Research On Beam Quality Characteristics Of Supercontinuum Source

To promote the engineering application of supercontinuum light sources,it is necessary to conduct in-depth research on its beam characteristics,clarify the influence of beam quality on the design of optical systems,and provide theoretical guidance for light source optimization and engineering applications.The thesis takes the supercontinuum light source generated by the Photonic Crystal Fiber（PCF）as the research object,theoretically analyzes and experimentally measures its beam quality after being output by a variety of pigtail fibers and transmitted by a fiber power combiner.The beam expansion effect of the supercontinuum light source when propagating through the atmosphere is numerically simulated,and the influence of atmospheric turbulence is analyzed.The main work of the paper is as follows:1.The beam far-field divergence angles of the supercontinuum output by various fibers are calculated and measured.The supercontinuum spectrum ranges from 0.5 to2.3μm,covers almost the entire supercontinuum that can be generated by quartz-based PCF.The calculation results show that the divergence angle of the supercontinuum directly output from the PCF increases monotonously with the increase of the wavelength;the divergence angle of the supercontinuum after the transmission through HI1060,SMF-28 and 10/130 fibers increases firstly and decreases then with the increase of the wavelength,but the turning point is different for the three kinds of fibers;for PCF and the above three types of fibers,the beam parameter product of the supercontinuum increases monotonously with the wavelength.Two cameras with different response bands are used to measure the divergence angle of the supercontinuum output from the fibers,the results are roughly consistent with the calculated results.The paper also measured the divergence angle of the supercontinuum transmitted through a variety of large core fibers.The results show that the far-field divergence angle has no obvious relationship with the wavelength,but changes within a certain range.2.The effect of 7x1 fiber power combiner on the beam quality of supercontinuum laser in a range of 0.5 to 2.3μm is numerically calculated and analyzed.The results show that as the length of the output fiber changes,the value of M²oscillates within a certain range for a specific wavelength.When the length of the output fiber is fixed,whether only one input fiber is injected with laser or all input fibers are injected with lasers,the M²of supercontinuum exhibits a nonlinear change with the increase of the wavelength,and continues to oscillate in the entire spectral range,but as a whole,the M²value first declines and then elevates with the wavelength increasing.The effect of changing the structure of the beam combiner on the quality of the transmitted supercontinuum beam is studied.The results show that changing the waist region length of the combiner can change the corresponding optimal working wavelength;In the case of injecting only the LP₀₁mode,within the core diameter range selected in the simulation,increase the core diameter of the input fiber can effectively improve the overall beam quality of the supercontinuum.In the subsequent verification experiment,for the case where the single arm of the beam combiner is injected with laser,the far-field divergence angle corresponding to multiple wavelengths of supercontinuum with the spectral range of 0.5 to 2.3μm after being transmitted through a 7x1 fiber power combiner was measured by the camera as the beam quality.The measured divergence angle first decreases but then increases with a prolonging wavelength,which is basically consistent with the calculated curve.3.Considering that the different spectral components of the supercontinuum source have different divergence angles,the beam transmission characteristics of the supercontinuum laser in atmospheric turbulence are studied.Numerical simulation of the influence of different intensity turbulence on the beam expansion under the Von Karman atmospheric power spectrum model shows that:the stronger the turbulence intensity,the more obvious the beam expansion effect of the supercontinuum laser.The short wavelength part of the supercontinuum is more likely affected by the turbulence than the long wave length part.The relationship between the elevation angle of supercontinuum laser transmission and the beam width is calculated,that is,as the angle increases,the spot size first decreases rapidly,then remains almost unchanged.The effect of turbulence on beam expansion in three cases are specifically analyzed,including vertical transmission,slant transmission and vertical transmission of supercontinuum laser.