| 摘要 |
PROBLEM TO BE SOLVED: To provide a beam splitter that exhibits high reflectance to a certain wavelength and very high transmittance to a wavelength to be transmitted highly, and has a small light loss. SOLUTION: A coating film Fb that selectively exhibits high reflectance to a certain wavelength (2ωwave) is formed on one surface 11b of a parallel flat plate 111, and a coating film Fa which reflects a wavelength (ωwave) to be transmitted highly is formed on the other surface 11a of the plate 111. Then, very high transmittance is realized by setting the reflectance of the surface 11a to a value which is nearly equal to that of the surface 11b and constituting the plate 111 carrying the coating films Fb and Fa to function as an etalon to the wavelength (ωwave) to be transmitted highly.
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