| 摘要 |
PURPOSE:To improve the reproduction level of a track signal by interposing a lens which forms the beam shaping optical system between a coupling lens for luminous flux emitted by a semiconductor laser and an objective lens, and making a spot circular. CONSTITUTION:Light projected on a cross section theta' parallel to the faying surface of the semiconductor laser is passed through the coupling lens 2 to obtain a parallel beam phi', which is diverged by a concave lens 9 and made into a parallel light beam B' through a convex lens 10. Then, a nearly circular spot is formed on an optical recording disk 6 through the objective lens 3. Light from a surface theta'' perpendicular to the faying surface of the semiconductor laser 1 has no power, so a parallel beam is obtained between the lenses 2 and 5. The power of the surface theta'' is within the range shown by an expression. In the expression, M is the power of the lenses 9 and 11 and fc and f0 are focal lengths of the lenses 2 and 3; and theta'' is the angle of divergence of the surface theta'' and NA0 is the numerical aperture of the lens 3. Consequently, the spot is corrected into a circle and the reproduced signal from the track signal is improved.
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