Tham khảo tài liệu 'laser pulse phenomena and applications part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Laser Beam Diagnostics in a Spatial Domain 231 focusing optic having a focal length of m is assumed. The change in defocus shown in Fig. 14 was used in the calculations. The monochromatic electric field distributions in the focal plane were calculated in steps of 2 nm over the spectral range and a focus-shift technique was used to calculate the spatial and temporal intensity distributions at different positions. In Fig. 15 the x-axis of each plot means the time and the y-axis shows the vertical location at each position. Also in the figure by broadening the temporal duration of the focal spot the laser pulse having chromatic aberration showed a very different focusing property than the laser pulse without chromatic aberration. The calculations showed that the peak intensity of the focal spot with chromatic aberration was reduced to about 39 of that of the focal spot without chromatic aberration. The non-symmetric property of the intensity distributions against the focal plane originated from the non-symmetric profile of the laser spectrum. Moreover with the chromatic aberration a double-peak structure was observed in several positions near the focal plane. However the double-peak structure disappeared recovering to the original pulse duration when the laser pulse propagated a long distance from the focal plane. Thus the Fourier transform approach seems to be very useful in calculating the intensity distributions near the focal plane and finally determining the beam propagation properties in both cw and pulsed lasers. Position z from mm mm 0 mm the focal plane mm mm Without Chromatic Aberration With Chromatic Aberration Fig. 15. Spatial and temporal intensity distributions of focal spots at different positions near the focal plane. 4. Beam quality factor Definition of beam radius The propagation of the laser beam can be partially characterized by the beam width and the divergence. In other words defining the beam quality factor is closely .
