Gain flattening fibre filters using phase-shifted long period fibre gratings

The transmission characteristics of phase-shifted long period gratings (LPGs) are examined theoretically by a combination of coupled-mode theory and transfer matrix formulation. It is shown that the transmission spectrum of such a grating can be significantly controlled by the phase shift which allows the shape of the spectrum to be tailored to meet the requirements of many kinds of gain flattening. By choosing the phase shift and the index change appropriately, a phase-shifted LPG is designed to flatten the spectrum of an Er:silica superfluorescent fibre source.