| 摘要 |
An optical head device having a broadband phase plate of transforming at least three linearly polarized light beams having different wavelengths into circularly polarized light beams, is obtained. A broadband phase plate comprising two phase plates 9A and 9B laminated with e.g. an adhesive agent so that their optical axes are crossed, wherein provided that wavelengths of linearly polarized incident light beams are λ1, λ2 and λ3 (λ1<λ2<λ3), at least one or both of the two phase plates has such a phase difference characteristics that a ratio R(λ1)/R(λ3) and a ratio R(λ2)/R(λ3) between retardation values of the wavelengths are smaller than 1 and R(λ1)/R(λ3) is smaller than R(λ2)/R(λ3), is integrated in the optical head device.
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