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 high er order gratings can cause unwanted resonances. We solve this problem by f irst measuring the photosensitivity of the glass and then scanning the fibe r across an ultraviolet light beam at a variable speed under computer contr ol. The resulting spectra are free from extraneous resonances caused by har monics of the fundamental grating period.