Phase-shifted distributed-feedback (PSDFB) lasers with multiple phase-shift regions are proposed to reduce the effect of spatial hole burning in conventional PSDFB lasers where a single phase shift leads to a highly nonuniform axial distribution of the mode intensity. We analyze multiple phase-shift PSDFB lasers by solving numerically the coupled-wave equations. Our results show that the use of even two phase-shift regions can substantially reduce the axial inhomogeneity of the mode intensity. Although the gain margin is generally reduced by the use of multiple phase shifts, it is still large enough for most practical purposes if the phase shifts are suitably optimized.

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The thermal conductivities of single-crystal Li, Na, K, and Ag P-alumina have been measured in the temperature range of = 0.1-300 K. The thermal conductivity is very sensitive to the cation present and is indicative of a broad band of excitations {&10 " J, 5 10" Hz) which scatter thermal phonons. The results are compared with a tunneling-states theory of disordered materials and with published data from specific-heat and light-scattering measurements.

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