| The application of coherent laser radar in the fields of military,environmental science and unmanned driving has made it a hotspot for researchers.Due to the safety of the human eye,low transmission loss,compact system structure,small in size,and easy to transport.The 1550 nm narrow linewidth single-frequency pulsed fiber laser is widely used as a source of coherent laser radar.Therefore,this paper has carried out research work on continuous and pulsed 1550 nm single-frequency laser.Firstly,based on the quasi-three-level structure of erbium ions,the rate equation model of Er-ion quasi-three-level system is established by analyzing the energy level transition mechanism of erbium ions,which provides a theory for the experiment of erbium-doped fiber seed laser.The method of short-cavity method,saturable absorber(SA)selection method and multi-ring cavitys method for obtaining single-frequency laser in fiber laser is introduced in detail.By comparing the advantages and disadvantages of each mode selection method,the method of saturable absorber is selected to obtain a single-frequency laser.The single longitudinal mode erbium-doped fiber laser is simulated by using Optisystem software.The simulations of two resonant cavity structures,without saturable absorber and saturable absorber,are carried out.The effects of gain fiber length,coupling-output ratio and saturable absorber length on laser power and oblique efficiency are discussed.The mode-selection effect of the saturable absorber is simulated by Matlab,and the relationship between the length of the saturable absorber and the output single longitudinal mode is obtained.Then,a single longitudinal mode erbium-doped fiber laser structure is built.The output power,spectral characteristics,single longitudinal mode operation characteristics and laser line width characteristics of the laser are analyzed.The effect of the gain fiber length and the output coupling ratio on the laser characteristics are verified.Three methods for measuring the laser linewidth in narrow linewidth lasers are introduced and compared,and the laser linewidth is measured by the delayed self-heterodyne detection method.A stable single-frequency continuous laser output with an output power of 9 mW,a linewidth of 2.1 kHz,and a center wavelength of 1550 nm is obtained.The stability of the seed light mode is continuously observed within one hour,and it is found that the beat signal was stable without mode hopping.Secondly,an experimental study of single-frequency pulsed laser amplification is carried out.The output characteristics of the pre-amplified structure are analyzed and a single longitudinal mode laser with a power of 89 mW and a line width of 2.8 kHz is obtained.On the basis of the above research work,the experimental research on the pulse amplification structure is carried out.The pre-amplified single longitudinal mode laser is chopped by an acousto-optic frequency shifter,and the pulse light output with the repetition frequency of 10 kHz and the pulse width of 300 ns is obtained.After a first-stage erbium-doped fiber amplification,a single-frequency pulsed laser with a pulse width of 330 ns and a peak power of 1.21 W is obtained at a repetition rate of 10 kHz.Finally,an experimental study on the frequency-locked amplification structure of the ring cavity is carried out.The output characteristics of the seed light structure are analyzed and a single longitudinal mode 1549.76 nm laser with a power of 495 mW and a line width of 3.2 kHz is obtained.The output characteristics of the ring cavity amplifier stage are measured and analyzed.When the seed light is not injected,a 1550.25 nm laser with a power of 476 mW and a beam quality of 2.66 is obtained.At this time,the laser is in a multi-longitudinal mode operation state.When seed light is injected,a single longitudinal mode 1549.85 nm laser output with a power of 468 mW and a line width of 3.5 kHz is obtained. |
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