Achromatic expansion prism for magneto-optical drive

An achromatic expansion prism in an optical disk drive wherein a light beam emitted from a light source is directed toward an information-containing surface of an optical disk and reflected therefrom to a detector, the prism comprising:

• a first prism with a first dispersion value, the light beam being incident on an entrance surface of the first prism at a first angle and exiting the first prism through an outlet surface;
• and
• a constant deviation prism fixed to the outlet surface and having a second dispersion value, the constant deviation prism including:
• a junction surface fixed to the outlet surface through which the light beam from the first prism is transmitted, the junction surface being coated with a semi-transparent reflective film for reflecting a portion of the light beam;
• a reflecting surface for reflecting the light beam entering the constant deviation prism toward the optical disk;
• a transmitting surface through which the light beam is transmitted from the reflecting surface to the optical disk in a first direction;
• and
• an exit surface through which the light beam reflected from the optical disk is transmitted to the detector in a second direction;
• wherein the angle between the first and second directions is constant irrespective of the first angle, and wherein the first and second dispersion values are selected such that the first and second directions are independent of wavelength of the light beam.

An optical system for a magneto-optical disk drive for reading and writing information on a magneto-optical disk, the optical system comprising:

• a laser producing a light beam for reflecting on a surface of the disk;
• a detector for detecting the light beam reflected from the disk;
• and
• an achromatic expansion prism disposed between the laser and the disk, the achromatic expansion prism comprising:
• a first prism with a first dispersion value, the light beam being incident from the laser on an entrance surface of the first prism at a first angle and exiting the first prism through an outlet surface;
• and
• a constant deviation prism fixed to the outlet surface and having a second dispersion value, the constant deviation prism receiving the light beam through the outlet surface, and reflecting the light beam in a first direction toward the disk and further reflecting the light beam in a second direction to the detector, the constant deviation prism employing a plurality of surfaces thereof to effect the reflecting, wherein the angle between the first and second directions is constant irrespective of the first angle, and wherein the first and second dispersion values are selected such that the first and second directions are independent of wavelength of the light beam.