Fiber grating and method for making the same
A femtosecond laser radiation or a picosecond laser radiation output from light source 6 is split into a first beam reflected to an angle of 90 degrees by a beam splitter 7 and a straightly advancing second beam cut out by the beam splitter 7. The first beam is reflected at an angle of 90 degrees by a second reflecting mirror 9, reflected again at an angle of 90 degrees by a fourth reflecting mirror 11, and is collimated by a second lens 15 to be irradiated onto an optical fiber core wire 13 to be written. The second beam is reflected at an angle of 90 degrees by a first reflection mirror 8, reflected again at an angle of 90 degrees by a second reflection mirror 10, and is collimated by a first lens 14 to be irradiated onto the optical fiber core wire 13 to be written. The femtosecond laser output from the light source 6 is split into two by the beam splitter 7, which interfere with each other in the vicinity of the core of the optical fiber core wire 13 to generate a change in the refractive indices of the glass depending on the light intensity distribution of the interference fringes, such that the gratings are written in the core 1.
1 . Fiber gratings characterized in that it comprises a core made from silica glass free of germanium and a cladding surrounding the outer periphery thereof, and that said core has gratings written in by irradiating femtosecond laser or picosecond laser radiations.
2 . Fiber gratings as claimed in claim 1 , which is characterized in that said core is made from a silica glass containing 100 to 1000 ppm of fluorine.
3 . Fiber gratings as claimed in claim 1 , which is characterized in that said cladding is made from a silica glass containing 1000 to 7000 ppm of fluorine, or a silica glass containing 2000 to 10000 ppm of boron.
4 . Fiber gratings as claimed in claim 1 , which is characterized in that said cladding is made from an ultraviolet-transmitting resin.
5 . Fiber gratings as claimed in claim 1 , which is characterized in that said cladding comprises plural hollow holes in parallel with the optical axis.
6 . Fiber gratings as claimed in claim 1 , which is characterized in that said cladding is provided with a protective coating layer on the outer periphery thereof.
7 . A method for fabricating fiber gratings, characterized in that it comprises irradiating an interference light generated by interfering two coherent femtosecond laser radiations or picosecond laser radiations to an optical fiber comprising a core made from silica glass free of germanium and a cladding surrounding the outer periphery thereof, thereby writing the gratings in said core.
8 . A method for fabricating fiber gratings as claimed in claim 7 , which is characterized in that a flat part is provided to the outer surface of said cladding, and that said interference light is irradiated to said flat part.
9 . A method for fabricating fiber gratings as claimed in claim 7 , which is characterized in that a protective coating layer is provided to the outer periphery of said cladding, and that said interference light is irradiated from the outer side of said protective coating layer.