The Distributed feedback (DFB) semiconductor lasers are widely used in coherent, high speed, long distance optical communications systems due to their attractive properties such as wavelength stability and narrow spectral width. In such systems, external optical feedback happens when a small fraction of the laser output re-enters into the laser cavity from an optical component such as the edge of optical fiber. It is well-established that laser operation could be affected negatively by a small amount of optical feedback. Never theless, semiconductor lasers with optical feed back generate a chaotic output which has been used for many important practical applications like secure communication systems and random number generation. Recently, laser designer emphasis on designing semiconductor lasers with high tolerance to optical feedback.