| 摘要 |
PROBLEM TO BE SOLVED: To make possible thinning a middle wavelength area unusable for multi-separation of a wavelength multiplex optical signal by providing the area that a refractive index difference between a minimum refractive index and a maximum refractive index increases from its end part toward the central part on both end parts of a grating area. SOLUTION: The area that the refractive index difference between the minimum refractive index and the maximum refractive index in a periodical refractive index change increases from the end part of the grating area toward the central part is provided on both end parts of the grating area. In such a manner, by changing the refractive index difference of both end parts of the grating area in the longitudinal direction, the middle wavelength area is thinned. Further, by forming the changes of the refractive index differences of both end parts of the grating area symmetrically to the central part of the grating, an excellent reflectance characteristic is obtained. For instance, the area that the refractive index difference by 1000 layers much is changed linearly is provided on both end parts of the grating area. |
