MASK FOR FORMING CHIRP GRATING AND FORMATION OF CHIRP GRATING

PROBLEM TO BE SOLVED: To make it possible to easily form chirped fiber grating having an arbitrary wavelength characteristic by using the same phase mask. SOLUTION: N pieces of grating stripe 53-1 to 53-N constituting a phase grating 52 of the mask are arrayed in such a manner that the X-direction spacings in a Y-direction top end side position are constant at a small set value and that the X-direction spacings in a Y-direction bottom end side position are constant at a large set value. The optical fibers 1 which are the objects for formation of the chirp gratings are extended in the X-direction and are intersected with the respective grating stripes. The optical fibers are so arranged as to pass the first grating stripe portion 54-1 between the first grating stripe 53-1 and second grating stripe 53-2 at the left end. Only the first grating stripe portions are irradiated with a UV laser beam. Next, the optical fibers are moved in parallel in the Y-direction and are so positioned as to pass the second grating stripe portion 54-2 between the second grating stripe portion 53-2 and the third grating stripe 53-3. Only the second grating stripe portion is irradiated with the UV laser beam.