Patent ID: 7184214

Claim:
A diffraction grating element, comprising: given first-third planes disposed parallel with each other in order, a first medium having an index of refraction n 1 provided at the outer side than the first plane being in contact with the first plane, a second medium having an index of refraction n 2 and a third medium having an index of refraction n 3 , n 3 <n 2 disposed alternately in a predetermined direction parallel with the first plane between the second plane and the third plane being in contact with the second plane and the third plane to constitute a diffraction grating, a fourth medium having an index of refraction n 4 provided at the outer side than the third plane being in contact with the third plane, and a fifth medium having an average index of refraction n 5 provided between the first plane and the second plane being in contact with the first plane and the second plane, wherein given that the average index of refraction between the second plane and the third plane is n av , the average index of refraction n 5 of the fifth medium satisfies a relational expression of “n 1 <n 5 <n av ” or “n av <n 5 <n 1 ”.