Two plate-like beam splitting device

An optical had comprising:

• a light emitting element for emitting a beam;
• a condensing element for condensing a beam onto an information recording medium and for transmitting the beam reflected by the information recording medium;
• a first plate beam-splitting element having a first optical element,
• said first beam-splitting element transmitting at least a part of a beam entering vertically from said light emitting element towards said condensing element,
• said first optical element having a splitting ratio for P polarization different from that for S polarization,
• said first optical element splitting at least a part of a beam reflected by the information recording medium and entering to said first optical element at a side thereof opposing said condensing element, the beam split by said first optical element propagating along a direction different from that of the beam entering said first optical element;
• a second plate beam-splitting element comprising a second optical element provided between said first plate beam-splitting element and said light emitting element in parallel to the first optical element of said first plate beam-splitting element,
• said second plate beam-splitting element being arranged so that the beam emitted by said light emitting element enters vertically to said second optical element,
• said second optical element splitting the beam having been reflected by the information recording medium and having been split by said first optical element along the direction different from that of the beam entering said first optical element into at least two beams having polarization planes perpendicular to each other;
• a pair of photodetectors for detecting beams provided at positions to detect the beams split by said second beam-splitting element.