| 摘要 |
PROBLEM TO BE SOLVED: To prevent contamination by facilitating replacement of an atmosphere around an optical element. SOLUTION: An antireflection film 22 comprising a film of magnesium fluoride (MgF2) is formed on the surface of a BO lens and six linear grooves each having a width of 0.1 mm and a depth of about 0.5μm which is about equal to the depth of a step in a BO element are provided radially from the center to the outer periphery of the BO lens 1. A separately prepared shielding substrate 26 covered with the antireflection film 22 on each side is fitted over the BO lens. A ring-shaped sealant 27 is interposed between the substrates 11, 26 and is bonded to an outer peripheral portion of each substrate 11, 26 which is 10 mm wide. The sealant 27 is provided with an opening 28 for evacuation and gas replacement of the closed space obtained. |
