Patent ID: 7542398

Claim:
An aberration correcting element disposed on an optical path within anoptical head and carrying out at least one of recording and reproduction of an information signal with respect to an optical recording medium including a light transmission layer on a recording layer where the information signal is recorded, the aberration correcting element comprising: a refractive index adjustment unit configured to adjust refractive index on the basis of applied voltage with an electrode for applying the voltage; electrodes, which make a pair, one at each side of the refractive index adjustment unit, wherein a transmitted beam is caused to generate phase distribution represented by the following phase distribution formula: phase distribution(r)=A(−r 4 )−B(−r 2 ) for each of the radius r, phase distribution corresponding to the A(−r4) term in the phase distribution formula is generated by one electrode, and phase distribution corresponding to the B(−r 2 ) term in the phase distribution formula is generated by the other electrode, when radius of beam spot of a beam converged onto the recording layer is assumed to be r, and variables different from each other are assumed to be A and B.