Beam diameter expansion of a multi-beam optical information system

A plural-beam optical head comprising:

• a monolithic semiconductor laser array having a plurality of light emitting points arranged in a predetermined direction
• a collimator lens for making parallel a plurality of laser beams emitted from said plurality of light emitting points, each laser beam having a diameter;
• an objective lens for imaging said plurality of parallel laser beams on an optical information recording medium;
• and
• means, disposed between said collimator lens and said objective lens, for expanding the diameter of each of said plurality of parallel laser beams in the predetermined direction of said plurality of light emitting points;
• wherein the following condition is satisfied:
• (d.multidot.f.sub.0)/(f.sub.col .multidot.m)<20 .mu.m,
• where d is the maximum spacing between adjacent ones of said plurality of light emitting points, f.sub.col is the focal length of said collimator lens, f.sub.0 is the focal length of said objective lens, and m is the expansion magnification of said means for expanding the diameters of said laser beams.