Patent ID: 7019904

Claim:
A diffraction grating element, comprising: given first-fourth 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 fourth plane being in contact with the fourth plane, 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, and a sixth medium having an average index of refraction n 6 provided between the third plane and the fourth plane being in contact with the third plane and the fourth plane, wherein given that an 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 ”, and the average index of refraction n 6 of the sixth medium satisfies a relational expression of “n 4 <n 6 <n av ” or “n av <n 6 <n 4 ”.