Variable focal length lens

Claim 1

An optical system provided with a variable focal length lens and for being in an optical information readout apparatus, comprising: supplying means for supplying a light beam; a polarization beam splitter for directing a first light beam to an information recording plate, said first light beam being polarized in a first direction; variable focal length lens means disposed in a light path between said supplying means and said beam splitter, said lens means including means for rotating a polarization plane of light and a birefringent lens for providing different focal lengths according to the polarization plane of the light from said rotating means; control means for making the polarization plane of the light from said birefringent lens the same with that of said first light beam, irrespective of the state of the polarization plane of the light from said rotating means, and causing said first light beam to enter said beam splitter; a λ/4 plate disposed in a light path between said beam splitter and said information recording plate, said λ/4 plate converting said first polarized light beam from said beam splitter to circularly polarized beam, and said λ/4 plate causing said first light beam to enter said information recording plate, and said λ/4 plate converting a second light beam reflected by said information recording plate to a second light beam polarized in a second direction whose polarization plane is rotated by 90° relative of that of said first light beam and directing said second light beam to said beam splitter; a photodetector for receiving said second light beam from said beam splitter; and information processing means for converting a signal from said photoconductor to information.