Phase-shifted long-period gratings by refractive index-shifting

We show that phase shifts in long-period gratings can be obtained by means of refractive index-shifting whereby discontinuities in the differential effective mode index are locally created using, for example, thermally induced effects based on resistive heating. These phase shifts can be made tunable by controlling the amount of the discontinuity. As an additional example, we also illustrate the use of refractive index-shifted LPGs for flattening the spectral output from an Er-doped superfluorescent fiber source.