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Accumulative roll bonding (ARB) is a kind of severe plastic deformation process which can produce high strength metals with ultra-fine grained microstructure. Mechanical properties will be increased due to the ultra-fine grain structures. In this study, the ARB process is used, the snap–stack working to reduplicate Al (ASM-1100)/Mg (AZ31) alloy which is chosen and then made thinner and longer by rolling and repeating the processes. Steps of 12 layers are created. The ARB process creates a multilayer compound between Al/Mg layers with excellent bonding characteristics and fine grained microstructure. The bonding condition became ascendant gradually and increased from one to three cycles. The grain sizes of Al and Mg alloys were 875 nm and 656 nm after three cycles. The hardness of the Al and Mg alloys were raised to HV42 and HV90 after three cycles.

Threshold current and quantum efficiency of the surface-emitting laser diodes were calculated on the basis of a planar carrier diffusion model with cylindrical boundary and analytical electromagnetic field distribution in either a uniform core waveguide or Vainshtein's open cavity model. Surface recombination at the boundary of the quantum-well active region was much greater than the bulk recombination in both structures unless cavity diameter was greater than 50 /spl mu/m. Diffraction loss was also significant in the open cavity structure as cavity diameter decreased. Reducing the surface recombination velocity leads to the reduction of threshold current and enhancement of differential quantum efficiency. In order to further reduce that current, using a buried heterostructure with a vertical waveguide structure is suggested.

Based on the open resonator theory, a model of a complex coupling coefficient suitable for the analysis of an ultrashort external-cavity laser diode is presented. It is shown that the effective reflectivity of the short external-cavity laser diode calculated by using this model is a function of the size of the laser-emitting area and is significantly influenced by the phase-modification factor as well as by the amplitude-reduction factor of the complex coupling coefficient. The calculated output power characteristics show good qualitative agreement with experimental observation. The necessary condition that the optical disk head using an external-cavity laser diode does not fail to detect the recorded signal for a broad range of flying height is also derived. >

Asymmetry in optical power versus external-cavity length characteristics has been observed in a short-external-cavity laser-diode configuration. It is shown that this phenomenon can be explained only by introducing a complex coupling coefficient, representing the coupled field amplitude into the laser diode at each reflection. This supports the open resonator model description of the short-external-cavity laser diode. Such an asymmetry effect should be recognized and properly dealt with in any laser-diode sensor. >

Under a high-gain operating condition, the presence of a multiplication process in the InGaAs(P) regions of an InP/InGaAsP/InGaAs avalanche photodiode (APD) having a structure of separated absorption and multiplication regions could lead to significant enhancement of the avalanche buildup time. As a result, the bandwidth of the device could be reduced considerably. The dependence of the avalanche multiplication factor and the intrinsic response time on the reverse bias voltage, the heterointerface field, the doping concentrations, and the width of the InP layer is examined in detail for the case in which hole injection is assumed. It is shown, for example, that for a fixed value of doping concentrations, reduction of the excess noise factor and enhancement of the gain-bandwidth product of the device can be achieved at the same time by a proper increase of the width of the InP layer. >