Optical reproducing apparatus

Claim 1

An optical reproducing apparatus, comprising: a dual grating provided with a substrate, a first diffraction grating on one surface of said substrate and a second diffraction grating on the other surface of said substrate, said first grating receiving an incident light reflected from an optical recording medium and separating said incident light into a first transmitted light and a first diffracted light, said second grating receiving said first transmitted light and said first diffracted light through said substrate and discharging a second transmitted light and a second diffracted light; means for detecting a tracking error signal by receiving one of said second transmitted light and said second diffracted light; and means for detecting a focusing error signal by receiving the other of said second transmitted light and said second diffracted light, wherein said tracking error signal detecting means and said focusing error signal detecting means are disposed approximately in a same plane, pitch distributions of said first grating and said second grating being prescribed by the following inequalities (1) and (2): ##EQU10## where dx: x-direction pitch of said first grating, dy: y-direction pitch of said first grating, dx': x-direction pitch of said second grating, dy': y-direction pitch of said second grating, t: distance between said first grating and said second grating, l 0 : x-direction vector of said incident light, m 0 : y-direction vector of said incident light, n 0 : z-direction vector of said incident light, λ: wavelength of said incident light, Δλ: amount of wavelength fluctuation, T: z-direction distance between said second grating and said plane on which said detectors are disposed, and 2μ m: allowable wavelength fluctuation limit of a light spot on said plane on which said detectors are disposed.