Photoinduced Modifications in Fiber Gratings Inscribed Directly by Infrared Femtosecond Irradiation

The structure of fiber Bragg gratings inscribed point-by-point by an infrared femtosecond laser is studied by quantitative phase microscopy. Results show that these gratings present a central region with a depressed refractive index surrounded by an outer corona with increased refractive index. The refractive index profile suggests the presence of microvoids embedded in a region of the core