Patent ID: 7139126

Claim:
An optical element, comprising: a diffraction grating by which both of a first laser beam and a second laser beam are diffracted, wherein a fifth spherical aberration SA5 1 , a seventh spherical aberration SA7 1 , and a ninth spherical aberration SA9 1 of the first laser beam, and a fifth spherical aberration SA5 2 of the second laser beam satisfy in the following conditional expressions: 0.005<|SA5 1 −SA5 2 |<0.015 [λrms] 0.001<|SA5 1 |<0.015 [λrms] (SA5 1 ) 2 +(SA7 1 ) 2 +(SA9 1 ) 2 <0.0002 [λrms] 2 .