Continuously distributed phase shifts by chirped distributed-feedback gratings for 1.55 μm distributed-feedback lasers

DFB lasers with axially distributed phase shifts implemented by distributed feedback (DFB) gratings with axially varied pitch lengths (chirped 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 design goals can be reached such as minimum threshold gain, reduced axial photon field inhomogeneities, specific spectral positions of the oscillating mode or increased gain margin. The results are obtained for two different ways of implementing DFB gratings with axially varied pitch length such as bent waveguides on homogeneous grating fields or fractal gratings. For identical chirping functions, both implementation possibilities show identical device properties if homogeneous coupling is assumed and bending losses remain negligible. Finally, the influence of facet phase statistics on the device yield is studied