| 摘要 |
Generally, in an optical system for recording and reproducing an optical information medium, a semi-conductor laser is used as a light source, an approximately parallel luminous flux is obtained by a collimator lens, and thereafter it is converged on a recording surface by an objective lens. If the semi-conductor laser causes a mode hop to abruptly change an oscillating wavelength, an error results when the objective lens has a chromatic aberration. A collimator lens with a reverse chromatic abberation can be used so that various conventional objective lens may be utilized as they are without any error as described.
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