FIBER GRATING

PROBLEM TO BE SOLVED: To provide a fiber grating having high reliability by enabling the easily writing of Bragg gratings without entailing the degradation in a transmission characteristic and in addition, enabling not only the stabilization of the transmission characteristic but the stabilization of a temp. characteristic. SOLUTION: A core which is codoped with a prescribed concn. of Sn and Al in addition to Ge of the ordinary concn. equal to the concn. thereof of the cores of the optical fibers to be connected is used as a core 2. A primary coating layer 4 is applied on the glass part F of the core and clad 3 by a UV transmission type resin which is cured by absorbing the UV rays of a wavelength region shorter than 240 nm or a wavelength region longer than 270 nm while allowing the transmission of UV rays of 240 to 270 nm. Writing of the Bragg gratings is executed on the core by irradiating the core with the UV rays from the outer side of the primary coating layer 4. A secondary coating layer 7 is applied on the primary coating layer 4 by a resin having a negative coefft. of linear expansion so as to negate to suppress the elongation and shrinkage occurring in the position coefft. of linear expansion of the glass part accompanying a temp. change.