Study On All-fiber Format Single-frequency Tm-doped Fiber Laser And Intra-cavity Optical Fiber Sensing Technology

High-power single-frequency fiber lasers and the intracavity fiber sensors based on high-performance laser technology are of great interest for their wide and significant applications in hyperfine spectral analysis,high-speed long-distance coherent communication,high-accuracy remote sensing,and in high-sensitivity gas concentration detection,high-resolution temperature measurement and so on.In this thesis,we focused on the all-fiber-based high-power single-frequency Tm-doped fiber laser source and the application of all-fiber-based intracavity sensors in gas concentration detection and temperature measurement.The work includes following parts.1.An all-fiber format 2?m Tm-doped MOPA system is developed.We first made a DBR single-frequency fiber laser at 1925 nm with 35-mW output power.The spectral linewith was less than 50 kHz and the RIN was-120 dB/Hz.Then it was amplified to45 W through a three-stage cladding-pump MOPA system,with corresponding slope efficiencybeing 43.3%.The signal linewidth at the maximum output power is 56 kHz,and the central frequency and the peak power of the RIN are 860 kHz and-90 dB/Hz,respectively.It is indicated that the central frequency of the RIN is determined by the laser upper-level lifetime of the active fiber while the linewidth has a relationship with the SNR.The increase of ASE results in the linewidth broadening by decreasing the SNR.2.The thermal distribution of double-cladding active fiber is analyzed.Taking Tm-doped fiber as an example,the internal thermal distribution analytic expression of active fiber was obtained based on the rate equations of the amplifier and the thermal conductive equation.Then we analyzed the influences of the inner cladding shape and the injected signal power on the extraction efficiency and the thermal distribution in the transverse cross-section and along the longitudinal direction.The results can be meaningful for the optimization of the high-power fiber amplifiers.3.Single-frequency fiber laser is employed in high-resolution temperature measurement.An intracavity temperature sensor based on tunable single-frequency ring fiber laser is designed and employed to realize the temperature measurement with a range from-5oC to 60oC.The corresponding sensitivity is about 9.65 pm/oC with a measurement error less than 2.9%.Considering the linewidth of the output signal is within 1 kHz,the optical heterodyne method can be used to improve the theoretical resolution limit of the sensor to up to 8.4×10^-6oC.4.The intracavity sensor is applied in gas concentration detection.We design a new type of intracavity sensor based on the wavelength-tunable Sagnac loop filter and the mode competition phenomena in the Tm-doped ring fiber laser,which can realize automatic channel-switching CO₂ gas sensing with high sensitivity.The intracavity sensor is experimentally demonstrated employing a high-accuracy tunable F-P filter operating at 1.5?m region to sweep the output wavelength of ring fiber laser,realizing the acetylene concentration detection at 1532.83 nm and 1534.10 nm.The corresponding sensitivities are 398 ppm and 1905 ppm,respectively.Both of the two channels have the measurement errors less than 7.2%.5.We developed a multipoint intracavity gas sensing network with commercial DWDMs.The acetylene concentration sensing network with standard DWDMs contains three detection channels at 1530.37,1532.80 and 1536.71 nm,respectively.With a piece of hollow-core photonic crystal fiber used as the gas cell,the sensitivity for acetylene concentration detection is 100 ppm.Moreover,the detection efficiency is enhanced by 6.5 times via using the voltage gradient method to control the F-P filter.