Condensing optical system with corrected chromatic aberration, and information recording/reproducing apparatus using this optical system

Claim 1

An optical information recording/reproducing apparatus comprising: means for directing a radiation beam to a recording medium, wherein said directing means includes a collimator lens for forming a parallel beam, an objective lens for receiving the parallel beam to focus it on the recording medium and a beam splitter disposed in a path of the parallel beam between said collimator lens and said objective lens; and means for detecting a beam from the recording medium, said means including a photodetector means and a condensing optical system for condensing the beam from the recording medium onto said photodetector means, and said condensing optical system is so arranged as to receive a beam reflected by the recording medium, and made generally parallel by said objective lens through said beam splitter said condensing optical system including a first lens group and a second lens group, said first lens group having a positive refracting power ψ p , where ψ p =1/(f p /F)=F/f p and f p is a focal length of a first group positive lens, and being arranged at a side of the recording medium and said second lens group having a negative refracting power ψ n , where ψ n =1/(fn/F)=F/fn and fn is a focal length of a second group negative lens, and being arranged at a side of said photodetector means, wherein the following relation is met: ##EQU3## where F is a focal length of said condensing optical system, ν p is Abbe's number of said first lens group and ν n is Abbe's number of said second lens group.