Gain flattening filter, and method for manufacturing gain flattening filter

A method for manufacturing a gain flattening filter, the method comprising:

• fusing a pair of second optical fibers onto both ends of a first optical fiber in a length direction, the first optical fiber being formed with a slanted refractive index grating and including a core, a first cladding surrounding the core from outside in a radial direction and containing a photosensitive material whose refractive index increases upon irradiation with light having a specific wavelength, and a second cladding surrounding the first cladding from outside in the radial direction, and the pair of second optical fibers each having a mode field diameter smaller than a mode field diameter of the first optical fiber,
• wherein the fusing includes heating the first optical fiber and the pair of second optical fibers such that temperatures of the pair of second optical fibers are higher than temperatures of both the ends of the first optical fiber, and a tensile stress remaining in the core is released at both end portions of the first optical fiber to reduce average mode field diameters at both the end portions of the first optical fiber.