Fabrication of long-period fiber gratings with no harmonics

We demonstrate a method to fabricate spectrally clean long-period gratings in optical fibers. Because the photosensitivity of glass is nonlinear, even a sinusoidal light pattern will generate harmonic gratings, and these higher order gratings can cause unwanted resonances. We solve this problem by first measuring the photosensitivity of the glass and then scanning the fiber across an ultraviolet light beam at a variable speed under computer control. The resulting spectra are free from extraneous resonances caused by harmonics of the fundamental grating period.