Apodized fiber gratings for DWDM using variable efficiency phase masks

Of the various applications of fiber Bragg gratings, their use as spectral filters is perhaps the most common. While the most important reasons why Bragg gratings are preferred over interference filters are cited as low insertion loss and polarization insensitivity, an often overlooked, but very important feature of Bragg gratings is the control over filter profile. Bragg gratings can easily be designed to have high or low reflectance, narrow or wide bandwidth, and none or any chirp. All these features make Bragg gratings a very attractive filter for DWDM applications such as channel multiplexing/demultiplexing or channel add/drop. While most requirements for DWDM applications such as low PDL, low insertion loss and low PMD are usually met by Bragg gratings, high channel isolation due to narrow bandwidth gratings is still a topic of investigation. The lower limit on the bandwidth is usually imposed by the existence of side lobes found in any grating filter. While it is well known that narrow bandwidth gratings can be achieved simply by apodizing the grating, a simple apodization yields a self induced chirp leading to resonance on the blue side of the filter. Strasser et al. (1996) reported that such effects can be eliminated by raising the average index of the grating such that it is constant along the grating. This phenomenon is illustrated using simulation tools.