CONTINUOUSLY DISTRIBUTED PHASE-SHIFTS BY CHIRPED DISTRIBUTED-FEEDBACKGRATINGS FOR 1.55 MU-M DISTRIBUTED-FEEDBACK LASERS

DFB lasers with axially distributed phase shifts implemented by distri buted feedback (DFB) gratings with axially varied pitch lengths (chirp ed DFB gratings) have been investigated. It is shown that distributed phase shifts can be used to counteract Spatial hole burning and reduce the inhomogeneity of the axial photon-density distribution. Different total amounts of phase shift and axial extensions of the phase shift section have been compared by appropriate chirping functions, and also to the behaviour of abrupt phase shifts. It is shown that different d esign goals can be reached such as minimum threshold gain, reduced axi al photon field inhomogeneities, specific spectral positions of the os cillating mode or increased gain margin. The results are obtained for two different ways of implementing DFB gratings with axially varied pi tch length such as bent waveguides on homogeneous grating fields or fr actal gratings. For identical chirping functions, both implementation possibilities show Identical device properties if homogeneous coupling is assumed and bending losses remain negligible. Finally, the influen ce of facet phase statistics on the device yield is studied.