Research Of Fiber Laser Polarization Compensation And Frequency-Doubling Technology Based On Axicon

The high power and efficiency fiber laser is leading a new wave of researches in fields of laser technology since the birth of semiconductor-laser pumping technology and it has been widely used in industrial processing,medical cosmetology,scientific research and other fields.In some specific fields such as coherent combination,nonlinear optics,fiber communication the linearly polarized laser with high extinction ratio is usually required.The linearly polarized fiber laser can meet this requirement and is very attractive but due to the complex manufacturing process of polarization-maintaining devices,high price,easily influenced by external disturbances and other limitations the development speed of the polarization-maintaining fiber laser has been much slower than that of the non-polarization-maintaining fiber laser.In accordance with this present situation this paper presents a terminal polarization-compensating system which realizes beam shaping using axicons and transform arbitrary polarization laser into linearly polarized laser using spatial coherent beam combining technology.The principle of the beam transformation in the process of the Gaussian beam passing through axicons is discussed and analyzed.And the numerical model is established.The simulation reveals that the system can transform the input naturally polarized laser into linearly polarized output laser without changing the input power and beam quality.On the basis the paper studies the working performance and double frequency efficiency of the system under different situations.The main contents of the paper are listed as followed:1.The paper reviews the history of the fiber laser's development,describes its current situation.The application value of the linearly polarized laser is also introduced.The power restrictions of the linearly polarized fiber lasers have been illustrated.The assumption is proposed that enabling non-polarization-maintaining fiber laser to output linearly polarized laser with high extinction ratio using the spatial polarization-compensating system and shows its importance in broadening the applications of the non-polarization-maintaining fiber lasers.Some methods that generate hollow beam and the applications of it are also briefly introduced.2.The beam transformation of the Gaussian beam passing through single axicon is theoretically discussed and simulated separately using the Huygens-Fresnel diffraction method and two-beam interference method.The beam transformation of the Gaussian beam passing through double axicons is also discussed and simulated by using the theory of light-field propagation.The paper analyses the influence of the distance between bottom surfaces of two axicons and the waist radius of the incident Gaussian beam on the near and far field intensity distribution of the transformed beam.The assumption and structure diagram is proposed.The working principle is analyzed and simulated.The feasibility of the system is verified theoretically.3.The feasibility of the polarization-compensating system is verified separately by three kinds of incident laser which are respectively low-power continuous laser,medium-power continuous laser and high-power pulsed laser.The near and far field intensity distribution,M~2 and far field divergence angle of the Gaussian beam,hollow beam and combined beam are measured using beam quality analyzer and intensity distribution analyzer.What is find is that the M~2 of the combined beam increases a lot but the far field divergence angle keeps well.The power-conversion efficiency of the system is measured and calculated.The reason behind the power loss is analyzed.At last the working stability of the system is simply tested.4.The principle of frequency doubling using nonlinear crystal and character of KTP crystal are briefly introduced.The linearly polarized laser outputted from the polarization-compensating system is used to carry out the one-way frequency-doubling experiment.Two lasers are used in the experiment.One has low average power and high peak power and the other has high average power and low peak power.The SHG efficiency under different power-levels is obtained and the reasons behind the difference between SHG efficiency using the two lasers are analyzed.The intensity distribution of the Gaussian beam,hollow beam and combined beam before and after the SHG experiment is measured and the scene drawing is obtained.The reason behind the difference between the intensity distribution before and after the SHG experiment and its possible causes are analyzed.