Considering the difference in the divergence angle of different spectral components of supercontinuum laser, this paper researched the beam expansion effect of supercontinuum laser in atmospheric turbulence. The simulated laser transmission is carried out in the Von Karman model. The influence of turbulence of different intensities on the beam expansion is compared, the influence of turbulence on supercontinuum laser beam expansion with vertical, slant and horizontal transmission are also studied. The results show the beam expansion effect of strong turbulence on the short- wavelength part of supercontinuum laser is more obvious than that of moderate turbulence and weak turbulence. As the intensity of turbulence increases, the spot of the short-wavelength part increases rapidly first and then increases slowly and steadily, the spot of the long-wavelength part keeps almost unchanged at first, and also then increases slowly and steadily. The relationship between the elevation angle of laser beam emission and the beam width is calculated, and three cases are specifically analyzed. In the cases of vertical transmission and slant transmission, the beam width is related to the characteristics of the light source after collimation, which increases with the increase of the wavelength. When the laser is transmitted horizontally, the beam width has been greatly expanded, especially in the short-wavelength part. As the angle increases, the radius of the spot decreases rapidly first and then keeps almost unchanged. The radius of the spot of the short wavelength part reduces a larger proportion than that of the long-wavelength part.
We numerically calculated and analyzed the influence of a 7 × 1 fiber power combiner on supercontinuum (SC) laser beam quality in the range of 0.5 to 2.3 μm. The calculation results show the M2 oscillates within a range for a specific wavelength as the length of the output fiber changes. When the length of the output fiber is fixed, whether single input fiber or all fibers are injected with SC lasers, the M2 of the SC shows a nonlinear change and oscillates continuously throughout the entire spectrum, while it first decreases and then increases as the wavelength increases as a whole. Analysis of the numerical results shows that by changing the type of input fiber and the taper waist structure, the beam quality of both long-band and short-band parts of the SC laser can be improved. To verify theoretical calculations, the divergence angle of SC was measured as the beam quality after being transmitted by one 7 × 1 fiber power combiner in the spectral range from 0.5 to 2.3 μm. The divergence angle first decreases but then increases with a prolonging wavelength, and the curve of the experimental result renders a basic match with the calculation. To the best of our knowledge, currently no overall evaluation is reported on the beam quality of combined SC sources.
For the collimation and focusing of supercontinuum, the divergence angle is an important parameter. In order to obtain the far-field beam divergence angle corresponding to multiple wavelengths of the supercontinuum in the wide spectrum range of 0.5-2.3μm, two types of cameras are used to collect the far-field spot image. The range covers the supercontinuum that PCF can generate. The relationship between the beam quality/divergence angle and the wavelength is summarized. The experimental values are in good agreement with the theoretical values. To the best of our knowledge, it is the first report to measure the divergence angle of supercontinuum in such a wide range of wavelength. The divergence angle of the supercontinuum after transmission through a section of passive optical fiber with a core diameter of 10μm was also measured. The results show that the divergence angle of supercontinuum variation with wavelength exhibits different characteristics for the 10μm core fiber and the PCF.
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