Research On High Power Narrow Linewidth Fiber Laser For Long-range Coherent Detection

High-power narrow-linewidth fiber lasers are widely used in long-range coherent detection fields such as synthetic aperture lidar,coherent laser ranging,coherent laser velocity measurement,and fiber sensing because of their high output power and narrow spectral linewidth.The 1.5 ?m band is the eye-safe band,one of the atmospheric windows,and can obtain a relatively large field of view when imaging with synthetic aperture radar.Therefore,the 1.5?m high-power narrowlinewidth fiber laser has become a research hotspot in the field of fiber lasers.In the application of long-distance coherent detection,the output power and laser linewidth of the laser determine the coherent detection distance and measurement resolution of the system,so how to achieve high-power narrow linewidth laser output has important research significance and value.In addition,due to the influence of laser frequency stability and noise,even in the range of narrow linewidth laser coherent length,it is not necessarily possible to obtain ideal high-quality coherent imaging.Therefore,it is of great significance to study the influence of laser signal frequency stability and noise on coherent detection resolution to improve laser performance and coherent detection resolution.This paper focuses on the research of high power narrow linewidth laser,including the accurate measurement of narrow linewidth laser linewidth,the influence of amplified spontaneous emission(ASE)in fiber amplification on laser linewidth,and the amplification of narrow linewidth laser high power fiber.On this basis,combined with the application of high power narrow linewidth laser,the effects of frequency stability and noise on imaging resolution in coherent detection are studied.The specific research contents are as follows:1.The basic principle of measuring laser linewidth with time-delay auto heterodyne method is introduced,and numerical calculations are carried out.The limitations of the delay self-heterodyne method in the measurement of narrow linewidth laser linewidth and the reasons for the inaccuracy of laser linewidth measurement below k Hz are studied.On this basis,a laser linewidth measurement method based on long-fiber measurement data numerical calculation combined with short-delay fiber demodulation is proposed.This method can effectively solve the Gaussian broadening of the center frequency of the photocurrent power spectrum during delay self-heterodyne linewidth measurement.The laser with linewidth below k Hz can be measured accurately.2.The influence of Amplified spontaneous emission(ASE)on the laser during fiber amplification is studied.It is found that the increase of ASE in the process of fiber amplification will lead to the broadening of laser linewidth measurement results.The action mechanism of ASE on laser is analyzed,the corresponding theoretical model is established for numerical calculation,and the correctness of the theory is verified by experiments.The results show that ASE is superimposed on the laser signal in the form of noise,which makes the measurement result of laser linewidth broaden.3.A high-power narrow-linewidth fiber laser based on the main oscillation power amplification(MOPA)technology was developed.During the amplification process,the ASE is suppressed and the threshold of Stimulated Brillouin scattering(SBS)is increased.A 1560 nm high power narrow linewidth continuous laser with 51.3 W,0.865 k Hz linewidth and polarization extinction ratio greater than 20 d B is obtained.4.The effects of frequency stability and noise of narrow linewidth laser on imaging resolution are studied.A mathematical model of laser light field including time-varying center frequency,random frequency and random phase is established,and its influence on the lateral resolution of coherent imaging is analyzed.It is found that imaging the target object in the coherent length range,due to the change of center frequency,random phase and random frequency,will make the resolution of imaging result worse,false target appears,and even can not be detected.When the center frequency offset is 15 k Hz,the time-varying frequency is 10 k Hz,the standard deviation of random frequency is 10 k Hz,and the standard deviation of random phase is 0.1 rad,with the increase of detection distance from 0.1 km to 40 km,the interference spectrum width of echo and reference light increases from 0.0129 k Hz to 4.2194 k Hz.When the laser wavelength is 1.5 ?m and the transverse velocity of the target is 100 m/s,the corresponding lateral resolution is reduced from 96.75 ?m to 1.27 m.