| Brillouin fiber lasers with narrow linewidth,low threshold,high optical signal-to-noise ratio and high sensitivity have been widely used in optical fiber wireless systems,optical communication systems,slow/fast optical technology,optical bistable,distributed/point fiber,sensors and microwave photonics.In particular,multi-wavelength Brillouin erbium-doped fiber lasers have become the first choice for high-sensitivity fiber sensors due to their unique advantages of high signal-to-noise ratio and narrow linewidth.In previous studies,researchers can improve the characteristics of lasers by using first-order or high-order Stokes.The current research have no a way to directly control the Brillouin frequency shift interval for laser temperature sensing,and the development of high-sensitivity fiber optic sensors still has technical bottlenecks that need to be resolved to further improve the accuracy,especially for sensing applications.The number of wavelengths output by the laser and the power flatness are one of the reasons that affect the sensitivity of the fiber laser sensor.Both of them are ultimately limited by the gain spectrum of amplified spontaneous emission(ASE),which greatly limits the sensitivity of the sensor.Therefore,targeted design is needed to achieve a large number of light waves output by the laser and equal power balance.Therefore,a Brillouin fiber laser sensor with three Brillouin frequency spacing for temperature measurement is proposed and studied in detail.Based on the existing fiber laser sensing,the method of high-order Stokes wave sensing is proposed,and the high-order Brillouin frequency shift is demodulated by the way of beat frequency,and the temperature parameters are accurately measured.Then the sensing system based on self-excited Brillouin fiber laser is developed.For the number of wavelengths and the power stability of laser outputs,a novel ASE gain spectrum flattening scheme based on the unpumped Erbium-doped fiber Sagnac loop is proposed.The realization of this scheme is expected to develop a novel fiber laser sensor based on multi-wavelength Brillouin fiber laser,and provide a new method for high sensitivity fiber sensingThe specific research work in this paper is as follows:(1)The characteristics and applications of Brillouin erbium-doped fiber laser sensor and erbium-doped fiber amplifier are briefly described.Several common technical schemes of controlling Brillouin spacing to improve strain accuracy and achieve ASE gain spectrum flatness are introduced.Finally,the main content and innovation of this thesis are described.(2)The basic principles of Brillouin erbium-doped lasers are introduced successively,including nonlinear Brillouin scattering,basic principles of Brillouin fiber lasers,basic theories of high-order Stokes waves for temperature sensing,and ASE gain spectrum.(3)A method of directly controlling the Brillouin frequency interval for laser temperature sensing is proposed.The 10 km test fiber FUT1 and the 5km test fiber FUT2 are used as the first,second,and third-order Stokes gain media,respectively.Controlling the pump power and EDFA power,the sensitivities obtained are about 1.036 MHz/°C,2.006 MHz/°C and3.104 MHz/°C,which finally increases the temperature sensitivity by 3 times.(4)The proposed Sagnac loop with unpumped erbium-doped fiber makes the flatness of the ASE gain spectrum at the transmission end of the ring reach ±1.225 d B,covering almost the entire C-band,with a flat range of 36.5nm.In the 16 nm and 14 nm wavelength ranges of the reflection port and the transmission port,the partial flatness of ASE is ±0.15 d B and±0.145 d B,respectively.If the gain flatter is widely used in wavelength division multiplexing and multi-wavelength lasers,more laser outputs with the same power and narrow linewidth can be obtained. |
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