hot topic in recent years.Fiber lasers are important components in optical fiber technologies because of their small volumes and flexible tunabilities.However,the existing fiber lasers have many drawbacks,such as less output parameters,poor independent control of parameters,insufficient tunable functions,and polarization interferences between different optical devices.Similarly,optical fiber sensors have a lot of virtues such as simple structures,miniaturization,anti-electromagnetic interferences and high sensitivity characteristics.Consequently,optical fiber sensors have been also widely studied in the field of sensing for measuring external physical quantities such as temperature,strain,refractive index,etc.In this aspect,how to further improve the sensitivities of optical fiber sensors has always been one of the most concerned issues in optical fiber technologies.To solve the above problems of optical lasers and improve the sensitivities of optical fiber sensors,this thesis proposes to introduce Sagnac rings into optical fiber lasers as filters and use Sagnac rings for optical fiber sensing.The properties of the proposed lasers and sensors are comprehensively studied.The detailed contents are as follows:Firstly,the properties of Sagnac rings are theoretically investigated by utilizing the optical Jones matrix.As a result,characteristics of polarization-independent,transmissions,and reflections of Sagnac rings are obtained.The corresponding numerical and experimental studies confirm the theoretical results and it shows that Sagnac rings possess tunable functions.Secondly,use the Sagnac ring as a filter,a tunable multi-wavelength erbium-doped fiber laser(TMWEDFL)based on Sagnac loop filter and nonlinear optical loop mirror(NOLM)is proposed.The results of simulations and experiments indicate that the proposed optical fiber laser can have a multi-parameter output and abundant tunable functions.Specifically,the spectral range and the number of wavelengths can be changed by adjusting the NOLM.The experimentally obtained maximum shift of the spectral range is 8.88 nm and the number of wavelength is 11;And by changing the polarization state of polarization maintaining fiber(PMF)in the Sagnac ring,the output wavelength can be tuned precisely in a free spectral range.Due to the polarization independent of the Sagnac ring and the polarization state of the NOLM can be considered as the incident polarization sate of the Sagnac ring,adjusting NOLM has no impact on the output of the whole system.It results in a quite stable output wavelength and therefore the polarization interferences are avoided.In addition,the number of the output wavelengths can be controlled independently by tuning the power of pumping.Thirdly,the thesis proposes a cascade Sagnan loop optical fiber sensor for strain measuring based on the Vernier effect.Compared with traditional optical fiber sensor,the sensitivity of the proposed optical fiber is apparently improved since the cascaded Sagnac loops provide a Vernier amplification effect.The experimental result shows that the maximum sensitivity of such optical fiber sensor can reach 4.43 nm/u? indicating a much higher measurement accuracy.