Misalignment of parabolic mirror in lightcraft vehicle

In lightcraft vehicle system, a ringy laser beam is focused by a 90 degree off-axis annular parabolic mirror. In this paper, methods are applied combining the geometrical optics and the physical optics. The effects of the angular misalignment of the optical axis and divergent angle on the focal power density are analyzed. Ray tracing shows that a pair of vertical focal lines is produced due to the angular misalignment. Basing on this, and from the theory of diffraction optics, we can know that the incident Gaussian beam will become the beam that is similar to the elliptical Gaussian mode for the misalignment. Moreover, the focal power density is decreased due to the angular misalignment. The parabolic mirror is very sensitive to misalignment of the incident beam, and the misalignment angle on the order of magnitude of mrad magnitude is enough to decrease peak power density down to the half. The tolerance of the misalignment angle down rapidly with the incident beam diameter increasing. When the incident angle has a divergent angle, coma is produced that also decrease the power density at focal plane.