Research on ultra-fast optical bistable devices with multi-dimentional quantum well structure

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We have investigated the following three topics as a basic research on ultra-fast optical bistable devices with a multi-dimentional quantum well structure.(1) Optical nonlinearities of Al_2O_3 glasses with CdS microcrystallitesWe fabricated Al_2O_3 glasses with CdS microcrystallites by means of the sol-gel method, and investigated their linear and nonlinear optical characteristics. The shift in the wavelength of the absorption edge through a quantum size effect was clearly observed for the CdS-doped Al_2O_3 prepared by changing the annealing temperature. A large nonlinear absorption and refractive index were obtained because a high CdS microcrystallite concentration was achieved through the sol-gel method.(2) Mode-locking of laser diodesTwo types of method for active mode locking of laser diodes (LDs) have been investigated. The first method is a pulse generation from optically coupled LDs. Two LDs, haveing an anti-reflection coating on one facet, were mutually optically coupled, and f … More orm an external cavity (cavity length L_<e x t>). By setting the phase difference in the moduration currents to pi rad (frequency- c/2L_<e x t>) injected into the two LDs, a pulse duration of as short as 7.2ps has been achieved with relatively high optical power (peak power - 170mW) from 1.3 mum InGaAsP/InP LDs. The second is a pulse generation from an external cavity LD by optical pulse pumping. An optical pulse train generated by a gain-switched LD was injected into a LD with an external cavity (slave LD) which was biased just below the laser threshold. The wavelength of the injected optical pulse was chosen to be slightly shorter than that of the slave LD. When the repetition rate of the optical injection pulse was nearly equal to the round trip time of the external cavity, the slave LD showed stable mode locking with the shorter pulse duration than that of pumping pulse.(3) Polarization-bistable laser diodesWe have proposed a new from of pitchfork-bifurcation-like polarization bistability in laser diodes (LD's) that has major speed advantages over conventional S-shaped polarization bistability. From the rate-equation analysis, it has been shown that flip-flop operation with bit rates of higher than 50Gbits/s may be achieved. Less