Gain-flattening filter design using rotationally symmetric crossed gratings

We present a method for designing gain-flattening filters using rotationally symmetric crossed gratings. This involves adjusting the fiber and grating parameters to obtain desirable coupling interactions between the forward-propagating core mode and the backward-propagating cladding modes. A chirped grating with an optimized envelope of the grating strength with axial position is then used to obtain an appropriate transmission spectrum. Here, we demonstrate that the gain spectrum of a typical erbium-doped fiber amplifier could be equalized using this method. The main advantages of this type of grating are its polarization insensitivity and low back reflection.