Optically reproducing and tracking system for spiral information track

An optical reproducing system for optically reproducing information stored in a record medium comprising:

• a source of monochromatic light having a wavelength .lambda.;
• a rotating disc-shaped record medium having a spiral information track formed of a series of pits, each of said pits having a depth of .lambda./4, and upon which said information is stored;
• a collecting lens disposed in a position for focussing said monochromatic light from said source onto said disc-shaped record medium;
• an optical detector means having a plurality of optical detection portions, each of said plurality of portions producing an electrical signal corresponding to the intensity of light falling thereon;
• said detector means disposed in a position whereby said collecting lens focusses said monochromatic light reflected from and diffracted by said spiral information track of said disc-shaped record medium onto said optical detector means in a diffraction pattern including a principal maximum due to a diffraction component of the zero order and secondary maxima on either side of said principal maximum due to diffraction components of the first order;
• a signal generator means connected to said optical detector means for generating both a reproduced signal corresponding to said information stored upon said spiral information track and a control signal indicative of the position of said focussed monochromatic light relative to said spiral information track;
• and
• a moving means connected to said source of monochromatic light, said collecting lens, said optical detector means and said signal generator means for moving said source of monochromatic light, said collecting lens and said optical detector means in response to said control signal, wherein said focussed monochromatic light follows said spiral information track of said disc-shaped record medium.