US Patent 7385761 Diffraction grating device and optical apparatus

A diffraction grating device for splitting or coupling light beams permits the divergence of the light beams to be minimized easily. A first light beam is incident on a diffraction grating from the side thereof facing the inside of the device, and a second light beam is incident on the diffraction grating from the side thereof facing air. The diffraction grating transmits the first light beam by diffraction of the minus first order so that it travels in the reverse direction along the optical path of the second light beam before incidence, and transmits the second light beam by diffraction of the zero order. The second light beam, transmitted by diffraction of the zero order, spreads over a certain width of wavelengths, but does not diverge even after diffraction.