Optical system for collimating elliptical light beam and optical device using the same

An optical device for reading/writing to an optical disk, comprising:

• an optical system comprising:
• a light source, for providing an elliptical light beam defining different diverging angles in different directions, wherein any cross-section of the elliptical light beam emitted from the light source defines a long axis and a short axis which are perpendicular to each other;
• a collimator, for converting the diverging elliptical light outputted from the light source 21 into a parallel elliptical light beam;
• and
• at least one prism, adapted for narrowing the long axis of the parallel elliptical light beam and remain the short axis of the parallel elliptical light beam unchanged, each of the at least one prism having a first surface and a second surface defining an acute angle, wherein the first surfaces is perpendicular to the parallel elliptical light beam, so as to allow the parallel elliptical light beam to perpendicularly pass through the first surface and reach the second surface;
• a semi-reflective mirror, for allowing light beams from a first direction to pass therethrough and for reflecting light beams from a second direction, the second direction being substantially perpendicular to the first direction;
• an object lens for focusing parallel light beams to a point on an optical disk;
• a collimator for collimating light beams passed therethrough;
• and
• an optoelectronic detector, for receiving a light beam, detecting information from the light beam, converting the information into electronic signals, and outputting the electronic signals.