PRODUCTION OF OPTICAL FILTER AND OPTICAL FILTER PRODUCING APPARATUS

PROBLEM TO BE SOLVED: To make it possible to form a FBG (fiber Bragg grating) of a grating length longer than 100 mm. SOLUTION: A fiber holder 10 is a planar body to be installed with photosensitive fibers 20 and is formed with grooves 22 to be fitted with these photosensitive fibers 20 on its front surface 10a to form a spiral shape. A phase mask 12 is a planar body transparent to UV light. Plural pieces of recessed party are arrayed on its main surface 12a so as to have a spiral shape flush with the plane shape of the grooves. The main surface 12a thereof is disposed to face a fiber holder so as to be parallel with the front surface 10a. A rotary stage 14 rotates the fiber holder 10 relative to an axis 24 passing the center of the spiral shape. The photosensitive fibers are irradiated with the UV light via the phase mask. A light quantity control section selectively changes the quantity of the UV light cast to the photosensitive fibers according to the positions. The laser light source 30 is a light source to generate pulse light. A frequency modulation section 18 for controlling the pulse repeating frequency of the pulse light is disposed as the light quantity control section.