A method for forming a diffraction grating in an optical fiber. First and second ultrashort laser pulses are generated. The first and second ultrashort laser pulses are directed to intersect and form an interference pattern. The interference pattern includes periodic intensity variations which cause corresponding periodic changes in an index of refraction of the optical fiber. The ultrashort laser pulses and the optical fiber are configured to be self-focused by the cladding and core of the optical fiber.