| 摘要 |
A holographic head including a collimating element for receiving a coherent beam of radiation non-circular in cross section, an objective element for receiving a radiation beam and focusing it to a spot, and a holographic unit also disposed in the path of the beam. The holographic unit includes a first hologram element for passing the beam at a predetermined diffractive order and sign. The holographic unit further includes a second hologram element inclined at an angle to the first hologram element for providing a beam substantially circular in cross section and at the same diffractive order and opposite sign to compensate for dispersion. The first and second holograms may additionally compensate for collimating or objective element aberrations, respectively.
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