| 摘要 |
PROBLEM TO BE SOLVED: To solve problems of a conventionally known technique such that many layers are required to obtain an antireflection layer made of an optical multilayered film having, for example, <=0.2% reflectance in a wide wavelength range from 1.3μm to 1.55μm. SOLUTION: The optical multilayered film is designed under such conditions that the reflectance for light on the face where light enters and exits is minimum for at least for two wavelengths while the reflectance in the region between the above wavelengths is not specified. The antireflection layer with the minimum reflectance at 1300±50 nm as the first wavelength and at 1550±50 nm as the second wavelength according to the above conditions can be obtained from an optical multilayered film 20 having a four-layer structure.
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